Note: Descriptions are shown in the official language in which they were submitted.
TITLE: FRACKING METHOD FOR FRACKING INTERVALS OF A
HORIZONTAL DRILLING ZONE IN A SWEET SPOT RANGE BASED ON
MEASUREMENTS OF RESISTIVITY AND NEUTRON LOGGING DATA IN
THE HORIZONTAL DRILLING ZONE
BACKGROUND OF THE INVENTION
The present invention disclosed herein relates to a well logging technology
for
underground natural resources such as shale gas, and more particularly, to a
method
for estimating slowness, Young's modulus, Poisson's ratio, and brittleness of
a
horizontal drilling zone in a sweet spot range at a shale gas play using
resistivity and
neutron logging data measured during a horizontal drilling procedure in a
sweet spot
range, and selecting fracking intervals (or fracturing spots) of the
horizontal drilling
zone in the sweet spot range using the estimated values.
Due to rapid increases in energy demand and the price rise of oil and gas in
the
last decade, the importance of developing unconventional gas such as shale
gas, tight
gas, and coaled methane is increasing in the modern oil industry. For example,
shale
gas is being actively produced at Barnett, Haynesville, Woodford, and
Eagleford of
North America. Shale gas is expected to occupy a large proportion of harvested
fossil
fuels in the future, and thus resource development and investment enterprises
and
research institutes are showing much interest in shale gas.
The reason shale gas has become commercially producible is found in a
technological
advance that can economically perform horizontal drilling that is essential in
the
development of shale gas, together with the situation of the market which
shows
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increases in the demand for energy and the price of oil. Horizontal drilling
refers to a
technology of drilling in a horizontal direction along a sweet spot that is a
reservoir.
FIG. 1 is a view illustrating reserve characteristics and drilling development
of
conventional gas and unconventional gas such as shale gas, and FIG. 2 is a
view
illustrating a shale gas play viewed from the top.
Referring to FIGS. 1 and 2, while conventional resources such as oil and gas
exist in reservoir rocks having high porosity and permeability, shale gas
exists in
compact shale having very low porosity and permeability. Also, while
conventional
oil and gas are concentrated in a specific region, shale gas is widely
distributed in a
horizontal direction along a shale layer. Accordingly, as shown in FIG. 2,
horizontal
drilling needs to be performed in a plurality of branches from a single
vertical
production well. That is, in order to effectively produce shale gas, not only
does the
shale reservoir layer need to be artificially fractured, the fracturing needs
to be
performed at a plurality of points at short intervals along the horizontal
drilling zone.
Specifically, for the economical production of unconventional gas, the
fracture
design, on which spots of the horizontal drilling zone should be fractured,
remains a
very important issue.
Cipolla et al. (2011) analyzed a hydraulic perforation effect by performing
production logging on an existing hydraulic perforated horizontal borehole.
FIG. 3
shows production logging results on four horeholes, where the horizontal axis
denotes
a percentage with respect to the total production and the vertical axis
denotes hydraulic
perforated clusters. In the boreholes No. 1 to 3, it can be seen that the
clusters
contributing to the actual gas production are only a few. As a result of the
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production logging on 100 horizontal boreholes or more, it was reported that
no gas
is produced in about 5% of the entire section and about 60% of the gas is
produced in
only about 40% of the entire section. As shown in the graph of FIG. 3, the
most
important factor for maximizing the production of shale gas is an optimal
fracture
design of a shale layer that includes shale gas.
The most important aspect of fracture design is to find spots where the
reserve
amount of gas is large and the brittleness is high. However, since shale
layers
containing unconventional gas have very low porosity and permeability unlike
typical
conventional gas containing structures, there are limitations in applying
typical well
logging techniques, which emerge as the biggest challenge facing modern
unconventional gas development.
SUMMARY OF THE INVENTION
The present invention may provide a method capable of economically and
commercially producing shale gas through optimal fracture design of a
horizontal
drilling zone within a sweet spot range where unconventional shale gas exists.
The
present invention may also provide a method of estimating the elastic modulus
and
brittleness of a stratum that are key to the fracture design and selecting
fracking
intervals of a horizontal drilling zone based on these estimated values.
Embodiments of the present invention provide methods of estimating
slowness, Young's modulus, Poisson's ratio, and brittleness of a horizontal
drilling
zone formed by extending, in a planar direction, a sweet spot in a vertical
drilling
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zone using data which are obtained by vertically drilling a reservoir rock
layer and
performing geophysical well logging, wherein resistivity logging and neutron
logging
are used.
In some embodiments, there is a method of fracturing intervals of a horizontal
drilling zone in a sweet spot range based on measurements of a reservoir rock
layer,
the method including:
measuring values representing a vertical resistivity, a vertical slowness
and a vertical neutron log value of a vertical drilling zone to obtain a
measured
vertical resistivity, a measured vertical slowness and a measured vertical
neutron log value;
determining a resistivity baseline, a slowness baseline and a neutron
baseline for the vertical drilling zone using the measured vertical
resistivity,
the measured vertical slowness and the measured vertical neutron log value;
determining a first proportional factor determined based on
relationships between the measure vertical resistivity and the resistivity
baseline, the measured vertical slowness and the slowness baseline, and the
measured vertical neutron log value and the neutron baseline;
measuring values representing a horizontal resistivity and a horizontal
neutron log value at a horizontal drilling zone to obtain a measured
horizontal
resistivity and a measured horizontal neutron log value, without conducting
sonic measurements along the horizontal drilling zone;
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estimating a horizontal slowness of a horizontal drilling zone using the
measured horizontal resistivity, the measured horizontal neutron log value and
the first proportional factor;
selecting a fracking interval of the horizontal drilling zone using the
estimate of the horizontal slowness; and
fracturing the selected fracking interval.
In further embodiments, determining the first proportional factor further
includes:
determining a first relative quantification of differences between the
measured vertical resistivity and the measure vertical slowness according to
depth and the resistivity baseline and the slowness baseline;
determining a second relative quantification of differences between the
measured vertical resistivity and measured vertical neutron log value
according
to depth and the resistivity baseline and the neutron baseline; and
determining the first proportional factor based on a correlation between
the first relative quantification and the second relative quantification
according
to depth.
