Calculation method for water saturation of graphitized low-resistivity shale reservoir

Abstract

The invention discloses a method for calculating the water saturation of a graphitized low-resistance shale reservoir, which comprises the following steps of: collecting a logging curve of a research area, and carrying out water saturation experiments on rock samples with different graphitization degrees; carrying out low resistivity degree division on a research area, wherein the research area comprises an ungraphitized A area, a weak graphite B area and a graphitized C area; for the area A and the area B, respectively performing curve fitting based on the water saturation and the logging curve organic carbon content obtained by the experiment, and establishing a corresponding water saturation calculation model; aiming at the C region, performing multivariate fitting based on the water saturation obtained by the experiment and the density, Poisson's ratio and longitudinal and transverse wave velocity ratio of the logging curve, and establishing a corresponding water saturation calculation model; and calculating the water saturation of the graphitized low-resistivity shale reservoir of the target interval based on the model. According to the method, the shale water saturation is calculated on the basis of low resistivity degree division of the graphitized low-resistivity shale, so that the calculated water saturation is more accurate.

Description

Technical Field

[0001] The invention relates to the technical field of shale oil and gas exploration and development application, and in particular to a method for calculating water saturation of a graphitized low-resistance shale reservoir. Background Art

[0002] In the evaluation of the "seven properties" of shale gas reservoirs (lithology, geochemical characteristics, physical properties, gas content, logging attributes or electrical properties, brittleness, and geostress characteristics), the resistivity characteristics of shale gas reservoirs are important evaluation contents. Shale gas reservoirs are rich in organic matter and have high gas content. The response characteristics of their resistivity logging curves are: the double lateral deep and shallow resistivity curves basically overlap, and the resistivity is medium and low. Locally, high values ​​appear due to the increase in gas content, sand, ash, and organic matter content. However, many low-resistance phenomena are encountered in the actual shale gas reservoir logging process, resulting in large errors between the calculation results and the actual results using conventional saturation calculation models such as Archie formula, double water model, W-S model, etc., which brings difficulties to the logging evaluation of this type of shale reservoirs, and also brings troubles to the identification of the gas content of shale reservoirs.

[0003] For conventional sandstone reservoirs, Archie's formula, Simandoux's formula, Total-Shale's formula and Indonesian equation are commonly used to calculate water saturation. The pore type, wetting phase type, gas storage state, etc. of shale gas reservoirs are quite different from those of conventional sandstone. The key to applying Archie's formula is how to reasonably determine the saturation index, cementation index, formation water resistivity and empirical coefficient, which generally requires a sufficient number of laboratory analysis data for calibration. Applying Simandoux's formula or Total-Shale's formula is essentially taking into account the influence of mud on the basis of Archie's formula under pure rock conditions.

[0004] Over-mature organic matter can cause a significant decrease in the resistivity of shale gas reservoirs. In the exploration and development of shale gas in southern Sichuan, low resistivity caused by over-maturity of organic matter has been seen in some areas. Generally speaking, as the thermal maturity of shale increases, organic matter first degrades into kerogen, and kerogen produces methane gas in the subsequent changes. As the temperature increases, kerogen continues to change, gradually turning into a carbonaceous residue with low hydrogen content, and finally into graphite (i.e. carbonization). As the relative content of graphitization increases, the resistivity of the shale reservoir gradually decreases. Therefore, the resistivity of the shale reservoir can be used to quickly and intuitively evaluate the degree of graphitization. At the same time, for graphitized low-resistance shale reservoirs, accurately obtaining the reservoir water saturation is also very important in the evaluation of shale gas reservoirs.

