Quantitative evaluation method for sealing performance of ultra-deep water and ultra-shallow layer cover layer

Abstract

The invention relates to the technical field of oil-gas exploration, in particular to an ultra-deep water and ultra-shallow layer cover layer sealing performance quantitative evaluation method which comprises the following steps: S1, collecting a cover layer sample, and recording the cover layer burying depth; s2, measuring the minimum principal stress and the maximum principal stress of the cap layer sample, and calculating the ratio of the minimum principal stress to the maximum principal stress; s3, determining the cap layer density of the cap layer sample, the formation water density of the cap layer sample and the gas density of the cap layer sample under the burying condition; and S4, calculating the maximum air column height capable of being plugged by the cap layer sample according to the cap layer burying depth, the ratio of the minimum principal stress to the maximum principal stress, the cap layer density, the formation water density and the gas density. The method provided by the invention overcomes the defect that the prior art is not suitable for the research of the cover layer under the ultra-deep water and ultra-shallow layer conditions. The method provided by the invention can be used for quantitatively evaluating the height of the plugging air column of the high-pressure, low-temperature, high-water-content and weakly-consolidated cover layer.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration, and more specifically, to a quantitative evaluation method for the sealing performance of ultra-deepwater and ultra-shallow caprocks. Background Art

[0002] The cap rock plays an important role in the process of oil and gas accumulation. As a vertical barrier to oil and gas reservoirs, it has always attracted much attention. The essence of the cap rock vertical barrier to oil and gas reservoirs is that the cap rock strength is greater than the maximum net buoyancy of oil and gas. Ultra-deepwater and ultra-shallow natural gas exploration has been at the forefront of marine oil and gas exploration at home and abroad in recent years. Ultra-deepwater refers to water depths greater than 1500m, and ultra-shallow refers to strata with a burial depth of less than 300m (measured from the seabed to the depth). The cap rock developed in ultra-deepwater and ultra-shallow layers has low consolidation strength and high water content. Previous researchers generally believed that it has poor sealing properties and is difficult to form large and medium-sized gas reservoirs. In order to implement the exploration prospects of ultra-deepwater and ultra-shallow oil and gas resources, it is very necessary to carry out quantitative evaluation of the cap rock.

[0003] The prior art discloses a method for predicting the maximum closed hydrocarbon column height of a cap layer, comprising the following steps: performing a breakthrough pressure test on a cap layer and a reservoir under experimental conditions, obtaining the cap layer capillary pressure and the reservoir capillary pressure by correcting the cap layer breakthrough pressure and the reservoir breakthrough pressure, and calculating the difference between the two to obtain the capillary pressure difference; obtaining the starting pressure gradient that needs to be overcome for formation water to flow in the cap layer; combining the capillary pressure difference of the hydrocarbon-water interface in the cap layer and the reservoir and the starting pressure gradient that needs to be overcome for fluid flow in the cap layer to obtain the relationship between the hydrocarbon column height and the cap layer thickness, and obtaining the predicted height of the maximum closed hydrocarbon column of the cap layer.

[0004] However, the above method is mainly used to study the mudstone caprock (consolidated rock) of conventional oil and gas reservoirs, and is not applicable to the caprock with high water content, weak consolidation and unaphthenic under ultra-deep water (representing high pressure) and ultra-shallow layer (representing low temperature). The currently disclosed prior art also does not involve a quantitative evaluation method for the sealing of caprocks with high water content and low consolidation degree under ultra-deep water and ultra-shallow layer conditions. Summary of the invention

[0005] In view of the problem that the above-mentioned existing technologies are not suitable for the study of cap rocks under ultra-deep water and ultra-shallow conditions, the present invention provides a quantitative evaluation method for the sealing of ultra-deep water and ultra-shallow cap rocks, which can quantitatively evaluate the height of the gas column that can be sealed by high-pressure, low-temperature, high-water-content, and weakly consolidated cap rocks.

