Deep gas reservoir in-situ stress sensitivity test method

Abstract

The invention discloses a deep gas reservoir in-situ stress sensitivity test method, which belongs to the field of petroleum and natural gas engineering, and comprises the following steps: firstly, preparing a rock core according to reservoir characteristics, and wrapping a steel sleeve on the outer layer of a standard rock core to replace a salt-gypsum layer; in order to simulate the influence of the stress borne by an upper salt-gypsum layer and a local impermeable area of a gas reservoir on a reservoir, a 1mm steel sleeve is additionally arranged to replace the salt-gypsum layer, and an embedded steel column replaces the local impermeable area. The stress sensitive characteristics of different reservoir characteristics can be reflected, the overburden rock pressure and the pore pressure are recovered to the original state of the reservoir, the overburden pressure and the pore pressure borne by a deep gas reservoir rock skeleton are both consistent with the in-situ state, and return pressure is reduced step by step to simulate gas reservoir depletion exploitation. By means of the method, the permeability change rule during actual reservoir depletion exploitation can be more accurately reflected, the working system is better optimized, and the oil and gas productivity is better predicted.

Description

Technical Field

[0001] The invention belongs to the field of petroleum and natural gas engineering, and in particular relates to an in-situ stress sensitivity testing method for deep gas reservoirs. Background Art

[0002] With the deepening of gas field exploration, a series of deep gas reservoirs have been discovered in my country in recent years. In the process of exploitation of such deep high-pressure gas reservoirs, as the formation pressure decreases, the effective stress on the rock skeleton and particles will increase, causing the rock pore structure to deform, thereby changing the rock permeability. Studying the variation law of reservoir rock permeability with effective stress under different effective stresses is of great significance for gas reservoir reserve calculation, production dynamic prediction, capacity analysis, testing and the formulation of production work systems. However, due to the limitations of experimental equipment, the existing stress sensitivity test method cannot restore the ultra-high pressure of more than 100MPa in the reservoir, and there is a large difference between the stress of the rock skeleton or particles under the original reservoir conditions, especially for the case of high plastic mineral content, so the stress sensitivity is often overestimated or underestimated, and the work system optimization and capacity prediction errors are large. Especially for ultra-high pressure deep gas reservoirs, the initial effective stress under in-situ conditions is higher, and the rock skeleton deformation or particle contact is more complex. Some reservoirs have salt-gypsum layers and local impermeable areas, which support the overlying rock formations and can reduce the effective stress of the reservoir. In order to more truly reflect the law of reservoir permeability changes during reservoir exploitation, better optimize the working system and accurately predict production capacity, it is necessary to establish a testing method that can more accurately reflect the in-situ permeability changes during deep gas reservoir exploitation, and to transform the core to reflect the influence of salt-gypsum layers and local impermeable areas on reservoir stress. Summary of the invention

[0003] In order to overcome the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide an in-situ stress sensitivity testing method for deep gas reservoirs to solve the technical problems in the prior art that stress sensitivity is overestimated or underestimated, and there are large errors in working system optimization and production capacity prediction.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A deep gas reservoir in-situ stress sensitivity testing method, comprising:

[0006] S1: Prepare cores; obtain overburden pressure and pore pressure of the target layer;

[0007] S2: Determine the effective stress of the rock skeleton in the initial state and the step length of the confining pressure increase and the step length of the back pressure increase according to the overburden pressure and pore pressure of the target layer;

[0008] S3: Raise the overburden pressure and initial pore pressure of the core to the original formation state and calculate the original state permeability;

[0009] S4: according to the confining pressure rise step length and pore pressure rise step length calculated in S2, the initial pore pressure value at the outlet is gradually reduced to simulate the depletion mining process, and the permeability corresponding to different pore pressures is calculated;

[0010] S5: Compare the original permeability with the permeability corresponding to different pore pressures to obtain the loss rate and determine the stress sensitivity.

[0011] Preferably, in S1, the core includes: a core simulating the stress effect of a thin salt-gypsum layer on a reservoir, a core simulating the stress effect of a thick salt-gypsum layer on a reservoir, and a core simulating the stress effect of a thin salt-gypsum layer and a local impermeable area on a reservoir.

