Method for determining gas saturation of shale

Abstract

The invention discloses a shale gas saturation determination method, and belongs to the field of shale gas reservoir exploration and development, and the method comprises the following steps: obtaining the gas saturation data of each shale layer according to the resistivity, neutron content and uranium content data of a to-be-measured well by using a multiple linear regression model of the shale gas saturation. According to the model, firstly, resistivity, neutron content and uranium content data of each shale layer are obtained; the method comprises the following steps: obtaining gas saturation data by using an existing rock core direct determination method, and inputting the obtained data into a multiple regression model for regression analysis. The cost for obtaining the resistivity, the neutron content and the uranium content is far lower than the cost for directly measuring the rock core, so that the cost for determining the gas saturation by the method is lower; in addition, the resistivity, the neutron content and the uranium content selected by the model are in strong correlation with the gas saturation, so that the model established according to the data is also accurate; the method achieves the effects of low cost and high accuracy at the same time.

Description

Technical Field

[0001] The invention belongs to the field of shale gas reservoir exploration and development, and in particular relates to a method for determining shale gas saturation. Background Art

[0002] The mineral composition of shale reservoirs is complex, especially continental shales, which have variable lithology, contain a lot of pyrite, and have widespread development of foliation and microcracks, which have a significant impact on the logging resistivity curve, and the resistivity curve shows a non-negligible decrease. The traditional Archie formula is less applicable in the calculation of shale gas saturation, and it is difficult to accurately determine the gas saturation of shale gas reservoirs.

[0003] The existing methods for determining the gas saturation of shale gas reservoirs are mainly direct core measurement and dipole acoustic longitudinal and transverse wave time difference method. Among them, the direct core measurement method uses oil-based mud to maintain pressure and coring, and then the sample is wax-sealed at the well site. The irreducible water saturation of the reservoir is directly measured by the instrument, and the irreducible water saturation is then used to indirectly obtain the gas saturation of the gas reservoir. Due to the complexity of the pressure coring process, the high cost of coring, and generally less sampling, a shale gas field will only be designed to obtain this information during the initial drilling of the exploration. The cost of using this method to determine the gas saturation of each well in the gas reservoir is extremely high.

[0004] The dipole acoustic longitudinal and transverse time difference method uses acoustic waves and natural gamma to build a formula to calculate gas saturation. Acoustic waves are subject to many interference factors. When the lithology and porosity change, the acoustic wave curve will be affected significantly. Natural gamma logging can only reflect the total effect of all radioactive nuclides in the formation, and cannot distinguish the types and contents of radioactive nuclides contained in the formation. It is mostly used to distinguish lithology and estimate mud content, and cannot reflect the organic matter content in the rock. Therefore, the calculation accuracy is not high.

[0005] In summary, the existing method of directly measuring gas saturation using cores has the problem of high cost, or the method of measuring gas saturation using the dipole acoustic longitudinal and transverse wave time difference method has the problem of low accuracy. Summary of the invention

[0006] The object of the present invention is to provide a method for determining the gas saturation of shale, so as to solve the technical problems of high cost or low precision in the prior art methods for determining the gas saturation.

[0007] To achieve the above object, the present invention provides a technical solution for determining the gas saturation of shale:

[0008] A method for determining shale gas saturation comprises the following steps: obtaining logging data of a well to be tested, determining resistivity, neutron content and uranium content data of a shale layer to be tested according to the logging data, inputting the acquired data into a multiple linear regression model of shale gas saturation, and the model outputs corresponding gas saturation data; the multiple linear regression model of shale gas saturation is obtained by using logging data of a coring well and a core sample of the coring well, obtaining the resistivity, neutron content and uranium content of the coring well through the logging data of the coring well, obtaining gas saturation data of each shale layer according to the core sample of the coring well, and fitting the obtained gas saturation data of each shale layer with the resistivity, neutron content and uranium content of the corresponding layer section of the coring well.

