Note: Descriptions are shown in the official language in which they were submitted.
CA 02508404 2005-05-31
Description of the Preferred Embodiment
The present invention comprises an improved subsurface tool 1 which is adapted
to be
included integrally in the bottomhole assembly of a drill string, as near to
the bit 2 and the
bit saver sub 3, as is practicable, which is capable of identifying non-
framework rocks in
real time. The tool 1 is suspended in the bore hole 4, by means of the drill
string 5. The
tool 1 is provided with a telemetry means 6, which transmits data from a
plurality of sensor
devices within the toot 1, to a telemetry receiving means 7 at the surface.
Data from the
surface receiver 7 is transferred to a data processing apparatus 8. Data is
simultaneously
transferred from the driller's console 26 to the data processing apparatus 8.
The
processing apparatus 8 integrally includes a visual display means 9, which
provides the
operator with various selectable images or pages. The primary page includes a
plurality of
plotted lines that are representative of the various geophysical and drilling
parameters
available to the processing apparatus 8 of the invention. The processing
apparatus 8
further provides an operator with aids to data analysis, interactively through
means of an
integral keyboard device 10 and a mouse and cursor control system 11. The
processing
apparatus 8, incorporates an operating system running a dedicated computer
program that
automatically provides the operator with, for example, continuous statistical
information of
each available physical parameter over a selected interval, including
variance, maximum
value, minimum value, mean value, and the magnitude and direction of any
continuous
slope defined by the data. The processing apparatus 8 of the invention aids
the operator in
judging the degree of symmetry of a given physical parameter over a given
interval and
about a given midpoint by means of an inversion overlay function.
The ultimate objective of the analyses is to compute the degree to which a
given shale,
identified and analyzed during the time of its penetration by the bit,
conforms to the
characteristics of a non-framework shale. The degree of conformity is
presented as a
simple mathematical probability. The presence of a non-framework rock informs
the driller,
in a timely manner, that an abnormally high pore system pressure is about to
be
encountered. The data received by the processing apparatus 8 and the
assessment of
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conformity are recorded on a permanent medium such as a compact disk writer
27, and
presented as a paper log by a dedicated plotting apparatus 12.
The tool 1 of the invention illustrated in Figure 1 is substantially a
modified version of
known logging while drilling tools, comprising a cylindrical body of high
density metal alloy,
provided with an axial passageway, permitting the movement of drilling mud
through the
core of the tool 1. Specific mod~cations of the tool 1 include three strain
gauges. One
strain gauge 13 is mounted parallel to the long axis of the tool 1 so as to
continuously
measure the vertical strain experienced by the body of the tool 1. The tool 1
is further
provided with a second strain gauge 14 mounted at a right angle to the axis of
the tool 1 so
as to continuously measure the lateral strain experienced by the body of the
tool 1. The
tool 1 is further provided with a third strain gauge 15 mounted at forty five
degrees to the
axis of the tool 1 so as to continuously measure the torque experienced by the
body of the
tool 1.
Another modification of the tool 1 of the invention is related to the means
provided to
measure the sonic velocity of the rock formations exposed in the borehole 4.
The
measuring means are modified from known devices in two aspects. Firstly, the
vertical
separation of the acoustic source 16 and the acoustic receivers 17 is greatly
reduced on
the body of the tool 1 of the invention in order to increase vertical
resolution. This permits
the tool 1 of the invention to more reliably detect the very thin central high
sonic velocity
spike that is peculiar to a non-framework shale. Secondly, the receivers are
of a type and
disposition that enable the collection of shear wave data as well as
compressive wave
data. These two data streams permit some measure of the plasticity of the
shale to be
calculated by the processing apparatus 8. Certain portions of non-framework
shales are
much more plastic than common shales.
The tool 1 of the invention is further provided with means to continuously
measure natural
gamma radiation emanating from the rocks of the open borehole 4. The spectral
gamma
radiation detector 19, counts separately, the gamma rays in the three energy
ranges that
coincide with the three common elements that emit gamma radiation; potassium,
uranium,
and thorium.
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The tool 1 of the invention is further provided with a neutron source 20 and a
neutron
detector 21 which continuously measure neutron capture cross-section in the
wall rocks of
the open borehole 4.
The tool 1 of the invention is further provided with a gamma ray source 22 and
two gamma
ray detectors 22 and 24 to continuously measure the electron density and
photoelectric
absorption factor of the wall rocks of the open borehole 4.
