A method for identifying a tight sand gas layer

Abstract

This invention relates to a method for identifying tight sandstone gas reservoirs, belonging to the field of oil and gas reservoir exploration and development technology. The method, based on original drilling data (field logging data and gas testing data), calculates the tight gas content index through parameter reconstruction. This index, geologically significant as a characterizing index of reservoir gas content, then establishes a correspondence between fluid properties in the study area and the tight gas content index and average sonic transit time. Finally, the fluid properties of the target layer are determined based on the tight gas content index and average sonic transit time. This method provides effective guidance for oil and gas exploration, evaluation, and development, improving the efficiency of drilling oil, gas, and water layer identification, reducing identification difficulty, and offering advantages such as simple operation, straightforward calculation, no need for highly skilled field personnel, high interpretation accuracy, and low processing time.

Description

Technical Field

[0001] This invention relates to a method for identifying tight sandstone gas layers, belonging to the field of oil and gas reservoir exploration and development technology. Background Technology

[0002] There are many methods for identifying gas-bearing layers. Qualitative identification methods mainly utilize the influence of the presence of natural gas on logging curves such as neutron, acoustic, and density logs, using curve overlay methods to identify gas-bearing layers. Semi-quantitative methods mainly include various cross-plot methods. Quantitative methods include a variety of methods such as P-wave time difference ratio method, apparent fluid identification index method, formation gas-bearing index method, lithology-density logging identification method, etc. Among these, quantitative methods can identify gas-bearing layers more objectively and accurately.

[0003] The P-wave transit time difference ratio method is a method of identification using sound waves. The "P-wave transit time difference ratio" is defined as the measured P-wave transit time difference minus the synthetic P-wave transit time difference, which is derived from neutron porosity. Theoretically, when the measured P-wave transit time difference is greater than the synthetic P-wave transit time difference, the sound wave difference is greater than zero, indicating a gas layer; when the measured P-wave transit time difference is equal to the synthetic P-wave transit time difference, the sound wave difference is zero, indicating a non-gas layer. However, in practice, this method often leads to some false positives and false negatives.

[0004] The apparent fluid index method and the formation gas index method identify gas-bearing layers by utilizing the different response characteristics of gas layers on density logging and sonic logging curves. This method is based on the "digging effect" of gas layers. Therefore, when the "digging effect" is significant, this method is very effective in identifying gas layers; however, when the "digging effect" is not significant, the identification results are unsatisfactory, often resulting in misidentification or omission of gas layers. Furthermore, since the formulas use fluid parameters and framework parameters, the values ​​of these two parameters also affect the identification results (Zhou Shouxin, 2004). A common drawback of these two methods is the high rate of misidentification or omission, and their inability to distinguish between gas and water layers.

[0005] The lithology-density logging identification method uses lithology logging data and density logging data to identify gas-bearing reservoirs. Lithology logging measures the photoelectric absorption cross-sectional index of the formation, while density logging measures the formation fluid density. Through computer processing of the logging data, the apparent skeleton volume photoelectric absorption index and apparent skeleton volume density are obtained. The apparent skeleton volume photoelectric absorption index of a gas-bearing reservoir is lower than the skeleton volume photoelectric absorption index, and the apparent skeleton density is lower than the skeleton volume density; both are related to effective porosity and gas saturation. It is worth noting that when barite is added to the drilling mud, the logging values ​​will be affected, and this method will not be applicable.

[0006] Identifying gas reservoirs in tight sandstone is far more challenging than in conventional sandstone. Due to the extremely poor porosity and permeability of tight sandstone reservoirs, many conventional identification methods are ineffective in identifying gas-bearing layers in tight sandstone. Therefore, considering the strong heterogeneity and weak conventional hydrocarbon shows of tight sandstone, there is an urgent need to develop a gas reservoir identification method suitable for tight sandstone. Summary of the Invention

[0007] The purpose of this invention is to provide a method for identifying tight sandstone gas layers, which can solve the problem that conventional gas layer identification methods have poor performance when used for identifying tight sandstone gas layers.

[0008] To achieve the above objectives, the technical solution adopted by the tight sandstone gas layer identification method of the present invention is as follows:

[0009] A method for identifying tight sandstone gas reservoirs includes the following steps:

[0010] (1) Obtain logging data of the test well section and the target layer section in the study area, as well as the fluid properties of the test well section; the logging data includes total hydrocarbon value, resistivity and sonic transit time; the fluid properties are one or any combination of gas layer, gas-bearing layer, water layer and dry layer.