[0009] The beneficial effect is: the resistivity, neutron content and uranium content data of the well to be measured are obtained, and the model can determine the gas saturation data of the well to be measured according to the resistivity, neutron content and uranium content of the well to be measured. First, the existing logging data of the coring well is obtained, and the resistivity, neutron content and uranium content data of each shale layer are obtained according to the obtained data; then the cores of each shale layer are obtained, and the gas saturation data of each core is obtained by using the existing core direct measurement method, and the multivariate linear regression model of shale gas saturation is obtained by fitting the obtained coring well data. Since the cost of obtaining resistivity, neutron content and uranium content is much lower than the cost of directly measuring gas saturation of the core, the cost of determining gas saturation by this method is low; in addition, the resistivity, neutron content and uranium content selected by the model are strongly correlated with gas saturation, so the model established based on the data is also accurate; this method of determining gas saturation achieves the effects of low cost and high accuracy at the same time.

[0010] As a further improvement, the core sample of the coring well refers to: selecting the core of the shale layer development section of the coring well, obtaining the relevant data of the shale layer corresponding to the core, and when any data in the relevant data is abnormal data, the sample data is eliminated, and the remaining core sample data is used as sample data for fitting; the relevant data include TOC, gas content, whole rock analysis and water saturation.

[0011] The beneficial effect is that the data with abnormal correlation data is eliminated, making the model obtained based on the core data more universal.

[0012] As a further improvement, the process of fitting the multivariate linear regression model of shale gas saturation is as follows: the resistivity, neutron content and uranium content of the coring well are taken as the genesis terms of gas saturation, the gas saturation data of each shale layer and the corresponding resistivity, neutron content and uranium content are taken as a sample data, and multiple sample data of the coring well are regressed and analyzed to establish a multivariate linear regression model of gas saturation.

[0013] The beneficial effect is: the sample data is input into the multivariate linear regression model for regression analysis, the correlation coefficients and residuals of the various causal terms in the model are obtained, and the output is used to establish a multivariate linear regression model of gas saturation. The model can calculate the gas saturation data of the well according to the resistivity, neutron content and uranium content of the well to be measured.

[0014] As a further improvement, the multivariate linear regression model is expressed as:

[0015] Sg=a×Rd+b×CNL+c×U+d

[0016] Among them, Sg is gas saturation, a, b, c, d are correlation coefficients, Rd is resistivity, CNL is neutron content, and U is uranium content.

[0017] As a further improvement, the process of determining the gas saturation data of each shale layer using core samples is as follows: firstly, water saturation data is obtained based on the core samples, and then the gas saturation data is inferred using the water saturation data.

[0018] The beneficial effect is that the volatilization rate of oil and gas is much higher than that of water, so the water saturation of the extracted core is directly measured, and then the gas saturation is inferred from the water saturation, making the gas saturation obtained from the core more accurate.

[0019] As a further improvement, the process of determining the resistivity, neutron content and uranium content data of each shale layer based on logging data is as follows: the logging data includes resistivity logging curves, neutron logging curves and natural gamma spectrum logging curves; the resistivity of each shale layer is determined based on the resistivity logging curve, the neutron content of each shale layer is determined based on the neutron logging curve, and the uranium content of each shale layer is determined based on the natural gamma spectrum logging curve.

[0020] As a further improvement, the division of the shale layer is based on: obtaining the lower limit of the effective thickness of the shale in the coring well, and using the lower limit of the effective thickness of the shale to divide the shale layer in the coring well. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the flow of the method for determining the gas saturation of shale in the present invention;

[0022] Figure 2 A well logging data diagram for the method for determining shale gas saturation in the present invention;

[0023] Figure 3 This is a comparison diagram of water saturation of the first section of well A in the method for determining gas saturation of shale in the present invention;

[0024] Figure 4This is a cross-plot of water saturation of the first section of Well A in the method for determining shale gas saturation in the present invention. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0026] Example of method for determining shale gas saturation:

[0027] Take Well A in the work area as an example and use Well A as the coring well. Figure 2 The well logging, imaging logging, coring data and core analysis water saturation sample distribution of the shale development layer of the Qianfoya Formation in Well A are shown. A model for calculating gas saturation was established using this method based on the relevant data of Well A.