[0012] Preferably, the core for simulating the influence of the thin salt-gypsum layer on the stress of the reservoir is selected from a natural standard core, which is polished to a diameter of 2.2-2.3 cm; a steel sleeve is wrapped around the core to form a core of natural standard size, and the space between the steel sleeve and the core is filled with glue.

[0013] Preferably, the core for simulating the influence of thick salt-gypsum layer on the stress of reservoir is selected from natural standard core, which is polished to a diameter of 2.1-2.2 cm; the core is wrapped with a steel sleeve to form a core of natural standard size, and the space between the steel sleeve and the core is filled with glue.

[0014] Preferably, the core simulating the influence of thin salt gypsum layer and local impermeable area on the stress of reservoir is selected from natural standard core, which is polished to a diameter of 2.2-2.3 cm, and 2-4 through holes are drilled in the cross section of the core; and steel columns of diameter are embedded in the core holes, and the steel columns and the core are filled with glue; the core is wrapped with a steel sleeve to form a core of natural standard size, and the space between the steel sleeve and the core is filled with glue.

[0015] Preferably, in S1, the overburden pressure of the test target layer is calculated based on the rock density, and the pore pressure of the test target layer is calculated based on the formation pressure coefficient measured during the drilling process, so as to determine the initial state of in-situ stress sensitivity.

[0016] Preferably, the overburden pressure of the test target layer is calculated based on the rock density, and the pore pressure of the test target layer is calculated based on the formation pressure coefficient measured during the drilling process, which specifically includes:

[0017] First, use formula (1) to calculate the gravity of the overlying rock strata borne by the skeleton:

[0018] P0=ρ r gD (1)

[0019] The pore pressure is calculated based on the formation pressure coefficient:

[0020]

[0021] Where P0 is the overburden pressure; ρ r is the rock density; g is the gravitational acceleration; D is the target layer depth; P p is the pore pressure of the target layer; χ is the formation pressure coefficient.

[0022] Preferably, in S2, the effective stress of the rock skeleton in the initial state is determined according to the overburden pressure and pore pressure of the target layer, specifically:

[0023] σ i =P0-P p (3)

[0024] Where P0 is the overburden pressure; ρ r is the rock density; σ i is the effective stress on the rock skeleton in the initial state.

[0025] Preferably, in S2, the confining pressure increase step length and the back pressure increase step length are determined according to the overburden pressure and the pore pressure of the target layer, specifically:

[0026]

[0027]

[0028] Where ΔP c is the step length of the confining pressure increase; ΔP b is the step length of back pressure rise; N is the number of steps; P0 is the overburden pressure; ρ r is the rock density.

[0029] Preferably, in S4, the overburden pressure and the initial pore pressure rise to the original formation state, and the gas is injected at a constant pressure at the injection end according to the increased overburden pressure and the increased pore pressure, and the inlet pressure is 1-2 MPa higher than the back pressure; until the overburden pressure and the initial pore pressure rise to the original formation state, and stand still for 45-50 hours.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention discloses a method for testing the in-situ stress sensitivity of deep gas reservoirs. First, cores are prepared according to reservoir characteristics, which can reflect the stress sensitivity characteristics of different reservoir characteristics, and the overburden stratum pressure and pore pressure are restored to the original state of the reservoir, so that the overburden pressure and pore pressure of the rock skeleton of the deep gas reservoir are consistent with the in-situ state, and the back pressure is reduced step by step to simulate the exhaustion exploitation of gas reservoirs. This method can more accurately reflect the permeability change law during the actual reservoir exhaustion exploitation, better optimize the working system and predict the oil and gas production capacity.

[0032] Furthermore, in order to simulate the influence of partial stress on the reservoir borne by the salt-gypsum layer in the upper part of the gas reservoir, a steel casing is wrapped around the outer layer of the standard core to replace the salt-gypsum layer. In order to simulate the influence of stress borne by the salt-gypsum layer and the local impermeable area in the upper part of the gas reservoir on the reservoir, a 1-2 mm steel casing is added to replace the salt-gypsum layer, and a steel column is embedded to replace the local impermeable area. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0035] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0036] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0037] Aiming at the problem that the existing deep gas reservoir stress sensitivity testing method cannot truly restore the permeability change during reservoir development, the present invention proposes a new in-situ stress sensitivity testing method.