[0006] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0007] A method for quantitatively evaluating the sealing performance of ultra-deepwater and ultra-shallow caprocks comprises the following steps:

[0008] S1: Collect cover layer samples and record the burial depth of the cover layer;

[0009] S2: measuring the minimum principal stress and the maximum principal stress of the cover layer sample, and calculating the ratio of the minimum principal stress to the maximum principal stress;

[0010] S3: determining the cap layer density of the cap layer sample, the formation water density of the cap layer sample, and the gas density of the cap layer sample under burial conditions;

[0011] S4: Calculate the maximum gas column height that can be blocked by the cap layer sample according to the burial depth of the cap layer, the ratio of the minimum principal stress to the maximum principal stress, the density of the cap layer, the formation water density and the gas density.

[0012] Preferably, in step S1, the pressure and temperature at the depth of the caprock sample are obtained by a pressure sampling method. The pressure sampling method can directly obtain data from the original location, provide real pressure and temperature information, ensure that the experimental results are consistent with the actual situation, and compared with theoretical calculations or indirect measurements, the actual sampling data can more accurately reflect the environmental conditions at the depth of the sample, so as to simulate more realistic environmental conditions in the subsequent process.

[0013] Preferably, in step S2, the minimum principal stress and the maximum principal stress of the caprock sample are measured using a triaxial stress test instrument. The triaxial stress test instrument can accurately measure the stress-strain relationship of the sample under different stress conditions, thereby more accurately determining the minimum principal stress and the maximum principal stress, and the triaxial stress test instrument can be adjusted and simulated according to needs to actual underground conditions, including different confining pressures and axial stresses, and provide detailed information on the stress state of the caprock sample in a real environment, which is conducive to accurate analysis of the experimental results.

[0014] Preferably, in step S1, the pressure and temperature at the depth of the cap layer sample are also measured; in step S2, the confining pressure and temperature of the triaxial stress test instrument are respectively set to the pressure and temperature at the depth of the cap layer sample to simulate the high pressure and low temperature environment of the cap layer.

[0015] Preferably, in step S2, before measuring the minimum principal stress and the maximum principal stress of the cap layer sample, water is first added to the cap layer sample. Since multiple samples need to be obtained during actual sampling, sometimes due to cost constraints, it is impossible to obtain all samples through the closed sampling method, and the process of obtaining the cap layer samples and transporting the samples from the sea to the laboratory takes a long time. The cap layer samples will evaporate and lose some water, which will affect the accuracy of the measurement results. Therefore, it is necessary to add water to the cap layer sample to improve the accuracy of the measurement results.

[0016] Preferably, water is added to the cap layer sample according to the true moisture content of the cap layer sample until the moisture content of the cap layer sample is the same as the true moisture content. It can be understood that the true moisture content of the cap layer sample is the moisture content of the original stratum of the cap layer sample. When the moisture content of the cap layer sample reaches the true moisture content, it means that the cap layer sample has been restored to its original state, and the measurement result will be more accurate at this time.

[0017] Preferably, the actual moisture content is measured based on the cover layer sample obtained by the closed sampling method. That is, the cover layer sample is obtained by the closed sampling method, and then the moisture content of the cover layer sample obtained by the closed sampling is measured. The closed sampling method can effectively avoid contamination or external interference of the sample during sampling and transportation, and maintain the original moisture content of the sample. The moisture content of the sample obtained by measuring this method is highly consistent with the actual moisture content of the cover layer and is more referenceable.

[0018] Preferably, in step S2, the supplemented water is distilled water. Distilled water does not contain salt and other chemical components, can effectively avoid contamination of the cover layer sample, and helps to maintain the original properties of the sample.

[0019] Preferably, in step S3, the gas density of the caprock sample under burial conditions is obtained through a variable temperature-pressure field gas density experiment.

[0020] Preferably, in step S4, the maximum air column height that can be blocked by the cap layer sample is calculated as follows:

[0021]

[0022] In the formula, H g is the maximum gas column height that can be blocked by the caprock, m; σ3 and σ1 are the minimum and maximum principal stresses when the caprock breaks under high pressure, low temperature and high water content conditions, kpa; ρ r is the caprock density under high pressure, low temperature and high water content conditions, g / cm 3 ρ w is the formation water density, g / cm 3 ρ g is the density of natural gas under high pressure and low temperature conditions, g / cm 3 ;H b is the seabed burial depth of the caprock, m.