[0028] Specifically, Figure 1 As shown in FIG. 1 , the method for determining shale gas saturation includes the following steps:

[0029] 1) Obtain the logging data of the core well and the lower limit of the effective thickness of shale. The logging data include resistivity logging curve, neutron logging curve and natural gamma ray spectrum logging curve.

[0030] Obtain conventional logging data, imaging logging interpretation data and core description data of the coring section of the first section of the Qianfoya Formation (3372.74-3409.2m) of Well A. Obtain the lower limit data of shale effective thickness through the "Specifications for Estimation of Shale Gas Resources and Reserves".

[0031] 2) Use the shale effective thickness lower limit to divide the reservoir of the wells to be logged, determine the resistivity of each shale layer according to the resistivity logging curve, determine the neutron content of each shale layer according to the neutron logging curve, and determine the uranium content of each shale layer according to the natural gamma ray spectrum logging curve. The process of using the shale effective thickness lower limit to divide the reservoir of the coring well is as follows: divide the shale effective thickness according to the effective thickness lower limit standard, remove the samples of the interlayer section, and complete the reservoir division.

[0032] Specifically, the effective thickness of shale is divided according to the lower limit data of the effective thickness of shale (3382.9-3394.1m, 3396.6-3397.4m, 3400.9-3403.9m), and the samples of the interlayer section are eliminated (3394.1m-3396.6m, 3397.4m-3400.9m). There are 18 gas saturation sample points in the effective layer section, and these 18 sample points are studied and analyzed. Then, based on the data of Well A obtained above, the resistivity, neutron content and uranium content data of each shale layer are obtained.

[0033] 3) Select core samples of each shale layer of the well to be tested, and obtain the corresponding gas saturation data based on the core samples. The basis for selecting core samples of each shale layer of the well to be tested is: select the core of the shale development section of the well to be tested, obtain the relevant data of the reservoir corresponding to the core, and when any of the relevant data is abnormal data, remove the sample data and use the remaining core as the core sample; the relevant data include TOC, gas content, whole rock analysis and water saturation.

[0034] Specifically, the shale section of the Qianfoya Formation in Well A is a semi-deep lake subfacies, with gray-black to black laminar felsic shale. This section of shale is in the same sedimentary facies belt and has stable lithology. The rock samples of the shale development section (3380.72m-3403.9m) of Well A were selected as core samples for research. Figure 2 As shown in the figure, the TOC, gas content, whole rock analysis and water saturation data of the 18 sample points in step 2) corresponding to the layer were sorted out from the well logging data of Well A, and no abnormalities were found. The cores of the 18 sample points were all used as sample cores for research.

[0035] The gas saturation data corresponding to the core is obtained according to the existing core direct measurement method. The process of obtaining the corresponding gas saturation data according to the core sample is: firstly obtain the water saturation data according to the core sample, and then use the water saturation data to calculate the gas saturation data. The sum of the gas saturation and the water saturation is 1.

[0036] 4) The resistivity, neutron content and uranium content are used as genesis items of gas saturation. The gas saturation data, resistivity, neutron content and uranium content of each reservoir are used as a sample data. Multiple sample data of the coring wells are regressed and analyzed to establish a multivariate linear regression model of gas saturation.

[0037] Specifically, this embodiment has 18 sample points, and the gas saturation data, resistivity, neutron content and uranium content of each sample point are taken as a sample data, and a total of 18 sample data are obtained. The 18 sample data are input into the multivariate linear regression model for regression analysis. The multivariate linear regression model is expressed by the formula:

[0038] Sg=a×Rd+b×CNL+c×U+d

[0039] Among them, Sg is gas saturation, a, b, c, d are correlation coefficients, Rd is resistivity, CNL is neutron content, and U is uranium content.

