Reservoir permeability prediction method considering salting-out damage

Abstract

The invention relates to the technical field of dynamic analysis of natural gas reservoirs, in particular to a reservoir permeability prediction method considering salting-out damage, which effectively realizes accurate calculation of reservoir permeability after salt deposition and timely measures the degree of damage to formation water salt deposition of a gas reservoir. The method can also be used for reservoir permeability time-varying gas reservoir numerical simulation considering formation water salt deposition damage, so that the reliability of gas reservoir numerical simulation research is greatly improved, and the problem that in the prior art, for a gas reservoir with salt deposition blockage damage in development, the damage condition of salt deposition to the reservoir permeability cannot be mastered in time, and the reservoir permeability time-varying gas reservoir numerical simulation research cannot be mastered in time is solved. And the technical problems of serious potential safety hazards and economic loss caused by the fact that the optimal opportunity for implementing a gas reservoir salt deposition treatment measure is missed are solved.

Description

Technical Field

[0001] The invention relates to the technical field of natural gas reservoir dynamic analysis, and in particular to a reservoir permeability prediction method taking salt precipitation damage into consideration. Background Art

[0002] As the exploration of oil and gas resources continues to steadily advance into deep and ultra-deep layers, more and more high-temperature and high-pressure gas reservoirs are discovered and developed. Compared with conventional gas reservoirs, due to the influence of high temperature and high pressure conditions, there is a drastic mass exchange and phase change between formation water and natural gas in the gas reservoir. Therefore, during the depletion and depressurization process, the formation water in the gas reservoir will evaporate in large quantities driven by phase change and be produced out of the ground with natural gas in the form of gas. The salt concentration in the formation water also increases significantly with the evaporation of water. In particular, high-temperature and high-pressure gas reservoirs with high-mineralization formation water may have the problem of salt blocking the reservoir around the well during development, resulting in a significant reduction in the reservoir seepage capacity, restricting the production capacity of the gas well, and even causing a "dead well" situation.

[0003] Effective permeability is an important parameter for characterizing reservoir seepage capacity and evaluating gas well production capacity. However, current research focuses on core experimental evaluation and salt damage mechanism, while there are few studies on quantitative prediction of reservoir permeability after formation water salting, and there is a lack of technical methods to effectively predict reservoir permeability after salting.

[0004] In summary, in the existing technology, for gas reservoirs that are damaged by salt blockage during development, the best time to implement salt control measures will be missed due to the failure to timely grasp the damage of salt to reservoir permeability, which will lead to serious safety hazards and economic losses. Summary of the invention

[0005] The purpose of the present invention is to provide a reservoir permeability prediction method taking into account salting-out damage, aiming to solve the technical problem that in the prior art, for gas reservoirs with salt blockage damage during development, the best time to implement salting control measures in the gas reservoirs will be missed due to the failure to timely grasp the damage of salting to the reservoir permeability, thereby causing serious safety hazards and economic losses.

[0006] To achieve the above object, the present invention adopts a reservoir permeability prediction method taking into account salt precipitation damage, comprising:

[0007] According to the distribution characteristics of gas reservoir physical properties, several standard gas reservoir cores with different physical properties were selected to carry out salt physical property damage experiments and measure the porosity and permeability of the cores under different salt levels.

[0008] Select a short core with physical properties close to the average value of the experimental core, and use the same sample preparation method as the salt physical property damage experiment, repeatedly salting, so that the core is completely blocked by salt and loses its seepage capacity;

[0009] Cut off the two ends of the core that are completely blocked by salt, leaving a core length of about 1 cm, and test whether it is completely blocked by salt by measuring the permeability of the retained core;

[0010] If it meets the requirements, it will be used to carry out high-pressure mercury injection and analyze the permeability distribution under high-pressure drive;

[0011] If it does not meet the requirements, re-prepare the rock sample and re-test it;

[0012] Based on the permeability loss law of rock pores completely blocked by salt under high pressure driving, a model suitable for the salt damage law of gas reservoir rocks was selected from the commonly used pore-permeability characterization mathematical models VP model and KC model.

[0013] According to the results of the salt damage experiment, a mathematical model for calculating the reservoir permeability in the salt-forming area was established by using multiple regression analysis method based on the initial physical properties of the rock and the amount of salt.

[0014] According to the initial physical properties and salt content of the reservoir around the wells logged in the gas reservoir, the permeability of the salt-forming area of ​​the reservoir around the wells logged in the gas reservoir is calculated by using the established mathematical model of reservoir permeability after salt formation.