[0005] Shi Wenrui et al. used the ratio of shale gas organic matter background value to the measured or calculated organic carbon content of the shale gas layer to obtain the water saturation of the shale gas layer, avoiding the influence of low resistivity caused by various factors. However, the shale organic matter background value and organic carbon content saturation index in the formula are difficult to obtain accurately, and the influence of the degree of graphitization is not considered. Summary of the invention

[0006] The purpose of the present invention is to provide a method for calculating the water saturation of a graphitized low-resistance shale reservoir, which can realize the calculation of the water saturation of the graphitized low-resistance shale based on the low resistivity degree classification, so that the calculated water saturation is more accurate.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0008] The present invention provides a method for calculating water saturation of a graphitized low-resistance shale reservoir, comprising:

[0009] Collect the formation resistivity, density, Poisson's ratio, longitudinal and transverse wave velocity ratio and organic carbon content logging curves in the study area, and select rock samples with different graphitization degrees in the study area to conduct water saturation experiments to obtain the water saturation of rock samples with different graphitization degrees;

[0010] The deep resistivity-natural gamma ray chart is used to divide the low resistivity degree of the study area, including the ungraphitized normal resistivity area A, the weakly graphitized low resistivity area B and the graphitized ultra-low resistivity area C.

[0011] For area A and area B, curve fitting was performed based on the correlation between the experimentally obtained water saturation and the organic carbon content at the corresponding depth point of the logging curve, and the water saturation calculation models for the ungraphitized normal resistivity area A and the weakly graphitized low resistivity area B were established respectively;

[0012] For the C zone, a multivariate fitting was performed based on the correlation between the experimentally obtained water saturation and the density, Poisson's ratio and P-wave velocity ratio at the corresponding depth point of the logging curve, and a water saturation calculation model for the graphitized ultra-low resistivity C zone was established.

[0013] The well logging curve of the graphitized shale reservoir of the target layer section to be detected is obtained, the region to which it belongs is determined based on the absolute value of the reservoir resistivity, and the water saturation of the graphitized low-resistivity shale reservoir of the target layer section to be detected is calculated using the water saturation calculation model of the region; the region includes an ungraphitized normal resistivity region A, a weakly graphitized low resistivity region B, and a graphitized ultra-low resistivity region C.

[0014] Furthermore, the low resistance refers to that the resistivity collected by the logging curve is less than 10Ω·m.

[0015] Furthermore, the use of the deep resistivity-natural gamma ray plate to divide the low resistivity degree of the study area includes:

[0016] The formation resistivity is divided into the non-graphitized normal resistivity zone A when the formation resistivity is greater than 10Ω·m, the weakly graphitized low resistivity zone B when the formation resistivity is between 5-10Ω·m, and the graphitized ultra-low resistivity zone C when the formation resistivity is less than 5Ω·m.

[0017] Furthermore, for area A and area B, the least square method was used for curve fitting.

[0018] Furthermore, the water saturation calculation model of the non-graphitized normal resistivity zone A and the weakly graphitized low resistivity zone B is in the form of an exponential function.

[0019] Furthermore, the water saturation calculation model of the graphitized ultra-low resistivity C zone is in the form of a multivariate linear equation.

[0020] The beneficial effects of the present invention are:

[0021] The present invention realizes the calculation of shale water saturation on the basis of low resistivity degree classification of graphitized low-resistance shale, so that the calculated water saturation is more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A flow chart of a method for calculating water saturation of a graphitized low-resistance shale reservoir provided by the present invention;

[0023] Figure 2 A low resistivity level division chart of deep resistivity-natural gamma in an embodiment of the present invention;

[0024] Figure 3 This is a water saturation chart of the normal resistivity A zone without graphitization in the embodiment of the present invention;

[0025] Figure 4 This is a water saturation chart of weakly graphitized low resistivity B zone in an embodiment of the present invention;

[0026] Figure 5 This is a water saturation chart of the graphitized ultra-low resistivity C zone in the embodiment of the present invention;

[0027] Figure 6 This is a diagram showing the calculation effect of water saturation of Well X in an embodiment of the present invention;

[0028] Figure 7 This is a diagram showing the calculation effect of water saturation of Well Y in an embodiment of the present invention;

[0029] Figure 8 This is a diagram showing the calculation effect of water saturation of well Z in an embodiment of the present invention. DETAILED DESCRIPTION

[0030] The present invention is further described below. The following examples are only used to more clearly illustrate the technical solution of the present invention, and are not intended to limit the protection scope of the present invention.

[0031] See also Figure 1 The present invention provides a method for calculating water saturation of a graphitized low-resistance shale reservoir, comprising the following steps:

[0032] S1. Collect logging curves of deep resistivity, density, Poisson's ratio, P-wave velocity ratio and organic carbon content in the study area;

[0033] It should be noted that all logging curves in the study area can be directly collected through logging data.