In further embodiments, the measured horizontal resistivity and the measured
horizontal neutron log value are obtained by geophysical well logging.
In further embodiments, determining the first proportional factor using the
correlation between the first relative quantification and the second relative
quantification according to depth further includes:
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calculating an estimate of a first total organic carbon by depth using the
first relative quantification and a measure of a level of maturity of the
reservoir
rock layer;
calculating an estimate of a second total organic carbon by depth using
the second relative quantification and the measure of the level of maturity of
the reservoir rock layer; and
determining the first proportional factor based a correlation between
the first and second total organic carbon estimates.
In further embodiments, the level of maturity of the reservoir rock layer is
obtained by performing a geochemical test on a drilling core acquired from a
sweet
spot range of the vertical drilling zone.
In further embodiments, the first proportional factor provides an approximate
constant correlation between the first total organic carbon and the second
total organic
carbon.
In further embodiments, the method further includes:
calculating Young's modulus for the sweet spot range in the vertical
drilling zone using data obtained by performing geophysical well logging on
the vertical drilling zone;
determining a relational correlation Y between the slowness and the
Young's modulus in the sweet spot range; and
estimating the Young's modulus of the horizontal drilling zone using
the relational correlation Y.
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In further embodiments, the relational correlation Y provides an approximately
proportional expression of a correlation between the slowness obtained in the
sweet
spot range of the vertical drilling zone and the Young's modulus.
In further embodiments, the method further includes:
calculating the Poisson's ratio for the sweet spot range in the vertical
drilling zone using data obtained by performing geophysical well logging on
the vertical drilling zone;
determining a relational correlation P between the slowness and the
Poisson's ratio in the sweet spot range; and
estimating Poisson's ratio of the horizontal drilling zone using the
relational correlation P and the slowness of the horizontal drilling zone.
In further embodiments, the relational correlation P provides a correlation
between the slowness obtained in the sweet spot range of the vertical drilling
zone and
the Poisson's ratio.
In further embodiments, the method further includes:
calculating Young's modulus and Poisson's ratio for the sweet spot
range in the vertical drilling zone using data obtained by performing
geophysical well logging on the vertical drilling zone;
determining a relational correlation Y between the slowness and the
Young's modulus in the sweet spot range;
determining a relational correlation P between the slowness and the
Poisson's ratio in the sweet spot range; and
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estimating brittleness of the horizontal drilling zone using the slowness
of the horizontal drilling zone and relational correlation Y to estimate
Young's
modulus of the horizontal drilling zone and using relational correlation P to
estimate Poisson's ratio of the horizontal drilling zone.
In further embodiments, the relational correlation Y is an approximately
proportional expression of a correlation between slowness obtained in the
sweet spot
range of the vertical drilling zone and Young's modulus.
In further embodiments, the relation correlation P is an approximately
proportional expression of a correlation between slowness obtained in the
sweet spot
range of the vertical drilling zone and Poisson's ratio.
In further embodiments, the method further includes:
obtaining a measured total organic carbon of a drilling core through a
geochemical test performed on the drilling core acquired from the sweet spot
range of the vertical drilling zone;
determining a second proportional factor between a first total organic
carbon measured by the geophysical well logging and the measured total
organic carbon of the drilling core; and
estimating a reserve amount of shale gas in the horizontal drill zone by
adjusting a horizontal total organic carbon of the horizontal drilling zone by
the first proportional factor and the second proportional factor.
In further embodiments, the method further includes:
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obtaining a measured total organic carbon of a drilling core through a
geochemical test performed on the drilling core acquired from the sweet spot
range of the vertical drilling zone;
determining a third proportional factor between a value obtained by
adding an average correction factor to the first total organic carbon and the
measured total organic carbon of the drilling core; and
estimating a reserve amount of shale gas in the horizontal zone by
adjusting a horizontal total organic carbon of the horizontal zone by the
first
proportional factor and the third proportional factor.
In further embodiments, the method further includes:
obtaining a measured total organic carbon of a drilling core through a
geochemical test performed on the drilling core acquired from the sweet spot
range of the vertical drilling zone;
determining a fourth proportional factor between a value obtained by
adding an average correction factor to the second total organic carbon and the
measured total organic carbon of the drilling core; and
estimating a reserve amount of shale gas in the horizontal drill zone by
adjusting a horizontal total organic carbon of the horizontal zone by the
fourth
proportional factor.
In further embodiments, the method further includes:
obtaining a total organic carbon of a drilling core through a
geochemical test performed on the drilling core acquired from the sweet spot
range of the vertical drilling zone;
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CA 2867583 2019-06-06
determining a fifth proportional factor between the second total organic
carbon and the measured total organic carbon of the drilling core; and
estimating a reserve amount of shale gas in the horizontal drill zone by
adjusting a horizontal total organic carbon of the horizontal drilling zone by
the fifth proportional factor.
In further embodiments, the measure of the level of maturity of the reservoir
rock layer is obtained by performing a geochemical test on a drilling core
acquired
from a sweet spot range of the vertical drilling zone.
In further embodiments, the proportional factor provides an approximately
constant correlation between the first total organic carbon and the second
total organic
carbon in the vertical drilling zone according to depth.
In further embodiments, the method further includes:
calculating Young's modulus for the sweet spot range in the vertical
drilling zone using data obtained by performing geophysical well logging on
the vertical drilling zone;
determining relational correlation Y between the slowness and the
Young's modulus in the sweet spot range;
estimating the Young's modulus of the horizontal drilling zone using
the relational correlation Y; and
wherein relational correlation Y is an approximately proportional
expression between slowness obtained in the sweet spot range of the vertical
drilling zone and Young's modulus.