[0023] The beneficial effects of the present invention are as follows: on the one hand, the real water content of the cap rock is determined based on the closed sampling data, and then the physical simulation experimental device is set to the high-pressure and low-temperature environment of the ultra-deep water and ultra-shallow cap rock to measure the strength of the cap rock; on the other hand, the density of natural gas under high-pressure and low-temperature conditions is obtained according to the variable temperature-pressure field gas density experiment; based on the above two results, a calculation formula for the maximum gas column height that can be sealed in the ultra-deep water and ultra-shallow cap rock is proposed, and the maximum gas column height that can be sealed in the ultra-deep water and ultra-shallow cap rock is calculated according to the calculation formula, so as to realize the quantitative evaluation of the sealing property of the ultra-deep water and ultra-shallow cap rock. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a flow chart of a quantitative evaluation method for sealing properties of ultra-deepwater and ultra-shallow caprocks. DETAILED DESCRIPTION

[0025] The drawings are only for illustrative purposes and cannot be construed as limiting the present invention. To better illustrate the present embodiment, some parts of the drawings may be omitted, enlarged, or reduced, and do not represent the size of the actual product. For those skilled in the art, it is understandable that some well-known structures and their descriptions may be omitted in the drawings. The positional relationships described in the drawings are only for illustrative purposes and cannot be construed as limiting the present invention.

[0026] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "long", "short" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limitations on this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0027] The technical solution of the present invention is further described in detail below through specific embodiments and in conjunction with the accompanying drawings:

[0028] Example 1

[0029] The L area in the Qiongdongnan Basin, where the seawater depth exceeds 1500m, was taken as the study area, and the cap rock with a burial depth of about 200m was taken as the research object. The pressure measurement sampling data of this depth section were collected. Exploration practice shows that natural gas is developed in the reservoir beneath the cap rock, which means that even a cap rock with high water content, weak consolidation and undiagenetic properties can also have a certain degree of sealing under high pressure and low temperature conditions.

[0030] This embodiment uses a method for quantitatively evaluating the sealing properties of ultra-deepwater and ultra-shallow caprocks to study the L region. The method includes the following steps:

[0031] S1: Collect cover layer samples and record the burial depth of the cover layer;

[0032] S2: measuring the minimum principal stress and the maximum principal stress of the cover layer sample, and calculating the ratio of the minimum principal stress to the maximum principal stress;

[0033] S3: determining the cap layer density of the cap layer sample, the formation water density of the cap layer sample, and the gas density of the cap layer sample under burial conditions;

[0034] S4: Calculate the maximum gas column height that can be blocked by the cap layer sample according to the burial depth of the cap layer, the ratio of the minimum principal stress to the maximum principal stress, the density of the cap layer, the formation water density and the gas density.

[0035] Furthermore, in step S1, the pressure and temperature at the depth of the caprock sample are obtained by a pressure sampling method. The pressure sampling method can directly obtain data from the original location, provide real pressure and temperature information, ensure that the experimental results are consistent with the actual situation, and compared with theoretical calculations or indirect measurements, the actual sampling data can more accurately reflect the environmental conditions at the depth of the sample, so as to simulate more realistic environmental conditions in the subsequent process.

[0036] Furthermore, in step S2, the minimum principal stress and the maximum principal stress of the caprock sample are measured using a triaxial stress test instrument. The triaxial stress test instrument can accurately measure the stress-strain relationship of the sample under different stress conditions, thereby more accurately determining the minimum principal stress and the maximum principal stress, and the triaxial stress test instrument can be adjusted and simulated according to needs to actual underground conditions, including different confining pressures and axial stresses, and provide detailed information on the stress state of the caprock sample in a real environment, which is conducive to accurate analysis of the experimental results.

[0037] Furthermore, in step S1, the pressure and temperature at the depth of the cap layer sample are also measured; in step S2, the confining pressure and temperature of the triaxial stress test instrument are respectively set to the pressure and temperature at the depth of the cap layer sample to simulate the high pressure and low temperature environment of the cap layer.

