A method for determining the water saturation of a shale gas reservoir

Abstract

The present invention belongs to the field of oil and gas field development, and particularly relates to a method for determining the water saturation of a shale gas reservoir formation; the present invention obtains rock samples in the reservoir formation, measures the initial porosity and clay mineral content of the rock samples, measures the resistivity of formation water, continuously saturates the rock samples under pressure and measures the rock resistivity and porosity at different water saturations, obtains the volume fraction of clay minerals in the pores based on the measured initial porosity of the rock samples and the porosity at different water saturations, establishes a calculation model for the effective porosity of reservoir rocks, and combines it with the relationship between the resistivity of reservoir rocks and the water saturation of the reservoir formation to obtain a calculation model for the water saturation of the reservoir formation; the new method in the present invention can quickly determine the water saturation of a shale gas reservoir formation, optimize the production allocation strategy, and formulate a reasonable production plan.

Description

Technical Field

[0001] The present invention relates to a method for determining the water saturation of a shale gas reservoir, belonging to the field of oil and gas field development. Background Art

[0002] Shale gas reservoirs are an important area for the development of unconventional oil and gas resources in recent years and have broad development potential. However, due to the high content of clay minerals in the shale gas reservoir, especially when clay minerals encounter water, they will expand and occupy the pore space, and adsorbed water will also be formed in the reservoir, resulting in a large deviation in the calculation of the water saturation of the reservoir, making it difficult to determine the true water saturation of the reservoir and unable to effectively develop the gas reservoir. Therefore, considering the content of clay minerals in the shale gas reservoir to determine the water saturation of the shale gas reservoir is crucial for the formulation of development plans.

[0003] After extensive research, there are many measurement methods for determining the water saturation of the reservoir. The nuclear magnetic resonance logging method estimates the water saturation through relaxation time spectrum analysis, but its data interpretation is complex and requires calibration in combination with data. Especially in shale gas reservoirs, it is more difficult to determine the water saturation. The mercury injection method combines the capillary pressure model to estimate the water saturation, which has certain destructiveness and is difficult to reflect the influence of clay minerals on the water saturation. Due to the characteristics of shale gas reservoirs, these methods have their respective limitations. Therefore, considering the high content of clay minerals in the shale gas reservoir, a method for determining the water saturation of the shale gas reservoir that is easy to use and highly reliable is needed. Summary of the Invention

[0004] The object of the present invention is: to solve the problem of large deviation in the calculation of the water saturation of the shale gas reservoir. The present invention uses a means of combining experiments and theories to obtain rock samples in the reservoir, measure the initial porosity and clay mineral content of the rock samples, measure the resistivity of formation water, continuously saturate the rock samples under pressure and measure the resistivity and porosity of the rock samples at different water saturations, obtain the volume fraction of clay minerals in the pores based on the measured initial porosity of the rock samples and the porosity at different water saturations, establish a calculation model for the effective porosity of reservoir rocks, and combine the relationship between the resistivity of reservoir rocks and the water saturation of the reservoir to obtain a calculation model for the water saturation of the reservoir. The water saturation of the shale gas reservoir can be quickly obtained according to the volume fraction of clay minerals in the pores; the calculation is simple and highly reliable.

[0005] To achieve the above object, the present invention provides a method for determining the water saturation of a shale gas reservoir, the method comprising the following steps:

[0006] First step, drill representative shale plug samples from different shale gas reservoirs respectively, and after machine cutting, obtain 50 regular rock samples, wash them and dry them;

[0007] In the second step, helium is used to measure the porosity of the dried rock sample to obtain the initial porosity of the reservoir rock;

[0008] In the third step, the powder of the dried rock sample is subjected to mineral composition analysis and electron microscopy scanning to determine the clay mineral content and location in the reservoir rock;

[0009] In the fourth step, water samples are obtained from the formation, the collected formation water is treated, suspended solids are filtered out, and under standard conditions, a resistivity meter is used to measure the resistivity of the formation water sample;

[0010] In the fifth step, the rock sample is continuously saturated with formation water under pressure, and the rock sample is continuously taken out. The water saturation is calculated using the weighing method. Every time the water saturation of the rock sample increases by 15%, a voltage is applied to both ends of the rock sample and the current passing through the rock sample is measured to calculate the resistivity of the reservoir rock at different water saturations;

[0011] In the sixth step, for the same rock sample with different water saturations, after measuring the resistivity of the rock sample, the porosity at different water saturations is then measured using helium;

[0012] In the seventh step, based on the measured initial porosity of the reservoir rock and the porosity at different water saturations, the volume fraction occupied by clay minerals in the pore volume of the reservoir rock after each expansion is calculated;

[0013] In the eighth step, in the reservoir, the clay minerals in the pores start to expand when they encounter formation water, and their volume continuously increases, which will reduce the effective porosity of the reservoir. The larger the volume of clay mineral expansion, the lower the effective porosity. A calculation model for the effective porosity of reservoir rock can be established.

