Note: Descriptions are shown in the official language in which they were submitted.
CA 02302995 2000-03-24
Title: Method for measuring fracture porosity in coal seams using geophysical
logs
References Cited
U . S . Patents
5,663,4999/97 Semmelbeck et
al.
5,519,6685/96 Montaron
4,961,3437/90 Boone
4,716,9731/88 Cobern
European Patent
EP0363259 11/90 Luthi
Other References
Puri, R, King, G. E. , and Palmer, I . D . 1991, Damage to coal permeability
during
hydraulic fracturing, Proceedings of the 1991 Coalbed Methane Symposium. The
University of Alabama, Tuscaloosa, May 13-16, 1991.
CA 02302995 2000-03-24
Background of the Invention
The subject matter of the present invention relates to a method for
determining fracture
porosity in coals using existing wellbore induction logging data produced by
an
induction tool disposed in the well bore. The disclosed invention uses
conventional
well log data in an unconventional manner to determine new and useful
information
regarding wellbore formation properties, specifically the amount of fracture
porosity.
During the drilling of a wellbore, mud pumps introduce mud into the well in
order to
flush rock chips and other unwanted debris out of the wellbore. The mud is
introduced
into the wellbore under pressure, the mud pressure being slightly greater than
the
pressure of a formation traversed by the wellbore thereby preventing a
phenomenon
known as well blowout. The resultant differential pressure between the mud
column
pressure and the formation pressure forces mud filtrate into the permeable
formation,
and solid particles of the mud are deposited on the wellbore wall, forming a
mudcake.
The mudcake usually has a very low permeability, and once developed,
considerably
reduces the rate of further mud filtrate invasion into the wellbore wall. In a
region
very close to the wellbore wall, most of the original formation may be flushed
away by
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CA 02302995 2000-03-24
the mud filtrate. This region is known as the "flushed zone" or the "invaded
zone" . If
the flushing is complete, the flushed zone pore space contains only mud
filtrate.
Further out from the wellbore wall, the displacement of the formation fluids
by the mud
filtrate is less and less complete. This results in a second region, this
region undergoing
a transition from mud filtrate saturation to original formation water
saturation. The
second region is known as the "transition zone" . The extent or depth of the
flushed
and transition zones depends on many parameters. Among them is the type and
characteristics of the drilling mud, the formation porosity, the formation
permeability,
the pressure differential and the time since the well was first drilled. The
undisturbed
formation beyond the transition zone is known as the "uninvaded, virgin or
uncontaminated zone" .
FIGS. 1 and 2 show prior art representations of an invasion and resistivity
profile in a
water-bearing zone. FIG. 1, illustrates a cross section of a wellbore showing
the
locations of the flushed zone, the transition zone and the uninvaded zone
extending
radially from the wellbore wall. FIG. 2 illustrates a radial distribution of
formation
resistivity extending radially from the wellbore wall, into the flushed zone,
into the
transition zone, and into the uninvaded zone. Sometimes, in oil and gas
bearing
formations, where the mobility of the hydrocarbons is greater than that of the
water,
because of relative permeability differences, the oil or gas moves away faster
than the
interstitial water. In this case, there may be formed, between the flushed
zone and the
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CA 02302995 2000-03-24
uninvaded zone, an "annular zone or annulus", with a high formation water
saturation.
Annuli probably occur in most hydrocarbon bearing formations; and their
influence on
measurement depends on the radial location of the annulus and its severity.
The existence of these zones (the flushed, transition, annular and uninvaded
zones)
influence resistivity log measurements and therefore the accuracy of the
resistivity log
itself that is presented to a client. In it's conventional use the resistivity
log is utilized
by the client to determine if oil exists in the formation traversed by the
wellbore. The
client is mainly interested in the true and correct value of Rt, the
resistivity (reciprocal
of the conductivity) of the uninvaded zone, since high values of Rt indicate
the presence
of an insulator, possibly oil, in the formation. Conventionally, it is
therefore desirable
to correct for the effect of mud filtrate invasion on formation resistivity.
Conventionally, mud filtrate invasion analysis from resistivity logs is
attempted by
qualitative inspection of the separation between measurement displays
representing
different depths of investigation. The purpose of this analysis is to
determine the radial
geometric function of the logging tool response in order to correct for
invasion and
generate a more accurate value of Rt. However, in the disclosed invention, Rt
is of no
interest, but a new and novel use for the conventional depth of invasion
measurements
is disclosed.