In further embodiments, the method includes:
CA 2867583 2019-06-06
calculating the Poisson's ratio for the sweet spot range in the vertical
drilling zone using data obtained by performing geophysical well logging on
the vertical drilling zone;
determining a relational correlation P between the slowness and the
Poisson's ratio in the sweet spot range; and
estimating Poisson's ration of the horizontal drilling zone using the
relational correlation P and the slowness of the horizontal drilling zone; and
wherein relation correlation P is an approximately proportional
expression between slowness obtained in the sweet spot range of the vertical
drilling zone and Poisson's ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide a further understanding
of the present invention, and are incorporated in and constitute a part of
this
specification. The drawings illustrate exemplary embodiments of the present
invention and, together with the description, serve to explain principles of
the present
invention. In the drawings:
FIG. 1 is a schematic view illustrating existence characteristics and drilling
development of conventional gas and unconventional gas such as shale gas;
FIG. 2 is a perspective view illustrating a shale gas play when viewed from
the
top;
FIG. 3 is a graph showing an effect of hydraulic perforation through
production
logging in a horizontal borehole (Cipolla et al., 2011):
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CA 2867583 2019-06-06
FIG. 4 is a schematic flowchart illustrating a method of estimating slowness,
Young's modulus, Poisson's ratio, and brittleness of a sweet spot horizontal
drilling
zone of a reservoir rock layer and a method of selecting fracking intervals
using the
estimated values according to the present invention;
FIG. 5 is a view illustrating contents of solid components and fluid
components
in a source rock including organic matters and a non-source rock;
FIG. 6 is a graph which Passey has made by overlapping resistivity and sonic
logging data measured in a vertical drilling zone to describe the concept of
AlogR;
FIG. 7 is a graph illustrating a relationship between AlogR and total organic
carbon (TOC) disclosed in the Passey's paper;
FIG. 8 is a table showing a potential for the TOC of a reservoir layer, which
is
quoted from "The Relationship Between Total Organic Carbon And Resource
Potential (Alexander et al., 2011)";
FIG. 9 is a view illustrating an applicability of Relational Equation 5 that
is an
element of the present invention:
FIG. 10 is a graph illustrating a correlation between slowness and Young's
modulus in a sweet spot range; and
FIG. 11 is a graph illustrating a correlation between slowness and Poisson's
ratio in a sweet spot range.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described below in
more detail with reference to the accompanying drawings. The present invention
may,
however, be embodied in different forms and should not be constructed as
limited to
the embodiments set forth herein. Rather, these embodiments are provided so
that this
disclosure will be thorough and complete, and will fully convey the scope of
the
present invention to those skilled in the art.
Hereinafter, it will be described about an exemplary embodiment of the present
invention in conjunction with the accompanying drawings.
The present invention relates to a method of estimating slowness, Young's
modulus, Poisson's ratio, and brittleness of a horizontal drilling zone in a
sweet spot
range at a shale gas play, and a method of determining a fracturing spot of
the
horizontal drilling zone using the estimated values.
The two most important factors in selecting a fracturing spot may be the
brittleness and the gas reserve amount of each region of a stratum.
Thus, the present invention provides a method of identifying the brittleness
of
a horizontal drilling zone. That is, the estimation of slowness of the
horizontal drilling
zone may be first performed, and then Young's modulus and Poisson's ratio may
be
estimated using the estimated slowness. When the Young's modulus and Poisson's
ratio are estimated, brittleness of the horizontal drilling zone may be
estimated using
the estimated Young's modulus and Poisson's ratio. In other words, Young's
modulus,
which is an elastic modulus, and Poisson's ratio are obtained by allowing the
slowness
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CA 2867583 2019-06-06
to be a common denominator, and, in turn, the brittleness may be obtained
using the
Young's modulus and Poisson's ratio.
In the present invention, the reserve amount of gas may be estimated at a
shale
gas horizontal drilling zone. Identifying the brittleness and the gas reserve
amount of
the horizontal drilling zone will make it possible to select an optimal
fracturing spot.
Hereinafter, the terms used herein will be defined as follows.
In this disclosure, the term 'horizontal drilling' does not denote only a
horizontal direction in terms of mathematical or physical meaning; rather
should be
construed as a relative concept to a vertical drilling and thus denotes a
drilling which
proceeds along a reservoir layer to be developed. That is, it should be
understood that
a horizontal drilling or horizontal drilling zone includes not only a
perfectly horizontal
plane but also an inclined plane with respect to a horizontal plane, and also
included a
curved surface as well as a flat surface. Similarly, it should be understood
that the
meaning of "vertical" in a vertical drilling does not denote an angle of 900
mathematically, but includes a drilling proceeding in a vertical or slightly
inclined
direction along the depth direction.
Prior to detailed description of the present invention, an overall process for
development of shale gas will be described in brief.
Generally, the development of shale gas begins with identifying a structure of
potential strata where gas may exist, by analyzing results of geophysical
prospecting
such as geological survey and elastic wave prospecting that are performed on
the
surface of the earth.
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When a proposed site is found by the geological survey and the physical
prospecting, the development prospect of the proposed site is evaluated
through
vertical exploratory drilling. In other words, a vertical borehole is drilled,
and
geophysical well logging and mud logging are then performed. The geophysical
well
logging refers to measuring of density, porosity, and permeability of a
stratum, and
sound wave speed in the stratum using various detectors. Examples of
geophysical
well logging may include sonic logging, density logging, and neutron logging.
The
mud logging refers to analyzing of rock fragments discharged together with the
mud
injected during excavation. The structure or ingredients of a stratum may be
identified
from the rock fragments sequentially discharged according to the depth. Also,
during
the vertical drilling, drilling cores may be collected from main sections
where shale
gas is expected to exist.
The drilling core analysis may be performed for the purpose of evaluation of a
gas content in shale and dynamic characteristics through various laboratory
tests for
analyzing an amount of gas, a type of gas, a level of maturity, an origin of
organic
matter. The test may be used for the analysis of the entire range of the
borehole by
drawing a relationship with the geophysical well logging data.
The geophysical well logging data basically serves to select a section where
shale gas is more likely to exist and determine which section will be
horizontally
drilled to meet the optimized production of shale gas.
When a promising section (hereinafter, referred to as "sweet spot") is
determined by the vertical prospecting drilling, horizontal drilling is
performed for
production of shale gas. Since shale gas is widely distributed along a
substantially
CA 2867583 2019-06-06
planar direction, the horizontal drilling may be essential. Meanwhile, an LWD
(Logging While Drilling)/MWD (Measurement While Drilling) technology by which
geophysical well logging is performed concurrently while horizontal drilling
is being
performed is being recently used. MWD means checking whether or not horizontal
drilling is accurately performed according to a designed pattern and
direction, i.e.,
along a horizontal drilling orbit while the horizontal drilling is being
performed. Along
with introduction of the LWD/MWD technology, the geophysical well logging data
about the horizontal drilling zone may be utilized to select fracturing spots
for
production of shale gas.
