Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS UTILIZING NMR MEASUREMENTS TO GATHER
INFORMATION ON A PROPERTY OF THE EARTH FORMATION SURROUNDING
A WELLBORE
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to a method and apparatus for
conducting drilling operations in an earth formation and, in the alternative,
for gathering
information on the properties or characteristics of the earth formation
surrounding a wellbore.
More particularly, the present invention relates to such an apparatus and
method that utilizes
nuclear magnetic resonance (NMR) measurements to determine, evaluate, predict,
or otherwise
gather certain properties of the earth formation.
[0002] In one preferred application of the invention described herein,
information
on the pore pressure in the formation surrounding the wellbore is derived from
NMR
measurements. Such pore pressure formation can play an important role in the
progress of the
drilling operation. For example, knowledge of the behavior of the pore
pressure within the
formation can help in optimizing the type and composition of the drilling
fluids used (more
commonly referred to as "mud" or "mud system"), particularly the fluid density
("mud weight").
Specifically, it is important during the drilling operation to avoid a large
pressure differential
between the mud column and the formation fluids. Excessive pressure in the mud
column can
lead to undesirable fracturing of the formation and to substantial loss of the
drilling fluid.
Reduced pressure in the mud column can, on the other hand, cause formation
fluid to enter and
disrupt the mud system. Both scenarios can lead to even more undesirable
consequences if the
formation fluids reach the surface in an uncontrolled manner, commonly known
as a "blowout."
[0003] Several techniques have been employed to estimate the pore pressure in
the
formation, but with varying degrees of success. For example, sonic and seismic
measurements
may be employed to deliver information on the pore pressure based on the
principle that the
speed of sound in a fluid increases with increasing pressure. Yet another
method of estimating
pore pressure is to measure the surface pump pressure and mud volume at
different pressures. In
any event, there has been no attempt or suggestion to use NMR measurement
techniques to
gather information on the pore pressure within the earth formation surrounding
a wellbore.
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[0004] It is known, however, that nuclear magnetic resonance (NMR)
measurements taken in a wellbore can provide different types of information
about a geological
formation. In the past, such measurements often were made after the wellbore
had been drilled.
Today it is possible to log NMR measurements while drilling (i.e., logging
while drilling or
LWD), thus saving time and providing valuable real-time information about the
earth formation
as drilling progresses. For example, such information can indicate the
fractional volume of pore
space, the fractional volume of mobile fluid, the total porosity of the
formation, permeability of
the formation, etc.
[0005] Several types of commercially available logging tools are employed to
perform the NMR measurements. These tools generally include one or more large
permanent
magnets or electromagnets for generating a static magnetic field, Bo, an
antenna placed
proximate the formation to be analyzed, and circuitry adapted to conduct a
sequence of RF
power pulses through the antenna to induce an RF magnetic field, 131, in the
formation. The
circuitry also includes a receiver adapted to detect signals induced in the
antenna as a result of
the RF pulse sequence. The induced signals can then be measured and processed
to provide the
desired information about the properties of the formation.
[0006] Typically, NMR logging tools are tuned to detect hydrogen resonance
signals (e.g., from either water or hydrocarbons) because hydrogen nuclei are
the most abundant
and easily detectable. In general, measurements of NMR related phenomena of
hydrogen nuclei
in the earth formation are performed by allowing some time for the static
magnetic field, Bo, to
polarize the spinning hydrogen nuclei of water and hydrocarbons in a direction
substantially
aligned with Bo. Then angle between the nuclear magnetization and the static
magnetic field, Bo
can be changed by applying a sequence of RF pulses to induce the RF field B1.
Commonly, the
pulse sequence employed includes a first RF pulse (i.e., the excitation pulse)
having a magnitude
and duration selected to re-orient the nuclear magnetization by about 90
degrees from the
orientation attained as a result of Bo (i.e., the initial transverse
magnetization). After a selected
time, a train of successive RF pulses is applied (i.e., inversion or
refocusing pulses), each of
which has a magnitude and a direction selected to re-orient the nuclear spin
axes by about 180
degrees from their immediately previous orientations. The frequency of the RF
field needed to
re-orient the nuclear magnetisation (i.e., the Larmor frequency) is related to
the amplitude of the
static magnetic field Bo by the gyromagnetic ratio y, which is unique to each
isotope.
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f
3
[0007] Due to inhomogeneities in the magnetic field B0, the spins in the
perpendicular plane (x,y-plane) typically lose their phase coherence rapidly
leading to a rapid
signal decay. After each of the 180 degree RF pulses the spins are re-oriented
in a way such that
they re-gain their phase coherence leading to the re-appearance of a signal-
the spin echo.
Measurement of the rate at which the spin echoes decay (i.e., the rate at
which the spinning
nuclei lose their alignment within the transverse plane) is referred to as a
relaxation, or T2
measurement. As is known in the art, the T2 measurement may be related to the
chemical and
physical properties of the earth formation. For example, hydrogen nuclei in
viscous oils have
relatively short relaxation times, whereas hydrogen nuclei in light oils have
relatively long
relaxation times. Similarly, hydrogen nuclei in free water typically have
longer relaxation times
than those in bound water (e.g., clay-bound water).
SUMMARY OF INVENTION
[0008] In one aspect of the invention, a method is provided for gathering
information on the pore pressure in an earth formation surrounding a wellbore.
The method
includes the initial steps of selecting at least one suitable property (e.g.,
porosity, permeability,
hydrogen index, drilling fluid composition, etc.) of the drilling environment
(which is defined by
the wellbore and the surrounding formation) and at least one NMR parameter
(e.g., T2
distribution) in an NMR measurement response. A suitable property is selected
for which values
over a plurality of wellbore depths can be correlated with the characteristics
or behavior of the
pore pressure in the earth formation. The method further includes conducting
an NMR
measurement at a plurality of wellbore depths, thereby generating an NMR
response from the
drilling environment. The measured values of the NMR parameter in the NMR
response are then
correlated with values of the suitable property. Next, the values of the
suitable property are
compared over the plurality of depths, and then the correspondence between the
property values
is correlated with the behavior of the pore pressure over the plurality of
depths. In this way, the
characteristics of the pore pressure in the earth formation over the plurality
of wellbore depths
are determined. In a variation of the inventive method, the values of the
selected NMR
parameter over a plurality of depths are also correlated with values of the
suitable property (to
first determine the behavior of the suitable property) and then the behavior
of the suitable
property is correlated with the behavior of the pore pressure.