Conventional log analysis techniques require correction for hydrocarbon
saturation in
the void spaces, and are complicated by depth based variation in the
hydrocarbon
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CA 02302995 2000-03-24
saturation gradient through the flushed zone/undisturbed zone interface that
may
confuse invasion character. Variations in drilling mud properties between
wells that
change the radial resistivity profile, and differences in the properties of
the formation
water can cause errors in conventional interpretation. As well, laboratory
measurements of fracture porosity in coal may not be applicable to the bulk
reservoir
properties due to sampling error, the inherent friability of coal, and the
sensitivity of
coal to changes in stress regime.
To correct these deficiencies in the prior art, the disclosed invention is
volume based
and requires no correction for hydrocarbon saturation or depth based
variations. As all
effectively connected fractures in coal are filled with water, hydrocarbon
saturation
variations are immaterial. Variations in mud properties are screened out or
are of no
impact to the disclosed invention, as the true value of Rt is irrelevant. As
well, data
used in the disclosed invention are collected from the formation in situ, with
the coals
under actual temperature and pressure conditions and are more representative
of the
bulk reservoir properties. Fracture porosity calculations in coal should be
performed in
the volume domain in accordance with the present invention. This volume domain
mud
filtrate invasion analysis minimizes the effect of all of these variables and
is useful for
comparing well to well and between zones within a well for determining
measures of
fracture porosity, and hence, methane production potential in the coal seams.
Fracture detection in coal seams is critical for the recovery of economic
quantities of
methane. Coal is a dual porosity medium, comprising a matrix containing
abundant
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micro-scale pores intersected by larger macro-scale fractures. The micro-scale
pores
are of the size that gas movement occurs via diffusion, resulting in a very
slow rate of
gas exchange per unit volume. The larger macro-scale fractures act as the
conduits for
connecting the gas-diffusing matrix to a well bore. For economic quantities of
methane
to be recoverable, extensive well-developed macro-scale fractures must be
present to
connect a large enough volume of coal matrix such that the total volume of gas
diffused
becomes significant. Thus, the detection of subsurface fracture systems is
critical for
delineating desirable locations for methane exploration.
After an exploration well is drilled, specialized tools are lowered down the
bore hole to
test and record the responses of the different rock formations to various
electrical,
acoustic and radioactive stimuli. This process is termed geophysical logging,
and the
recorded data are termed geophysical logs. In one petroleum producing region
of the
world, the Western Canada Sedimentary Basin, approximately 280,000 wells have
been
drilled to date, and geophysical logs exist for virtually all of them.
Geophysical logs
have been used extensively in the past in conventional oil and gas
exploration, but little
data exist on their use in fracture detection in coal.
Some highly specialized geophysical logs are able to detect fractures in coal
under very
specific conditions, but the data are prone to error and the logging
techniques have seen
limited use. Advancement in the art delineated by the disclosed invention is
that a large
portion of previously unused data can now be processed for a new and useful
result.
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Various geophysical techniques exist to detect mud filtrate invasion and/or
mudcake.
One such device is an electrical pad containing regularly spaced electrodes.
As the
pad moves across the target formation, variations between the voltages are
recorded,
detecting the existence of mudcake on the borehole wall. This device relies on
a solid
contact with the bore hole wall and any variations in the size of the hole can
disrupt its
operation. This is significant as, over time, coals tend to cave-in resulting
in rugose
and irregular bore holes, thus limiting the utility of the pad contact type
device.
Other types of electrical logging devices exist, but all have the goal of
determining the
rock properties away from the invasive and damaging effects of the well bore.
In
general, most of these devices are able to accurately detect the depth to
which the
drilling fluid has invaded. However, because of the complex geometry of the
pore
spaces in conventional clastic and carbonate reservoir rocks and variation in
hydrocarbon saturation, invasion has not been previously considered a
quantifiable
indicator of porosity.
U.S. patent # 5,663,449 discloses a method for estimating permeability using
geophysical well log data. This method interprets data from a mufti-array
induction
device having at least five resistivity measures for a given formation and
uses a variety
of complex estimates, measurements and calculations. The measurements required
include estimates of gas gravity, cementation factor, saturation exponent,
shale volume
and, and many others. The method requires a specialized logging apparatus to
generate
the required data and is unable to examine pre-existing data.
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CA 02302995 2000-03-24
European patent EP0363259 discloses a method for interpreting data from a
formation
micro-scanner, a pad contact type of device, to detect and estimate width of
fractures
intersecting a borehole. It is limited in use and unable to examine pre-
existing data.
U.S patent # 5,379,216 discloses a method and a highly specialized apparatus
for
measuring invading volumes of mud filtrate to determine relative measurements
of
permeability. However, this patent is limited to analysis of data generated by
its own
disclosed apparatus, and is unable to analyze pre-existing data for
indications of
fracture porosity.