[0034] S2. Select rock samples with different graphitization degrees in the study area to conduct water saturation experiments to obtain the water saturation of rock samples with different graphitization degrees;

[0035] It should be noted that the graphitization degree is determined according to the reservoir resistivity.

[0036] S3. Using the deep resistivity-natural gamma chart to divide the low resistivity level, it can be divided into three types: non-graphitized normal resistivity A zone, weakly graphitized low resistivity B zone and graphitized ultra-low resistivity C zone;

[0037] It should be noted that the resistivity <5 is defined as strong graphitization, 5-10 as weak and relatively high graphitization, and >10 as non-graphitization;

[0038] S4. For the A and B zones, based on the correlation between the water saturation obtained from the experiment in step S2 and the organic carbon content at the corresponding depth point of the well logging curve in step S1, the least square method is used for curve fitting to establish water saturation calculation models for the ungraphitized normal resistivity zone A and the weakly graphitized low resistivity zone B, respectively;

[0039] It should be noted that the water saturation calculation model of the ungraphitized normal resistivity A zone is:

[0040] S w =a*EXP(b*TOC);

[0041] Among them, a and b are coefficients, which are obtained by fitting the experimental water saturation and the organic carbon content calculated by well logging using the least squares method.

[0042] The water saturation calculation model of weakly graphitized low resistivity B zone is:

[0043] S w =c*EXP(d*TOC);

[0044] Among them, c and d are coefficients, which are obtained by fitting the experimental water saturation and the organic carbon content calculated by well logging using the least squares method.

[0045] S5. For the C zone, a multivariate fitting is performed based on the correlation between the water saturation obtained from the experiment in step S2 and the density, Poisson's ratio and P-wave velocity ratio collected by the well logging in step S1 to establish a water saturation calculation model for the graphitized ultra-low resistivity C zone;

[0046] It should be noted that the water saturation calculation model of the graphitized ultra-low resistivity C zone is:

[0047] S w =e*DEN+f*POIS+g*RCS+h;

[0048] Among them, e, f, g, and h are coefficients, which are obtained by multivariate fitting of experimental water saturation, density, Poisson's ratio, and P-wave and S-wave velocity ratios.

[0049] S6. Obtain the well logging curve of the graphitized shale reservoir of the target layer to be detected, determine the region to which it belongs (region A, region B, and region C defined in step S3) based on the absolute value of the reservoir resistivity, and calculate the water saturation of the graphitized shale reservoir of the target layer to be detected using the water saturation calculation model of the region.

[0050] Based on the above invention ideas, an embodiment of the present invention provides a method for calculating the water saturation of a graphitized low-resistivity shale reservoir, and the specific implementation process is as follows:

[0051] 1. Taking the shale gas formation in Sichuan Basin as the research area, collect logging curves such as deep resistivity, density, Poisson's ratio, longitudinal and transverse wave velocity ratio and organic carbon content in the research area;

[0052] 2. Select rock samples with different graphitization degrees in the study area to conduct water saturation experiments to obtain the water saturation of rock samples with different graphitization degrees;

[0053] 3. Use the logging characteristics of different graphitization degrees to establish deep resistivity-natural gamma charts, such as Figure 2 As shown in the figure, it can be divided into three types: non-graphitized normal resistivity A zone, weakly graphitized low resistivity B zone, and graphitized ultra-low resistivity C zone.

[0054] In this embodiment, the formation resistivity greater than 10Ω·m is defined as the non-graphitized normal resistivity A zone, the formation resistivity between 5-10Ω·m is defined as the weakly graphitized low resistivity B zone, and the formation resistivity less than 5Ω·m is defined as the graphitized ultra-low resistivity C zone;

[0055] 4. For area A, the correlation between the experimentally obtained water saturation and the organic carbon content at the corresponding depth point was curve fitted, and the least squares method was used to determine the unknown parameters, such as Figure 3 , and the water saturation calculation model of the ungraphitized normal resistivity A zone was obtained.