[0011] Obtain the total hydrocarbon values ​​of the gas-bearing test well section and the top adjacent stable mudstone layer in the target section within the study area;

[0012] (2) Calculate the average logging data of each test well section and the average logging data of the target layer section;

[0013] Calculate the average total hydrocarbons measured in gas from the top adjacent stable mudstone layer of each test well section and the average total hydrocarbons measured in gas from the top adjacent stable mudstone layer of the target section.

[0014] (3) Calculate the tight gas content index of each test well section according to Formula 1:

[0015] MQZS1=lg[(QT 11 ×LLD1) / (QT 12 ×s)] (1)

[0016] In Equation 1, MQZS1 is the tight gas content index of the test well section, and QT 11 LLD1 is the average total hydrocarbon value of the test well section, and QT is the average resistivity of the test well section. 12 The average total hydrocarbon content measured in the gas-bearing test well section is the value of the total hydrocarbon content measured in the adjacent stable mudstone layer at the top of the test well section.

[0017] Calculate the tight gas content index of the target layer according to Equation 2:

[0018] MQZS2=lg[(QT 21×LLD2) / (QT 22 ×s)] (2)

[0019] In Equation 2, MQZS2 is the tight gas content index of the target layer, and QT 21 LLD2 is the average total hydrocarbon concentration of the target layer, and QT is the average resistivity of the target layer. 22 The average total hydrocarbon content measured in the gas of the adjacent stable mudstone layer at the top of the target section;

[0020] In Equations 1 and 2, s is a fixed value greater than 0 and the unit of s is the same as the unit of resistivity.

[0021] (4) Based on the tight gas content index, average sonic transit time and fluid properties of each test well section in the study area, establish the correspondence between the fluid properties and the tight gas content index and average sonic transit time in the study area. Then, based on the tight gas content index and average sonic transit time of the target layer, determine the fluid properties of the target layer.

[0022] This invention provides a method for identifying tight sandstone gas reservoirs. Based on original drilling data (field logging data and gas testing data), it calculates the tight gas content index through parameter reconstruction. Geologically, this index characterizes the gas content of the reservoir; a higher index indicates a higher gas content, serving as a means of determining the gas content of tight reservoirs. Then, it establishes a correspondence between fluid properties in the study area and the tight gas content index and average sonic transit time. Finally, it determines the fluid properties of the target layer based on the tight gas content index and average sonic transit time. This method provides effective guidance for oil and gas exploration, evaluation, and development, improving the efficiency of drilling oil, gas, and water layer identification and reducing identification difficulty. The method is simple to operate and calculate, requiring no high level of expertise from field personnel. It provides a powerful tool for gas logging interpretation, independent of the service provider, offering advantages such as high interpretation accuracy and low time consumption.

[0023] In this invention, a stable mudstone layer refers to a layer with a thickness greater than 2 meters and a distribution area greater than 100 km² within the study area. 2 A mudstone layer with stable composition and stable thickness, where stable composition means that the mud content in the mudstone layer is greater than 70%, and stable thickness means that the thickness of the mudstone layer is greater than 2 meters.

[0024] Preferably, the unit of resistivity is Ω·m; the unit of acoustic transit time is μs / m; and the gas content index of the tight gas is a dimensionless value.

[0025] The corresponding relationship between the fluid properties in the study area and the tight gas gas-bearing index and the average acoustic time difference can be established in the form of a table, a picture or a mathematical formula. To make the operation simpler and the effect more intuitive, preferably, the method for establishing the corresponding relationship between the fluid properties in the study area and the tight gas gas-bearing index and the average acoustic time difference includes the following steps: taking the tight gas gas-bearing index of each gas testing interval as the ordinate and the average acoustic time difference of each gas testing interval as the abscissa, establishing a relationship chart of the tight gas gas-bearing index and the average acoustic time difference, then plotting the fluid properties of each gas testing interval on the chart, and then determining the area to which each fluid property belongs on the chart through cross-plot analysis.

