CA2867583A1 - 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 drillingzone - Google Patents

Abstract

The present invention relates to a method of estimating slowness, Young's modulus, Poisson's ratio, brittleness, and the reserve amount of gas of a horizontal drilling zone in a sweet spot range at a shale gas play using resistivity and neutron logging data, and a method of determining fracking intervals of the horizontal drilling zone using the estimated values. In the present invention, slowness of a sweet spot horizontal drilling zone is estimated using a combination of neutron logging and resistivity logging data without performing sonic logging on the sweet spot horizontal drilling zone. Also, elastic modulus and brittleness of the horizontal drilling zone are estimated based on the slowness, and the reserve amount of shale gas is estimated for each region of the sweet spot horizontal drilling zone.

Description

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 BASED ON
RESISTIVITY AND NEUTRON LOGGING DATA AND METHOD FOR
DETERMINING FRACKING INTERVALS OF A HORIZONTAL DRILLING
ZONE IN A SWEET SPOT RANGE AT A SHALE GAS PLAY USING THE
SAME
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 modem 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 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 boreholes, where the horizontal axis denotes a percentage with respect to the total production and the vertical axi.s 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

2 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 modem 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 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.

3 In some embodiments, methods of estimating slowness of a sweet spot horizontal drilling zone of a reservoir rock layer, according to the present invention, include:
(a) determining a resistivity baseline (RB), a slowness baseline (AtB) that is an inverse of a sound wave speed, and a neutron baseline (NB) using resistivity (Rv), slowness (Atv) that is an inverse of a sound wave speed, and neutron log value (Nv), which are measured at a vertical drilling zone, and then inputting the determined values to Relational Equations 1 and 2 below, AlogR_S = logl 0(RviRB) + a(Atv-AtB) ... Relational Equation 1 AlogR_D = log 10(Rv/RB) - b(Nv-NB) ... Relation Equation 2 where a in Relational Equation 1 and b in Relational Equation 2 are correction factors;
(b) calculating a first total organic carbon (TOC_S) of the vertical drilling zone through Relational Equation 3 below using the resistivity (Rv) and the slowness (Atv) obtained by performing the geophysical well logging on the vertical drilling zone, calculating a second total organic carbon (TOC_N) through Relational Equation 4 below using the resistivity (Rv) and the neutron log value (Nv), and deducing a proportional factor (Ni) between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N), c-dx ...
TOC_S = AlogR_Sx 10(tow Relational Equation 3 TOC_N = AlogR_Nx10("xl,om) Relational Equation 4 where, EOM denotes a level of maturity of a reservoir layer, and c and d are correction factors; and (e) deducing Relational Equation 5 below using Relational Equations I and 3 and the proportional factor (Ni), and estimating slowness (Atli) of the horizontal

4 drilling zone by inputting, into Relational Equation 5, TOC_NH obtained through AlogR_NH according to resistivity (RH) and neutron log value (NH) of the horizontal drilling zone which are obtained by geophysical well logging, AtH = AtB + [Ni x TOC_NHx10-("x wm)- loglO(RH/RB)Va ...Relational Equation 5.
In some embodiments, the level of maturity in Relational Equations 3 and 4 may be obtained by performing a geochemical test on a drilling core acquired from a sweet spot range of the vertical drilling zone.
In other embodiments, the proportional factor (N1) may deduce, as an approximate constant, a correlation between the first total organic carbon (TOC_S) and the second total organic carbon (Toc_N) in the vertical drilling zone, when X-axis represents a depth of the vertical drilling zone and Y-axis represents the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N). For example, the proportional factor (NI) may deduce a correlation between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N) in the vertical drilling zone when X-axis represents the second total organic carbon(TOC_N) and Y-axis represents the first total organic carbon (TOC_S).
In other embodiments of the present invention, methods of estimating Young's modulus of a sweet spot horizontal drilling zone of a reservoir rock layer, according to the present invention, include:
estimating slowness of the horizontal drilling zone by the aforesaid methods;
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, and deducing Relational Equation Y which mathematizes a correlation between the slowness and the Young's modulus in the sweet spot range; and