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[0009] In another aspect of the invention, the inventive method includes the
initial
steps of selecting at least one suitable property of the drilling environment,
whereby variations in
the suitable property over a wellbore depth interval can be correlated with
variations in the pore
pressure in the earth formation, and predicting a profile of the suitable
property over a wellbore
depth interval (e.g., through historical information or preliminary
measurements). Further, at
least one NMR parameter is selected, such that values of the NMR parameter
over the depth
interval can be correlated with values of the suitable property over the depth
interval. After
providing an NMR measurement apparatus, the drilling operation may then
commence so as to
initiate forming of the wellbore.
[0010] During drilling, the NMR measurement apparatus is operated at a depth
in
the wellbore to generate an NMR response from the drilling environment and to
account for the
NMR parameter in the NMR response. By repeating this procedure at a plurality
of wellbore
depths and providing values of the NMR parameter at these depths, an actual
profile of the
suitable property is established. Deviations of the actual profile from the
predicted profile may
then be correlated with variations in the pore pressure in the earth
formation.
[0011] In the above method, the suitable property selected may be, among other
things, porosity, permeability, hydrogen index, a drilling fluid property such
as composition, a
formation fluid property such as composition, or combinations of these. In one
specific
application, the suitable property selected is depth of fluid invasion, and
the NMR measurements
are directed to a near-wellbore region of the drilling environment.
[0012] In yet another aspect of the invention, the invention is directed to a
method
of drilling a wellbore in an earth formation. The method includes the steps of
commencing
drilling of the wellbore in the earth formation, using drilling fluid having a
fluid composition,
and while drilling, monitoring the pore pressure in the earth formation
surrounding the wellbore.
The monitoring step further involves selecting at least one suitable property
of the drilling
environment such that variations in the suitable property over wellbore depths
can be correlated
with variations in pore pressure in the earth formation. Then, NMR
measurements are obtained
at a plurality of wellbore depths, thereby generating an NMR response from the
surrounding
wellbore over the plurality of wellbore depths. From the NMR response, the
behavior of the
suitable property over the wellbore depths is determined and then, the
behavior of the suitable
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property over the wellbore depths is correlated with the
behaviour of the pore pressure in the earth formation.
[0013] Other aspects of the invention are
described in the Detailed Description, or specified in the
appended claims. For example, some embodiments of the
invention are also directed to a system and a tangible
medium suitable for use, or at least associated with, the
methods described above.
[0013a] According to one particular aspect of
the invention, there is provided a method of gathering
information on the pore pressure in an earth formation
surrounding a wellbore, the wellbore and the surrounding
earth formation defining a drilling environment, said method
comprising the steps of: (a) selecting at least one suitable
property of the drilling environment, whereby values of the
suitable property over a plurality of wellbore depths can be
correlated with the characteristics of the pore pressure in
the earth formation; (b) selecting at least one NMR
parameter of an NMR measurement response; (c) commencing a
drilling operation and conducting an NMR measurement at a
plurality of wellbore depths, thereby generating an NMR
response from the drilling environment; (d) correlating
values of the NMR parameter in the NMR response with values
of the suitable property; and (e) determining
characteristics of the pore pressure in the earth formation
over the plurality of wellbore depths by comparing values of
the suitable property over the plurality of depths, and
correlating the correspondence between the values with the
characteristics of the pore pressure over the plurality of
depths, including evaluating the drilling operation based on
the characteristics of the pore pressure.
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[0013b] A further aspect of the invention
provides a method of gathering information on the pore
pressure in an earth formation surrounding a wellbore, the
wellbore and the surrounding earth formation defining a
drilling environment, said method comprising the steps of:
(a) selecting at least one suitable property of the drilling
environment, whereby variations in the suitable property
over a wellbore depth interval can be correlated with
variations in the pore pressure in the earth formation;
(b) predicting a profile of the suitable property over a
wellbore depth interval; (c) selecting at least one NMR
parameter, such that values of the NMR parameter over the
depth interval can be correlated with values of the suitable
property over the depth interval; (d) providing an NMR
measurement apparatus; (e) commencing a drilling operation
so as to initiate forming of the wellbore; (f) during
drilling, lowering the NMR measurement apparatus to a depth
in the wellbore and operating the NMR measurement apparatus
to generate an NMR response from the drilling environment;
(g) accounting for the NMR parameter in the NMR response;
(h) repeating steps (f) and (g) at a plurality of wellbore
depths to provide values of the NMR parameter and
establishing therefrom, an actual profile of the suitable
property; and (i) correlating deviations of the actual
profile from the predicted profile with variations in the
pore pressure in the earth formation.
[0013c] There is also provided a method of
drilling a wellbore in an earth formation, the wellbore and
the surrounding earth formation defining a drilling
environment, the method comprising the steps of:
(a) commencing drilling of the wellbore in the earth
formation, using drilling fluid having a fluid composition;
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and (b) while drilling, monitoring the pore pressure in the
earth formation surrounding the wellbore, including
selecting at least one suitable property of the drilling
environment, such that variations in the suitable property
over wellbore depths can be correlated with variations in
pore pressure in the earth formation; obtaining NMR
measurements at a plurality of wellbore depths, thereby
obtaining an NMR response from the surrounding wellbore over
the plurality of wellbore depths; from the NMR response,
determining the behavior of the suitable property over the
wellbore depths; correlating the behavior of the suitable
property over the wellbore depths with the behavior of the
pore pressure in the earth formation; and further
comprising, after step (c), the step of adjusting the
drilling operation in response to variations in the pore
pressure as determined in step (b).
[0013d] In accordance with a still further
aspect of the invention, there is provided a method of
drilling a wellbore in an earth formation, the wellbore and
the surrounding earth formation defining a drilling
environment, the method comprising the steps of:
(a) selecting at least one suitable property of the drilling
environment, the suitable property being a property of
drilling fluid provided in a near-wellbore region of the
drilling environment during drilling, whereby variations in
the suitable property over a wellbore depth interval can be
correlated with variations in the pore pressure in the earth
formation; (b) selecting at least one NMR parameter, whereby
variations in the NMR parameter over the depth interval can
be correlated with variations in the suitable property;
(c) commencing a drilling operation so as to initiate
forming of the wellbore; (d) during drilling, conducting NMR
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measurements at a plurality of wellbore depths to generate
an NMR response from the near-wellbore region;
(e) correlating values of the NMR parameter in the NMR
response with values of the suitable property; and
(f) determining the behavior of the pore pressure in the
earth formation by comparing values of the suitable property
over the depth interval.