[0040] After the regression analysis is completed, the data of a, b, c, and d are output, where a = 0.050, b = -119.913, c = 1.934, and d = 73.371. The multivariate linear regression model of shale gas saturation is established based on this data. The multivariate linear regression model of shale gas saturation is expressed by the formula:

[0041] Sg=0.050×Rd-119.913×CNL+1.934×U+73.371

[0042] Among them, Sg is gas saturation, Rd is resistivity, CNL is neutron content, and U is uranium content.

[0043] 5) The above-mentioned multiple linear regression model of shale gas saturation is used to output the corresponding gas saturation according to the resistivity, neutron content and uranium content of the well to be measured. Specifically, the logging data of the well to be measured is first obtained, and the resistivity, neutron content and uranium content data of the shale layer to be measured are determined according to the logging data. The obtained data are input into the multiple linear regression model of shale gas saturation, and the model outputs the corresponding gas saturation data.

[0044] like Figure 3 , Figure 4 As shown, Figure 3 and Figure 4 The well logging water saturation in is equal to 1 minus the well logging gas saturation data obtained according to this model. Since gas saturation and water saturation are two strongly correlated data, the existing measured water saturation is compared with the well logging water saturation inferred by this application, and the comparison result is equivalent to the result of comparing the measured gas saturation with the well logging gas saturation of this application. Specifically, the gas saturation of the shale gas reservoir is calculated according to the multivariate linear regression model of the gas saturation, and the gas saturation range is [38%, 51%]. The calculation results of the model are compared with the results of the core measurement. The absolute error is in the range of [-9.1%, 5.7%], that is, the relative error is less than 12%, which can meet the estimation needs.

[0045] Since the cost of obtaining resistivity, neutron content and uranium content is much lower than the cost of directly measuring gas saturation of core, the cost of determining gas saturation by this method is low; in addition, the genesis items resistivity, neutron content and uranium content selected by the model are strongly correlated with gas saturation, so the model established based on this genesis is also accurate; this method of determining gas saturation achieves the effects of low cost and high accuracy at the same time.

[0046] Finally, the reasons why the present application selects resistivity, neutron content and uranium content as the genesis items of gas saturation are explained. The present invention comprehensively applies neutron logging, resistivity logging and natural gamma ray spectroscopy to calculate gas saturation, and removes the influence of lithology and physical property changes on the calculation results through the comprehensive response of the three logging curves, thereby improving the accuracy and reliability of the calculation results. The relationship between the three logging methods and gas saturation is introduced below.

[0047] Resistivity logging: Shale reservoirs belong to sedimentary rocks. There are pores between rock particles, which are filled with formation water. The salts contained in the water are in ionic state. Under the action of an external electric field, they have strong conductivity and low resistivity. However, when shale contains oil and gas, since oil and gas are almost non-conductive, the resistivity of the rock containing oil and gas is higher than the resistivity of the rock when it is completely water-containing. Therefore, in a specific rock sample, the rock resistivity increases with the increase of oil and gas saturation.

[0048] Neutron logging: During neutron logging, neutrons are injected into the material, and fast neutrons collide with the target nucleus, gradually reducing their energy and speed, and finally becoming thermal neutrons. Due to the different hydrogen content of the formation, the formation has different deceleration capabilities for fast neutrons, and the intensity of thermal neutrons detected by the detector is also different. Compared with the formation water in the shale pores, the hydrogen index of natural gas in the shale reservoir is very low. The higher the natural gas content in the formation, the lower the hydrogen index, which is converted into a lower thermal neutron intensity received by the detector, and the lower the neutron value. Therefore, neutron logging can sensitively reflect the changes in gas saturation in the formation, and the neutron value is negatively correlated with the gas saturation.