5 then inputting the slowness into Relational Equation Y to estimate Young's modulus of the horizontal drilling zone.
In some embodiments, when X-axis represents slowness and Y-axis represents Young's modulus, Relational Equation Y may deduce, as an approximate proportional expression, a correlation between the slowness obtained in the sweet spot range of the vertical drilling zone and the Young's modulus.
In still other embodiments of the present invention, methods of estimating Poisson's ratio of a sweet spot horizontal drilling zone of a reservoir rock layer, according to the present invention, include:
estimating slowness of the horizontal drilling zone by the aforesaid methods;
calculating the Poisson's ratio for the sweet spot range in the vertical drilling zone using data obtained by performing geophysical well logging data on the vertical drilling zone, and deducing Relational Equation P which mathematizes a correlation between the slowness and the Poisson's ratio in the sweet spot range; and 1 5 then inputting the slowness into Relational Equation P to estimate Poisson's ratio of the horizontal drilling zone.
In some embodiments, when X-axis represents slowness and Y-axis represents Poisson's ratio, Relational Equation P may deduce, as an approximate proportional expression, a correlation between the slowness obtained in the sweet spot range of the vertical drilling zone and the Poisson's ratio.
In even other embodiments of the present invention, methods of estimating brittleness of a sweet spot horizontal drilling zone of a reservoir rock layer, according to the present invention, include:
estimating the Young's modulus and Poisson's ratio of the horizontal drilling zone using the Young's modulus estimating method and the Poisson' ration

6 estimating method as described above; and then estimating the brittleness of the horizontal drilling zone using the Young's modulus and Poisson's ratio of the horizontal drilling zone estimated by Relational Equations Y and P.
In yet other embodiments of the present invention, methods of selecting fracking intervals of a sweet spot horizontal drilling zone of a reservoir rock layer, according to the present invention, include selecting fracking intervals of the horizontal drilling zone formed by extending, in a planar direction, a sweet spot in a determined vertical drilling zone which is determined using data obtained by vertically drilling a reservoir rock layer and performing geophysical well logging, wherein the fracking intervals are achieved by estimating the brittleness of the horizontal drilling zone and a reserve amount of shale gas in the horizontal drilling zone, and the brittleness of the horizontal drilling zone is estimated based on the slowness, Young's modulus, and Poisson's ratio estimated by the above-described methods.
In some embodiments, the estimating of the reserve amount (TOC_I-l) of shale gas in the horizontal drill zone may use any one of four Relational Equations (Relational Equations H1 to H4) below.
in further embodiments, the estimating of the reserve amount (TOC H) of shale gas in the horizontal drill zone may include: obtaining a total organic carbon (TOC_C) of a drilling core by performing a geochemical test on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N3) between a value obtained by adding an average correction factor (V) to a first total organic carbon (TOC_S) measured by the geophysical well logging and the total organic carbon (TOCS) of the drilling core, TOC_C = N3 x (TOCS + V)

7 TOC = N3xN1xTOC NH ...Relational Equation HI
As seen from Relational Equation HI, the reserve amount (TOC_H) of shale gas in the horizontal drill zone may be estimated by multiplying a total organic carbon (TOC_NH) of the horizontal drilling zone by the proportional factor (N3) and the proportional factor (Ni) between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N) of the vertical drilling zone.
In other embodiments, the estimating of the reserve amount (TOC_H) of shale gas in the horizontal drill zone may include: obtaining a total organic carbon (TOC_C) of a drilling core by performing a geochemical test on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N2) between a first total organic carbon (TOC_S) measured by the geophysical well logging and the total organic carbon (TOC_C) of the drilling core, TOC C = N2xTOC S
TOCH = NI x N2xTOC_NH ...Relation Equation H2 As seen from Relational Equation H2, the reserve amount (TOC_H) of shale gas in the horizontal drill zone may also be estimated by multiplying a total organic carbon (TOCN11) of the horizontal drilling zone by the proportional factor (N2) and the proportional factor (NI) between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N) of the vertical drilling zone.
In still other embodiments, the estimating of the reserve amount (TOC_H) of shale gas in the horizontal drill zone may include: obtaining a total organic carbon (TOC_C) of a drilling core by performing a geochemical test on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N4) between a value obtained by adding an average correction

