A method for determining the upper water saturation value of a tight gas reservoir using two-phase permeability

Abstract

This invention discloses a method for determining the upper limit of water saturation in tight gas reservoirs using two-phase permeability analysis, comprising: 1. Selecting several rock samples for gas-water two-phase permeability analysis, and establishing a gas-water phase permeability curve based on the relative permeability of the water phase, the relative permeability of the gas phase, and the water saturation value of each rock sample; 2. Based on the gas-water phase permeability curve, first reading the water saturation value of each rock sample when the relative permeability of the water phase is a set value A, then reading the relative permeability of the gas phase corresponding to the same water saturation value of each rock sample, and determining the upper limit of water saturation for each rock sample accordingly; 3. Calculating the average value of the upper limit of water saturation for each rock sample, and the obtained average value is the upper limit of water saturation for the tight gas reservoir. This invention can accurately determine the upper limit of water saturation in tight gas reservoirs using relatively simple means, simplifying the determination process and improving accuracy compared to existing technologies, providing an accurate basis for the rational exploitation of tight gas reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field exploration and development technology, specifically relating to a method for determining the upper limit of water saturation in tight gas reservoirs using two-phase permeability. Background Technology

[0002] When a tight gas reservoir contains multiphase fluids, the relative permeability of each phase increases with the increase of the saturation of that phase. Generally, tight gas reservoirs consist of two phases: a gas phase and a water phase. During gas extraction, water extraction should be minimized; therefore, it is necessary to determine the upper limit of water saturation for tight gas reservoirs.

[0003] Currently, the main methods for determining the upper limit of water saturation in tight gas reservoirs include the analogy method, the pore saturation relation method, the semi-permeable diaphragm capillary pressure method, and the two-phase permeability method. Among these,

[0004] The analogy method is a method of determining the upper limit of water saturation of a tight gas reservoir by borrowing the existing upper limit of water saturation of similar tight gas reservoirs. However, the prerequisite is that there must be a similar tight gas reservoir and an existing upper limit of water saturation.

[0005] The porosity-saturation relation method first determines the lower limit of porosity, and then uses the porosity-saturation relation to determine the upper limit of water saturation. However, it requires that the lower limit of porosity has been determined and that the porosity-saturation relation has good correlation.

[0006] The semi-permeable diaphragm capillary pressure method first involves drawing tangents from the middle and end segments of the capillary pressure curve. The saturation value at the intersection of these tangents on the capillary pressure curve is the upper limit of water saturation. However, the difficulty lies in determining the tangent.

[0007] Two-phase permeability methods have previously used the crossover point method (which considers the water saturation at the crossover point of two-phase permeability as the upper limit of water saturation), the inflection point method (which considers the water saturation at the inflection point of two-phase permeability as the upper limit of water saturation), and the residual gas method (which considers the water saturation when the relative permeability of the gas phase is zero as the upper limit of water saturation). However, all three methods have technical problems of poor accuracy and are now rarely used.

[0008] Furthermore, no simple and accurate method for determining the upper limit of water saturation in tight gas reservoirs has been found in the existing technology. Summary of the Invention

[0009] The purpose of this invention is to solve the above-mentioned problems existing in the prior art and to provide a method for determining the upper limit of water saturation of tight gas reservoirs by two-phase permeability. This invention can accurately determine the upper limit of water saturation of tight gas reservoirs with relatively simple means. Compared with the prior art, it simplifies the determination process, improves accuracy, and provides an accurate basis for the rational exploitation of tight gas reservoirs.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] A method for determining the upper limit of water saturation in tight gas reservoirs using two-phase permeability, characterized by comprising the following steps:

[0012] Step 1: Select several rock samples for gas-water two-phase permeability analysis, obtain the relative permeability of the water phase, the relative permeability of the gas phase, and the water saturation value of each rock sample, and establish a gas-water phase permeability curve based on the relative permeability of the water phase, the relative permeability of the gas phase, and the water saturation value of each rock sample.

[0013] Step 2: Based on the gas-water phase permeability curve, first read the water saturation value of each rock sample when the water phase relative permeability is the set value A, and then read the gas phase relative permeability value corresponding to the same water saturation value of each rock sample.

[0014] Among the gas phase relative permeability values ​​of each rock sample read, if the gas phase relative permeability value of any rock sample is greater than or equal to the set value B, then the corresponding water saturation value will be used as the upper limit of water saturation for that rock sample.

