Method for determining biogenic silica content of shale gas reservoir based on litho-density logging data

Abstract

The application discloses a method for determining biological silicon content in shale gas reservoirs based on lithology density logging data, and solves the problems of complex cost, inability to meet field scale, regional, low-cost evaluation and large calculation error of existing biological silicon content logging calculation methods. The method comprises the following steps: obtaining logging lithology density DEN, logging photoelectric absorption cross-section index Pe and logging natural gamma GR, establishing a ternary calculation equation based on the lithology density logging data, and then calculating the biological silicon content Si_toc in the shale gas reservoirs of the well to be interpreted. The method can be applied in coring wells, and has universal application value for all shale gas wells, and has smaller error and lower cost.

Description

Technical Field

[0002] This invention relates to a method for well logging interpretation, characteristic analysis, and potential for modification of shale gas reservoirs in the field of oil and gas exploration and development, specifically a method for determining the biogenic silica content of shale gas reservoirs based on lithology density logging data. Background Technology

[0004] Biogenic silica in shale gas reservoirs, also known as biogenic silica, significantly influences shale porosity development and fracturing properties, serving as a crucial indicator for both shale gas enrichment and fracturing production. Typically, biogenic silica-rich intervals are preferred target formations for fracturing, facilitating high oil and gas production and enabling commercially viable development. In oil and gas exploration and development, the biogenic silica content in shale gas reservoirs is also referred to as the biogenic silica mass fraction, or simply biogenic silica content.

[0005] The most widely used method for quantitative evaluation of biogenic silica content in shale gas reservoirs, both domestically and internationally, is calculation based on the content of silicon and aluminum elements, referred to as the silicon-aluminum elemental method. There are generally two types of methods for obtaining the content of silicon and aluminum: one is elemental analysis of shale gas reservoir core samples. Due to the difficulty and high cost of drilling and coring, this method has a high overall cost, resulting in fewer cored wells in actual production. This also makes it difficult to reflect the overall distribution of biogenic silica in shale gas reservoirs within a region solely through core sample analysis, affecting the effectiveness of regional evaluation. The second method is to obtain the content of silicon and aluminum elements in the formation through elemental logging, thereby calculating the biogenic silica content of the shale gas reservoir. The second method can obtain the content of silicon and aluminum elements in continuous formation profiles within the wellbore. However, due to the high cost of logging instruments and operations, as well as significant safety risks in horizontal wells, this logging method has not been widely used in shale gas evaluation, failing to achieve the goal of calculating and comprehensively evaluating the biogenic silica content of all shale gas wells within a block.

[0006] Invention CN111663940A discloses a method for calculating biogenic silica in shale reservoirs. This method determines the biogenic silica content by establishing a linear relationship between biogenic silica content and well logging curves. The well logging parameters used in this method are natural gamma ray (GR), sonic transit time (AC), lithological density (DEN), and neutron porosity (CNL). Although this method can quickly and easily perform continuous analysis of biogenic silica content in shale reservoirs, it requires a large amount of well logging data and has a high implementation cost. Summary of the Invention

[0008] The purpose of this invention is to address the aforementioned technical situation by providing a method for determining the biogenic silica content of shale gas reservoirs that is simple to operate, widely applicable, and can be used not only in core wells but also in non-core wells. This method has universal application value, smaller errors, and lower costs.

[0009] The objective of this invention is achieved by a method for determining the biogenic silica content of shale gas reservoirs based on lithology density logging data. This method establishes a ternary calculation equation based on lithology density logging data (DEN), photoelectric absorption cross section index (Pe), and natural gamma ray (GR) obtained from logging data, and then calculates the biogenic silica content (Si_toc) of the shale gas reservoir in the well to be interpreted.

[0010] Specifically, it includes the following steps:

[0011] 1) Obtain the biogenic silica content (Si_toc1) of shale gas reservoirs from core samples taken from core wells within the work area through experimental analysis;

[0012] 2) Obtain the logging lithological density DEN1, logging photoelectric absorption cross section index Pe1, and logging natural gamma ray GR1 at the corresponding depth points of the core samples from the core well;

[0013] 3) Using a ternary linear fitting method, the biogenic silica content Si_toc1 of the shale gas reservoir obtained from core experimental analysis was fitted with the corresponding well logging lithological density DEN1, well logging photoelectric absorption cross section index Pe1, and well logging natural gamma ray GR1. This determined the model coefficients and correlation coefficient R of the ternary calculation equation Si_toc1=a·DEN1+b·Pe1+c·GR1+d for calculating the biogenic silica content of the shale gas reservoir in the studied area.