[0026] Preferably, the fluid properties include gas layers and gas-bearing layers; the area where the fluid property is a gas layer is the area where the tight gas gas-bearing index ≥ m1 and the average acoustic time difference ≥ n1; the area where the fluid property is a gas-bearing layer is the area where m2 ≤ tight gas gas-bearing index < m1 and n2 ≤ average acoustic time difference or the area where m1 < tight gas gas-bearing index and n2 ≤ average acoustic time difference < n1; where, m1 is the minimum value of the tight gas gas-bearing index of the gas testing intervals where the fluid property is a gas layer, m2 is the minimum value of the tight gas gas-bearing index of the gas testing intervals where the fluid property is a gas-bearing layer, n1 is the minimum value of the average acoustic time difference of the gas testing intervals where the fluid property is a gas layer, n2 is the minimum value of the average acoustic time difference of the gas testing intervals where the fluid property is a gas-bearing layer; m2 < m1, n2 < n1.

[0027] Preferably, the fluid properties of the target interval are determined by Method A or Method B:

[0028] Method A includes the following steps: comparing the tight gas gas-bearing index and the average acoustic time difference of the target interval with the thresholds of the areas to which each fluid property belongs. When both the tight gas gas-bearing index and the average acoustic time difference of the target interval are within the thresholds of the area to which a certain fluid property belongs, the fluid property of the target interval is the same as the certain fluid property;

[0029] Method B includes the following steps: plotting the tight gas gas-bearing index and the average acoustic time difference of the target interval on the chart. When the plotted point is located in the area to which a certain fluid property belongs, the fluid property of the target interval is the same as the certain fluid property. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic flow chart of the tight sandstone gas layer identification method for the embodiment;

[0031] Figure 2 It is a schematic diagram of the fluid property identification chart established in the embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0033] Example

[0034] Taking a tight gas field as an example, the test well sections of wells numbered XX1, XX2, XX3, XX4, XX5, XX6, and XX7 are selected, and the designed perforation section of the new well numbered XX8 is taken as the target layer. The tight sandstone gas layer identification method in this embodiment is as follows: Figure 1 As shown, the specific steps include:

[0035] (1) Collect field measurement data and field test data to determine the total hydrocarbon data, logging resistivity, sonic transit time data and field gas test data; the gas test data are fluid properties, including gas layer, gas-bearing layer, water layer and dry layer;

[0036] (2) Calculate the average total hydrocarbons measured in gas at all depths between the top and bottom of each test well section, which is the average total hydrocarbons measured in gas at each test well section, QT. 11 The average resistivity of all deep sections between the top and bottom of each test well section is calculated, which is the average resistivity LLD1. The average acoustic transit time of all deep sections between the top and bottom of each test well section is calculated, which is the average acoustic transit time AC1.

[0037] Calculate the average total hydrocarbons measured in gas at all depths within the adjacent stable mudstone layer at the top of each test well section. This average total hydrocarbons measured in gas at the top of each test well section are QT. 12 ;

[0038] The average total hydrocarbon (TH) of the gas-based total hydrocarbon (TH) measurements for all depths between the top and bottom of the target layer is calculated; this average TH is the TH value for the target layer. 21 The average resistivity of all deep segments between the top and bottom of the target layer is calculated, which is the average resistivity LLD2. The average acoustic time difference of all deep segments between the top and bottom of the target layer is calculated, which is the average acoustic time difference AC2.

[0039] The average total hydrocarbon (TH) measured at all depths within the adjacent stable mudstone layer at the top of the target interval is calculated; this average TH is the total TH measured at all depths within the adjacent stable mudstone layer at the top of the target interval. 22 ;

[0040] The calculation results show that AC2 = 256 μs / m;

[0041] (3) Calculate the tight gas content index of each test well section by reconstructing the parameters according to Equation 1:

[0042] MQZS1=lg[(QT 11×LLD1) / (QT 12 ×s)] (1)

[0043] In Equation 1, MQZS1 is the tight gas content index of the test well section, and QT 11 LLD1 is the average total hydrocarbon value of the test well section, and LLD1 is the average resistivity of the test well section, in Ω·m and QT. 12 The average total hydrocarbon content measured in the gas-bearing test well section is given by s = 1 Ω·m; the calculation results are shown in Table 1.

[0044] Table 1. Well numbers, top depth, bottom depth, logging averages, MQZS1, and fluid properties of each test well section in a tight gas field.