U.S. patent 4,961,343 discloses a method for determining permeability of a
subsurface
earth formation in real time during drilling operations through monitoring
volumes of
drill fluid lost into the formation and volumes of gas liberated. Geophysical
log
responses are not used. As well, this patent is limited in utility as no means
of
examining pre-existing data is disclosed.
Summary of the invention
This invention relates methods of detecting fracturing in rock using
geophysical logs,
and in particular to the use of electrical type logging devices. The disclosed
invention
seeks to remedy these deficiencies in the prior art of fracture detection in
coal through a
method that incorporates previously unused data into a new and useful result.
Coals are
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uniquely suited to this method, as the fractures tend to occur in a regularly
spaced
orthogonal geometry. This type and pattern of fracturing simplifies the
determination
of invading and invaded volumes.
As well, coals comprise a special case where only fractures that are
effectively
connected to the borehole are available to invasion of drilling fluids.
Fracture porosity
is then directly related to the volume of coal effectively connected for gas
diffusion,
and therefore, is a major indicator of economic methane production. The
disclosed
invention represents a significant advancement in the art as previously by-
passed
reservoirs of methane can now be found.
The disclosed invention screens existing geophysical well logs to ensure
reliable data by
discarding wells where the resistivity of the drilling mud (Rm) is less than
1Ø
Experience has shown that below this value, induction logs are affected by the
conductivity of the drilling mud and unreliable values of depth of invasion
are
produced. A second screening procedure involves the examination of the
borehole
caliper log. This log measures the size of the borehole. Measurements of the
borehole
diameter that exceed 200 % of the bit size are considered unreliable and
screened out.
Measurements of the thickness of the coal seam of interest, the bit size and
the depth of
invasion of drilling fluids define an invaded volume of coal.
From records of the characteristics of the drilling fluid, a measure of the
amount of
fluid available to create this invasion can be made. The volume of fluid
available for
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invasion is then divided by the volume of the invaded rock. The resulting
volume
fraction equals the effective void space occupied by the invading fluid.
In coal, this volume fraction of effective void space is fracture porosity, as
only
fractures are able to accept invading fluids; the matrix is impermeable. The
disclosed
invention outlines a new, useful and unconventional method for interpreting
previously
unused data and delineating methane exploration targets.
Brief Description of the Drawings
FIGS. 1 and 2 show prior art representations of an invasion and resistivity
profile in a
water-bearing zone. FIG. 1, illustrates a cross section of a wellbore showing
the
locations of the flushed zone, the transition zone and the uninvaded zone
extending
radially from the wellbore wall. FIG. 2 illustrates a radial distribution of
formation
resistivity extending radially from the wellbore wall, into the flushed zone,
into the
transition zone, and into the uninvaded zone. FIG. 3 illustrates a schematic
of a typical
well log header, showing various data collected from drilling and logging
operations.
FIG. 4 illustrates the bore hole caliper and induction geophysical logs for a
coal
interval. FIG. 5 illustrates an industry-standard interpretation chart
provided by a well
log service company, in this case Schlumberger Corporation. This chart can be
used to
determine depth of mud filtrate invasion.
CA 02302995 2000-03-24
Description of the Preferred Embodiment
FIG. 3 shows a schematic of typical log header of an induction type
geophysical log
records data used in this method. The following data collected from such a log
header
are tabulated in Table 1:
Resistivity of the mud 3.0 ohmm at~l5 Celsius
(Rm)
Fluid Loss (Water Loss 7.0 cm'
W.L)
_
Bore Hole size (Bit 200 mm
Size)
Table 1 - Data collected from a well log header
A first data screening procedure is done. The resistivity of the mud is
greater than 1.0
ohmm at 15° Celsius. The bore hole caliper size is less than 200% of
the bit size. The
data are thus far deemed acceptable for use.
FIG. 4 illustrates the caliper log and induction geophysical log for a coal
interval. The
coal is present from 453.0 m to 457.6 meters. The single solid line in the
left track is
the borehole caliper. The resistivity measurements are recorded on the right
hand
track.
In the right track, the solid line represents the shallow-reading resistivity
device, the
dotted line represents the medium-reading resistivity device and the long
dashed line
represents the deep-reading resistivity device. The resistivity is recorded in
ohm-
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meters on a logarithmic scale. From this log, the following data are recorded
in Table
2:
Bore Hole Caliper Size 220 mm
Coal Seam Thickness 4.6 meters
Deep Resistivity (RID) 100 ohmm
Medium Resistivity (RIM)113 ohmm
Shallow Resistivity 550 ohmln
(RSH)
Table 2- Data collected from the caliper log and induction log
A second data screening procedure is done at this time. The bit size from
Table 1 is
compared to the bore hole caliper size from Table 2 and the bore hole caliper
size is
less than 200% of the bit size. The data are deemed acceptable for use.