[0014] P e =P t (1-V clay )

[0015] In the formula, P e is the effective porosity of the reservoir rock, with the unit of %; P t is the initial porosity of the reservoir rock, with the unit of %; V clay is the volume fraction of clay minerals in the rock pores, with the unit of %;

[0016] In the ninth step, there is a negative correlation between the resistivity of reservoir rock and the water saturation of the reservoir. A relational formula between the resistivity of reservoir rock and the water saturation of the reservoir can be established.

[0017]

[0018] In the formula, R t is the resistivity of the reservoir rock, with the unit of Ω·m; Sw is the reservoir water saturation, in %; R w is the formation water resistivity, in Ω·m; P is the reservoir rock porosity, in %; a, m, n are coefficients, dimensionless;

[0019] Step 10: In the reservoir pores, considering the influence of clay minerals on the pore effectiveness, the reservoir rock porosity is now the effective porosity of the reservoir rock. Combining with the calculation model of the effective porosity of the reservoir rock, the calculation model of the reservoir water saturation is obtained as

[0020]

[0021] where S w is the reservoir water saturation, in %; R w is the formation water resistivity, in Ω·m; R t is the reservoir rock resistivity, in Ω·m; P t is the initial porosity of the reservoir rock, in %; V clay is the volume fraction of clay minerals in the rock pores, in %; a, m, n are coefficients, dimensionless.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) The method has high reliability and a wider application range; (2) The method is convenient and effective, with high working efficiency; (3) Comprehensive calculation of multiple factors, with high calculation result accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the drawings:

[0024] Figure 1 is the overall technical roadmap of the method.

[0025] Figure 2 is the variation diagram of the rock resistivity.

[0026] Figure 3 is the variation diagram of the reservoir water saturation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The present invention will be further described below in conjunction with the embodiments and the drawings;

[0028] The present invention provides a method for determining the water saturation of a shale gas reservoir Figure 1 which is the overall technical roadmap of the method. The method includes the following steps:

[0029] First step: Representative shale plug samples were drilled from different shale gas reservoir formations. After machine cutting, 50 regular rock samples were obtained. After washing and drying, a total of 294 groups of experiments were conducted. We selected rock samples from 3 different gas reservoirs for illustration. The methods for the rest are the same and will not be elaborated one by one.

[0030] Second step: The initial porosity of the reservoir rock was obtained by measuring the porosity of the dried rock samples using helium.

[0031] Third step: The mineral composition analysis and electron microscope scanning of the dried rock sample powder were carried out to determine the clay mineral content and location in the reservoir rock. Through electron microscope scanning observation, clay minerals are all present in the pore space.

[0032] Fourth step: Water samples were obtained from the formation, and the collected formation water was processed to filter out suspended solids. Under standard conditions, the resistivity of the formation water sample was measured using a resistivity meter. The measurement results of the 3 rock samples are shown in Table 1.

[0033] Table 1 Physical properties of rocks and formation water

[0034]

[0035] Fifth step: The rock samples were continuously saturated with formation water under pressure, and the rock samples were continuously taken out and the water saturation was calculated using the weighing method. For every 15% increase in the water saturation of the rock sample, a voltage was applied across both ends of the rock sample and the current passing through the rock sample was measured to calculate the resistivity of the reservoir rock at different water saturations.

[0036] Sixth step: For the same rock sample at different water saturations, after measuring the resistivity of the rock sample, the porosity at different water saturations was then measured using helium. The test results are shown in Table 2.

[0037] Table 2 Resistivity and porosity of rock samples at different water saturations

[0038]

[0039] Seventh step: According to the measured initial porosity of the reservoir rock and the porosity at different water saturations, the volume fraction of the clay minerals in the pore volume of the reservoir rock after each expansion was calculated, as shown in Table 3.

[0040] Table 3 Volume fraction of clay minerals

[0041]

[0042] In the eighth step, in the reservoir, the clay minerals in the pores start to swell when encountering formation water, and their volume continuously increases, which will reduce the effective porosity of the reservoir. The larger the swelling volume of the clay minerals, the lower the effective porosity. A calculation model for the effective porosity of reservoir rocks can be established.

[0043] P e = P t (1 - V clay )

[0044] In the formula, P e is the effective porosity of reservoir rocks, with the unit of %; P t is the initial porosity of reservoir rocks, with the unit of %; V clay is the volume fraction of clay minerals in the rock pores, with the unit of %.

[0045] In the ninth step, there is a negative correlation between the resistivity of reservoir rocks and the water saturation of the reservoir. As the water saturation increases, the resistivity of reservoir rocks begins to decline and the decline trend slows down. The resistivity changes of 3 rock samples are shown in Figure 2 , and a relational formula between the resistivity of reservoir rocks and the water saturation of the reservoir can be established.