[0045]

[0046] The tight gas content index of the target layer is calculated by reconstructing the parameters according to Equation 2:

[0047] MQZS2=lg[(QT 21 ×LLD2) / (QT 22 ×s)] (2)

[0048] In Equation 2, MQZS2 is the tight gas content index of the target layer, and QT 21 LLD2 is the average total hydrocarbon concentration in the target layer, and LLD2 is the average resistivity of the target layer, in Ω·m and QT. 22 The average total hydrocarbon content measured in the gas of the adjacent stable mudstone layer at the top of the target interval is s = 1 Ω·m; the calculated result is: MQZS2 = 2.94;

[0049] (4) According to Table 1, when the fluid property is a gas layer, the value range of MQZS1 is 3.18 to 3.50, and the value range of AC1 is 238 to 243.8 μs / m; when the fluid property is a gas-bearing layer, the value range of MQZS1 is 2 to 3.49, and the threshold range of AC1 is 222 to 263.3 μs / m.

[0050] Based on the above calculation results, since the target layer has MQZS2 = 2.94 and AC2 = 256 μs / m, and comparing the correspondence between the above fluid properties and the tight gas index and the average acoustic transit time, it can be concluded that the fluid properties of the target layer are gas-bearing layers.

[0051] To more clearly and comprehensively distinguish and determine the fluid properties of the target interval, a relationship chart of the tight gas gas-bearing index MQZS1 and the average acoustic time difference AC1 of each gas testing interval can be established with the calculated tight gas gas-bearing index of each gas testing interval as the ordinate and the average acoustic time difference of each gas testing interval as the abscissa. Then, the fluid property data obtained from gas testing of each gas testing interval are plotted on the chart, and the chart can be divided into three zones: gas zone, gas-bearing zone, and dry zone, thus obtaining the fluid property identification chart. The result is as Figure 2 shown;

[0052] The gas zone is the area where the tight gas gas-bearing index ≥ m1 and the average acoustic time difference ≥ n1. Among them, m1 is the minimum value of the tight gas gas-bearing index of the gas testing interval with the fluid property of gas zone, and n1 is the minimum value of the average acoustic time difference of the gas testing interval with the fluid property of gas zone. That is, the threshold of the gas zone is: MQZS ≥ 3.18 and AC ≥ 238 μs / m;

[0053] The gas-bearing zone is the area where m2 ≤ tight gas gas-bearing index < m1 and n2 ≤ average acoustic time difference, or m1 < tight gas gas-bearing index and n2 ≤ average acoustic time difference < n1. Among them, m1 is the minimum value of the tight gas gas-bearing index of the gas testing interval with the fluid property of gas zone, m2 is the minimum value of the tight gas gas-bearing index of the gas testing interval with the fluid property of gas-bearing zone, n1 is the minimum value of the average acoustic time difference of the gas testing interval with the fluid property of gas zone, and n2 is the minimum value of the average acoustic time difference of the gas testing interval with the fluid property of gas-bearing zone. That is, the threshold of the gas-bearing zone is: 2.00 ≤ MQZS < 3.18 and 222 μs / m ≤ AC, or 3.18 < MQZS and 222 μs / m ≤ AC < 238 μs / m;

[0054] The dry zone is the area where the tight gas gas-bearing index is less than n1 and the average acoustic time difference is less than n2. That is, the threshold of the dry zone is: MQZS < 2.00, or AC < 222 μs / m;

[0055] Then, by plotting the tight gas gas-bearing index MQZS2 (2.94) and the average acoustic time difference AC2 (256 μs / m) of the target interval on the above chart, it can be found that the plotted point of the target interval falls into the gas-bearing zone. Therefore, the fluid property of the target interval is gas-bearing;

[0056] It is also possible to compare the tight gas gas-bearing index MQZS2 (2.94) and the average acoustic time difference AC2 (256 μs / m) of the target interval with the thresholds of each fluid property obtained above, and then determine the fluid property of the target interval.

[0057] Application Example

[0058] To evaluate the accuracy of the tight sandstone gas layer identification method of the present invention, a gas test was conducted on the new well with well number XX8 in the embodiment. The results showed that the actual gas production of the new well was 2153.5 m³ / day. 3 The results are consistent with the predictions, further confirming that the threshold range identified by the tight sandstone gas layer identification method of the present invention is correct.