[0056] In this embodiment, the water saturation S of the ungraphitized normal resistivity A zone is w The fitting relationship is:

[0057] S w =63.767*EXP(-0.124*TOC) (1)

[0058] Where TOC is the organic carbon content and EXP() is the exponential function.

[0059] The correlation coefficient of the least squares modeling in this example reached 0.71, which verified the accuracy of the model in predicting water saturation.

[0060] According to the well logging resistivity, the resistivity of the target layer, Wufeng Formation-Longmaxi Formation 1, is >10Ω·m. Based on the definition of low resistivity (RT<10Ω·m) and the typical characteristics of resistivity after typical graphitization (RT<5Ω·m), it is determined that graphitization has basically not occurred in Well X.

[0061] The water saturation of the target layer section X well is calculated based on the water saturation calculation model of the non-graphitized normal resistivity A zone. Figure 6 This is the calculation effect diagram of water saturation of Well X. Figure 6 The first channel is the geological layer, the second channel is the conventional gamma curve (natural gamma and deuranium gamma), the third channel is the depth channel, the fourth channel is the lithology channel, the fifth channel is the density, compensated neutron and compensated acoustic wave curve channel, the sixth channel is the deep lateral and shallow lateral curve channel, the seventh channel is the total hydrocarbon and methane curve channel, the eighth channel is the organic carbon content curve channel, and the ninth channel is the water saturation curve channel ( Figure 6 The water saturation curve calculated by well logging is the water saturation curve calculated according to the model of formula (1), and the scattered points are the water saturation obtained by experiment. Figure 6 It can be seen that the water saturation calculated by the model of formula (1) is consistent with the water saturation obtained by experiment and has high accuracy.

[0062] 5. For area B, the correlation between the experimentally obtained water saturation and the organic carbon content at the corresponding depth calculated by logging was curve fitted, and the least squares method was used to determine the unknown parameters, such as Figure 4 , which is used as the water saturation calculation model for the weakly graphitized low resistivity B zone.

[0063] In this embodiment, the correlation coefficient of the least squares modeling is as high as 0.83, which verifies the accuracy of the model in predicting water saturation.

[0064] In this embodiment, the water saturation S of the weakly graphitized low resistivity B zone is w The fitting relationship is:

[0065] S w =85.402*EXP(-0.151*TOC) (2)

[0066] According to the definition of low resistivity (RT<10Ω·m) and the typical characteristics of resistivity after typical graphitization (RT<5Ω·m), the resistivity of the target layer of Well Y, Wufeng Formation-Long-1 sub-member, is between (5-10Ω·m), which indicates that a certain degree of graphitization has occurred in Well Y.

[0067] The water saturation of the target layer section Y well is calculated based on the water saturation calculation model of the weakly graphitized low resistivity B zone mentioned above. Figure 7 This is the calculation effect diagram of water saturation of well Y. Figure 7 The first channel is the geological layer, the second channel is the conventional gamma curve (natural gamma and deuranium gamma), the third channel is the depth channel, the fourth channel is the density, compensated neutron, compensated acoustic wave curve channel, the fifth channel is the deep lateral and shallow lateral curve channel, the sixth channel is the organic carbon content curve channel, and the seventh channel is the water saturation curve channel ( Figure 7 The water saturation curve calculated by logging is the water saturation curve calculated according to the model of formula (2), and the scattered points are the water saturation obtained by experiment. Figure 7 It can be seen that the water saturation calculated according to the model of formula (2) is consistent with the experimental water saturation and has high accuracy.

[0068] 6. For the C zone, the correlation between the water saturation obtained from the experiment and the logging compensation density, Poisson's ratio, and the ratio of longitudinal and transverse wave velocities is used to perform curve fitting; and the least squares method is used to determine the unknown parameters, which is used as the water saturation calculation model for the graphitized ultra-low resistivity C zone.

[0069] See also Figure 5 ,In this embodiment, the correlation coefficient of modeling using the least square method is 0.67, proving the accuracy of the model in predicting water saturation.

[0070] In this embodiment, the water saturation S of the graphitized ultra-low resistivity C zone is w The fitting relationship is:

[0071] S w =120.4*DEN+610.8*POIS-75*RCS-297.2 (3)

[0072] Where DEN is the compensation density, POIS is the Poisson's ratio, and RCS is the ratio of longitudinal and transverse wave velocities.