8 factor (V) to a second total organic carbon (TOC_N) measured by the geophysical well logging and the total organic carbon (TOC_C) of the drilling core, TOC_C = N4x(TOC_N + V) TOC_H = N4xTOC_NH ...Relational Equation H3 As seen from Relational Equation H3, the reserve amount (TOC_H) of shale gas in the horizontal drill zone may be estimated by multiplying a total organic carbon (TOC_NH) of the horizontal drilling zone by the proportional factor (N4).
In even other embodiments, the estimating of the reserve amount (TOC H) of shale gas in the horizontal drill zone may include: obtaining a total organic carbon (TOC_C) of a drilling core by performing a geochemical test on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N5) between a second total organic carbon (TOC_N) measured by the geophysical well logging and the total organic carbon (TOC C) of the drilling core, TOC C = N5xTOC N
TOC_H = N5xTOC_N11 ...Relational Equation H4 As seen from Relational Equation H4, the reserve amount (TOC_H) of shale gas in the horizontal drill zone may be estimated by multiplying a total organic carbon (TOC_NH) of the horizontal drilling zone by the proportional factor (N5).
All of the proportional factors used in Relational Equations H2 and H4 exclude the average correction factor (V) related to the average amount of shale gas in the stratum. Not considering the average correction factor may cause the reliability in calculation of the reserve amount of shale gas in the horizontal drilling zone to be deteriorated. However, it is possible to know whether the reserve amount of shale gas in the horizontal drilling zone is relatively large or small even in the case

9 of not using the average correction factor, and therefore, results from Relational Equations H2 and H4 may be utilized as meaningful data in selecting a fracturing spot of the horizontal drilling zone.
In yet other embodiments, a level of maturity in Relational Equations 3 and 4 may be obtained by performing a geoehemical test on a drilling core acquired from a sweet spot range of the vertical drilling zone.
In further embodiments, the proportional factor (Ni) may deduce, as an approximate constant, a correlation between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N) in the vertical drilling zone when X-axis represents a depth of the vertical drilling zone and Y-axis represents the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N). For example, the proportional factor (Ni) may deduce a correlation between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N) in the vertical drilling zone when X-axis represents the second total organic carbon(TOC_N) and Y-axis represents the first total organic carbon (TOC_S) 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);
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.

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 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 90 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.
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 planar direction, the horizontal drilling may be essential. Meanwhile, an LWD
(Logging While Diilling)/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 tow 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 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.
BIym (E - Enlin)/(Emax - Emin)... Reference Equation 1 Blp[i = (csmax - (5)/(umax - amin) ... Reference Equation 2 131t,00 = (BIym + BIpR)/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 Iwo 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.
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 = pxVs2x(3Vp2-4Vs2)/(Vp2-Vs2) ... Reference Equation 4 o = 1/2x(Vp2-2Vs2)/(Vp2-V2) ... 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.
Hereinafter, with reference to the accompanying drawings, description will be sequentially given of a method of estimating slowness of a sweet spot horizontal 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 Al.ogR 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 of a 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 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 epithermai 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.
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 1 and 2 for sonic logging.
The Relational Equation 1 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/RB) + a(Atv-AtB) ... Relational Equation 1 AlogR_N = logio(Rv/RB) - b(Nv-NB) Relational Equation 2 where a in Relational Equation 1 and b in Relational Equation 2 are correction factors.
The Relational Equation I 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 AtB.
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 N13.
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 100u.sec/ft, respectively.
A
separation degree of the resistivity from the slowness by depth was quantified into AlogR.
In the present invention, AlogR of Passcy 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 At 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 I
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 I 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_Sx10("xwm) ... Relational Equation 3 TOC_N = AlogR_Nx I e-dxL0M) 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_ N
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., 0=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 geoehemical 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 (TOCN) 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.

In the present invention, therefore, a proportional factor Ni (TOC_S=N1xTOC_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 NI are deduced, slowness AtH of the horizontal drilling zone is calculated through Relational Equation 5 below.
A tn = At + [N1><TOC No10-(c-cixL0M).. logio(RH/RB)Va ... Relational Equation 5 Relational Equation 5 uses Relational Equations 1 and 3 and the proportional factor NI. The slowness baseline AtB 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 AtH of 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 NH 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 (R14/R8) - b(NH-NB) ... Application of Relational Equation 2 to horizontal drilling zone.
TOC_NH = 0(c-dxL0M) Application of Relational Equation 4 to horizontal drilling zone.
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 NI
between the first total organic carbon TOC_S and the second total organic carbon TOCN
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 I 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 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 At of the horizontal drilling zone.
In principle, in order to apply the dlogR 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 intactly 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 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.
E = oxVs2x(3Vp2-4Vs2)/(Vp2-Vs2) Reference Equation 4 o = 1/2x(Vp2-2Vs2)/(Vp2-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 11, 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 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 are 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.
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 - EmitY(Emax - Einin) =.. Reference Equation I