[0014] In the formula: the dimensionless density of well logging lithology, DEN1, is g / cm³. 3 The photoelectric absorption cross section index Pe1 of the well logging has dimensions of b / e, the natural gamma ray GR of the well logging has dimensions of API, and the biogenic silica content Si_toc1 obtained from the experimental analysis of the core sample has dimensions of %.

[0015] 4) Obtain the logging lithological density (DEN), logging photoelectric absorption cross section index (Pe), and logging natural gamma (GR) of the shale gas reservoir section of the well to be interpreted in the work area through lithological density logging data;

[0016] 5) Input the logging lithological density DEN, logging photoelectric absorption section index Pe, and logging natural gamma ray GR of the well to be interpreted from step 4) into the calculation model in step 3) to calculate the biogenic silica content Si_toc of the shale gas reservoir of the well to be interpreted;

[0017] 6) Output the calculation results.

[0018] In step 2), based on the lithological density logging data of the core wells in the work area, the logging lithological density DEN1, logging photoelectric absorption cross section index Pe1, and logging natural gamma ray GR1 at the corresponding depth points of the core samples are obtained, and the data corresponding to abnormal layers with logging photoelectric absorption cross section index Pe1 greater than the upper limit of the area are removed.

[0019] In step 2), the upper limit of the photoelectric absorption cross section index Pe1 in the Jiaoshiba, Pingqiao and Hongxing blocks of the Sichuan Basin is 8.0b / e. Core samples exceeding this upper limit are data corresponding to abnormal layers.

[0020] Preferably, in step 3), R > 0.7 is considered to meet the requirement.

[0021] This invention overcomes the problems of high cost, narrow applicability, and large limitations of traditional methods in obtaining biogenic silicon content. Compared with the patented method for calculating biogenic silicon in shale reservoirs (CN111663940A), it requires less data, only using lithological density DEN, photoelectric absorption cross section index Pe, and natural gamma ray GR parameters from the lithological logging series, resulting in lower cost and facilitating rapid on-site evaluation.

[0022] This invention has been applied to 80 wells in multiple shale gas blocks, including Jiaoshiba, Pingqiao, and Hongxing in the Sichuan Basin. The calculated biogenic silica content of the shale gas reservoir is close to that obtained from experimental analysis of core samples, with an average error of no more than 12%. It can meet the needs of field shale gas reservoir logging interpretation, characteristic analysis, and potential for modification evaluation. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the workflow of the present invention.

[0025] Figure 2 This is a diagram illustrating an application example of the S shale gas field W well in this invention;

[0026] Figure 3 This is a 45° cross plot of the biogenic silica content from the core experimental analysis of Well W in the S shale gas field of this invention and the biogenic silica content calculated by the model.

[0027] Figure 4 This is a diagram illustrating an application example of the R well in the S shale gas field according to the present invention. Detailed Implementation

[0028] Reference Figure 1 The specific steps of this invention are as follows:

[0029] 1) The biogenic silica content Si_toc1 of shale gas reservoirs obtained through experimental analysis of core samples from core wells within the work area;

[0030] 2) Based on the lithological density logging data of the core wells in the work area, obtain the logging lithological density DEN1, logging photoelectric absorption cross section index Pe1, and logging natural gamma ray GR1 at the corresponding depth points of the core samples, and remove the data corresponding to the abnormal layers where the logging photoelectric absorption cross section index Pe1 is greater than the upper limit value of the region. For the Jiaoshiba, Pingqiao, and Hongxing blocks in the Sichuan Basin, the upper limit value of the logging photoelectric absorption cross section index is taken as 8.0b / e.