[0013e] According to another aspect of the
invention, there is provided a system for gathering
information on the pore pressure in an earth formation
surrounding a wellbore, the wellbore and the surrounding
earth formation defining a drilling environment, the system
comprising: an NMR measurement apparatus for conducting an
NMR measurement at a plurality of wellbore depths and for
receiving an NMR response from the near wellbore region of
the drilling environment; from the values of the NMR
parameter over the plurality of depths, determining the
values of the suitable property over the plurality of
depths; a microprocessor disposed in communication with the
NMR measurement apparatus so as to receive NMR response data
therefrom, the microprocessor including an executable
program configured for: selecting a suitable property of the
drilling fluid in the drilling environment, such that values
of the suitable property over a plurality of wellbore depths
can be correlated with the characteristics of the pore
pressure in the earth formation; receiving measured values
of the at least one NMR parameter in the NMR response over
the plurality of wellbore depths; and a tangible medium for
tracking the values of the suitable property over the
plurality of wellbore depths, include displaying variations
in the values of the suitable property and indicating
variations in the pore pressure at wellbore depths
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corresponding to the displayed variations in the suitable
property.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] For a more complete understanding of
embodiments of the present invention, reference is now made
to the following descriptions taken in conjunction with the
accompanying drawings, in which:
[0015] FIG. 1 is a simplified schematic of a
wellbore and a system for conducting NMR measurements in the
wellbore;
[0016] FIG. 2 is a simplified circuit diagram
for use with the system of FIG. 1;
[0017] FIG. 3 is a flow chart illustrating a
method for gathering pore pressure information according to
an embodiment of the invention;
[0018] FIG. 3A is a representative well log
suitable for use with various methods according to an
embodiment of the invention;
[0019] FIG. 4 is a flow chart illustrating a
method of conducting a drilling operation according to an
embodiment of the invention;
[0020] FIG. 5 is a flow chart illustrating an
alternative method according to an embodiment of the
invention; and
[0021] FIG. 6 is a flow chart illustrating yet
another alternative method according to an embodiment of the
invention.
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DETAILED DESCRIPTION OF THE INVENTION
[0022] In one aspect of the invention, a method
is provided that utilizes nuclear magnetic resonance
measurements (NMR) to evaluate, determine, predict, or
otherwise characterize the pore pressure profile (pore
pressure with respect to depth) in the earth formation
surrounding a wellbore. Such pore pressure information may
be gathered during or
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simultaneously with the drilling operation (e.g., measurement/logging while
drilling) or after the
drilling operation is completed. In another aspect of the invention, the
method utilizes pore
pressure information gathered in this manner during the actual conduct of the
drilling operation.
In the case of an LWD application, pore pressure information gathered during
the drilling
operation can provide important benefits, including improving the safety and
efficiency of the
drilling operation. As discussed below, the discovery of an overpressure or
underpressure
condition may warrant altering the drilling operation in a number of ways,
including calling for
an immediate stop to the drilling operation to address a blowout risk and/or
for an adjustment to
the mud system.
[0023] To facilitate description of the various aspects of the invention, the
Figures,
and the descriptions thereof, focus on exemplary methods of gathering
information on a property
of the earth formation surrounding the wellbore, particularly the pore
pressure. It should be
understood, however, that the scope of the invention extends beyond these
exemplary methods,
and that various aspects of the inventive methods or the methods themselves
are suitable for
other applications related to the gathering of information on the drilling
environment and/or the
drilling operation. In any event, FIGS. 1 and 2 illustrate an exemplary NMR
measurement
apparatus capable of performing the NMR measurements utilized in these
methods. FIGS 3-6
are provided to help illustrate exemplary methods of gathering pore pressure
information or of
conducting drilling operations utilizing such NMR measurements and pore
pressure information,
each of which embodies various aspects of the invention.
[00241The simplified schematic of FIG. 1 depicts an NMR measurement apparatus
in
the form of a wireline conveyed logging tool 10. The logging tool 10 is
designed for
investigating one or more earth formations or zones within a formation 12
traversed by, or
otherwise located in the vicinity of a wellbore 14. In a typical application,
the logging tool 10 is
suspended in the wellbore 14 on an armored cable 16, the length of which
substantially
determines the relative depth of the logging tool 10. The cable length is
controlled by any
suitable means, such as a drum and winch arrangement 18 at the surface.
[0025]The NMR measurement apparatus or logging tool 10 may be any suitable
downhole NMR logging device adapted for either wireline logging applications
or logging-
while-drilling (LWD) applications. As will become apparent, the method of the
present
invention is equally adapted for either application. The logging tool 10
includes a permanent
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magnet, an electromagnet, or a magnet array for generating a static magnetic
field B0 in a volume
of interest 19 in the formations, one or more- RF antennae (e.g., solenoid
antennae, loop
antennae, saddle antennae, etc.), and circuitry configured to produce pulses
of RF power to
induce the RF magnetic field B, in the formations and for receiving the spin
echoes detected
from the formations.
[0026] A surface recording system 20 provides electrical power to the logging
tool
10, and signals detected by the tool 10 are returned to system 20 for
recording and interpretation
via the armored cable 16. Typically, the surface recording system 20 maintains
a log of the
detected spin echoes with respect to the depth of the logging tool 10. In the
embodiment
illustrated, output signals representative of depth are provided by a cable
length measuring
encoder 22. Alternatively, the surface recording system 20 may maintain a log
of the detected
spin echoes with respect to time. Later, the time-based measurements may be
correlated with a
log of depth measurements such that depth-based measurements may be derived.
[0027] FIG. 2 illustrates, in simplified block diagram form, an exemplary
embodiment of downhole circuitry associated with the logging tool 10 and
configured to produce
the RF pulses and detect the spin echoes. In general, the circuitry generates
an RF signal which
is transmitted through an antenna to induce an RF magnetic field in the earth
formations. Spin
echo signals generated as a result of the RF magnetic field are detected by
the antenna and are
either stored or transmitted back to the earth's surface for logging by the
surface recorder system
20. One skilled in the art would appreciate that other embodiments of
circuitry may be used
without departing from the scope of the invention.
[0028] As shown in FIG. 2, the downhole circuitry includes a processor
subsystem
210 having associated memory, timing circuitry, interfaces, and selected
peripheral devices (not
separately shown). The processor subsystem 210 is coupled with a telemetry
circuitry 212, that
communicates with the surface recording system 20. The processor subsystem 210
may include
or may be operatively associated with programmable means for executing
processes that convert
NMR data into useful information on the properties of the drilling
environment.