[0038] Furthermore, in step S2, before measuring the minimum principal stress and the maximum principal stress of the cap layer sample, water is first added to the cap layer sample. Since multiple samples need to be obtained during actual sampling, sometimes due to cost constraints, it is impossible to obtain all samples through the closed sampling method, and the process of obtaining the cap layer samples and transporting the samples from the sea to the laboratory takes a long time. The cap layer samples will evaporate and lose some water, which will affect the accuracy of the measurement results. Therefore, it is necessary to add water to the cap layer sample to improve the accuracy of the measurement results.

[0039] Further, water is added to the cap layer sample according to the true moisture content of the cap layer sample until the moisture content of the cap layer sample is the same as the true moisture content. It can be understood that the true moisture content of the cap layer sample is the moisture content of the original stratum of the cap layer sample. When the moisture content of the cap layer sample reaches the true moisture content, it means that the cap layer sample has been restored to its original state, and the measurement result will be more accurate at this time.

[0040] Furthermore, the actual moisture content is measured based on the cap layer sample obtained by the closed sampling method. That is, the cap layer sample is obtained by the closed sampling method, and then the moisture content of the cap layer sample obtained by the closed sampling is measured. The closed sampling method can effectively avoid contamination or external interference of the sample during sampling and transportation, and maintain the original moisture content of the sample. The moisture content of the sample obtained by measuring this method is highly consistent with the actual moisture content of the cap layer, and is more referenceable.

[0041] Specifically, in step S4, the maximum air column height that can be blocked by the cap layer sample is calculated as follows:

[0042]

[0043] In the formula, H g is the maximum gas column height that can be blocked by the caprock, m; σ3 and σ1 are the minimum and maximum principal stresses when the caprock breaks under high pressure, low temperature and high water content conditions, kpa; ρ r is the caprock density under high pressure, low temperature and high water content conditions, g / cm 3 ρ w is the formation water density, g / cm 3 ρ g is the density of natural gas under high pressure and low temperature conditions, g / cm 3 ;H b is the seabed burial depth of the caprock, m.

[0044] The working principle or workflow of this embodiment is as follows:

[0045] 1) Collect cap rock samples at different water depths above 1500m and at different depths of about 200m, and measure the pressure P and temperature T at the depth of the cap rock samples. In this experiment, the cap rock samples were taken at a depth of about 200m on the seabed, with an average pressure of 17.5Mpa and an average temperature of 16℃;

[0046] 2) Set the confining pressure and temperature of the triaxial stress test instrument to 17.5Mpa and 16℃ respectively. A small amount of the caprock samples in this experiment are cores, and most of them are coring from the wellbore. The core data was obtained under closed conditions, and the water content is more realistic, while the coring from the wellbore does not meet the closed conditions. Therefore, it is necessary to use the core water content as a standard and add an appropriate amount of distilled water to the wellbore coring to make its water content consistent with that of the core. In this experiment, the water content is uniformly set to 35%. Based on the triaxial stress experiment, it is known that is 0.22. Based on the well logging curve, we know that the caprock density ρ r 2.16 g / cm 3 , formation water density ρ w 1.04g / cm 3 , natural gas density ρ g 0.21 g / cm2 under temperature and pressure conditions in step 1) 3 , seabed depth H b 200m;

[0047] 3) Quantitative evaluation of the sealing performance of ultra-deepwater and ultra-shallow caprocks based on the formula for the maximum gas column height that can be blocked by the caprock:

[0048]

[0049] It can be seen that the caprock at a seafloor depth of about 200 m in the study area can block the highest gas column H g It is 59.37m. At present, the actual maximum effective closure height of gas reservoirs in ultra-deep water and ultra-shallow layers in the Qiongdongnan Basin at this depth is 58m, which verifies the correctness of this formula.