When horizontal drilling is completed, casing and grouting are performed.
Thereafter, specific spots of the horizontal drilling zone are artificially
fractured
through hydraulic fracture or the like. As described above, this is because
shale gas
exists in a form of free gas or adsorbed gas in a compact and dense shale
layer having
very low porosity and permeability and it is thus necessary to open a sort of
passage
for allowing shale gas to be discharged into the horizontal drilling hole
through
artificial fracturing.
Furthermore, since the shale layer has low porosity and permeability, shale
gas
in the shale layer does not smoothly move through pores. Accordingly, the
hydraulic
perforation needs to be performed in a plurality of spots. When the hydraulic
perforation is completed, shale gas may be produced in earnest.
As described above, the present invention relates to selection of fracturing
spots during the development process of shale gas which was roughly reviewed
above.
The most important issue for the commercial development of shale gas may
consist in
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whether or not economical production is possible, which, in turn, relates to
how much
the efficiency of the hydraulic perforation can increase. That is, the
essential point for
realizing commercial production is to select an interval or spot of the
horizontal
drilling zone which is easily fractured in comparison with others by the
hydraulic
perforation.
The fracturing efficiency greatly depends on the brittleness of rocks. The
Brittleness Index (BI) is determined by Young's modulus that is a dynamic
elastic
modulus of rock and Poisson's ratio, as shown in Reference Equations 1 to 3,
and is
not an absolute value but a relative value indicating a relative degree in a
target section.
Blym = (E - Emin)/(Ema. - Emin)... Reference Equation 1
BipR=(:imax - 6)/(Gmax - amin) ... Reference Equation 2
BILoG = (Blym + BlpR)/2 ... Reference Equation 3
Reference Equation 1 expresses the brittleness according to Young's modulus
(E); Reference Equation 2 expresses the brittleness according to Poisson's
ratio (a);
and Reference Equation 3 expresses the final brittleness of rocks as a mean
value of
two brittleness values.
Young's modulus denotes a resistive degree against a deformation when an
external force is applied. When Young's modulus increases, the stiffness may
increase.
Also, when Poisson's ratio representing a deformation ratio between horizontal
and
vertical directions by an external force decreases, the stiffness may
increase.
Accordingly, high Young's modulus and low Poisson's ratio mean a small
deformation
when an external force is applied. In this case, when a force having a
magnitude
greater than a certain magnitude is applied, cracks easily occur.
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Equation for obtaining Young's modulus is expressed as Reference Equation
4, and equation for obtaining Poisson's ratio is expressed as Reference
Equation 5. As
seen from Reference Equations 4 and 5, both of Young's modulus and Poisson's
ratio
are determined by the speed of sound waves (P wave and S wave) propagated in
rocks
and the density of rocks.
E = p xVs2x (3 Vp2-4V,2)/(Vp2- Vs2) ... Reference Equation 4
a = 1/2x(vp2_2vs2)/(vp2_vs2s
) Reference Equation 5
In Reference Equation 4, p is the density of a rock.
As shown in Reference Equations 1 to 5, the speed of a sound wave needs to
be measured to identify the brittleness of rocks at each spot in the
horizontal drilling
zone. However, in the horizontal drilling zone, sonic logging for directly
measuring
the speed of the sound wave is not performed. Since the sonic logging spends
much
time and cost for analysis and process of data, the sonic logging is performed
only on
several horizontal drilling zones for the purpose of sampling after the
horizontal
drilling zone has been determined for production of shale gas, and is not
performed on
most of the horizontal drilling zones.
The present invention provides a method of identifying the brittleness of
rocks
when only neutron logging and resistivity logging data are obtained without
sonic
logging data in a sweet spot horizontal drilling zone. Furthermore, the
present
invention provides a method of estimating the reserve amount of shale gas in
each
region of the horizontal drilling zone.
I lereinafter, with reference to the accompanying drawings, description will
be
sequentially given of a method of estimating slowness of a sweet spot
horizontal
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drilling zone, a method of estimating Young's modulus and Poisson's ratio
based on
the estimated slowness, a method of estimating brittleness based on the
estimated
Young's modulus and Poisson's ratio, and finally a method of selecting
fracking
intervals based on the estimated brittleness and the gas reserve amount.
FIG. 4 is a schematic flowchart illustrating a method of estimating slowness,
Young's modulus, Poisson's ratio, and brittleness of a sweet spot horizontal
drilling
zone of a reservoir rock layer and a method of selecting fracking intervals
using the
estimated values according to the present invention.
The method of estimating of slowness begins with determining a baseline from
a AlogR relational equation which is proposed through the Passey's paper
"Practical
Model for Organic Richness from Porosity and Resistivity Logs (1990)". The
AlogR
relational equation may be deduced from an existing research result regarding
an
aspect in which a source rock including a large amount of organic matters that
are
sources of oil or gas changes while undergoing thermal maturity.
FIG. 5 is a view illustrating contents of solid components and fluid
components
in a source rock including organic matters and a non-source rock. A indicates
a non-
source rock, and B indicates a form of a pre-source rock. Also, C indicates a
thermally
matured source rock under the temperature and pressure of the underground.
Referring to FIG. 5, non-source rocks consist fa solid matrix, and pores
which
are filled with underground water. The source rocks prior to maturity consist
of a solid
matrix, solid organic matters, and pores which are filled with water. When the
source
rock changes to C state through thermal maturity, a portion of the organic
matters may
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change into a gaseous state or liquefied hydrocarbon, i.e., fluid such as gas
or oil,
replacing a portion of the pores filled with water.
Based on this phenomenon, Passey has proposed that a region where the
organic matters of source rocks are transformed into oil or gas may be
estimated from
changes in resistivity and porosity (slowness of sonic logging, density and
porosity of
neutron logging). The porosity is deeply correlated with slowness through
sonic
logging, density through density logging, and neutron porosity through neutron
logging.