[0029] The pulse forming circuitry includes a variable frequency oscillator
214
that, under control of the processor subsystem 210, generates an alternating
RF signal at the
desired frequency. The output of the oscillator 214 is coupled to a phase
shifter 216, that
provides for control of pulse phases, and a modulator 218, both of which are
under control of the
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I
processor subsystem 210 to produce the desired pulse phases of the RF field.
The output of the
modulator 218 is coupled, via a power amplifier 220, to an RF antenna 222. A Q-
switch 22.4
optionally can be provided to damp the RF antenna system to reduce antenna
ringing.
[0030] In the circuitry shown, the antenna 222 both transmits the RF pulses to
induce the RF field in the formations and detects the echo signals resulting
from application of
the RF field. Thus, as shown in FIG. 2, the antenna 222 also is coupled with a
receiver section
through a duplexer or switch 226, the output of which is coupled with a
receiver amplifier 228.
During transmitting and damping modes, the switch 226 protects the receiver
amplifier 228 from
the high power pulses which pass to the RF antenna 222. During the receiving
mode, the
duplexer 226 acts as a low impedance connection from the antenna 222 to the
receiver amplifier
228. The output of the receiver amplifier 228 is coupled with a dual phase-
sensitive detector
230, which also receives, as a reference, a signal derived from the oscillator
signal. The output
of the detector 230 is coupled to an analog-to-digital converter 232, the
output of which is a
digital signal representative of the detected NMR signals.
[0031] It should be understood that, although the logging tool 10 is shown as
an
integral or unitary device in FIG. 1, it may alternatively comprise separate
components and may
be combinable with other logging tools. Further, while a wireline tool is
illustrated in FIG. 1,
alternative forms of physical support and a communicating link with the
surface can be used, for
example, in an LWD application. Still further, the digital signals
representative of the detected
spin echoes may be transmitted to the surface recording system 20 while the
tool 10 is downhole.
Alternatively, the digital signals may be stored in memory by the processor
subsystem 210 and
later retrieved when the tool 10 has returned to the surface.
[0032] Using a logging tool such as the tool depicted in FIG. 1, several NMR
parameters can be measured from which properties of the drilling environment
may be derived.
For example, most NMR logging instruments are configured to measure the spin-
lattice
(longitudinal) relaxation times (TI) and/or spin-spin (transverse) relaxation
times (T2) of
hydrogen nuclei. Such measurements may be acquired by first polarizing the
nuclei by exposure
to a static magnetic field Bo and then applying an RF pulse (the
initialization pulse) tuned at the
Lan-nor frequency of the nuclei of interest and calibrated in length to
achieve a 90 degree rotation
of the spin magnetization.
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[0033] An LWD tool suitable for use with the present invention is described in
U.S. Patent No. 6,246,236
[0034] Referring now to FIG. 3, a flow chart 300 is provided that describes in
general terms a method of gathering information on the pore pressure in the
earth formation
surrounding the wellbore or, at least, in the formation wherein the wellbore
is to be located. The
method involves measurement of the NMR response from the area or region
defined by the
wellbore and the surrounding formation ("the drilling environment") over a
depth interval. The
drilling environment may be contained, within a single homogeneous formation
or zone or pass
through a plurality of formation or zones. As further explained through the
various examples of
the inventive method, the pore pressure in the earth formation adjacent the
wellbore affects
certain properties of the drilling environment. Under this premise, the method
according to the
invention employs NMR measurement techniques to evaluate certain properties of
the drilling
environment over a wellbore depth interval, and from this evaluation, derives
information
concerning the pore pressure over the same wellbore depth interval.
[0035] A preferred initial step in the inventive method is to select one or
more
suitable properties (chemical or physical) of the drilling environment (302).
The suitable
property must be one for which the measured values or behavior over a given
wellbore depth
interval can be directly or indirectly related to the values or behavior of
the pore pressure over
the same wellbore depth interval. In particular, it has been determined that,
for certain
properties, variations in their values over the depth interval can be
attributed to variations in the
pore pressure. For example, the behavior of porosity over certain intervals of
wellbore depth
may be related to the behavior of pore pressure in the formation over the same
depth interval.
Generally, porosity will decrease with increasing depth as the higher pressure
tends to compact
the formation. A reversal of this general trend or profile, indicating a sharp
increase in porosity
in an otherwise homogeneous zone, may indicate a zone of increased pressure.
[0036] Furthermore, the porosity of the formation (or total porosity) as
discussed
above may be separated into two components: the bound fluid volume (BFV) and
the free fluid
volume (FFV). In respect to the inventive method, either of two components may
be selected
independently as the suitably property of the drilling environment. BFV refers
to the relative
share of the total porosity that is bound, whereas the FFV is the share that
is not bound. More
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specifically, BFV represents the percent of the porosity (i.e., of the total
porosity) that is in close
interaction with the solid grains, especially clay particles. In many
applications, it will be
convenient, and even advantageous, to monitor BFV and/or FFV along with total
porosity. In
any event, as used herein, the term porosity may mean total porosity, BFV,
FFV, or
combinations of any or all of these.
[0037] Like total porosity values, BFV values tend to decrease with increasing
depth, especially in shale formations, but increases in an overpressure zone.
Thus, a reversal of
this tend, i.e., an increase in BFV in an otherwise homogeneous zone, may
indicate a sudden
increase in pore pressure.
[0038] Porosity is one of several properties of the drilling environment
determined
to be suitable for the method illustrated in FIG. 3. Other properties
contemplated for use with
the method according to the invention include: permeability; pore size (as
determined by
restricted diffusion); drilling fluid. properties, including composition;
formation fluid properties,
including composition; drilling fluid/formation fluid invasion
characteristics; and combinations
of these properties.
[0039] As another initial step, the method preferably includes selecting a
suitable
NMR parameter (304). As explained above, NMR measurements generate a response
that is
unique to the environment or object onto which the NMR signal is targeted.
That NMR response
may be characterized by a unique set of parameters, although manipulation of
the response may
be required in order to observe and better evaluate certain parameters. A
variety of suitable
techniques are known to achieve this purpose. In the application of the
inventive method, one or
more of these parameters (or NMR data) is identified as having values that
have some relation to
the values or behavior of the selected suitable property over a given wellbore
depth interval.
More particularly, the selected NMR parameter is one characterized by values
over a given
wellbore depth interval that have some correlation with the values or behavior
of the selected
property of the drilling environment over the same depth interval and, thus,
from which the
behavior or values of the pore pressure may be derived.
[0040] With respect to porosity, the preferred NMR parameter for use is the T2
distribution in the NMR response. The NMR T2 distributions of brine are
correlated to pore size
distributions. Specifically, shorter relaxation times are correlated with
better compaction or
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smaller pore size. The T1 distribution (not affected by
diffusivity) may also be selected, possibly in addition to
T2 distribution.