[0015] Combining the original physical properties of the gas reservoir and the calculated permeability of the salt-forming area of ​​the reservoir around the wells in the gas reservoir, the equivalent seepage resistance principle is used to calculate the reservoir permeability after salt formation in the gas reservoir.

[0016] Among them, the length of the short core selected, which is close to the average value of the physical properties of the experimental core, is 2 to 3 cm.

[0017] The core being completely blocked by salt and losing its seepage capacity means that the core permeability after salt blocking is less than 0.1% of the initial permeability.

[0018] Among them, according to the law of seepage capacity loss after the rock pores are completely blocked by salt under the analysis of permeability distribution under high pressure drive, the method of selecting a model suitable for the law of salt damage of gas reservoir rocks from the commonly used pore-permeability characterization mathematical models VP model and KC model is based on the following selection criteria:

[0019] When the large pore throats of the core are completely blocked under high pressure driving and still have a certain seepage capacity, it means that the KC model is more consistent with the law of salt damage in gas reservoir rocks. The KC model is expressed as follows:

[0020]

[0021] In the formula, k is the permeability, φ is the porosity, the superscripts c and d are unknown exponents, and the subscript 0 indicates the initial state;

[0022] When the macropore throats of the core are completely blocked under high pressure driving and have no seepage capacity, it means that the VP model is more consistent with the law of salt damage in gas reservoir rocks. The VP model is expressed as follows:

[0023]

[0024] Wherein, the superscript n is an unknown index, and the subscript cc represents the critical state when the rock pore space is just completely blocked.

[0025] Among them, the permeability of the salt-forming area of ​​the reservoir around the wells in the gas reservoir is calculated by combining the original physical properties of the gas reservoir and the equivalent seepage resistance principle. The radial composite model formula is selected to calculate the permeability of the reservoir after salt formation in the gas reservoir, which is expressed as follows. The permeability of the reservoir after salt formation within the well control range of the wells in the gas reservoir is calculated:

[0026]

[0027] Where: kr is the reservoir permeability after salt formation, re is the well control radius, rw is the wellbore radius, and rs is the radius of the salt formation range around the well.

[0028] The reservoir permeability prediction method considering salting-out damage of the present invention effectively realizes the accurate calculation of reservoir permeability after salting-out, and the degree of salting-out damage of formation water in gas reservoir in time. In addition, the reservoir permeability prediction method considering salting-out damage proposed by the present invention can also be used in the numerical simulation of gas reservoirs with time-varying reservoir permeability considering salting-out damage of formation water, thereby greatly improving the reliability of gas reservoir numerical simulation research. In this way, the technical problem that the gas reservoirs with salting-out blockage damage during development in the prior art cannot be timely understood due to the damage of salting-out to reservoir permeability, which will lead to missing the best time to implement gas reservoir salting control measures, thereby causing serious safety hazards and economic losses is solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 This is a diagram of the high-pressure mercury injection analysis results after complete salt blockage of the present invention. DETAILED DESCRIPTION

[0031] Embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0032] See also Figure 1 , Figure 1 This is the result of high pressure mercury injection analysis after complete salt blockage of the present invention

[0033] In the present invention, the X gas reservoir is a typical deep weak-edge water gas reservoir, the formation water salt concentration is about 25%, and the soluble salt in the formation water is close to saturation in the original state; the original formation pressure of the gas reservoir is about 40MPa, the formation temperature is about 120°C, the main effective porosity range of the gas reservoir is 10% to 17%, and the main effective permeability is 0.2 to 28mD; in the middle and late stages of the development of the gas reservoir, most of the gas wells have serious salt blockage problems, which affects the normal production of the gas reservoir and causes relatively serious economic losses to the gas reservoir development enterprises.

[0034] Among them, Well A is one of the gas wells with the most serious salt blockage in the X gas reservoir, with a well control radius of about 200m. Before it was put into development, the effective permeability of the well control area of ​​Well A was 3.42mD and the porosity was 12.2%. In the middle and late stages of development, Well A was shut in due to salt blockage and a sharp drop in gas production. Test analysis shows that the salt range of Well A is 10 to 15m around the well, and the salt accounts for 8% to 12% of the reservoir pores.