Neutron logging means measuring density of hydrogen atom through
measuring induced activity by neutron which is artificially radiated at high
speed
(about 10000 km/sec). To be concrete, high speed neutron radiated from a
transmitter
loses its energy when it collides with an atomic nucleus of matter, and it
loses the most
energy if it collides with hydrogen atom which has similar weight to the
neutron. If
the neutron keeps losing energy and turns to a thermal neutron, several atomic
nucleuses of matters in stratum absorb the thermal neutron and emit gamma rays
of
high energy. Through measuring thermal neutron and epithermal neutron, density
of
hydrogen atom may be calculated. Since hydrogen atom in stratum is usually
contained
in liquid filling pores (e.g. underground water), so density of hydrogen atom
reflects
volume porosity of rocks.
As explained above, both resistivity and neutron are deeply correlated with
porosity. When porosity is high, neutron is low, and resistivity also becomes
low due
to water filled into the pores.
CA 2867583 2019-06-06
A non-source rock which has organic matters, or a source rock which has
organic matters but is not thermally matured yet consists of only two
components, a
solid matrix, and water filling pores. In such rocks, the increase and
decrease in
resistivity and porosity may show the same tendency. However, when the organic
matters change into gas or oil, the porosity of a rock does not significantly
change, but
replacement of the water filled in the pores with gas or oil causes
resistivity to
significantly increase. Accordingly, the increase and decrease in porosity and
resistivity may show different tendencies from each other.
Consequently, a reservoir layer with high contents of oil or gas may be
detected
by measuring resistivity and neutron log value according to the depth through
geophysical well logging performed on the vertical drilling zone; setting, as
baselines,
neutron log value and resistivity of a region where the increase and decrease
in neutron
and resistivity show the same tendency; and then identifying a region where
the
increase and decrease in neutron log value and resistivity show different
tendencies
according to the depth. Based on the above. Passey deduced following
Relational
Equations I and 2 for sonic logging.
The Relational Equation I relates to resistivity and slowness that is an
inverse
number of sound wave speed, and Relational Equation 2 relates to resistivity
and
neutron log value. The speed of sound wave has a very good correlation with
porosity,
and thus considered as a term regarding porosity.
AlogR S = logio(Rv/Rn) + a(Atv-Ate) ... Relational Equation 1
AlogR_N = logio(Rv/Rn) - b(Nv-Nn) ... Relational Equation 2
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where a in Relational Equation 1 and b in Relational Equation 2 are correction
factors.
The Relational Equation 1 relatively quantifies differences between the
resistivity Rv and slowness Atv measured according to the depth of the
vertical drilling
zone and the resistivity baseline RB and slowness baseline At. Likewise,
Relational
Equation 2 relatively quantifies differences between the resistivity Rv and
neutron log
value Nv at each spot of the vertical drilling zone and the resistivity
baseline RB and
neutron baseline NB.
FIG. 6 is a graph which Passey has made by overlapping resistivity and sonic
logging data measured in a vertical drilling zone to describe the concept of
AlogR by
Passey.
Referring to FIG. 6, X-axis and Y-axis are set such that the former represents
resistivity and slowness that is an inverse number of sound wave speed, and
the latter
represents the depth of the vertical drilling zone.
Although resistivity and slowness are expressed in different units, values of
a
region where the increase and decrease in resistivity and slowness show the
same
tendency may be set as baselines. Also, resistivity and slowness may be
plotted in the
same graph by adjusting respective scales for resistivity and slowness. In
FIG. 6, it
can be seen that, at a depth indicated as "Baseline Interval", slowness and
resistivity
almost coincide with each other on the graph. The slowness and resistivity in
this
section is set as baselines, respectively. In the graph of FIG. 6, the
baselines of
resistivity and slowness were set to 1 ohm-m and 104sec/ft, respectively. A
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separation degree of the resistivity from the slowness by depth was quantified
into
AlogR.
In the present invention, AlogR of Passey is used. In the present invention,
resistivity Rv, slowness that is an inverse number of sound wave speed Atv,
and
neutron log value Nv are measured according to the depth through geophysical
well
logging performed on the vertical drilling zone. In order to use AlogR of
Passey, the
resistivity and slowness of a region where the increase and decrease in
resistivity Rv
and slowness Atv show the same tendency are set as the resistivity baseline RB
and
slowness baseline Ate, respectively. Similarly, the resistivity and neutron of
a region
where the increase and decrease in resistivity Rv and neutron log value Nv
show the
same tendency are set as the resistivity baseline RB and neutron baseline NB,
respectively. These values obtained are then inputted into Relational
Equations 1 and
2. In Relational Equations 1 and 2, a and p are correction factors for scale
adjustment.
For reference, Passey has set a and b to 0.02 and 4.0, respectively; however,
these
correction factors may vary based on conditions of a stratum or geophysical
well
logging.
As described above, after Relational Equations 1 and 2 regarding AlogR are
set, data measured through geophysical well logging performed on the vertical
drilling
zone are used to calculate AlogR_S and AlogR_N according to the depth, and
these
calculated values are then inputted into Relational Equations 3 and 4 below to
thereby
calculate a first total organic carbon (TOC_S) and a second total organic
carbon
(TOC_N).
TOC_S = AlogR_S x1 0c-d Relational Equation 3
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TOC N = AlogR_Nx n0(c-d 4,0M) Relational Equation 4
Relational Equations 3 and 4 are used for calculating the total organic carbon
(TOC) by the depth of the vertical drilling zone. Relational Equation 3 uses
Relational
Equation I deduced through resistivity and slowness, and Relational Equation 4
may
use Relational Equation 2 deduced through resistivity and neuron. The first
total
organic carbon TOC_S and the second total organic carbon TOC _1\1 are values
calculated by multiplying AlogR_S and AlogR_N by a term regarding the level of
maturity (LOM) of the reservoir layer.
In Relational Equations 3 and 4, c and d are correction factors, e.g., c=2.297
and d=0.1688. These values may vary with the characteristics and conditions of
a
stratum. Also, the unit of the total organic carbon is percent by weight
(wt%). The
level of maturity of the reservoir layer may be obtained by a well-known
geochemical
test on a drilling core acquired from the vertical drilling zone.
FIG. 7 is a graph illustrating a relationship between AlogR and total organic
carbon (TOC) disclosed in the Passey's paper. Referring to FIG. 7, as AlogR
and the
level of maturity (LOM) of the reservoir layer increase, the total organic
carbon (TOC)
increases. This tendency can be sufficiently understood from the basic concept
regarding AlogR and the level of maturity (LOM) of the reservoir layer.