[0015] If the relative permeability of the gas phase of a rock sample is less than the set value B, the water saturation value corresponding to the relative permeability of the gas phase of the rock sample when the relative permeability of the gas phase is the set value B will be reread on the gas-water phase permeability curve, and the water saturation value will be used as the upper limit of the water saturation of the rock sample.

[0016] Step 3: Calculate the average value of the upper limit of water saturation for each rock sample. The average value obtained is the upper limit of water saturation for the tight gas reservoir.

[0017] After selecting rock samples in step 1, the relative permeability of the water phase and the relative permeability of the gas phase can be directly analyzed using a gas-water two-phase relative permeability test instrument under different water saturation conditions. Then, a cross-plot is plotted with water saturation as the abscissa (water saturation from 0% to 100%) and the percentages or decimals of the relative permeability of the water phase and the relative permeability of the gas phase as the ordinate to establish a gas-water phase permeability curve (relative permeability of the water phase and the relative permeability of the gas phase from 0 to 1).

[0018] The water saturation refers to the ratio of the volume of water in the pores of a rock sample to the total volume of rock pores.

[0019] The relative permeability of the water phase refers to the ratio of the permeability of water flowing simultaneously in a rock sample to the permeability of water flowing alone in the rock sample. When both gas and water phases flow simultaneously in a rock sample, the permeability of water flowing is lower than that of water flowing alone in the rock sample due to the resistance of gas. Therefore, the relative permeability of the water phase is between 0 and 1.

[0020] The relative permeability of the gas phase refers to the ratio of the permeability of gas flow to the permeability of gas flowing alone in a rock sample when both gas and water flow simultaneously. When both gas and water flow simultaneously in a rock sample, the permeability of gas flow is lower than that of gas flowing alone in the rock sample due to the resistance of water. Therefore, the relative permeability of the gas phase is between 0 and 1.

[0021] In step 1, the number of rock samples is 5-100, and samples are taken from different sampling depths.

[0022] In step 2, when rereading the relative permeability value of the gas phase of the rock sample, if the relative permeability value of the gas phase of the rock sample on the gas-water phase permeability curve is less than the set value B, then the rock sample is determined to be a non-reservoir, that is, the rock sample has no upper limit value of water saturation.

[0023] The aforementioned tight gas reservoir refers to a gas reservoir stored in a pressurized matrix with a permeability of less than or equal to 0.1 × 10⁻⁶. -3 μm 2 Natural gas in reservoirs such as tight sandstone and tight carbonate rocks; or natural gas with an air permeability of less than 1×10⁻⁶. -3 μm 2 Natural gas in reservoirs such as dense sandstone and dense carbonate rocks.

[0024] The overburden matrix permeability is a geophysical instrument used in the fields of chemistry, earth science, biology, and materials science. It simulates the ability of unfractured rocks to allow fluid passage under formation pressure in oil and gas reservoirs. The international standard unit is µm. 2 The fluid is usually air; it is a commonly used instrument in the petroleum industry, such as the fully automatic pressure-covered hole permeability measurement system.

[0025] Air permeability refers to the ability of a rock to allow fluid to pass through it under a certain pressure difference. It is a parameter characterizing the rock's ability to conduct fluids, and its international standard unit is µm. 2 The fluid in this context specifically refers to air; it is a commonly used instrument in the petroleum industry, such as the STY-Ⅲ type gas permeability meter.

[0026] By adopting the above technical solution, the beneficial technical effects of the present invention are:

[0027] 1. Based on the existing two-phase permeability experimental analysis results, this invention can directly read the upper limit of water saturation of rock samples, which is simple to operate. Since the relative permeability of the water phase at the sampling point is extremely low, the water droplets have not yet formed an intermittent water flow, and there is no gas-water mixing. The relative permeability of the gas phase is relatively high, which can produce gas. Therefore, it can quickly and effectively obtain the upper limit of reservoir water saturation, increasing the reliability of reserve calculation.

[0028] 2. This invention can accurately determine the upper limit of water saturation in tight gas reservoirs using relatively simple means. Compared with the prior art, it simplifies the determination process, improves accuracy, and provides an accurate basis for the rational exploitation of tight gas reservoirs.

[0029] 3. There are few existing methods for evaluating the upper limit of water saturation in reserve assessment. This method provides a more reliable, data-rich, and readily available method for evaluating the upper limit of water saturation.