Blra = - - amin) ... Reference Equation 2 BUG + BIpR)/2 ... Reference Equation 3 In Reference Equation I, E denotes Young's modulus at each spot of the horizontal drilling zone, and Enit, and E,õ 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, ci denotes Poisson's ratio at each spot of the horizontal drilling zone, and amin and cõõix 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 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 HI
to 114. 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_H of shale gas in the horizontal drilling zone may be expressed as Relational Equation 111 below.
TOC_H = N3 x N1xTOC_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 TOGS measured through geophysical well logging, which may be mathematically expressed as below.
TOCS = N3x(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.
TOC_ NH is obtained from AlogR_Nit 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 R1.4 and NH, by using the subscript of H.
AlogRNn = logio(RH/RB) - b(NH-NB) Toc_Nii = AlogR_NHX10("xL0M) 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_Na 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 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 1-13 specifying another method for estimating the reserve amount TOC_H of shale Ras does not need to use the proportional factor 1\11 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_Nli 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_N11 ...Relational Equation H3 Meanwhile, Relational Equations H2 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.
TOCH = N1 xN2xTOC_NH ...Relational Equation El2, TOC_C =
N2xTOC_S, Toc_H = N5 x TOC NH ...Relational EquationH4, TOC_C = N5xTOC_N
If the average correction factor of the reservoir layer is not considered as in Relational Equations H2 and 1-14, 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 H2 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 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 'Tracking 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.

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 the true spirit and scope of the present invention. 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 by the foregoing detailed description.

Claims (19)

WHAT IS CLAIMED IS:

1. A method of estimating slowness of a horizontal drilling zone formed by extending, in a planar direction, a sweet spot range in a vertical drilling zone which is determined using data obtained by vertically drilling a reservoir rock layer to perform geophysical well logging, the method comprising:
(a) determining a resistivity baseline (R B), a slowness baseline (.DELTA.t B) that is an inverse of a sound wave speed, and a neutron baseline (N B) using resistivity (Rv), slowness (.DELTA.t V) that is an inverse number of a sound wave speed, and neutron log value (N V), which are measured at a vertical drilling zone, and then inputting the determined values to Relational Equations 1 and 2 below .DELTA.logR_S = log10(R V/R B) + a(.DELTA.t V-.DELTA.t B) ... Relational Equation 1 .DELTA.logR_N = log10(R V/R B) - b(N V-N B) ... Relation Equation 2 where, a in Relational Equation 1 and b in Relational Equation 2 are correction factors;
(b) calculating a first total organic carbon (TOC_S) of the vertical drilling zone through Relational Equation 3 below using the resistivity (Rv) and the slowness (.DELTA.tV) obtained by performing the geophysical well logging regarding the vertical drilling zone, calculating a second total organic carbon (TOC_N) through Relational Equation 4 below using the resistivity (Rv) and the neutron log value (Nv), and deducing a proportional factor (N1) between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N), TOC_S = .DELTA.logR_S × 10(c-d×LOM) ... Relational Equation 3 TOC_N = .DELTA.logR_D× 10(c-d×LOM) ... Relational Equation 4 where, LOM denotes a level of maturity of a reservoir layer, and c and d are correction factors; and (c) deducing Relational Equation 5 below using Relational Equations 1 and 3 and the proportional factor (N1), and estimating slowness (.DELTA.t H) of the horizontal drilling zone by inputting, into Relational Equation 5, TOC_N H obtained through .DELTA.logR_N H according to resistivity (R H) and neutron log value (N H) of the horizontal drilling zone which are obtained by geophysical well logging, .DELTA.t H = .DELTA.t B + [N1×TOC_N H×10-(c-d×LOM)- log10(R
H/R B)/a ...Relational Equation 5.