At present, the measuring of hydrocarbon gas saturations of the drilling fluid
takes place at
the surface, specifically at the mud tank. Because the mud exiting from the
jets of the bit 2
takes hours to travel back to the surface, the mud saturation data can lag
many meters
behind the advance of the borehole. To eliminate lag time, the tool 1 of the
invention is
provided with a pair of identical mud resistivity meters. The electrode pairs
are sheltered in
two small concavities, one pair on the internal surface and one pair 25 on the
external
surface of the tool 1. The electrodes are supplied with identical electric
potentials. The
pair of electrodes on the inner surface of the tool 1 is exposed to the mud
flow as it travels
down the interior passage of the tool. The pair 25 on the outer surface of the
tool 1 is
exposed to the mud flow just after it has swept past the cutting face. If
there are pores in
the rock, and they contain formation water that is more saline than the mud,
the electrode
pair on the outer surface will register a lower resistivity than the pair on
the interior surface.
If, however, there are pores in the rock that are hydrocarbon filled, the
electrode pair 25 on
the outer surface will register higher resistivity than the pair on the
interior surface. The
comparative resistivity method of measuring hydrocarbon saturation in the mud
system is
superior to the current technology because it provides instantaneous
information, and the
hydrocarbon saturation profile of a given rock unit is not distorted and
degraded by rising
several kilometers through a fluid mud column in the annulus 18 of a well that
is of a
complex and irregular shape.
OPERATION OF THE INVENTION
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In the preferred embodiment of the invention, the processing apparatus 8 of
the invention
incorporates a computer with an operating system running a dedicated computer
program
that provides a plurality of automated calculations and graphic displays. For
example, the
display apparatus 9 can be directed, by way of on-screen controls such as drop
down
menus, to provide graphic renderings of line curves that represent real-time
geophysical
data such as vertical acoustic velocity, or neutron capture cross-section,
combined with
real-time drilling data such as torque stress on the tool 1 of the invention,
as depicted in
Figure 2. The line curves can be of various colours and can be displayed
against well
depth, which can be depicted along the vertical axis of one page, on the
display apparatus
9.
The units of display for the various line curves can be controlled by the
operator by means
of drop down menus. For example, sonic velocity can be displayed in units of
meters per
second or feet per second, or as travel times such as microseconds per meter
or
microseconds per foot, or as sonic porosity. The calculation of sonic porosity
within a
non-framework shale is theoretically meaningless, however, a display in this
format
facilitates the comparison of sonic velocity data from the subject well with
similar data from
other wells that have used the porosity unit as an industry convention.
In the case of the natural spectral gamma ray data as captured by the gamma
radiation
detector 19, the operator may choose to display the combined radiation of all
energy levels
so as to define the extent of the shale mass under study. The operator may
choose to
allow the processing apparatus 8 to calculate a default shale line, or he may
enter an
alternate value by means of the keyboard device 10, or he may choose to
determine the
position of the shale line visually, manipulating the shale line interactively
by means of the
cursor and mouse device 11. As an aid to interpretation, the processing
apparatus 8 can
fll the area between the shale line and the line curve with a transparent
colour or a pattern.
Likewise, the operator can control the unit of display for the neutron capture
cross-section
data as derived from the neutron source 20 and neutron detector 21. The
operator can
select a line curve representing either neutron flux or neutron porosity. As
in the case of
sonic porosity, neutron porosity is meaningless with respect to non-framework
shafes but is
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helpful in comparing neutron flux data with similar data available from the
conventional logs
of other welts.
Furthermore, the operator can choose to display the gamma ray flux data
derived from the
gamma ray source 22 and detectors 23 and 24, as photoelectric effect, electron
density,
bulk formation density, or density porosity after having either assigned a
value for the grain
density of the shale or accepting a default value provided by the processing
apparatus 8 of
the invention.
One of the most definitive characteristics of non-framework shales is the
extreme
homogeneity in physical attributes such as breaking strength. The data from
the three
strain gauges 13, 14, and 15, which are mounted on the tool 1, can be
displayed
separately, or in combination on the display means 9 of the processing
apparatus 8.
While drilling, the variations of strain suffered by the tool 1 of the
invention are largely the
result of the friction between the teeth or buttons of the drill bit 2, and
the rock face that it is
working on. In typical clastic rocks, including shales with typically well
developed mineral
frameworks, the mechanical integrity of the rock is relatively variable. The
homogeneity of
non-framework shales leads to very low statistical variance in "bit bounce" or
vibration.