[0035] The present invention provides a reservoir permeability prediction method considering salt precipitation damage, comprising the following steps:

[0036] S101. According to the distribution characteristics of gas reservoir physical properties, five gas reservoir standard cores with different physical properties were selected to carry out salt physical property damage experiments, and the porosity and permeability of the cores after repeated salting for 4 times were measured;

[0037] For this specific implementation method: Preparation of rock samples with different salting degrees can be achieved by repeatedly adding saturated sodium chloride solution to the core and high-temperature air drying; in this step, the test results of the original porosity and permeability of cores 1 to 5 and the porosity and permeability after 4 salting are as follows:

[0038]

[0039] S102, select a short core with physical properties close to the average value of the above experimental core, named core No. 6, with a length of 2 to 3 cm, and prepare the sample by repeatedly applying saturated sodium chloride solution to the core and drying it with high temperature air for 6 times, so that the core is completely blocked by salt and loses its seepage capacity;

[0040] For this specific implementation method, making the core completely blocked by salt and losing its seepage capacity means that the core permeability after salt blocking is less than 0.1% of the initial permeability. The porosity and permeability test results of the No. 6 core after complete salt blocking are shown in the following table:

[0041] Core number Original porosity / % Original permeability / mD Porosity after complete salt deposition / % Permeability after complete salt deposition / mD 6 15.66 9.684 9.39 0.0574

[0042] S103, cutting and removing the two ends of the core that are completely blocked by salt, leaving the core with a length of about 1 cm, and testing whether the core is completely blocked by salt by measuring the permeability of the retained core;

[0043] If it meets the requirements, it will be used to carry out high-pressure mercury injection and analyze the permeability distribution under high-pressure drive;

[0044] If it does not meet the requirements, re-prepare the rock sample and re-test it;

[0045] For this specific implementation method, the results of high-pressure mercury injection analysis show that after the pore throats smaller than 0.63 μm are completely blocked by salt, they can no longer provide seepage capacity, and the pore throats of 2.5 to 6.3 μm are still the main seepage channels. In this step, the porosity and permeability test results of the middle part of the No. 6 core after complete salt blockage are as follows: Figure 1 As shown:

[0046] Core number Original porosity / % Original permeability / mD Porosity after complete salt deposition / % Permeability after complete salt deposition / mD 6 in 15.66 9.684 9.68 0.0752

[0047] S104. Based on the permeability loss law after the rock pores are completely blocked by salt formation obtained by analyzing the permeability distribution under high pressure driving, a model suitable for the salt formation damage law of gas reservoir rocks is selected from the commonly used pore-permeability characterization mathematical models VP model and KC model;

[0048] According to this specific implementation method, since the large pore throats of the core still have a certain seepage capacity after being completely blocked under high pressure driving, the KC model is more in line with the law of salt damage of gas reservoir rocks; that is, it is expressed as formula 1:

[0049]

[0050] Where k is the permeability, φ is the porosity, the superscripts c and d are unknown exponents, and the subscript 0 indicates the initial state.

[0051] S105. Based on the results of the salt-forming physical property damage experiment, a mathematical model for calculating the reservoir permeability in the salt-forming area is established by using a multiple regression analysis method based on the initial physical properties of the rock and the amount of salt formed;

[0052] For this specific implementation, when the index d is 2, the model regression effect is good; therefore, the mathematical model of the reservoir permeability in the salt-forming area of ​​the measured gas reservoir is expressed as Formula 2:

[0053]

[0054] S106. Based on the initial physical properties and salt content of the reservoir around the wells in the gas reservoir, the established mathematical model of reservoir permeability after salt formation is used to calculate the permeability range of the salt formation area of ​​the reservoir around Well A in the gas reservoir as 1.36-1.87 mD;

[0055] S107, combining the original physical properties of the gas reservoir and the calculated permeability of the salt-forming area of ​​the reservoir around the wells in the gas reservoir, and using the equivalent seepage resistance principle, calculate the reservoir permeability after the gas reservoir is salted;

[0056] According to this specific implementation method, the equivalent seepage resistance principle is adopted to calculate the permeability of the reservoir after salt formation in the gas reservoir. The equivalent seepage resistance principle is adopted and the radial composite model formula is selected, which is expressed as Formula 3 to calculate the permeability of the reservoir after salt formation within the well control range of Well A in the gas reservoir 1.72-2.26 mD:

[0057]

[0058] Where: kr is the reservoir permeability after salt formation, re is the well control radius, rw is the wellbore radius, and rs is the radius of the salt formation range around the well.