[0046]

[0047] In the formula, R t is the resistivity of reservoir rocks, with the unit of Ω·m; S w is the water saturation of the reservoir, with the unit of %; R w is the resistivity of formation water, with the unit of Ω·m; P is the porosity of reservoir rocks, with the unit of %; a, m, n are coefficients, with the unit of dimensionless.

[0048] In the tenth step, in the reservoir pores, considering the influence of clay minerals on the pore effectiveness, the porosity of reservoir rocks is the effective porosity of reservoir rocks at this time. Combining with the calculation model of the effective porosity of reservoir rocks, the calculation model of the water saturation of the reservoir is obtained as

[0049]

[0050] In the formula, S w is the water saturation of the reservoir, with the unit of %; R w is the resistivity of formation water, with the unit of Ω·m; R t is the resistivity of reservoir rocks, with the unit of Ω·m; P t is the initial porosity of reservoir rocks, with the unit of %; V clay is the volume fraction of clay minerals in the rock pores, with the unit of %; a, m, n are coefficients, with the unit of dimensionless.

[0051] Based on the actual on-site logging data, we obtained the values of coefficients a, m, and n. According to the data obtained from the previous experiments and using the reservoir water saturation calculation model, we can calculate the reservoir water saturation. The calculation results are shown in Table 4. As the volume fraction of clay minerals increases, the reservoir water saturation also increases, as shown in Figure 3 .

[0052] Table 4 Calculation Results of Reservoir Water Saturation

[0053]

[0054] Compared with the prior art, the present invention has the following beneficial effects: (1) The method has high reliability and a wider application range; (2) The method is convenient and effective, with high working efficiency; (3) Comprehensive calculation of multiple factors, with high accuracy of calculation results.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: Modifications or equivalent replacements can still be made to the present invention without departing from the spirit and scope of the present invention. Any modification or partial replacement within the scope of the present invention should be covered by the scope of the claims of the present invention.

Claims

1. A method for determining the water saturation of a shale gas reservoir formation, characterized in that, The method includes the following steps: S001 Drill representative shale plug samples from different shale gas reservoir formations respectively. After machine cutting, 50 regular rock samples are obtained, washed and then dried; S002 Measure the porosity of the dried rock samples using helium to obtain the initial porosity of the reservoir rock; S003 Analyze the mineral composition of the dried rock sample powder and conduct electron microscopy scanning to determine the clay mineral content and location in the reservoir rock; S004 Obtain water samples from the formation, treat the collected formation water, filter to remove suspended solids, and measure the resistivity of the formation water sample using a resistivity meter under standard conditions; S005 Continuously saturate the rock samples with pressurized formation water, continuously take out the rock samples and use the weighing method to calculate their water saturation. Every time the water saturation of the rock sample increases by 15%, apply a voltage across the two ends of the rock sample and measure the current passing through the rock sample to calculate the resistivity of the reservoir rock at different water saturations; S006 For the same rock sample at different water saturations, after measuring the resistivity of the rock sample, then use helium to determine its porosity at different water saturations; S007 Calculate the volume fraction of the clay minerals in the pore volume of the reservoir rock after each expansion based on the measured initial porosity of the reservoir rock and the porosity at different water saturations; S008 In the reservoir, the clay minerals in the pores start to expand when encountering formation water, and their volume continuously increases, which will reduce the effective porosity of the reservoir. The larger the volume of the clay mineral expansion, the lower the effective porosity. A calculation model for the effective porosity of the reservoir rock can be established; P e = P t (1 - V clay ) Wherein, P e is the effective porosity of the reservoir rock, in %; P t is the initial porosity of the reservoir rock, in %; V clay is the volume fraction of clay minerals in the rock pores, in %; S009 There is a negative correlation between the resistivity of the reservoir rock and the water saturation of the reservoir. A relationship formula between the resistivity of the reservoir rock and the water saturation of the reservoir can be established; where, R t is the resistivity of the reservoir rock, with the unit of Ω·m; S w is the water saturation of the reservoir, with the unit of %; R w is the resistivity of the formation water, with the unit of Ω·m; P is the porosity of the reservoir rock, with the unit of %; a, m, n are coefficients, with the unit of dimensionless; S010 In the pores of the reservoir, considering the influence of clay minerals on the pore effectiveness, the porosity of the reservoir rock at this time is the effective porosity of the reservoir rock. Combining with the calculation model for the effective porosity of the reservoir rock, the calculation model for the water saturation of the reservoir is obtained as follows: Wherein, S w is the water saturation of the reservoir, in %; R w is the resistivity of formation water, in Ω·m; R t is the resistivity of reservoir rock, in Ω·m; P t is the initial porosity of reservoir rock, in %; V clay is the volume fraction of clay minerals in rock pores, in %; a, m, and n are coefficients, dimensionless.

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