[0030] 4. In practical applications, the present invention takes less than 1 minute to determine the upper limit of water saturation for a sample. Therefore, the upper limit of water saturation for an oil and gas reservoir can be determined in less than 2 hours, which is extremely fast and greatly improves efficiency. Attached Figure Description

[0031] Figure 1 This is a flowchart of the present invention.

[0032] Figure 2 This is a graph showing the gas-water phase permeability curve of the R3 sample in the Sha-2 section of the Bajiaochang gas field reservoir.

[0033] Figure 3 This is a graph showing the gas-water phase permeability curve of the R1 sample in the Sha-2 section of the Bajiaochang gas field reservoir.

[0034] Figure 4 This is a graph showing the gas-water phase permeability curve of the R7 sample in the Sha-2 section of the Bajiaochang gas field reservoir. Detailed Implementation

[0035] Example 1

[0036] This embodiment provides a method for determining the upper limit of water saturation in tight gas reservoirs using two-phase permeability. This method is applicable to tight gas reservoirs located in overlying matrices with permeability less than or equal to 0.1 × 10⁻⁶. -3 μm 2 Natural gas in reservoirs such as tight sandstone and tight carbonate rocks; or natural gas with an air permeability of less than 1×10⁻⁶. -3 μm 2 Natural gas in reservoirs such as tight sandstone and tight carbonate rocks.

[0037] The overburden matrix permeability is a geophysical instrument used in the fields of chemistry, earth science, biology, and materials science. It simulates the ability of unfractured rocks to allow fluid passage under formation pressure in oil and gas reservoirs. The international standard unit is µm. 2 The fluid is usually air; it is a commonly used instrument in the petroleum industry, such as the fully automatic pressure-covered hole permeability measurement system.

[0038] Air permeability refers to the ability of a rock to allow fluid to pass through it under a certain pressure difference. It is a parameter characterizing the rock's ability to conduct fluids, and its international standard unit is µm. 2 The fluid in this context specifically refers to air; it is a commonly used instrument in the petroleum industry, such as the STY-Ⅲ type gas permeability meter.

[0039] This method mainly establishes a gas-water phase permeability curve based on the existing two-phase permeability experimental analysis results. Then, based on this gas-water phase permeability curve, the upper limit of water saturation of the rock sample can be directly read, which not only simplifies the process of determining the upper limit of water saturation but also improves the accuracy.

[0040] Specifically, such as Figure 1 As shown, the method includes the following steps:

[0041] Step 1: Select several rock samples for gas-water two-phase permeability analysis to obtain the relative permeability of the water phase, the relative permeability of the gas phase, and the water saturation value of each rock sample. Based on the relative permeability of the water phase, the relative permeability of the gas phase, and the water saturation value of each rock sample, establish a gas-water phase permeability curve.

[0042] It should be noted that the number of rock samples in this step is usually 5-100, taken from different sampling depths. After selecting the rock samples, a gas-water two-phase relative permeability test instrument (such as the QSXS-III type gas-water relative permeability instrument) can be used to directly analyze the water phase relative permeability and gas phase relative permeability values ​​of each rock sample under different water saturation conditions. Then, a cross-plot is plotted with water saturation as the abscissa (water saturation from 0% to 100%) and the percentage or decimal of the water phase relative permeability and gas phase relative permeability values ​​as the ordinate to establish a gas-water phase permeability curve (water phase relative permeability and gas phase relative permeability values ​​from 0 to 1).

[0043] In addition, the water saturation refers to the ratio of the volume of water in the pores of the rock sample to the volume of rock pores.

[0044] The relative permeability of the water phase refers to the ratio of the permeability of water flowing simultaneously in a rock sample to the permeability of water flowing alone in the rock sample. When both gas and water phases flow simultaneously in a rock sample, the permeability of water flowing is lower than that of water flowing alone in the rock sample due to the resistance of gas. Therefore, the relative permeability of the water phase is between 0 and 1.

[0045] The relative permeability of the gas phase refers to the ratio of the permeability of gas flow to the permeability of gas flowing alone in a rock sample when both gas and water flow simultaneously. When both gas and water flow simultaneously in a rock sample, the permeability of gas flow is lower than that of gas flowing alone in the rock sample due to the resistance of water. Therefore, the relative permeability of the gas phase is between 0 and 1.