[0059] The method for predicting reservoir permeability taking into account salting-out damage of the invention is used to effectively realize the accurate calculation of reservoir permeability after salting-out, as well as the degree of salting-out damage to gas reservoir formation water in time. In addition, the method for predicting reservoir permeability taking into account salting-out damage proposed by the invention can also be used in numerical simulation of gas reservoirs with time-varying reservoir permeability taking into account salting-out damage to formation water, thereby greatly improving the reliability of numerical simulation research on gas reservoirs. In this way, the technical problem in the prior art that for gas reservoirs with salting-out blockage damage during development, the best time to implement gas reservoir salting control measures will be missed due to the inability to timely grasp the damage of salting to reservoir permeability, thus causing serious safety hazards and economic losses is solved.

[0060] What is disclosed above is only a preferred embodiment of the present invention, and it certainly cannot be used to limit the scope of rights of the present invention. Ordinary technicians in this field can understand that all or part of the processes of the above embodiment and equivalent changes made according to the claims of the present invention still fall within the scope of the invention.

Claims

1. A reservoir permeability prediction method considering salt precipitation damage, characterized in that: include According to the distribution characteristics of gas reservoir physical properties, several standard gas reservoir cores with different physical properties were selected to carry out salt physical property damage experiments and measure the porosity and permeability of the cores under different salt levels. Select a short core with physical properties close to the average value of the experimental core, and use the same sample preparation method as the salt physical property damage experiment, repeatedly salting, so that the core is completely blocked by salt and loses its seepage capacity; Cut off the two ends of the core that are completely blocked by salt, leaving a core length of about 1 cm, and test whether it is completely blocked by salt by measuring the permeability of the retained core; If it meets the requirements, it will be used to carry out high-pressure mercury injection and analyze the permeability distribution under high-pressure drive; If it does not meet the requirements, re-prepare the rock sample and re-test it; Based on the permeability loss law of rock pores completely blocked by salt under high pressure driving, a model suitable for the salt damage law of gas reservoir rocks was selected from the commonly used pore-permeability characterization mathematical models VP model and KC model. According to the results of the salt damage experiment, a mathematical model for calculating the reservoir permeability in the salt-forming area was established by using multiple regression analysis method based on the initial physical properties of the rock and the amount of salt. According to the initial physical properties and salt content of the reservoir around the wells logged in the gas reservoir, the permeability of the salt-forming area of ​​the reservoir around the wells logged in the gas reservoir is calculated by using the established mathematical model of reservoir permeability after salt formation. Combining the original physical properties of the gas reservoir and the calculated permeability of the salt-forming area of ​​the reservoir around the wells in the gas reservoir, the equivalent seepage resistance principle is used to calculate the reservoir permeability after salt formation in the gas reservoir.

2. The reservoir permeability prediction method considering salting-out damage according to claim 1, characterized in that: The length of the short core selected to be close to the average value of the physical properties of the experimental core is 2 to 3 cm.

3. The reservoir permeability prediction method considering salting-out damage according to claim 2, characterized in that: The core is completely blocked by salt and loses its seepage capacity means that the core permeability after salt blockage is less than 0.1% of the initial permeability.

4. The reservoir permeability prediction method considering salting-out damage according to claim 3, characterized in that: According to the law of seepage capacity loss after rock pores are completely blocked by salt under high pressure driving, the model suitable for the law of salt damage of gas reservoir rocks is selected from the commonly used pore-permeability characterization mathematical models VP model and KC model. The selection basis is: When the large pore throats of the core are completely blocked under high pressure driving and still have a certain seepage capacity, it means that the KC model is more consistent with the law of salt damage in gas reservoir rocks. The KC model is expressed as follows: In the formula, k is the permeability, φ is the porosity, the superscripts c and d are unknown exponents, and the subscript 0 indicates the initial state; When the macropore throats of the core are completely blocked under high pressure driving and have no seepage capacity, it means that the VP model is more consistent with the law of salt damage in gas reservoir rocks. The VP model is expressed as follows: Wherein, the superscript n is an unknown index, and the subscript cc represents the critical state when the rock pore space is just completely blocked.

5. The reservoir permeability prediction method considering salting-out damage according to claim 4, characterized in that: Combining the original physical properties of the gas reservoir and the calculated permeability of the salt-forming area of ​​the reservoir around the wells in the gas reservoir, the equivalent seepage resistance principle is used to calculate the permeability of the reservoir after salt formation in the gas reservoir. The radial composite model formula is selected as follows to calculate the permeability of the reservoir after salt formation within the well control range of the wells in the gas reservoir: Where: kr is the reservoir permeability after salt formation, re is the well control radius, rw is the wellbore radius, and rs is the radius of the salt formation range around the well.

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