Method for calculating water saturation of graphitized low-resistance shale reservoirs

Abstract

The application discloses a kind of graphitization low resistance shale reservoir water saturation calculation method, comprising: collecting the well logging curve of study area, and the water saturation experiment is carried out to rock sample of different graphitization degree;The low resistivity degree of study area is divided, including ungraphitized A area, weak graphite B area and graphitization C area;For A area and B area, respectively based on the water saturation obtained by experiment and the organic carbon content of well logging curve is curve fitting, establishes corresponding water saturation calculation model;For C area, based on the water saturation obtained by experiment and the density of well logging curve, Poisson's ratio and ratio of longitudinal and transverse wave velocity are multiple fitting, establish corresponding water saturation calculation model;The graphitization low resistance shale reservoir water saturation of purpose layer is calculated based on the above model.The application realizes the graphitization low resistance shale in low resistivity degree division basis and carries out shale water saturation calculation, so that the water saturation calculated more accurately.

Description

TECHNICAL FIELD

[0001] The present application relates to the exploration and development application technical field of shale oil and gas, in particular to a calculation method of water saturation of graphitized low-resistance shale reservoir. BACKGROUND

[0002] In the evaluation of shale gas reservoirs "seven properties" (lithology, geochemical properties, physical properties, gas content, logging properties or electrical properties, brittleness, and stress characteristics), the resistivity characteristics of shale gas reservoirs are important evaluation content. Shale gas reservoirs are rich in organic matter and have high gas content. The resistivity logging curve response characteristics are as follows: the deep and shallow dual-lateral resistivity curves are basically coincident, the resistivity shows a medium-low value, and local high values appear due to the increase of gas content, sandy, calcareous, and organic matter content. However, in the actual shale gas reservoir logging process, many low-resistance phenomena are encountered, resulting in a large error between the calculation results and the actual results using conventional saturation calculation models such as Archie formula, dual-water model, and W-S model, which brings difficulties to the logging evaluation of such shale reservoirs, and also causes confusion in identifying the gas content of shale reservoirs.

[0003] For conventional sandstone reservoirs, Archie formula, Simandoux formula, Total-Shale formula, and Indonesian equation are commonly used to calculate water saturation. The pore types, wetting phase types, and gas occurrence states of shale gas reservoirs are quite different from those of conventional sandstones. The key to applying Archie formula is how to reasonably determine the saturation exponent, cementation exponent, formation water resistivity, and empirical coefficient, which generally requires a sufficient number of laboratory analysis data for calibration. The application of Simandoux formula or Total-Shale formula essentially considers the influence of argillaceous matter based on the Archie formula under pure rock conditions.

[0004] Over-mature organic matter can cause a significant decrease in the resistivity of shale gas reservoirs. In the shale gas exploration and development in southern Sichuan, low-resistance phenomena caused by over-mature organic matter have been observed in some areas. Generally, as the thermal maturity of shale increases, organic matter is first degraded into kerogen, and kerogen produces methane gas during the subsequent change process. With the increase of temperature, kerogen continuously changes, gradually transforms into low-hydrogen carbon residue, and finally converts into graphite (i.e., carbonization). As the relative content of graphitization increases, the resistivity of shale reservoirs gradually decreases. Therefore, the degree of graphitization can be quickly and intuitively evaluated through the resistivity of shale reservoirs. At the same time, for graphitized low-resistance shale reservoirs, it is also very important to accurately obtain the water saturation of the reservoir in the evaluation of shale gas reservoirs.

[0005] Shi W et al. obtained the water saturation of shale gas layer by using the ratio of the background value of shale gas organic matter to the measured or calculated organic carbon content, which avoids the influence of low resistance caused by various factors, but the background value of shale organic matter and the saturation index of organic carbon content in the formula are difficult to accurately obtain, and the influence of graphitization degree is not considered. SUMMARY

[0006] The present application aims to provide a method for calculating the water saturation of graphitized low-resistance shale reservoirs, which can calculate the water saturation of graphitized low-resistance shale based on the low-resistance degree division, so that the calculated water saturation is more accurate.