From the data in Table 2 above, the following ratios are calculated and
recorded in
Table 3:
RIM/RID ( 110/ 100) 1.13
RSHIRID (550/ 100) 5.5
Table 3 -Ratios of resistivity curves
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With these ratios, it is possible to determine the depth of mud filtrate
invasion using
industry-standard interpretation charts provided by well-log service
companies. FIG. 5
illustrates one such chart, as published by Schlumberger Corporation. By
plotting the
ratios from Table 3 onto the chart in FIG. 5, a depth of invasion of mud
filtrate (di) of
0.75 meters is determined.
With knowledge of the depth of invasion, the bit size (bts) and the thickness
(th) of the
coal seam, the volume of invaded coal can be determined. This calculation is
made by
determining the volume of a cylinder defined by the diameter of the bit plus
the depth
of invasion and the thickness of the coal seam. As coal seams tend to cave
over time,
the bit size is most indicative of borehole size in the critical few hours
after bit
penetration. Once this volume is determined, the volume of the borehole is
subtracted
to yield a volume of invaded coal (VIC). The calculation is outlined in
Formula 1.
VIC = (((di + btsl2)2 x 17 x th) - (btsl2)z x II x th (Formula 1)
Substituting the values di = 0.75, bts = 0.2 m and th = 4.6 m, it is
determined that
VIC= 10.3 m3.
The next step is to determine the amount of fluid available to create this
invaded
volume of coal. From Table 1, fluid loss (or water loss, W.L.) is listed at
7.0 cm3.
This volume is determined from a standard American Petroleum Institute (API)
test
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which uses a filter of 45.8 cm2. This volume of fluid is lost through this
filter in 30
minutes under a pressure of 689.5 Kpa (100 psi). A 689.5 Kpa differential
serves as a
reasonably good proxy to relative pressures between the invaded coal seam and
the
invading column of drilling fluid.
Dividing the fluid loss value by the area of the filter results in a volume
per cmz yields a
Standardized Fluid Loss (SFL). This is outlined in Formula 2:
SFL = Fluid loss/ API filter Area
(Formula 2)
By substituting 7.0 for Fluid loss and 45.8 cm2 for filter area, the
Standardized Fluid
Loss can be determined to be 0.15284 cm3/cmz in a 30 minute period.
The next step involves determination of the amount of time available for
invasion to
occur. This value is controlled by the sensitivity of the coal seam to
formation damage.
Research (Puri, et al. 1991) has shown that formation damage to coals can
occur in
about 24 hours (1440 minutes), after which permeability is effectively
destroyed.
Formula 3 is an American Petroleum Institute standard formula for calculating
total
volume of fluid passing through a mudcake in a given time and illustrates this
calculation.
Qt = WL x (tl30) "2 (Formula 3)
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Substituting t = 1440 minutes (24 hours), it is calculated that the volume of
invading
fluid available is 48.5 cm3 in 24 hours. To correct for the API filter size,
48.5 cm3 is
substituted into Formula 2, thus yielding a time corrected Standardized Fluid
Loss
(SFLT~) of 1.06 cm3/cm2. With SFLT~ determined, it is now possible to
calculate the
total volume of invaded filtrate by taking into account the bit size and the
thickness of
the coal seam.
From the bit size and the coal seam thickness, the surface area of the
borehole (SAbh)
can be calculated. This calculation is outlined in Formula 4.
SAbh = bts x IIx th (Formula 4)
By substituting bts =0.2 m (from Table 1) and th= 4.6 m (from Table 2), SAbh
can
be calculated as 2.89 mz or 28,900 cm2. With SAbh and SFLT~ determined, the
volume
of invading fluid (VIF) can be calculated. Formula 5 illustrates the
calculation:
VIF = SAbh x SFLT~ (Formula 5)
Substituting SAbh = 28,900 cm2 and SFLc,. = 1.06 cm3/cm2 yields a value of VIF
=
30,634 cm3 or 0.0306 m3 .
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With VIC and VIF now known, the volume fraction of void space (porosity)
occupied
by the invading fluid in the invaded coal can be calculated. As only fractures
are
available for invasion in coal, the resulting value is fracture porosity
(~F~c).
Formula 6 outlines the calculation of ~F~c.
~F~c = VIF~VIC (Formula 6)
By substituting VIF = 0.0306 m3 and VIC = 10.3 m3, ~Fn~c can be determined to
be
0.297 % .
16