2. The method of claim 1, wherein the level of maturity of Relational Equations 3 and 4 is obtained by performing a geochemical test on a drilling core acquired from a sweet spot range of the vertical drilling zone.

3. The method of claim 1, wherein the proportional factor (N1) deduces, as an approximate constant, a correlation between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N) in the vertical drilling zone, when X-axis represents a depth of the vertical drilling zone and Y-axis represents the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N).

4. A method of estimating Young's modulus of a horizontal drilling zone formed by extending, in a planar direction, a sweet spot range in a vertical drilling zone which is determined using data obtained by vertically drilling a reservoir rock layer to perform geophysical well logging, the method comprising:
estimating slowness of the horizontal drilling zone by the method of claim 1;
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, and deducing Relational Equation Y which mathematizes a correlation between the slowness and the Young's modulus in the sweet spot range; and then inputting the slowness of the horizontal drilling zone into Relational Equation Y to estimate Young's modulus of the horizontal drilling zone.

5. The method of claim 4, wherein, when X-axis represents slowness and Y-axis represents Young's modulus, Relational Equation Y deduces, as an approximately proportional expression, a correlation between the slowness obtained in the sweet spot range of the vertical drilling zone and the Young's modulus.

6. A method of estimating Poisson's ratio of a horizontal drilling zone formed by extending, in a planar direction, a sweet spot range in a vertical drilling zone which is determined using data obtained by vertically drilling a reservoir rock layer to perform geophysical well logging, the method comprising:
estimating slowness of the horizontal drilling zone by the method of claim 1;
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, and deducing Relational Equation P which mathematizes a correlation between the slowness and the Poisson's ratio in the sweet spot range; and then inputting the slowness of the horizontal drilling zone into Relational Equation P to estimate Poisson's ratio of the horizontal drilling zone.

7. The method of claim 6, wherein, when X-axis represents slowness and Y-axis represents Poisson's ratio, Relational Equation P deduces a correlation between the slowness obtained in the sweet spot range of the vertical drilling zone and the Poisson's ratio.

8. A method of estimating brittleness of a horizontal drilling zone formed by extending, in a planar direction, a sweet spot range in a vertical drilling zone which is determined using data obtained by vertically drilling a reservoir rock layer to perform geophysical well logging, the method comprising:
estimating slowness of the horizontal drilling zone by the method of claim 1;
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, and deducing Relational Equation Y which mathematizes a correlation between the slowness and the Young's modulus in the sweet spot range and Relational Equation P which mathematizes a correlation between slowness and Poisson's ratio in the sweet spot range;
inputting the slowness of the horizontal drilling zone into Relational Equations Y and P to estimate Young's modulus and Poisson's ratio of the horizontal drilling zone; and estimating brittleness of the horizontal drilling zone using the Young's modulus and Poisson's ratio of the horizontal drilling zone estimated by Relational Equations Y and P.

9. The method of claim 8, wherein, when X-axis represents slowness and Y-axis represents Young's modulus, Relational Equation Y deduces, as an approximately proportional expression, a correlation between slowness obtained in the sweet spot range of the vertical drilling zone and Young's modulus.

10. The method of claim 8, wherein, when X-axis represents slowness and Y-axis represents Poisson's ratio, Relational Equation P deduces, as an approximately proportional expression, a correlation between slowness obtained in the sweet spot range of the vertical drilling zone and Poisson's ratio.

11. A method of selecting fracking intervals of a horizontal drilling zone formed by extending, in a planar direction, a sweet spot range in a vertical drilling zone which is determined using data obtained by vertically drilling a reservoir rock layer to perform geophysical well logging, wherein the fracking intervals are achieved by estimating the brittleness of the horizontal drilling zone and a reserve amount of shale gas in the horizontal drilling zone, the estimating of the brittleness of the horizontal drilling zone, comprising:
(a) estimating slowness of the horizontal drilling zone by the method of claim 1;
(b) estimating Young's modulus and Poisson's ratio using the slowness; and (c) estimating brittleness using Young's modulus and Poisson's ratio.