[0046] Step 2: Based on the gas-water phase permeability curve, first read the water saturation value of each rock sample when the water phase relative permeability is the set value A, and then read the gas phase relative permeability value corresponding to the same water saturation value of each rock sample.

[0047] Among the gas phase relative permeability values ​​read from each rock sample, if the gas phase relative permeability value of any rock sample is greater than or equal to the set value B, then the corresponding water saturation value will be used as the upper limit of water saturation for that rock sample.

[0048] If the relative permeability of the gas phase of a rock sample is less than the set value B, the water saturation value corresponding to the relative permeability of the gas phase of the rock sample when the relative permeability of the gas phase is the set value B will be reread on the gas-water phase permeability curve, and this water saturation value will be used as the upper limit of the water saturation of the rock sample.

[0049] In addition, when rereading the relative permeability value of the gas phase of the rock sample, if the relative permeability value of the gas phase of the rock sample on the gas-water phase permeability curve is less than the set value B, that is, if there is no data with a relative permeability value of the gas phase of the rock sample greater than or equal to the set value B, then the rock sample is determined to be a non-reservoir, that is, the rock sample is determined to have no upper limit value of water saturation.

[0050] It should be noted that the theoretical basis for determining the upper limit of water saturation in tight gas reservoirs is as follows: Vertically, the water saturation of a tight gas reservoir is a function of altitude, meaning that water saturation gradually decreases from bottom to top. Tight gas reservoirs possess a gas-water transition zone. The water saturation variation zone between bound water saturation and residual gas saturation in the two-phase permeability curve falls within the flowable stage of the gas-water two-phase fluids, precisely corresponding to the water saturation variation zone of the gas-water transition zone in the tight gas reservoir. The critical water saturation is located within the flowable stage of the gas-water two-phase fluids and is close to the bound water saturation level. Since the gas and water permeability in tight gas reservoirs is extremely low, for a small amount of mobile water to prevent continuous flow, the gas phase permeability must be much greater than the water phase permeability. Gas phase flow must dominate, meaning the relative permeability of the gas phase must be at least an order of magnitude greater than the relative permeability of the water phase. Therefore, the setpoint A is usually set to 0.01, the setpoint B is set to 0.1, and the relative permeability of the water phase is required to be no greater than 0.01 and the relative permeability of the gas phase is required to be no less than 0.1.

[0051] Step 3: Calculate the average value of the upper limit of water saturation for each rock sample. The average value obtained is the upper limit of water saturation for the tight gas reservoir.

[0052] Example 2

[0053] This embodiment verifies the method described in Embodiment 1, as follows:

[0054] 1. Thirteen rock samples were taken from different sampling depths in the tight gas reservoir of the Sha-2 member of the Bajiaochang gas field, and the gas-water phase permeability analysis data shown in Table 1 below were obtained.

[0055] Table 1. Statistical table of upper limit of water saturation obtained by gas-water phase permeability method in Sha-2 section gas reservoir of Bajiaochang gas field.

[0056]

[0057] 2. Permeability analysis data of gas-liquid phase in sample R3 (see...) Figure 2 The water saturation value SW, with a relative permeability of 0.01 in the aqueous phase, is 64.2%. The corresponding relative permeability value Krg in the gas phase is 0.2228. Since Krg ≥ 0.1, the upper limit of water saturation for sample R3 is 64.2% (see Table 1).

[0058] 3. Permeability analysis data of gas-liquid phase in sample R1 (see...) Figure 3 The water saturation value SW, with a relative permeability of 0.01 in the aqueous phase, is 67.2%. The corresponding relative permeability value Krg in the gas phase is 0.066. Since Krg < 0.1, the water saturation value with a relative permeability of 0.1 in the gas phase needs to be read again. This water saturation value SW is 57.5%. Therefore, the upper limit of water saturation for sample R1 is 57.5% (see Table 1).

[0059] 4. In the gas-liquid phase permeability analysis data of sample R7 (see...) Figure 4 Its maximum relative permeability in the gas phase is 0.047 × 10⁻⁶. -3 μm 2 All are less than 0.1×10 -3 μm 2 Therefore, the sample was determined to be non-reservoir, and the upper limit of water saturation could not be determined (see Table 1).