[0031] 3) Using the shale gas reservoir biogenic silica content model Si_toc1=a·DEN1+b·Pe1+c·GR1+d, the shale gas reservoir biogenic silica content Si_toc1 obtained from the core sample experiment is fitted with the well logging lithology density DEN1, well logging photoelectric absorption cross section index Pe1, and well logging natural gamma ray GR1 of the corresponding well using the least squares method to determine the model coefficients a, b, c, d, and the correlation coefficient R. R>0.7 is considered to meet the requirements.

[0032] 4) Obtain the logging lithological density (DEN), logging photoelectric absorption cross section index (Pe), and logging natural gamma (GR) of the shale gas reservoir section of the well to be interpreted in the work area through lithological density logging data;

[0033] 5) Based on the shale gas reservoir biogenic silica content calculation model determined in step 3), the logging lithology density DEN, logging photoelectric absorption section index Pe, and logging natural gamma ray GR of the well to be interpreted in step 4) are substituted into the model, and the ternary calculation equation Si_toc=a·DEN+b·Pe+c·GR+d is used to calculate the shale gas reservoir biogenic silica content Si_toc of the well to be interpreted.

[0034] 6) Output the calculation results.

[0035] The invention is described in detail below using a specific example of the W well in the S shale gas field of the Sichuan Basin.

[0036] 1) Through experimental analysis of 287 shale core samples from 4 core wells in the S shale gas field, the biogenic silica content Si_toc1 of the shale gas reservoir in this gas field was obtained;

[0037] 2) Based on the lithological density logging data from the above four core wells, the logging lithological density (DEN1), logging photoelectric absorption cross section index (Pe1), and logging natural gamma (GR1) at corresponding depth points were obtained for 287 shale core samples. Data corresponding to anomalous intervals where the photoelectric absorption cross section index (Pe1) was greater than the regional upper limit of 8.0 b / e were removed. Among the 287 shale core samples, 29 core samples had Pe1 values ​​greater than 8.0 b / e, which were considered anomalous intervals.

[0038] 3) After removing the biogenic silica content (Si_toc1), logging lithological density (DEN1), logging photoelectric cross section index (Pe1), and logging natural gamma (GR1) of the 29 shale cores, the Si_toc1 of a total of 258 shale cores were compared with DEN1, Pe1, and GR1 using the model Si_toc1=a•DEN1+b•Pe1+c•GR1+d. The model coefficients were a=-29.486, b=-5.315, c=-0.036, and d=119.252. The ternary equation was Si_toc1=-29.486•DEN1-5.315•Pe1-0.036•GR1+119.252, with a correlation coefficient R of 0.847.

[0039] 4) Obtain the logging lithological density (DEN), logging photoelectric absorption cross section index (Pe), and logging natural gamma (GR) from the lithological density logging data of the well to be interpreted, W well.

[0040] 5) Based on the shale gas reservoir biogenic silica content calculation model determined in step 3), the logging lithology density DEN, logging photoelectric absorption section index Pe, and logging natural gamma ray GR of well W to be interpreted in step 4) are substituted into the model. The ternary calculation equation Si_toc1=-29.486•DEN-5.315•Pe-0.036•GR+119.252 is used to calculate the biogenic silica content Si_toc of the shale gas reservoir in well W.

[0041] 6) Output the calculation results: the biogenic silica content of the shale gas reservoirs in the 3291.4–3363.0 m interval of the Wujiaping and Maokou Formations in Well W (see...). Figure 2 (Abbreviated as biogenic silica content). The calculated biogenic silica content of the shale gas reservoir was compared with the core biogenic silica content obtained from experimental analysis of lithological samples from this well. A 45° cross plot of the two was established. The data points of the cross plot are relatively evenly distributed on both sides of the 45° line (see...). Figure 3 The average relative error is 2.8%, and the average absolute relative error is 10.5%, which can meet the needs of field shale gas reservoir logging interpretation, reservoir characteristic analysis, and compressibility evaluation.

[0042] Example 2: Well R in the S Shale Gas Field

[0043] 1) Well R and Well W in Example 1 both belong to the S shale gas field. The core wells used for modeling are the same, so the model coefficients are the same. The model coefficients in Example 1 can be used. The model coefficients are a=-29.486, b=-5.315, c=-0.036, d=119.252. The ternary calculation equation is Si_toc1=-29.486•DEN1-5.315•Pe1-0.036•GR1+119.252;

[0044] 2) Obtain the logging lithological density (DEN), logging photoelectric absorption cross section index (Pe), and logging natural gamma (GR) from the lithological density logging data of the well to be interpreted, R.