12. The method of claim 11, wherein the estimating of the reserve amount (TOC_H) of shale gas in the horizontal drill zone comprises:
obtaining a total organic carbon (TOC_C) of a drilling core through a geochemical test performed on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N3) between a value obtained by adding an average correction factor (V) to a first total organic carbon (TOC_S) measured by the geophysical well logging and the total organic carbon (TOC_C) of the drilling core, TOC_C = N3×(TOC_S + V) TOC_H = N3×N1×TOC_N H ...Relational Equation H1 where, N1 is a proportional factor between the first total organic carbon (TOC_S) and a second total organic carbon (TOC_N) of the vertical drilling zone;
and multiplying, as shown in Relational Equation H1, a total organic carbon (TOC_N H) of the horizontal drilling zone by the proportional factor (N3) and the proportional factor (N1).

13. The method of claim 11, wherein the estimating of the reserve amount (TOC_H) of shale gas in the horizontal drill zone comprises:
obtaining a total organic carbon (TOC_C) of a drilling core through a geochemical test performed on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N2) between a first total organic carbon (TOC_S) measured by the geophysical well logging and the total organic carbon (TOC_C) of the drilling core, TOC_C = N2×TOC_S
TOC_H = N1×N2×TOC_N H ...Relation Equation H2 Here, N1 is a proportional factor between the first total organic carbon (TOC_S) and a second total organic carbon (TOC_N) of the vertical drilling zone;
and multiplying, as shown in Relational Equation H2, a total organic carbon (TOC_ N H) of the horizontal drilling zone by the proportional factor (N2) and the proportional factor (N1).

14 The method of claim 11, wherein the estimating of the reserve amount (TOC_H) of shale gas in the horizontal drill zone comprises:
obtaining a total organic carbon (TOC_C) of a drilling core through a geochemical test performed on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N4) between a value obtained by adding an average correction factor (V) to a second total organic carbon (TOC_N) measured by the geophysical well logging and the total organic carbon (TOC_C) of the drilling core, TOC_C = N4× (TOC_N + V) TOC_H = N4×TOCN_ H ..Relational Equation H3; and multiplying, as shown in Relational Equation H3, a total organic carbon (TOC_ N H) of the horizontal drilling zone by the proportional factor (N4)

15. The method of claim 11, wherein the estimating of the reserve amount (TOC_H) of shale gas in the horizontal drill zone comprises obtaining a total organic carbon (TOC_C) of a drilling core through a geochemical test performed on the drilling core acquired from the sweet spot range of the vertical drilling zone and then deducing a proportional factor (N5) between a second total organic carbon (TOC_N) measured by the geophysical well logging and the total organic carbon (TOC_C) of the drilling core, TOC_C = N5×TOC_N

TOC_H = N5×TOC_N H ...Relational Equation H4; and multiplying, as shown in Relational Equation H4, a total organic carbon (TOC_N H) of the horizontal drilling zone by the proportional factor (N5).

16. The method of claim 11, wherein a level of maturity in Relational Equations 3 and 4 is obtained by performing a geochemical test on a drilling core acquired from a sweet spot range of the vertical drilling zone.

17. The method of claim 11, wherein the proportional factor (N1) deduces, as an approximate constant, a correlation between the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N) in the vertical drilling zone when X-axis represents a depth of the vertical drilling zone and Y-axis represents the first total organic carbon (TOC_S) and the second total organic carbon (TOC_N).

18. The method of claim 11, wherein in the estimating of Young's modulus and Poisson's ratio using the slowness, the estimating of Young's modulus comprises:
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 and deducing Relational Equation Y which mathematizes a correlation between the slowness and the Young's modulus in the sweet spot range;
inputting the estimated slowness of the horizontal drilling zone into Relational Equation Y to estimate Young's modulus of the horizontal drilling zone;
and by Relational Equation Y, deducing, as an approximately proportional expression, a correlation between the slowness obtained in the sweet spot range and the Young's modulus when X-axis represents slowness and Y-axis represents Young's modulus.

19. The method of claim 11, wherein in the estimating of Young's modulus and Poisson's ratio using the slowness, the estimating of Poisson' ratio comprises:
calculating 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 and deducing Relational Equation P which mathematizes a correlation between slowness and Poisson's ratio in the sweet spot range;
inputting the estimated slowness into Relational Equation P to estimate Poisson's ratio of the horizontal drilling zone; and by Relational Equation P, deducing, as an approximately proportional expression, a correlation between the slowness obtained in the sweet spot range and the Poisson' ratio when X-axis represents slowness and Y-axis represents Poisson's ratio.