[0041) Referring again to FIG. 3, the preferred
method calls for NMR measurements to be conducted at a
plurality of wellbore depths (e.g., over a wellbore depth
interval), thereby granting an NMR. response from the
drilling environment (306). The measurements may be taken
continuously through the depth interval, or periodically
over time or depth. Typically, a large amount of geological
information is available on the characteristics of the
various zones or formations in which the wellbore is to be
drilled. Of particular interest are the depths at which the
transitions between zones or formations are located. The
frequency and location of these transitions may dictate the
extent of the depth interval in step 306 and, in the case of
periodic measurements, the number of frequency of
measurements. Marked variations unrelated to overpressure
or underpressure in properties are common at such
variations; therefore, care must be taken not to
misinterpret or prematurely respond to such variations. In
any case, the depth interval referred to in step 306 may be
a discreet interval within the entire depth of the wellbore,
as for example within a homogenous zone, or extend the
entire wellbore depth encompassing several zones and
formations.
[0042) Moreover, the type of NMR measurement will
vary depending on the drilling environment property sought.
For example, when values of porosity or permeability are
targeted, the NMR measurement will be directed so as to
generate a response from deep within the surrounding earth
formation. NMR measurement techniques and equipment
11
CA 02450598 2006-07-26
79350-96
suitable for such measurements are described in U.S. Pat.
Nos. 6,246,236 and U.S. 6,232,778.
[0043] As further described below, NMR
measurements may also be directed into and round a region of
the drilling environment wherein the interface between
formation fluids and drilling fluids is normally found.
This region may be centered anywhere between 1" and 2N from
the outer face of the wellbore, and is commonly referred to
as the near-wellbore region. The near-wellbore region is
susceptible to invasion by drilling fluids, that displace or
mix with the formation fluid due to excess pressure in the
wellbore relative to the pressure in the formation. The
invasion front within the wellbore will, therefore, vary
with depth and differential pressure. NMR measurement
techniques and equipment directed to this region are
described in U.S. Pat. No. 6,246,236. These types of NMR
measurements may be utilized to gather information on
properties related to the drilling fluid, including the
drilling fluid composition and the degree and rate of
drilling fluid invasion into the region normally occupied by
formation fluid. Drilling fluid properties may also be
derived from measurements directed to the formation fluid
(i.e., formation fluid properties). Specifically, the
compositions and location of the formation fluid in the
near-wellbore region can reveal properties of the drilling
fluid. The utilization of these types of NMR measurements
in the present invention is described below with respect to
the alternative methods illustrated through FIGS. 5 and 6.
[0044] Preferably, values of the NMR parameter(s)
and the property of the drilling environment are outputted
onto a well log or well data screen such as the one depicted
12
CA 02450598 2006-07-26
79350-96
in FIG. 3A. As values of the NMR parameter are received by
the processor subsystem (see FIGS. 1 and 2), these values
become input to a computer software/program residing
thereon. The computer program may embody any one of several
known processes for correlating the NMR parameter values
with the values of the selected drilling environment
property (3:12). Preferably, execution of the software or
program generates values for the drilling environment
property over the depth interval. These values are then
outputted onto the well log 305 alongside the NMR parameter,
as shown in FIG. 3A. Examples of processes suitable for
correlating values of T2 distributions with values of
porosity or permeability are described in Society of
Petroleum Engineers paper nos. SPE30560 Processing of Data
from an NNR Logging Tool; 1995, R. Freedman, C. E. Morriss,
(Schlumberger Well Services) and SPE49010 Quantitative NMR
Interpretation; 1998, D. E. Cannon, C. C. Minh,
(Schlumberger Sugar Land Product Center); R. L. Kleinberg,
(Schlumberger-Doll Research),
[0045] As shown in PIG. 3, a plurality of NMR
measurements may have to be taken to establish a sufficient
number of values for the suitable property or in the
alternative, to establish some profile or trend over a
sufficiently indicative depth interval (step 308). This
allows the drilling operator or some other use or automated
process to compare the measured or actual values of the
suitable property over the depth interval, thereby
evaluating the behavior of the suitable property. of
particular interest is whether the measured values are
relatively constant or increase or decrease gradually, or
deviate sharply over depth.. How the values compare or
correspond with one another can then be correlated with the
behavior of the pore pressure. If, for example, the actual
12a
CA 02450598 2006-07-26
JUL-Cfb-bbbb lb:1J rKUl9 01JCJ-0""U
79350-96
values of porosity gradually increase with depth over a
known homogenous zone, the pore pressure may be determined
as also increasing with depth in the normal manner and
unaffected by sudden pressure changes. On the other hand,
if the otherwise gradual increase (the profile) is
interrupted by a section of sudden decrease or
12b
CA 02450598 2003-11-24
d 1
increase, the pore pressure over the depth interval may be determined as being
interrupted by an
overpressure or underpressure zone respectively. - In the flowchart of FIG. 3,
this comparison of
the property values over the depth interval amounts to a step of determining
or predicting the
characteristics of the pore pressure in the earth formation (312).
[0046] Preferably, the values of the NMR property and the property of the
drilling
environment are outputted onto a well log such as that depicted in FIG. 3A. In
an LWD
application, the values are preferably logged during drilling, such that the
property and, thus, the
pore pressure may be monitored throughout the drilling operation. During
drilling, the well log
may be available on a continuous paper print-out or a digital display, thereby
allowing for real-
time or nearly simultaneous monitoring. In alternative embodiments, the
logging software may
be further equipped with alarm or indicating means to monitor for certain
undesirable behavior
of the suitable property and the pore pressure or for values exceeding
predetermined levels of the
suitable property or pore pressure. For purposes of the present Detailed
Description, the term
"well log" shall mean any display or observable representation reflecting
values of the NMR
parameters, drilling environment properties, and other well information.
[0047] The well log of FIG. 3A is a simplified representation of a suitable
well log
350 for use with the inventive method, apparatus, and system. The well log 350
provides values
for Gamma Ray log and well depth in Tracks 1 and 2, respectively. The selected
NMR
parameter, T2 distribution, is displayed in Track 3. The suitable drilling
environment property
selected is porosity, the values of which are displayed in Track 4. In one
respect, the well log
350 is a simplified representation because, in a more common application,
various NMR
parameters may be logged along with the three common porosity properties and
perhaps
permeability. Such a suite of NMR data and NMR-derived property values may be
further
complemented with other physical measurements obtained through conventional
methods. In
this way, a more accurate or more fail-safe monitoring of the conditions at
the drilling
environment and of pore pressure conditions may be achieved. The manner by
which a method,
apparatus, or system may employ these various resources will become apparent
to one skilled in
the geophysical, petrophysical, or other relevant art upon reading the present
Detailed
Description and/or viewing the various accompanying Figures.