[0045] 3) Based on the shale gas reservoir biogenic silica content model in step 1), the logging lithology density DEN, logging photoelectric absorption section index Pe, and logging natural gamma ray GR of the well to be interpreted R determined in step 2) are substituted into the model. The ternary calculation equation Si_toc1=-29.486•DEN-5.315•Pe-0.036•GR+119.252 is used to calculate the biogenic silica content Si_toc of the shale gas reservoir in well W.

[0046] 4) Output the calculation results. The calculated biogenic silica content of the shale gas reservoirs in the Wujiaping Formation and Maokou Formation of Well R in the interval of 3598.0–3685.0 m is 0.6%–29.1% (see...). Figure 4 The biogenic silicon content (abbreviated as biogenic silicon content) has an arithmetic mean of 15.9%. Based on calculated data such as the biogenic silicon content of the Wujiaping Formation shale gas reservoir in Well R, the 3607.5–3625.0m section of the Wujiaping Formation was selected as the window for horizontal sidetracking. A 1500m long horizontal section was fractured and tested, achieving an open flow rate of 21.0 × 10⁻⁶ m. 4 m 3 / d, based on a yield of 6.0×10 4 m 3 / d production trial mining has been carried out continuously and stably for more than 160 days, with a cumulative output exceeding 1000×10 4 m 3 .

Claims

1. A method for determining biogenic silica content in a shale gas reservoir based on litho-density log data, characterized in that, The obtained logging lithology density DEN, logging photoelectric absorption cross section index Pe and logging natural gamma GR are used to establish a ternary calculation equation based on the logging lithology density data, and the shale gas reservoir biogenic silica content Si_toc of the well to be interpreted is calculated, which specifically includes the following steps: 1) Obtain the shale gas reservoir biogenic silica content Si_toc1 of the core sample obtained by experimental analysis of the core sample of the coring well in the work area; 2) Obtain the logging lithology density DEN1, logging photoelectric absorption cross section index Pe1 and logging natural gamma GR1 of the core sample corresponding to the depth point of the core sample of the coring well; 3) Use the ternary linear fitting method to fit the shale gas reservoir biogenic silica content Si_toc1 obtained by experimental analysis of the core sample and the logging lithology density DEN1, logging photoelectric absorption cross section index Pe1 and logging natural gamma GR1 of the corresponding well, to determine the shale gas reservoir biogenic silica content calculation model of the studied work area, i.e. the model coefficients and correlation coefficient R of the ternary calculation equation Si_toc1=a·DEN1+b·Pe1+c·GR1+d, In the formula, the dimension of logging lithology density DEN1 is g / cm 3 , the dimension of logging photoelectric absorption cross section index Pe1 is b / e, the dimension of logging natural gamma ray GR is API, the dimension of biological silicon content Si_toc1 obtained by core sample experimental analysis is %, and a, b, c and d are model coefficients. 4) Obtain the logging lithology density DEN, logging photoelectric absorption cross section index Pe and logging natural gamma GR of the shale gas reservoir section of the well to be interpreted in the work area through the logging lithology density data; 5) The logging lithology density DEN, logging photoelectric absorption cross section index Pe and logging natural gamma GR of the well to be interpreted in step 4) are brought into the calculation model in step 3) to calculate the shale gas reservoir biogenic silica content Si_toc of the well to be interpreted; 6) Output the calculation result.

2. The method for determining biogenic silica content of shale gas reservoirs based on litho-density log data as claimed in claim 1, wherein, In the step 2), the logging lithology density DEN1, logging photoelectric absorption cross section index Pe1 and logging natural gamma GR1 of the core sample corresponding to the depth point of the core sample of the coring well are obtained according to the logging lithology density data of the coring well in the work area, and the data corresponding to the abnormal layer section with the logging photoelectric absorption cross section index Pe1 greater than the upper limit value of the region are removed.

3. The method for determining biogenic silica content of shale gas reservoirs based on litho-density log data as claimed in claim 1, wherein, In the step 3), R>0.7 is considered to meet the requirements.

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