When the first total organic carbon (TOC_S) and the second total organic
carbon (TOC_N) are calculated, the first total organic carbon TOC_S and the
second
total organic carbon TOC_N show similar tendencies according to the depth, but
do
not completely coincide with each other.
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In the present invention, therefore, a proportional factor Ni
(TOC_S=NIxTOC_N) between the first total organic carbon and the second total
organic carbon, which is approximately reasonable along the overall depth of
the
vertical drilling zone is deduced.
As described above, after Relational Equations 1 to 4 and the proportional
factor N I are deduced, slowness AtH of the horizontal drilling zone is
calculated
through Relational Equation 5 below.
A tH = AtB + [NIxTOC_NHx10-(c-d'I-A)m)- logio(RH/RB)j/a ... Relational
Equation 5
Relational Equation 5 uses Relational Equations 1 and 3 and the proportional
factor Nl. The slowness baseline At and the resistivity baseline RB that are
determined in the foregoing Relational Equations may be intactly used, and
other
values may be replaced with the values of the horizontal drilling zone. That
is, the
value obtained by Relational Equation 5 is slowness Attiof the horizontal
drilling zone.
Also, resistivity RH is also a value obtained sequentially along the orbit
through
resistivity logging in the horizontal drilling zone. As described below, TOC
NH is a
value that is obtained by calculating AlogR_NH using neutron log value NB
obtained
through neutron logging in the horizontal drilling zone. That is, Relational
Equations
2 and 4 are applied to the horizontal drilling zone.
AlogR_NH = logio (RH/RB) - b(NH-NB) ... Application of Relational Equation
2 to horizontal drilling zone.
TOC_NH = A logR_NH xl(Y"KLOM) Application of Relational Equation 4 to
horizontal drilling zone.
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The Relational Equation 5 may carry two essential considerations, which will
be described in detail below.
As described above, the starting point of this research is that slowness that
is
an inverse number of sound wave speed in the sweet spot horizontal drilling
zone needs
to be provided for the optimal hydraulic perforation but cannot be measured
because
sonic logging is not performed in the horizontal drilling zone.
In order to overcome this limitation, the first idea that slowness can be
estimated using neutron log value, resistivity, and the proportional factor N1
between
the first total organic carbon TOC_S and the second total organic carbon TOC_N
is
deduced. That is, Passey used AlogR only in estimating the total organic
carbon, but
in the present invention, the idea that slowness can be estimated using AlogR
is
inversely deduced.
When slowness is expressed as a mathematical equation using Relational
Equations 1 and 3 and the proportional factor NI only in terms of mathematics,
all
parameters may become Rv and Nv obtained from logging in the vertical drilling
zone.
First of all, since the proportional factor NI uses sonic logging of the
vertical drilling
zone, this mathematical equation has no meaning.
Due to the second idea, Relational Equation 5 expressed for slowness is
meaningful.
The second idea is an extension of thought that the resistivity baseline RB
and
the slowness baseline AtB that are obtained from the vertical drilling zone
can be
intactly converted into baselines of the horizontal drilling zone while
changing all
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parameters of the mathematic equation expressed as slowness into logging
values of
the horizontal drilling zone.
That is, when Relational Equation 5 is set as described above by combining the
two ideas, resistivity and neutron can be sequentially obtained according to
the orbit
of the horizontal drilling zone only by performing resistivity logging and
neutron
logging without performing sonic logging, and then the resistivity and neutron
log
value may be substituted in Relational Equation 5 to estimate slowness Atli of
the
horizontal drilling zone.
In principle, in order to apply the AlogR technique to the horizontal drilling
zone, the slowness baseline and the resistivity baseline should be determined
by
performing geophysical well logging including sonic logging needs to be
performed
on the entire area of the horizontal drilling zone. However, this idea is
merely a
mathematical thought that discusses only the logical consistency. According to
a
scientific point of view, particularly, according to a point of view of
geology and
resource & petroleum engineering, which expresses phenomena occurring in the
real
natural world by use of mathematical tools, when considering the formation
process
of strata or the reserve conditions of gas, it can be seen that there is no
problem in
applying the baseline of the vertical drilling zone intaetly to the horizontal
drilling
zone.
That is, the scientific and actual reason why the baseline prepared in the
vertical
drilling zone can be intactly applied to Relational Equation 6 regarding the
horizontal
drilling zone is because the horizontal drilling zone is finally an planar
extension of
the sweet spot in the vertical drilling zone and the planar extension is not
long but
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limited to several kilometers. In other words, the criteria for the sweet spot
in the
vertical drilling zone may be similarly applied to the horizontal drilling
zone.
For example, as shown in FIG. 9, the horizontal drilling zone may be inserted
into the sweet spot region in the vertical drilling zone, and in this state,
it may be very
reasonable to use the baseline of the vertical drilling zone.
As described above, the reasonability of Relational Equation 5 may be
sufficiently accepted in viewpoints of geology and resource & petroleum
engineering.
Above all, it was verified that slowness calculated by Relational Equation 5
had a very
good correlation with slowness obtained through direct sonic logging in the
horizontal
drilling zone, through the empirical study.
As described above, after the slowness is estimated, Young's modulus and
Poisson's ratio are estimated.
That is, Young's modulus and Poisson's ratio by depth in the sweet spot range
may be calculated using the sound wave speed and the density that are obtained
by
geophysical well logging in the vertical drilling zone. For example, the
Young's
modulus calculation formula of Reference Equation 4 and the Poisson's ratio
calculation formula of Reference Equation 5 may be utilized.
For more readability of this disclosure, Reference Equations 4 and 5 will be
again described below.
As seen from Reference Equations 4 and 5, Young's modulus and Poisson's
ratio may be determined by the speeds of sound waves (P wave and S wave)
propagated in rocks and the density of rocks. In Reference Equation 4, p is
the density
of a rock.