[0048] In the Exemplary well log 350, the T2 and porosity values are logged
over
several known zones in the formation. The BFV values or profile 354 is logged
separately in
25198706.1 13
CA 02450598 2003-11-24
4 ,
Track 6. The values follow the expected profile (shown in dash lines 356)
through the first three
zones with the BFV values decreasing gradually with depth before approaching
zone transition
areas 358. These zone transition areas 358 are characterized, and thus
highlighted, by sudden
changes in T2 distribution and in FFV values (see 360). Gradual decrease in
BFV values is
interrupted, however, as drilling continues in the subsequent zones.
Specifically, the BFV values
increase sharply starting at a depth of about 14,000' thereby indicating a
sudden increase in pore
pressure. In the well log 350, this sudden increase is highlighted by flag 364
and flag 366. In
particular, flag 366 is an alert indication (e.g., red audible alarm) alarming
the user of the
possible overpressure condition.
[0049] In an LWD application, such an occurrence may warrant stoppage of the
drilling operation. In some applications, additional measurements or further
investigation may
be performed to confirm the overpressure condition. The overpressure condition
may then be
addressed by adjusting the drilling technique (for example, by increasing the
density of the
drilling fluid composition or reducing the pump flow rate).
[0050] Use of the well log 350 (and the flowchart of FIG. 3) provides an
example
of how the characteristics of the pore pressure over a depth interval may be
determined and how
such information may be utilized in the conduct of a drilling operation. To
highlight the change
in pore pressure, well log 350 is also provided with an extrapolation of the
predicted or normal
pressure values, thereby establishing the predicted or normal profile. The
predicted profile
appears as an extension in dash lines (365) to the actual profile of the
measured or actual BFV
values (the actual profile 354). When the actual profile 354 deviates from the
predicted profile
356, both profiles appear and remain on the log, and the difference
(represented as the area 364)
highlighted (e.g., in red). This allows the user to evaluate and even quantify
the degree by which
the actual values deviate from the otherwise normal values, thereby evaluating
the degree of
overpressure.
[0051] FIG. 4 provides a variation of the general method as described with
respect
to the flowchart FIG. 3. In particular, the method embodied in the flowchart
400 of FIG. 4
utilizes a comparison of a predicted profile of the drilling environment
property over a depth
interval with an actual profile as derived from NMR measurements. Moreover,
the flowchart
400 in FIG. 4 illustrates a method of drilling a wellbore utilizing NMR
measurements.
25198706.1 14
CA 02450598 2003-11-24
` a t
I~ I)
[0052] In an initial step 402 of the method, a suitable property of the
drilling
environment is selected. In accordance with the invention, the suitable
property selected is such
that variations in the property's values over a wellbore depth interval can be
correlated with
variations in the pore pressure of the formation. As mentioned above, a large
amount of
information may be known about the subject formation even before drilling and
before
conducting the NMR measurements. Form this body of historical information, an
expected or
predicted profile of the physical property over the given wellbore depth
interval is obtained
(414). The predicted profile represents the expected or normal behavior of the
property over a
depth interval, and serves as the baseline for monitoring the pore pressure.
Thus, in many
applications, the predicted profile represents the pore pressure conditions by
which the drilling
operation is designed. If the actual profile corresponds directly with the
predicted, the drilling
operation is likely to advance as planned.
[0053] Preferably, the predicted profile is illustrated in a well log 450 as
shown in
FIG. 3A. The predicted profile is provided over varying depths in the
wellbore, and over
different zones. It is typical that the profile in each zone will be unique
for that zone and perhaps
independent from the other zones. Thus, in one respect, the predicted profile
may be regarded as
a collection of predicted profiles over multiple zones.
[0054] In another initial step of the method, at least one suitable NMR
parameter is
selected (406). A suitable NMR parameter is one characterized by values over
the depth interval
which may be correlated with those of the selected suitable property. In
particular, the NMR
parameter is selected such that variations in its values over a wellbore depth
interval can be
correlated with variations in the property over the same depth interval. In
this way, the behavior
of the suitable property may be derived from the NMR measurements. If porosity
is selected as
the physical property, the T2 distribution in the NMR response is preferably
selected as the NMR
parameter.
[0055] After the above initial steps are completed, drilling of the wellbore
begins
(408). Then, using an NMR measurement apparatus (preferably an LWD tool in the
present
case), measurements are taken over a wellbore depth interval (410). As
mentioned previously,
the NMR measurements may, be made periodically at successive depth intervals
or at incremental
time periods, or continuously throughout a.wellbore depth interval. In respect
to the method of
25198706.1 15
CA 02450598 2003-11-24
FIG. 4, the referenced well bore depth interval may mean a depth interval
within a zone or
through a plurality of zones, or extend the entire wellbore depth.
[0056] The NMR response is preferably transmitted uphole via conventional
telemetry means and received by a processor subsystem of the surface recorder
system. As
discussed above with respect to the flowchart of FIG. 3, values of the
physical property are
derived from the measured NMR parameter values using a known correlation and
processing
technique. The values of the NMR parameter and the physical property are then
outputted onto
well log 450. By logging a number of values of the physical property over the
initial depth
intervals, the actual profile of the physical property is established (416).
[0057] As shown in FIG. 3A, the actual profile 354 may be logged in the same
track as the predicted profile 356, so that it overlies the predicted profile
356. In many
applications, this will greatly facilitate the subsequent steps of comparing
the two profiles (418)
and determining whether the actual profile deviates or varies from the
expected profile (420). If
the actual profile does indeed deviate or vary from the expected profile, the
deviation or variation
may be correlated with variations in the pore pressure (426) in accordance
with the present
invention. Typically, at such an occurrence, the well log (which will likely
include other NMR
data and measurements) will be analyzed further and perhaps, additional
measurements and tests
(e.g., from the surface) will be conducted. In this manner, the operator
determines or confirms
whether an overpressure zone has been detected (428).
[0058] Referring to the well log 450 of FIG. 4A, an overpressure or
underpressure
zone may be indicated by a sharp deviation from the predicted profile. If such
an
overpressure/underpressure situation is confirmed, the drilling operation may
be adjusted to
compensate or otherwise address the overpressure or underpressure zone (430).