[0073] According to the definition of low resistivity (RT<10Ω·m) and the typical characteristics of resistivity after typical graphitization (RT<5Ω·m), the resistivity of the Wufeng Formation-Long-1 submember of the target layer of Well Z is (<5Ω·m), which indicates that a certain degree of graphitization has occurred in Well Y.

[0074] The water saturation of the target layer Z well is calculated based on the water saturation calculation model of the graphitized ultra-low resistivity C zone mentioned above. Figure 8 This is the calculation effect diagram of water saturation of Well Z. Figure 8 The first track is the geological layer, the second track is the conventional gamma curve (natural gamma, deuranium gamma), the third track is the depth track, the fourth track is the density, compensated neutron, compensated acoustic wave curve, the fifth track is the deep lateral, shallow lateral curve, the sixth track is the Poisson's ratio, longitudinal and transverse wave velocity ratio curve, the seventh track is the water saturation curve ( Figure 8 The water saturation curve calculated by logging is the water saturation curve calculated according to the model of formula (3), and the scattered points are the water saturation obtained by experiment). Figure 8 It can be seen that the water saturation calculated by the model according to formula (3) is consistent with the water saturation obtained by experiment and has high accuracy.

[0075] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A method for calculating water saturation of graphitized low-resistance shale reservoir, characterized in that: include: Collect the formation resistivity, density, Poisson's ratio, longitudinal and transverse wave velocity ratio and organic carbon content logging curves in the study area, and select rock samples with different graphitization degrees in the study area to conduct water saturation experiments to obtain the water saturation of rock samples with different graphitization degrees; The deep resistivity-natural gamma ray chart is used to divide the low resistivity degree of the study area, including the ungraphitized normal resistivity area A, the weakly graphitized low resistivity area B and the graphitized ultra-low resistivity area C. For area A and area B, curve fitting was performed based on the correlation between the experimentally obtained water saturation and the organic carbon content at the corresponding depth point of the logging curve, and the water saturation calculation models for the ungraphitized normal resistivity area A and the weakly graphitized low resistivity area B were established respectively; For the C zone, a multivariate fitting was performed based on the correlation between the experimentally obtained water saturation and the density, Poisson's ratio and P-wave velocity ratio at the corresponding depth point of the logging curve, and a water saturation calculation model for the graphitized ultra-low resistivity C zone was established. The well logging curve of the graphitized shale reservoir of the target layer section to be detected is obtained, the region to which it belongs is determined based on the absolute value of the reservoir resistivity, and the water saturation of the graphitized low-resistivity shale reservoir of the target layer section to be detected is calculated using the water saturation calculation model of the region; the region includes an ungraphitized normal resistivity region A, a weakly graphitized low resistivity region B, and a graphitized ultra-low resistivity region C.

2. The method for calculating water saturation of a graphitized low-resistance shale reservoir according to claim 1, characterized in that: The low resistance refers to that the resistivity collected by the logging curve is less than 10Ω·m.

3. The method for calculating water saturation of a graphitized low-resistance shale reservoir according to claim 2, characterized in that: The use of deep resistivity-natural gamma ray charts to divide the low resistivity level of the study area includes: The formation resistivity is divided into the non-graphitized normal resistivity zone A when the formation resistivity is greater than 10Ω·m, the weakly graphitized low resistivity zone B when the formation resistivity is between 5-10Ω·m, and the graphitized ultra-low resistivity zone C when the formation resistivity is less than 5Ω·m.

4. The method for calculating water saturation of graphitized low-resistance shale reservoir according to claim 3, characterized in that: For area A and area B, the least square method was used for curve fitting.

5. The method for calculating water saturation of graphitized low-resistance shale reservoir according to claim 4, characterized in that: The water saturation calculation model of the non-graphitized normal resistivity A zone and the weakly graphitized low resistivity B zone is in the form of an exponential function.

6. The method for calculating water saturation of graphitized low-resistance shale reservoir according to claim 3, characterized in that: The water saturation calculation model of the graphitized ultra-low resistivity C zone is in the form of a multivariate linear equation.

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