[0007] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

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

[0009] Collecting the formation resistivity, density, Poisson's ratio, ratio of compressional wave to shear wave velocity and organic carbon content logging curves of the study area, and selecting rock samples with different graphitization degrees in the study area for water saturation experiments to obtain the water saturation of rock samples with different graphitization degrees;

[0010] Using the deep resistivity-natural gamma chart to divide the low-resistance degree of the study area, including non-graphitized normal resistivity A area, weakly graphitized low-resistance B area and graphitized ultra-low resistivity C area;

[0011] For A area and B area, the correlation between the water saturation obtained by the experiment and the organic carbon content of the corresponding depth point of the logging curve is fitted respectively, and the water saturation calculation model of non-graphitized normal resistivity A area and weakly graphitized low-resistance B area is established respectively;

[0012] For C area, the correlation between the water saturation obtained by the experiment and the density, Poisson's ratio and ratio of compressional wave to shear wave velocity of the corresponding depth point of the logging curve is multi-fitted, and the water saturation calculation model of graphitized ultra-low resistivity C area is established;

[0013] Obtain the graphitized shale reservoir logging curve of the target layer to be detected, determine the belonging area based on the absolute value of the reservoir resistivity, and calculate the water saturation of the graphitized low-resistance shale reservoir of the target layer to be detected by using the water saturation calculation model of the belonging area; the belonging area includes non-graphitized normal resistivity A area, weakly graphitized low-resistance B area and graphitized ultra-low resistivity C area.

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

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

[0016] The formation resistivity is divided into the non-graphitized normal resistivity zone A when it is greater than 10Ω·m, the weakly graphitized low resistivity zone B when it is between 5-10Ω·m, and the graphitized ultra-low resistivity zone C when it 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 region A and the weakly graphitized low resistivity region 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 based on the low resistivity degree classification of graphitized low-resistance shale, making the calculated water saturation more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0023] Figure 2 This is a low resistivity level classification chart for deep resistivity-natural gamma ray in an embodiment of the present invention;

[0024] Figure 3 This is a water saturation chart of the non-graphitized normal resistivity A zone in an embodiment of the present invention;

[0025] Figure 4 This is a water saturation plate of the 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 an embodiment of the present invention;

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

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

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

[0030] The application will be further described below. The following examples are only used to more clearly illustrate the technical solutions of the application, and cannot be used to limit the protection scope of the application.

[0031] Referring to Figure 1 The application provides a calculation method of water saturation of a graphitized low-resistivity shale reservoir, and comprises the following steps:

[0032] S1, collecting deep resistivity, density, Poisson ratio, P-wave / S-wave velocity ratio and organic carbon content and other logging curves of a research area;

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

[0034] S2, selecting rock samples with different graphitization degrees in the research area to perform water saturation experiments, and obtaining water saturation of the rock samples with different graphitization degrees;

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

[0036] S3, using a deep resistivity-gamma ray graph to divide the low-resistivity degree, which can be divided into three types of non-graphitized normal resistivity A area, weakly graphitized low-resistivity B area and graphitized ultralow-resistivity C area;

[0037] It should be noted that resistivity < 5 is defined as strong graphitization, 5-10 is defined as weak higher graphitization, and > 10 is defined as non-graphitization.

[0038] S4, for the A and B areas, based on the correlation between the water saturation obtained by the experiment in step S2 and the organic carbon content of the depth point corresponding to the logging curve in step S1, the least square method is used for curve fitting, and the water saturation calculation models of the non-graphitized normal resistivity A area and the weakly graphitized low-resistivity B area are respectively established.

[0039] It should be noted that the water saturation calculation model of the non-graphitized normal resistivity A area is:

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

[0041] Wherein, a and b are coefficients, which are obtained by least square fitting of experimental water saturation and logging calculated organic carbon content.

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

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

[0044] Where 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 zone C, 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 and S-wave velocity ratio collected from the well logging data in step S1 to establish a water saturation calculation model for the graphitized ultra-low resistivity zone C;

[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 a well logging curve of the graphitized shale reservoir of the target layer to be tested, 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 tested using a water saturation calculation model for the region.

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

[0051] 1. Taking the shale gas formation in the Sichuan Basin as the research area, collect logging curves such as deep resistivity, density, Poisson's ratio, P-wave velocity ratio and organic carbon content in the study 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 zone A, weakly graphitized low resistivity zone B, and graphitized ultra-low resistivity zone C.

[0054] In this embodiment, the formation resistivity is defined as the normal resistivity zone A without graphitization 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.

[0055] 4. For area A, the correlation between the experimentally obtained water saturation and the organic carbon content at the corresponding depth point was fitted with a curve, 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 in 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 embodiment reached 0.71, verifying the accuracy of the model in predicting water saturation.

[0060] Well logging resistivity indicates that the target interval, the Wufeng-Longmaxi Formation, has a resistivity greater than 10 Ω·m. Based on the definition of low resistivity (RT < 10 Ω·m) and the typical resistivity characteristics after graphitization (RT < 5 Ω·m), it was determined that graphitization was essentially non-existent in Well X.

[0061] The water saturation of the target interval, Well X, is calculated based on the water saturation calculation model for the ungraphitized normal resistivity zone A. 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 channels, the sixth channel is the deep lateral and shallow lateral curve channels, 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 saturations obtained by experiments. 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 well logging was fitted, and the unknown parameters were determined using the least squares method, such as Figure 4 , which is used as the water saturation calculation model of the weakly graphitized low resistivity B zone.