In some cases,
the drilling operation may be halted. In many instances, the
overpressure/underpressure zone
will be addressed by adjusting the fluid composition of the drilling fluid.
For example,
weighting agents may be added to increase the weight and fluid density. Also,
the pump flow
rate may be varied, the drilling rate may be reduced, or controlled drilling
implemented. In some
instances, drilling may be stopped and the casing may be set (sooner than
planned). In any of
these situations, the adjusted drilling will deviate from the original plan.
Additionally, various
measurements may be madec such as seismic surveys, to further evaluate the
drilling conditions.
25198706.1 16
CA 02450598 2003-11-24
= i
Thereafter, the drilling operation may be re-started (408), followed by
additional NMR
measurements being conducted at subsequent depths (410).
[0059] In a further aspect of the invention, the inventive method allows for
the
expected profile to be adjusted during the drilling operation (424), thereby
providing for a more
accurate monitoring of the drilling operation. Specifically, the expected
profile may be adjusted
by incorporating the actual measurements of the NMR parameter and suitable
property at
preceding depths. Preferably, the predicted profile is adjusted in real time
and simultaneous with
the logging of the actual profile. For example, the increase in porosity over
varying depths may
be sharper or more rapid than originally predicted due to unexpected changes
in the geological
model. In view of this, the predicted profile may be adjusted to reflect the
true profile.
[0060] In the case of certain physical properties such as porosity and
permeability,
the predicted profile may be obtained from historical information. For
example, the porosity of
the subject formation or zones may have been previously logged using surface
techniques such
as seismic or sonic measurements. Alternatively, the predicted profile may be
obtained from
historical information on similar zones or formations, particularly formations
surrounding offset
wells. Further yet, the predicted profile may be established by taking initial
measurements
within a homogeneous zone and extrapolating the initial profile over the rest
of the zone.
[0061] The property of the drilling environment selected may be one attributed
to the
drilling fluid, such as the drilling fluid composition. In several
applications, the drilling fluid
property selected concerns the interaction between the drilling fluid and the
formation fluid in
the near-wellbore region (e.g., the depth or rate of drilling fluid invasion).
In these cases, the
predicted or base profile may reflect a nearly constant value over a short
depth interval. The
inventive method is then employed to monitor for sharp deviations from this
constant profile. In
many of these cases, the predicted profile is established by taking
measurements at the well
surface using conventional means. Alternatively, measurements may be conducted
near or at the
surface utilizing NMR measurements of the drilling fluid or formation fluid.
The profile
obtained in this manner is then extrapolated over the wellbore depth interval
by way of a vertical
line (see, e.g., line 370 in well log 550). The flowcharts of FIGS. 5 and 6
focus on a variation of
the inventive method that utilizes methods in which the suitable property
selected concerns the
drilling fluid or formation fluid. In these instances, properties directed to
the formation fluid
25198706.1 17
CA 02450598 2003-11-24
content in the wellbore region is, in essence, a property of the drilling
fluid, because it normally
reveals something about the invasion front.
[0062] Referring now to the flowchart of FIG. 5, there is illustrated a method
of
conducting drilling operations. Specifically, an initial step of the method is
the selection of at
least one suitable property of the drilling environment provided in a near-
wellbore region of the
drilling environment (502). Specifically, the property selected is the depth
of formation fluid
invasion. During drilling, the balance of fluids inside the formation is
disturbed. Usually, the
pressure of the mud in the wellbore is higher than the pressure of the native
formation fluid
(overbalance drilling). This pressure deferential leads to an invasion of the
drilling fluid or mud
into the formation and, a replacement of the native formation fluid by mud
filtrate in a region
surrounding the wellbore (known as the invaded zone). This invaded zone is
within the near
wellbore region which is the subject of the NMR measurement. The invading mud
usually
contains fine particulate matter, that is quickly filtered out onto the sides
of the wellbore to form
a layer known as the "mudcake." The resulting mudcake build-up is a desirable
effect because it
reduces communication between the formation and the mud column, thereby
preventing or
hindering the further flow of mud into the formation. In an overpressured
zone, the mud will not
invade the formation but rather, the formation fluid may tend to enter the mud
column. This, of
course, is an undesirable result, because the mud system may be diluted
thereby changing the
mud weight and density.
[0063] Modem NMR measurement tools often employ a magnetic field that decays
from the tool away into the formation (known as a gradient design). Such a
tool allows an
operator to measure the NMR signal in a shell around the tool. The position of
the shell can be
chosen by selecting a suitable frequency fulfilling the larmor resonance
residence condition at
the desired distance (or depth) from the tool. It is, therefore, possible to
perform NMR
measurements at different depths of investigation. Because the pressure
deferential between the
mud and the formation fluid influences the invasion profile and speed,
measurement of the mud
filtrate content with respect to depth can give valuable information about the
invasion process
and, thus, about the pore pressure. In the alternative, measurement of changes
in the invasion
profile over time may further yield valuable information. A slower invasion
may indicate a
higher formation pressure and vice versa. Similar approaches may be adopted
using invasion
profiling from a different technique (e.g., resistivity) in combining this
with NMR derived rock
property measurements to estimate pore pressure. In any event, measurements of
the depth of
25198706.1 18
CA 02450598 2003-11-24
fluid invasion, or more particularly, variations in the depth of fluid
invasion over wellbore
depths, may be correlated with variations in the pore pressure in the
surrounding earth formation.
[0064] In another initial step of the inventive method of FIG. 5, at least one
NMR
parameter is selected (504), whereby variations in NMR parameter over the
depth interval can be
correlated with variations in the depth of fluid invasion. Preferably, a suite
of NMR parameters
will be selected. The NMR parameters will typically include T2 distributions
across the gradient
field, so as to distinguish between the presence or volume of native formation
fluid and invading
drilling fluid, such as mud filtrate. In this manner, the depth of invasion at
a particular depth
may be evaluated.
[0065] Over a wellbore depth interval, it is desirable for the depth of
invasion to
remain relatively constant. Accordingly, the predicted or base profile
obtained for the depth of
formation fluid invasion through the wellbore depth interval is a nearly
constant vertical line
(see, e.g., line 370 in FIG. 3A) (see step 506). It should be noted, however,
that it may be
common to see some (but not sudden) variations in the depth of formation fluid
invasion, due to
minor effects unrelated to overpressure or underpressure, e.g., increasing
depth and pressure
buildup.