[0050] The beneficial effects of this embodiment are as follows: on the one hand, the actual water content of the cap rock is determined based on the closed sampling data, and then the physical simulation experiment device is set to the high-pressure and low-temperature environment of the ultra-deep water and ultra-shallow cap rock to measure the strength of the cap rock; on the other hand, the density of natural gas under high-pressure and low-temperature conditions is obtained based on the variable temperature-pressure field gas density experiment; based on the above two results, a calculation formula for the maximum gas column height that can be sealed in the ultra-deep water and ultra-shallow cap rock is proposed, and the maximum gas column height that can be sealed in the ultra-deep water and ultra-shallow cap rock is calculated according to the calculation formula, which can realize the quantitative evaluation of the sealing performance of the ultra-deep water and ultra-shallow cap rock.

[0051] Example 2

[0052] This embodiment further supplements step S2 based on embodiment 1. In step S2, the supplemented water is distilled water. Distilled water does not contain salt and other chemical components, which can effectively avoid contamination of the cover layer sample and help maintain the original properties of the sample.

[0053] The other features, working principles and beneficial effects of this embodiment are consistent with those of Embodiment 1.

[0054] Example 3

[0055] This embodiment further supplements step S3 based on embodiment 2. In step S3, the gas density of the caprock sample under burial conditions is obtained through a variable temperature-pressure field gas density experiment.

[0056] The other features, working principles and beneficial effects of this embodiment are consistent with those of Embodiment 2.

[0057] Obviously, the above embodiments of the present invention are merely examples for clearly explaining the present invention, and are not intended to limit the implementation methods of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description, and it is not necessary and impossible to list all the implementation methods here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A quantitative evaluation method for sealing of ultra-deepwater and ultra-shallow caprocks, characterized in that: The following steps are involved: S1: Collect cover layer samples and record the burial depth of the cover layer; S2: measuring the minimum principal stress and the maximum principal stress of the cover layer sample, and calculating the ratio of the minimum principal stress to the maximum principal stress; S3: determining the cap layer density of the cap layer sample, the formation water density of the cap layer sample, and the gas density of the cap layer sample under burial conditions; S4: Calculate the maximum gas column height that can be blocked by the cap layer sample according to the burial depth of the cap layer, the ratio of the minimum principal stress to the maximum principal stress, the density of the cap layer, the formation water density and the gas density.

2. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 1, characterized in that: In the step S1, the pressure and temperature at the depth of the caprock sample are obtained by a pressure sampling method.

3. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 1, characterized in that: In the step S2, a triaxial stress test instrument is used to measure the minimum principal stress and the maximum principal stress of the cover layer sample.

4. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 3 is characterized in that: In the step S1, the pressure and temperature at the depth where the cap layer sample is located are also measured; in the step S2, the confining pressure and temperature of the triaxial stress test instrument are respectively set to the pressure and temperature at the depth where the cap layer sample is located.

5. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 1, characterized in that: In the step S2, before measuring the minimum principal stress and the maximum principal stress of the cover layer sample, water is first added to the cover layer sample.

6. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 5, characterized in that: Water is added to the cover layer sample according to the actual moisture content of the cover layer sample until the moisture content of the cover layer sample is the same as the actual moisture content.

7. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 6, characterized in that: The actual moisture content is measured based on the cover layer samples obtained by the closed sampling method.

8. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 5, characterized in that: In step S2, the supplemented water is distilled water.

9. The method for quantitatively evaluating the sealing property of ultra-deepwater and ultra-shallow caprocks according to claim 1, characterized in that: In step S3, the gas density of the caprock sample under burial conditions is obtained through a variable temperature-pressure field gas density experiment.

10. A method for quantitatively evaluating the sealing properties of ultra-deepwater and ultra-shallow caprocks according to any one of claims 1 to 9, characterized in that: In step S4, the maximum air column height that can be blocked by the cap layer sample is calculated as follows: In the formula, H g is the maximum gas column height that can be blocked by the caprock, m; σ3 and σ1 are the minimum and maximum principal stresses when the caprock breaks under high pressure, low temperature and high water content conditions, kpa; ρ r is the caprock density under high pressure, low temperature and high water content conditions, g / cm 3 ; ρ w is the formation water density, g / cm 3 ; ρ g is the density of natural gas under high pressure and low temperature conditions, g / cm 3 ;H b is the seabed burial depth of the caprock, m.

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