[0059] The tight sandstone gas layer identification method of the embodiment was applied to gas layer identification in the XX region, achieving an interpretation accuracy rate of 92.5%, significantly higher than the highest interpretation accuracy (60.2%) obtained by the earlier gas layer identification methods (Archie formula method and biporosity method) used in the region. Furthermore, compared to the time required to obtain interpretation results using the earlier gas layer identification methods, the tight sandstone gas layer identification method of the embodiment reduced the time required by 10 days, achieving significant economic and social benefits.

Claims

1. A method for identifying tight sandstone gas layers, characterized in that, Includes the following steps: (1) Obtain logging data of the test well section and the target layer section in the study area, as well as the fluid properties of the test well section; the logging data includes total hydrocarbon value, resistivity and sonic transit time; the fluid properties are one or any combination of gas layer, gas-bearing layer, water layer and dry layer. Obtain the total hydrocarbon values ​​of the gas-bearing test well section and the top adjacent stable mudstone layer in the target section within the study area; (2) Calculate the average logging data of each test well section and the average logging data of the target layer section; Calculate the average total hydrocarbons measured in gas from the top adjacent stable mudstone layer of each test well section and the average total hydrocarbons measured in gas from the top adjacent stable mudstone layer of the target section. (3) Calculate the tight gas content index of each test well section according to Formula 1: (1) In Equation 1, The tight gas content index of the test well section. This represents the average total hydrocarbon value of the test well section. This represents the average resistivity of the test well section. The average total hydrocarbon content measured in the gas-bearing test well section is the value of the total hydrocarbon content measured in the adjacent stable mudstone layer at the top of the test well section. Calculate the tight gas content index of the target layer according to Equation 2: (2) In Equation 2, The tight gas content index of the target layer. The average total hydrocarbon concentration in the target layer is given. The average resistivity of the target layer segment. The average total hydrocarbon content measured in the gas of the adjacent stable mudstone layer at the top of the target section; In Equations 1 and 2, s is a fixed value greater than 0 and the unit of s is the same as the unit of resistivity. (4) Based on the tight gas content index, average sonic transit time and fluid properties of each test well section in the study area, establish the correspondence between the fluid properties and tight gas content index and average sonic transit time in the study area, and then determine the fluid properties of the target layer based on the tight gas content index and average sonic transit time of the target layer. The method for establishing the correspondence between fluid properties and tight gas content index and average sonic transit time in the study area includes the following steps: using the tight gas content index of each test well section as the vertical axis and the average sonic transit time of each test well section as the horizontal axis, a relationship chart between the tight gas content index and the average sonic transit time is established; then the fluid properties of each test well section are projected onto the chart; and finally, the region to which each fluid property belongs is determined on the chart through cross-analysis.

2. The method for identifying tight sandstone gas layers as described in claim 1, characterized in that, The fluid properties include gas layers and gas-bearing layers; the fluid property is that the region to which the gas layer belongs has a tight gas content index. And the average time difference of sound waves is ≥ The region; the region to which the fluid properties belong (i.e., the gas-bearing layer) are located. Tight gas content index and The region of average sound wave time difference or Tight gas content index and Average time difference of sound waves The area; among which, This represents the minimum tight gas content index in the test well section where the fluid properties are gas-bearing. This represents the minimum tight gas content index in the test well section where the fluid properties are gas-bearing. This represents the minimum value among the average sonic transit times of the test well section where the fluid properties are gas-bearing. It is the minimum value among the average values ​​of acoustic transit time in the test well section where the fluid properties are gas-bearing. , .

3. The method for identifying tight sandstone gas layers as described in claim 1, characterized in that, The fluid properties of the target layer are determined by method A or method B: Method A includes the following steps: comparing the tight gas content index and average acoustic transit time of the target layer with the threshold values ​​of the regions to which each fluid property belongs; when the tight gas content index and average acoustic transit time of the target layer are both within the threshold values ​​of the regions to which a certain fluid property belongs, then the fluid property of the target layer is the same as that fluid property. Method B includes the following steps: projecting the tight gas content index and average acoustic transit time of the target layer onto the chart. When the projected point is located in the region of a certain fluid property, the fluid property of the target layer is the same as that fluid property.

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