[0066] Typically, the predicted or base profile may be established from
initial
NMR measurements near or at the surface, and then extrapolated over the
wellbore depth
interval. Upon establishing the predicted or base profile, drilling of the
wellbore commences
(508). In a subsequent step 510, NMR measurements are then conducted at a
plurality of
wellbore depths or depth interval, either continuously or periodically. As
mentioned above, the
NMR measurements are directed, preferably using a gradient type tool, to
generate an NMR
response from the near wellbore region. In particular, the NMR response will
come from the
near-wellbore region including at and around the invasion front. Upon
collection of a sufficient
number of NMR parameter values and depth of invasion values, the values of the
NMR
parameters and thus the depth of invasion, are compared over the wellbore
depth. Typically, this
information will be provided on a well log in the form of radial profile over
depth. More
particularly, the values are compared continuously throughout the drilling, so
as to monitor for
sharp deviations from the ba4e profile (512).
[0067] As indicated by step 514 in flowchart 500, an inquiry is made as to
whether
the values of the NMR parameters (or suitable property) at a current wellbore
depth have
25198706.1 19
CA 02450598 2003-11-24
~II
significantly increased from values at preceding wellbore depths (514). A
decrease in the values,
particularly a sudden decrease, generally indicates the possibility of an
overpressure zone. Thus,
the next step in the method is to further evaluate or confirm the possibility
of an overpressure
zone (516). If further evaluation confirms overpressurization, the drilling
operation may be
halted and/or the drilling variables adjusted (520), as discussed previously.
Further, if no
significant decrease or increase in the values are detected, the drilling
operation continues as
normal, and further additional NMR measurements are conducted.
[0068] If the pressure of the formation fluid is much higher than the pressure
of the
mud column (overpressurization), the formation fluid will flow into the mud
column. This
affects the NMR measurements similar to tool motion or diffusion.
Specifically, shortened T2
distributions may be detected as well as a change of the echo shape.
Accordingly, the T2
distributions and echo shapes are typically selected as the suitable NMR
parameter. With respect
to the echo shape, the effect of the lateral tool motion will be very small in
the inventive method,
because the LWD NMR tool is symmetric, such that the opposing effects on two
sides of the tool
cancel one another to a high degree. A flow of fluid inward (formation fluid
entering the
wellbore) or outward (fast invasion or loss of circulation), however, does not
lead to this
cancellation so that the effect on the echo shape will be more pronounced.
[0069] In the well log 350 of FIG. 3A, Track 6 provides a log of depths of
invasion
(DOT) over depth. Starting at about 14,000', the DOI increases sharply,
thereby indicating a
possible overpressure zone. This condition is highlighted by the overpressure
alarms 374 and
376.
[0070] Another way to detect invasion of fluid into the wellbore from the
formation is to analyze the mud or drilling fluid composition in the vicinity
of the tool. Since the
formation fluid has different NMR properties than the mud, dilution of the mud
by way of the
formation fluid may be determined easily, using conventional methods.
Furthermore, such
measurements could be performed very close to the NMR tool, such that the
signal to noise ratio
would be advantageously large. Early detection of changing mud properties
using this technique
provides valuable early warning of a possible "kick scenario," when formation
fluids begin to
enter the bore hole. The method embodied in the flowchart 600 of FIG. 6
illustrates the
gathering and use of this infdrmation.
25198706.1 20
1
CA 02450598 2003-11-24
1
[0071] Referring to flowchart 600, an initial step of the method is to select
a
property of the drilling environment, in this case a drilling fluid property
such as drilling fluid
composition (602). In one type of application, the NMR measurements focus on
the constituent
of the formation fluid (e.g., to evaluate the degree of invasion and/or
dilution by drilling fluid).
In another application, the NMR measurements focus on the constituents of the
drilling fluid
(e.g., to evaluate degree of invasion and/or dilution by formation fluid). In
most applications,
both the formation fluid and the drilling fluid areas of the near-wellbore
region will be targeted.
[0072] Next, a suitable NMR parameter is selected (604), one which may be
correlated with a drilling fluid property. Again, a number of NMR parameters
may be selected
including T2 distributions and echo shape (which may be regarded as one of
several T2
acquisition properties). The values of such NMR parameters over a wellbore
depth interval, or at
least the behavior or variation in the values thereof, relate to the
variations in the drilling fluid
composition. In a subsequent step of the method, a predicted or base profile
of the fluid property
is obtained (606). This predicted or base profile is considered to be made up
of "base" or
"normal" values of the NMR parameter. Preferably, the "base" values are
obtained near or at the
surface, and then extrapolated over the wellbore depth interval, When a radial
profile of the
formation fluid-drilling fluid is sought at varying depths, the "base or
normal profile" would be
one where such a profile is relatively the same through a given depth
interval.
[0073] Upon completion of the above initial steps, commencement of the
drilling
operation ensues (608). As in previous methods, NMR measurements are then
conducted at a
plurality of wellbore depths (610). These NMR measurements will be directed to
the near
wellbore region wherein the drilling fluid, mud filtrate, or formation fluid
is contained. The
objective of the NMR measurements is to provide monitoring of the NMR
parameters attributed
to the drilling fluid (the basic drilling fluid, the mud filtrate, or the
formation fluid).
[0074] With a sufficient number of NMR measurements, the actual values of the
NMR parameter (and thus the drilling fluid properties) may be compared with
the "normal" or
"base" values (612). This step may be regarded as equivalent to comparing the
actual profile of
the NMR parameter or drilling fluid property, with the predicted or base
profile, as described
previously with respect to the methods of FIG 4 and 5.
[0075] A subsequent operation 614 in the method, relies on the comparison of
the
actual and normal values. Specifically, the inquiry is whether the actual
values deviate sharply
25198706.1 21
CA 02450598 2003-11-24
1 '
from the normal values. If, indeed, this is the case, there exists a
possibility (618) of an
overpressure. If no sharp deviations are observed, the drilling operation may
be stopped and/or
the drilling variables adjusted (620), as discussed with previous methods.
With the proper
adjustment, the drilling operation and the monitoring of pore pressure
continues.
[0076] While the invention has been described with respect to a limited number
of
embodiments, those skilled in the geophysical, petrophysical, and other
relevant art, having the
benefit of this disclosure, would appreciate that other embodiments can be
devised which do not
depart from the scope of the invention as disclosed herein. For example, it is
contemplated that
various aspects of the inventive methods may be applied to other applications
concerning the
gathering of information on the drilling environment or formations surrounding
a wellbore.
These other methods may be directed to correlating to one or more NMR
parameters to pore
pressure behavior, or to another physical or chemical property of the drilling
environment.
Accordingly, the scope of the invention should be limited only by the appended
claims.
25198706.1 22