[0049] Uranium (natural gamma spectroscopy logging): Natural gamma spectroscopy logging is developed on the basis of natural gamma logging, which can quantitatively determine the contents of uranium, thorium and potassium. Generally speaking, ordinary clay rocks have high potassium and thorium contents, while uranium has relatively low contents compared to potassium and thorium. However, the uranium content will increase in a reducing environment. The uranium content in black marine shale can be as high as 100g / t, that is, the uranium content increases significantly when the clay is rich in organic matter. The reason is that organic matter in the rock plays an important role in the enrichment of uranium. Therefore, when the radioactive black shale rich in organic matter has the ability to generate and produce oil, it is characterized by high uranium on the energy spectrum curve. Among them, reservoirs with industrial oil and gas flow production capacity are typically characterized by high uranium characteristics. Therefore, the uranium content can directly reflect the content of organic matter in the formation, and is positively correlated with the content of organic matter, that is, the higher the organic matter content, the higher the uranium content.

[0050] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments without creative work, or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for determining shale gas saturation, characterized in that: The method comprises the following steps: Obtaining logging data of the well to be tested, determining the resistivity, neutron content and uranium content data of the shale layer to be tested based on the logging data, inputting the acquired data into a multivariate linear regression model of shale gas saturation, and the model outputs corresponding gas saturation data; The multivariate linear regression model of shale gas saturation is obtained by using the logging data of the coring well and the core samples of the coring well. The resistivity, neutron content and uranium content of the coring well are obtained by the logging data of the coring well. The gas saturation data of each shale layer are obtained according to the core samples of the coring well. The obtained gas saturation data of each shale layer are fitted with the resistivity, neutron content and uranium content of the corresponding layer section of the coring well.

2. The method for determining shale gas saturation according to claim 1, characterized in that: The core sample of the coring well refers to: selecting the core of the shale layer development section of the coring well, obtaining the relevant data of the shale layer corresponding to the core, and when any data in the relevant data is abnormal data, the sample data is eliminated, and the remaining core sample data is used as sample data for fitting; The relevant data include TOC, gas content, whole rock analysis and water saturation.

3. The method for determining shale gas saturation according to claim 1, characterized in that: The process of fitting the multivariate linear regression model of shale gas saturation is as follows: the resistivity, neutron content and uranium content of the coring well are obtained according to the logging data of the coring well; the resistivity, neutron content and uranium content of the coring well are used as the genesis items of gas saturation, the gas saturation data of each shale layer and the corresponding resistivity, neutron content and uranium content are used as a sample data, and multiple sample data of the coring well are regressed and analyzed to establish a multivariate linear regression model of gas saturation.

4. The method for determining shale gas saturation according to claim 3, characterized in that: The multiple linear regression model is expressed as: Sg=a×Rd+b×CNL+c×U+d Among them, Sg is gas saturation, a, b, c, d are correlation coefficients, Rd is resistivity, CNL is neutron content, and U is uranium content.

5. The method for determining shale gas saturation according to claim 1, characterized in that: The process of determining the gas saturation data of each shale layer using core samples is as follows: firstly, water saturation data is obtained based on the core samples, and then the gas saturation data is inferred using the water saturation data.

6. The method for determining shale gas saturation according to any one of claims 1 to 5, characterized in that: The process of determining the resistivity, neutron content and uranium content data of each shale layer according to the logging data is as follows: the logging data includes a resistivity logging curve, a neutron logging curve and a natural gamma spectrum logging curve; the resistivity of each shale layer is determined according to the resistivity logging curve, the neutron content of each shale layer is determined according to the neutron logging curve, and the uranium content of each shale layer is determined according to the natural gamma spectrum logging curve.

7. The method for determining shale gas saturation according to any one of claims 1 to 5, characterized in that: The basis for dividing the shale layer is: obtaining the lower limit of the effective thickness of the shale in the coring well, and using the lower limit of the effective thickness of the shale to divide the shale layer in the coring well.

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