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CA 2867583 2019-06-06
E = p xVs2x (3 Vp2-4Vs2)/(Vp2-Vs2) ... Reference Equation 4
= 1/2 x(Vp2-2Vs2)/(vpz_vs2) ... Reference Equation 5
Also, a correlation between Young's modulus and slowness by depth in the
sweet spot range and a correlation between Poisson's ratio and slowness may be
deduced. In the present invention, instead of calculating Young's modulus and
Poisson's ratio of the horizontal drilling zone by Reference Equations 4 and 5
using
the speed of sound wave, the correlations of Young's modulus and Poisson's
ratio with
respect to slowness obtained from the vertical drilling zone are deduced by
separate
Relational Equations, and then slowness estimated in the horizontal drilling
zone is
input into the relational equation to thereby estimate Young's modulus and
Poisson's
ratio.
FIG. 10 is a graph illustrating a correlation between slowness and Young's
modulus in sweet spot range, and FIG. 11 is a graph illustrating a correlation
between
slowness and Poisson's ratio in sweet spot range. In the graphs of FIGS. 10
and II,
X-axis represents slowness measured through geophysical well logging in the
sweet
spot range, and Y-axes respectively represent Young's modulus and Poisson's
ratio
obtained using the speed of sound wave and the density of a region where the
corresponding slowness is calculated.
In the present invention, the correlation between slowness and Young's
modulus is quantified as Relational Equation Y, and the correlation between
slowness
and Poisson's ratio is deduced as Relational Equation P. Since all Young's
modulus
and Poisson's ratio with respect to slowness may not completely coincide with
each
other, X and Y coordinates may be mathematically plotted to approximately
deduce
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CA 2867583 2019-06-06
Relational Equations Y and P. Relational Equations Y and P may be expressed as
a
linear function or a polynomial function.
The important point is that Relational Equations Y and P are not universal
concepts that arc generally applied to all reservoir layers, and are concepts
that should
be deduced by independently analyzing the correlation of slowness and Young's
modulus by each shale gas play or each vertical drilling zone as a smaller
unit area.
In the method of estimating Young's modulus and Poisson's ratio of the
horizontal drilling zone in the reservoir layer according to the present
invention.
Young's modulus and Poisson's ratio are finally estimated by substituting the
slowness
obtained from Relational Equation 5 at the horizontal drilling zone, in
Relational
Equations Y and P.
As also described above, since Relational Equations Y and P are deduced by
verifying the correlation between slowness and Young's modulus/Poisson's ratio
at the
sweet spot range in the vertical drilling zone, Relational Equations Y and P
may also
be sufficiently applied to the horizontal drilling zone. This is because the
horizontal
drilling zone is a planar extension of the sweet spot range within a certain
interval.
As described above, in the present invention, it is possible to estimate
slowness
of the horizontal drilling zone reliably by only using resistivity and neutron
logging
without performing direct sonic logging on the horizontal drilling zone.
Furthermore,
Young's modulus and Poisson's ratio of the horizontal drilling zone may be
estimated
using Relational Equations Y and P regarding slowness-Young's modulus and
slowness-Poisson's ratio, which are obtained by verification regarding the
vertical
drilling zone.
CA 2867583 2019-06-06
When Young's modulus and Poisson's ratio for the whole orbit of the horizontal
drilling zone according to the present invention are estimated, brittleness
regarding the
whole orbit of the horizontal drilling zone may be identified.
When Young's modulus and Poisson's ratio for the whole orbit of the horizontal
drilling zone are sequentially estimated, data regarding Young's modulus and
Poisson's
ratio may be calculated by Reference Equations 1 to 3 described above. For
more
readability of this disclosure, Reference Equations 1 to 3 will be again
described
below.
BIym = (E - Emm)/(Emax - Emm) ... Reference Equation 1
BIpu = (Gmax - o)/(arnax - Omin) ... Reference Equation 2
BIL =(Blym + Blpi0/2 ... Reference Equation 3
In Reference Equation 1, E denotes Young's modulus at each spot of the
horizontal drilling zone, and Emm and Emax denote the smallest value and the
greatest
value of Young's moduli measured over the whole the horizontal drilling zone,
respectively. Similarly, in Reference Equation 2, o denotes Poisson's ratio at
each spot
of the horizontal drilling zone, and omm and lama, denote the smallest value
and the
greatest value of Poisson's ratios measured over the whole the horizontal
drilling zone,
respectively. Since both Young's modulus and Poisson's ratio are expressed as
a
difference between the highest value and the lowest value at a certain
interval (i.e.,
horizontal drilling zone), Young's modulus and Poisson's ratio are relative
values.
As described above, according to the present invention, slowness, Young's
modulus, and Poisson's ratio at each spot in the horizontal drilling zone of
the reservoir
layer are sequentially estimated without performing sonic logging on the
horizontal
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CA 2867583 2019-06-06
drilling zone, and finally, brittleness at each spot of the horizontal
drilling zone can be
reliably calculated.
Brittleness identified at each spot of the horizontal drilling zone is
expected to
be very usefully utilized in designing efficient and economical hydraulic
perforation
in future.
Meanwhile, in the present invention, the reserve amount of shale gas is
estimated by each region of the horizontal drilling zone in order to select
fracturing
spots.
In the present invention, four methods are provided to estimate the reserve
amount of shale gas, each of which may be expressed as Relational Equation H1
to
H4. Among Relational Equations HI to H4, it is desirable to use Relational
Equations
HI and H3.
A first method of estimating the reserve amount TOC _11 of shale gas in the
horizontal drilling zone may be expressed as Relational Equation H1 below.
TOC_H = N3 xN 1 x TOC_Nii ...Relational Equation Hi
Here, N3 denotes a proportional factor between the total organic carbon
TOC_C of the drilling core and a value obtained by adding an average
correction factor
V to the first total organic carbon TOC_S measured through geophysical well
logging,
which may be mathematically expressed as below.
TOC_C = N3 x(TOC_S + V)
The total organic carbon TOC C of the drilling core is a value obtained by
performing a geochemical test on the drilling core acquired from the sweet
spot range
of the vertical drilling zone.
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CA 2867583 2019-06-06
TOC_ NH is obtained from AlogR_NH below. AlogR_NH intactly uses the
baselines for applying the logR technique to the vertical drilling zone as
described
below. The reason why the baselines can be intactly used is the same as the
reason
described in the method of estimating slowness of the horizontal drilling
zone.