The strain gauge data coming from the tool 1 is analyzed by the processing
apparatus 8,
and the statistical variance of the strain data is presented on the display
means 9, both
numerically and graphically. The graphic display can consist of the line curve
of the
original data, enclosed by two straight lines representing the standard
deviation of the data,
which is the mathematical inverse of variance. The space between the two
dashed lines
can be filled with transparent colour to visually emphasize the degree of
variance.
Furthermore, the operator can choose to display the rate of penetration (ROP)
data at a
sampling interval that is much reduced from that normally available directly
from the
driller's console. The operator of the processing apparatus 8 of the invention
is provided
with the option of displaying the ROP in units of feet per minute, metres per
minute,
minutes per foot, or minutes per metre. There is no range of ROP data values
that is
directly indicative of a non-framework shale, rather the statistical variance
in ROP data
diminishes markedly in such rocks. In addition, ROP data occasionally reveals
a very thin
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anomaly of a reduced rate of penetration at the axis or hydraulic mid-point of
a
non-framework shale. The metre thick anomaly or spike is a representation of
the dense,
impermeable central membrane of a non-framework shale that constitutes an
actively
functioning pressure seal.
If time permits, and there is some doubt that a given candidate shale is a
sealing
non-framework shale, drilling can be interrupted and the mud system circulated
until the
lowermost mud in the annulus reaches the surface. This procedure is referred
to as
circulating bottoms up. By this means, the mud gas concentration profile in
the shale
under study can be obtained from the standard mud tank sampling system. Again,
there is
no range of gas readings that is indicative of a non-framework shale, rather
there is
sometimes a special case of symmetry visible in the gas profile. Centered over
the
hydraulic mid-point or axis of the shale is a symmetric curve that appears to
be parabolic in
form and reaches its maximum value at the mid-point of the non-framework
shale. The two
lows on either side of the axis closely approach the zero gas line. This
pattern is unique to
non-framework shales. With conventional mud pit technology this gas pattern is
rarely
preserved. Furthermore, it normally can only be seen some hours after the
penetration of
the shale. The rarity of the pattern is due to poor preservation of the
pattern as the gas
moves several kilometers up the annulus. Any turbulence in the mud flow will
degrade the
pattern as it rises. Both the delay problem and the degradation problem are
avoided by
adopting the dual resistivity method of the invention.
Many of the data streams coming from the various sensors on the tool 1 display
a
symmetry about the central axis of the shale. To aid the operator in
recognizing symmetry,
the processing apparatus 8 can automatically display a second copy of any
given line
curve immediately adjacent to the original line curve, which is differentiated
from the
original by rendering it in a dissimilar colour. The second curve line is also
rendered
vertically inverted in relation to the original line curve and can be moved
about the display
means 9, by way of the cursor and mouse device 11. The operator can readily
judge the
symmetry of a given parameter by visually assessing the degree of correlation
between the
original and the inverted line curves. The operator has the option of
referring to a
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calculated coefficient of con-elation that is automatically provided by the
processing
apparatus 8 of the invention.
The visual display means 9, can be toggled by a command key on the keyboard
device 22
or by means of an on-screen cursor command, to display muttiple pages. One
page
provides the operator with an image of plotted line curves as well as several
aids to
analysis that include statistical variance, the magnitude and direction of
slope, and the
degree of symmetry, Another page, as depicted in Figure 3, can accept input
from an
operator. The input page consists of several arrays of radio buttons than can
be selected
or unselected by the operator, either by means of the cursor and mouse device
11, or by
means of the keyboard device 10. Each button has a predetermined numeric value
assigned to it. The operator can select only one of the radio buttons in any
particular array.
The selection of any radio button automatically provides the associated
numeric value to a
simple arithmetic accumulator. The accumulator calculates the product of the
numeric
values arising from the multiple button arrays. The product in the accumulator
can
represent the probability that any given shale is a non-framework shale that
is functioning
as a pressure seal.
As an example, one array, consisting of a single set of 3 radio buttons can be
reserved for
the evaluation of the variance in the torque data derived from the strain
gauge 16 mounted
at forty five degrees to the axis of the tool 1. Only one button can be
selected by the
operator to represent a judgment that the torque data displays low, medium, or
high
variance within the shale bed. The accumulator can receive a value that
diminishes as the
degree of variance increases. As a further example, another array consisting
of a single
set of 3 radio buttons can be reserved for the evaluation of the central sonic
spike seen in
many non-framework shales. If no central spike is visible, a low value can be
sent to the
accumulator. If a well developed central spike is present, a larger value can
be sent to the
accumulator than if a poorly developed spike is found. If none of the buttons
in any
particular array are selected, that array can automatically be excluded by the
processing
apparatus 8 when the overall probability is calculated.