[0060] 5. Similarly, the upper limit of water saturation was obtained from the gas-water phase permeability analysis data of the remaining 10 samples (R2, etc., where samples R7, R8, and R9 had no upper limit of water saturation). The arithmetic mean of the obtained upper limit of water saturation for the 10 samples (R1, etc.) was then calculated, and the average value was 62.9% (see Table 1). Therefore, the upper limit of water saturation for the tight gas reservoir in the Sha-2 member of the Bajiaochang gas field is 62.9%.

[0061] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All features or steps in the disclosed methods or processes may be combined in any way, except for mutually exclusive features and / or steps.

Claims

1. A method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability, characterized in that... Includes the following steps: Step 1: Select several rock samples for gas-water two-phase permeability analysis, obtain the relative permeability of the water phase, the relative permeability of the gas phase, and the water saturation value of each rock sample, and establish a gas-water phase permeability curve based on the relative permeability of the water phase, the relative permeability of the gas phase, and the water saturation value of each rock sample. Step 2: Based on the gas-water phase permeability curve, first read the water saturation value of each rock sample when the water phase relative permeability is the set value A, and then read the gas phase relative permeability value corresponding to the same water saturation value of each rock sample. Among the gas phase relative permeability values ​​of each rock sample read, if the gas phase relative permeability value of any rock sample is greater than or equal to the set value B, then the corresponding water saturation value is taken as the upper limit of water saturation of that rock sample. If the relative permeability of the gas phase of a rock sample is less than the set value B, the water saturation value corresponding to the relative permeability of the gas phase of the rock sample when the relative permeability of the gas phase is the set value B will be reread on the gas-water phase permeability curve, and the water saturation value will be used as the upper limit of the water saturation of the rock sample. The set value A is 0.01, the set value B is 0.1, and the relative permeability of the water phase cannot be greater than 0.01, and the relative permeability of the gas phase cannot be less than 0.1; Step 3: Calculate the average value of the upper limit of water saturation for each rock sample. The average value obtained is the upper limit of water saturation for the tight gas reservoir.

2. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 1, characterized in that: After selecting rock samples in step 1, the relative permeability of the water phase and the relative permeability of the gas phase of each rock sample under different water saturation conditions are directly analyzed using a gas-water two-phase relative permeability test instrument. Then, a cross plot is drawn with water saturation as the abscissa and the percentage or decimal of the relative permeability of the water phase and the relative permeability of the gas phase as the ordinate to establish a gas-water phase permeability curve.

3. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 2, characterized in that: The water saturation refers to the ratio of the volume of water in the pores of a rock sample to the total volume of rock pores.

4. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 2, characterized in that: The relative permeability of the water phase refers to the ratio of the permeability of water flowing simultaneously in a rock sample to the permeability of water flowing alone in the rock sample.

5. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 2, characterized in that: The relative permeability of the gas phase refers to the ratio of the permeability of gas flow to the permeability of gas flow alone in a rock sample when gas and water flow simultaneously.

6. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 1, characterized in that: In step 1, the number of rock samples is 5-100, and samples are taken from different sampling depths.

7. A method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to any one of claims 1-6, characterized in that: In step 2, when rereading the relative permeability value of the gas phase of the rock sample, if the relative permeability value of the gas phase of the rock sample on the gas-water phase permeability curve is less than the set value B, then the rock sample is determined to be a non-reservoir, that is, the rock sample has no upper limit value of water saturation.

8. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 1, characterized in that: The aforementioned tight gas reservoir refers to a gas reservoir stored in a pressurized matrix with a permeability of less than or equal to 0.1 × 10⁻⁶. -3 μm 2 Natural gas in tight sandstone or tight carbonate rock reservoirs; or natural gas with an air permeability of less than 1×10⁻⁶. -3 μm 2 Natural gas in dense sandstone and dense carbonate rock reservoirs.

9. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 8, characterized in that: The overburden matrix permeability is an earth exploration instrument used in the fields of chemistry, earth science, biology, and materials science. It simulates the ability of unfractured rocks to allow fluid to pass through under the formation pressure of oil and gas reservoirs and is measured using a fully automated overburden permeability measurement system.

10. The method for determining the upper limit of water saturation in a tight gas reservoir using two-phase permeability according to claim 8, characterized in that: The air permeability refers to the ability of a rock to allow fluid to pass through under a certain pressure difference. It is a parameter characterizing the rock's ability to conduct fluid and is measured using a gas permeability meter.

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