However, both parameters, resistivity and neutron log value, which are values
measured in the horizontal drilling zone, are respectively expressed as R11
and NH, by
using the subscript of H.
AlogR_NH = logio(RH/RB) - b(NH-NB)
TOC_NH = AlogR_NHXi 0(c-d,1,0M)
That is, the total organic carbon of the horizontal zone may be obtained by
measuring resistivity RH and neutron log value NH of the horizontal drilling
zone.
However, the total organic carbon TOC_NH of the horizontal zone is not
directly estimated as the reserve amount of shale gas, but is multiplied by
the
proportional factors NI and N3. The reason is as follows. The most reliable
value
regarding the reserve amount of shale gas is the total organic carbon TOC_C of
a
drilling core, which is a value obtained by performing a geochemical test on
the drilling
core acquired by directly drilling a stratum. Thus, the proportional factor is
set
between the total organic carbon TOC_C of the drilling core and the first
total organic
carbon TOC_S measured through geophysical well logging in the vertical
drilling
zone, and then multiplied by the proportional factor N2 between the first
total organic
carbon TOC_S and the second total organic carbon TOC_N that are already set.
Resultantly, the reserve amount TOC_H of shale gas in the horizontal drilling
zone
may be obtained. However, it is desirable to add the average correction factor
V to the
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CA 2867583 2019-06-06
first total organic carbon before the proportional factor is obtained. This is
because
the first total organic carbon is a relative value with respect to the whole
range in the
vertical drilling zone, and thus the amount of shale gas that is generally
reserved in the
reservoir layer should be included. The correction factor V may be set to
about 0.8
wt%. Consequently, the proportion factor N3 becomes a proportional factor
between
the total organic carbon TOC_C of the drilling core and the value obtained by
adding
the average correction factor V to the first total organic carbon TOC_S of the
vertical
drilling zone.
Meanwhile, Relational Equation H3 specifying another method for estimating
the reserve amount TOC_H of shale gas does not need to use the proportional
factor
NI between the first total organic carbon TOC_S and the second total organic
carbon
TOC_N. That is, after the proportional factor N4 between the second total
organic
carbon TOC_N and the total organic carbon TOC_C of the drilling core in the
vertical
drilling zone is directly deduced, the reserve amount of shale gas TOC_H may
be
obtained by directly multiplying the proportional factor N4 by the total
organic carbon
TOC_NH of the horizontal drilling zone. In this case, the proportional factor
N4 is
deduced after adding the average correction factor V. The proportional factor
N and
the estimation of the reserve amount of shale gas may be further understood by
equations below.
TOC_C = N4x(TOC_N + V)
TOC_H = N4xTOC_NH ...Relational Equation H3
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Meanwhile, Relational Equations 112 and H4 described below differ from
Relational Equations HI and 1-13 only in the proportional factor. In this
case, the
average correction factor is not considered in the proportional factor.
TOC_H = N 1 xN2xTOC_NH ...Relational Equation 1-12, TOC_C =
N2xTOC_S,
TOC H = N5 xTOC_Nn ...Relational EquationH4, TOC_C = N5xTOC_N
If the average correction factor of the reservoir layer is not considered as
in
Relational Equations H2 and H4, the reliability in calculation of the reserve
amount of
shale gas may be reduced. However, even though the average correction factor
is not
used, it is possible to identify whether the reserve amount of shale gas in
each range
of the horizontal drilling zone is relatively large or small, results from
Relational
Equations 112 and H4 may be utilized as meaningful data in selection of the
fracturing
spot of the horizontal drilling zone.
For reference, in development of shale gas, the reserve amount of shale gas in
the horizontal drilling zone may be classified according to the conditions
proposed in
the table of FIG. 8. The table of FIG. 8 is quoted from ''The relationship
between total
organic carbon and resource potential (Alexander et al., 2011)".
As describe above, in the present invention, fracturing spots having good
brittleness and much shale gas thereunder may be selected by estimating
brittleness of
the horizontal drilling zone and the reserve amount of shale gas. Once the
brittleness
and the reserve amount of shale gas are identified, selection of the
fracturing spots
based on certain criteria may be sufficiently achieved in practical point of
view.
According to the conditions and situations of the shale gas play, the reserve
amount of
CA 2867583 2019-06-06
shale gas may be ignored, and the hydraulic perforation spots may be selected
only in
consideration of brittleness, and vice versa logically.
However, in any situation, according to the present invention, it is very
significantly useful to identify the brittleness of the horizontal drilling
zone and the
reserve amount of shale gas in selection of the fracturing spots.
In this disclosure, the terms "fracturing spot" and "fracking intervals" are
used
together; however, since hydraulic perforation is performed on a certain
interval inside
a packer after the packer is installed, the expression "interval" is used.
However, since
brittleness of rocks according to the present invention is sequentially
measured by each
spot through geophysical well logging, the expression "spot" is used.
Although AlogR value is described based on slowness, slowness is an inverse
number of wave sound speed, and therefore aforesaid Relational Equations may
be
naturally developed based on the speed of sound wave, which is also included
in the
scope of the present invention.
According to an embodiment, spots having high brittleness in a horizontal
drilling zone can be efficiently estimated by estimating a slowness value,
Young's
modulus, and Poisson ratio in the horizontal drilling zone where shale gas is
concentrated, and thus fracking intervals can be efficiently selected.
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CA 2867583 2019-06-06
Particularly, the brittleness of each spot of the horizontal drilling zone can
be
estimated using only resistivity and neutron logging, without performing sonic
logging
which is restrictively performed due to economical limitations despite it
being the most
essential factor in determining brittleness.
Also, since the reserve amount of shale gas in the horizontal drilling zone
can
be estimated using a baseline used for the application of the AlogR method in
a vertical
drilling zone, fracturing spots of the horizontal drilling zone may be
effectively
selected.
The above-disclosed subject matter is to be considered illustrative, and not
restrictive, and the appended claims are intended to cover all such
modifications,
enhancements, and other embodiments, which fall within meaning of the claims
as
understood by the person skilled in the art. Thus, to the maximum extent
allowed by
law, the scope of the present invention is to be determined by the broadest
permissible
interpretation of the following claims and their equivalents, and shall not be
restricted
or limited only to the exemplary embodiments set out in the foregoing detailed
description.
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