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

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

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

[0066] According to the definition of low resistance (RT<10Ω·m) and the typical characteristics of resistivity after typical graphitization (RT<5Ω·m), the resistivity of the target interval of the Y well is (5-10Ω·m), and it is determined that the Y well has undergone a certain degree of graphitization.

[0067] Based on the water saturation calculation model of the weakly graphitized low resistivity B area described above, the water saturation of the target interval of the Y well is calculated. Figure 7 The water saturation calculation effect diagram of the Y well. Figure 7 In the figure, the first track is a geological sublayer, the second track is a conventional gamma curve (natural gamma and deuranium gamma), the third track is a depth track, the fourth track is a density, compensated neutron, and compensated acoustic curve track, the fifth track is a deep lateral and shallow lateral curve track, the sixth track is an organic carbon content curve track, and the seventh track is a water saturation curve track. Figure 7 In the figure, the well logging calculated water saturation curve is the water saturation curve calculated according to the model of formula (2), and the scattered points are the experimental water saturations. Figure 7 As can be seen from the figure, the calculated water saturation calculated according to the model of formula (2) has good consistency with the experimental water saturation and high accuracy.

[0068] 6. For the C area, the correlation between the experimental water saturation and the logging compensated density, Poisson's ratio, and the ratio of P-wave velocity to S-wave velocity is used for curve fitting, and the least square method is used to determine the unknown parameters, which are used as the water saturation calculation model of the graphitized ultra-low resistivity C area.

[0069] Referring to Figure 5 In this embodiment, the correlation coefficient of the least square modeling is 0.67, which proves the accuracy of the model in predicting water saturation.

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

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

[0072] Wherein, DEN is the compensated density, POIS is the Poisson ratio, and RCS is the ratio of P-wave and S-wave velocity.

[0073] According to the definition of low resistance (RT<10Ω·m) and the typical characteristics of resistivity after typical graphitization (RT<5Ω·m), the resistivity of the Z well target layer Wufeng group-Longyi 1 subzone is (<5Ω·m), and it is determined that the Y well has a certain degree of graphitization.

[0074] Based on the above-mentioned water saturation calculation model of the graphitized ultra-low resistivity C zone, the water saturation of the target layer of the Z well is calculated. Figure 8 The water saturation calculation effect diagram of the Z well. Figure 8 The first track is the geological sublayer, 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, and compensated acoustic curve track, the fifth track is the deep lateral and shallow lateral curve track, the sixth track is the Poisson ratio and the ratio of P-wave and S-wave velocity curve track, and the seventh track is the water saturation curve track. Figure 8 The well logging calculated water saturation curve is the water saturation curve calculated according to the model of formula (3), and the scattered points are the experimental water saturation). From Figure 8 As can be seen from the above, the water saturation calculated according to the model of formula (3) and the experimental water saturation have good consistency and high accuracy.

[0075] Finally, it should be pointed out that the above specific embodiments are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to examples, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present application, and they should be covered in the scope of the claims of the present application.

Claims

1. A method for calculating water saturation of graphitized low-resistivity shale reservoir, characterized in that: include: Collect formation resistivity, density, Poisson's ratio, P-wave velocity ratio, and organic carbon content logging curves in the study area. Also, select rock samples with different degrees of graphitization in the study area for water saturation experiments to obtain the water saturation of rock samples with different degrees of graphitization. The deep resistivity-natural gamma ray chart was used to divide the low resistivity degree of the study area into the following categories: non-graphitized normal resistivity zone A, weakly graphitized low resistivity zone B, and graphitized ultra-low resistivity zone 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 well logging curve. Calculation models for water saturation in Area A (non-graphitized normal resistivity) and in Area B (weakly graphitized low resistivity) were established, respectively. For the C zone, a water saturation calculation model for the graphitized ultra-low resistivity C zone was established by performing multivariate fitting based on the correlation between the experimentally obtained water saturation and the density, Poisson's ratio, and P-wave and S-wave velocity ratio at the corresponding depth points of the well logging curve. A well logging curve of the graphitized shale reservoir of the target layer to be tested is obtained, and the region to which it belongs is determined based on the absolute value of the reservoir resistivity. The water saturation of the graphitized low-resistivity shale reservoir of the target layer to be tested is calculated using a water saturation calculation model for 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-resistivity shale reservoir according to claim 1, characterized in that: The low resistance refers to the resistivity of the well logging curve being less than 10Ω·m.

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

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

5. The method for calculating water saturation of a graphitized low-resistivity shale reservoir according to claim 4, characterized in that: 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.

6. The method for calculating water saturation of a graphitized low-resistivity 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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