[0038] A deep gas reservoir in-situ stress sensitivity testing method, comprising:

[0039] S1: Prepare cores; obtain overburden pressure and pore pressure of the target layer;

[0040] S2: Determine the effective stress of the rock skeleton in the initial state and the step length of the confining pressure increase and the step length of the back pressure increase according to the overburden pressure and pore pressure of the target layer;

[0041] S3: Raise the overburden pressure and initial pore pressure of the core to the original formation state and calculate the original state permeability;

[0042] S4: according to the confining pressure rise step length and pore pressure rise step length calculated in S2, the initial pore pressure value at the outlet is gradually reduced to simulate the depletion mining process, and the permeability corresponding to different pore pressures is calculated;

[0043] S5: Compare the original permeability with the permeability corresponding to different pore pressures to obtain the loss rate and determine the stress sensitivity.

[0044] The invention discloses a method for testing the in-situ stress sensitivity of deep gas reservoirs. First, cores are prepared according to reservoir characteristics, which can reflect the stress sensitivity characteristics of different reservoir characteristics, and the overburden stratum pressure and pore pressure are restored to the original state of the reservoir, so that the overburden pressure and pore pressure of the rock skeleton of the deep gas reservoir are consistent with the in-situ state, and the back pressure is reduced step by step to simulate the exhaustion of gas reservoirs. This method can more accurately reflect the permeability change law during actual reservoir exhaustion exploitation, better optimize the working system and predict oil and gas production capacity.

[0045] As a preferred solution, in S1, the core includes: a core simulating the stress effect of a thin salt-gypsum layer on a reservoir, a core simulating the stress effect of a thick salt-gypsum layer on a reservoir, and a core simulating the stress effect of a thin salt-gypsum layer and a local impermeable area on a reservoir.

[0046] As a preferred solution, the core simulating the influence of the thin salt-gypsum layer on the reservoir stress is selected from a natural standard core with a diameter of 2.5 cm, which is polished to a diameter of 2.2-2.3 cm. The core is wrapped with a steel sleeve with a diameter of 2.5 cm and a thickness of 1-2 mm, and the space between the steel sleeve and the core is filled with glue;

[0047] Further preferably, the core for simulating the influence of the thick salt-gypsum layer on the stress of the reservoir is a natural standard core with a diameter of 2.5 cm, which is polished to a diameter of 2.1-2.2 cm; the core is wrapped with a steel sleeve with a diameter of 2.5 cm and a thickness of 1-2 mm, and the space between the steel sleeve and the core is filled with glue;

[0048] Further preferably, for the core simulating the influence of the thin salt-gypsum layer and the local impermeable area on the stress of the reservoir, a natural standard core with a diameter of 2.5 cm is selected, and its diameter is polished to 2.2-2.3 cm, and 2-4 through holes are drilled in the cross section of the core; and steel columns of diameter are embedded in the core holes, and the steel columns and the core are filled with glue; the core is wrapped with a steel sleeve with a diameter of 2.5 cm and a thickness of 1-2 mm, and the space between the steel sleeve and the core is filled with glue.

[0049] In order to simulate the influence of partial stress on the reservoir borne by the salt-gypsum layer in the upper part of the gas reservoir, a steel casing is wrapped around the outer layer of the standard core to replace the salt-gypsum layer. In order to simulate the influence of stress borne by the salt-gypsum layer and the local impermeable area in the upper part of the gas reservoir on the reservoir, a 1-2 mm steel casing is added to replace the salt-gypsum layer, and a steel column is embedded to replace the local impermeable area.

[0050] As a preferred solution, in S1, the overburden pressure of the test target layer is calculated based on the rock density, and the pore pressure of the test target layer is calculated based on the formation pressure coefficient measured during the drilling process, so as to determine the initial state of in-situ stress sensitivity.

[0051] As a preferred solution, the overburden pressure of the test target layer is calculated based on the rock density, and the pore pressure of the test target layer is calculated based on the formation pressure coefficient measured during the drilling process, specifically including:

[0052] First, use formula (1) to calculate the gravity of the overlying rock strata borne by the skeleton:

[0053] P0=ρ r gD (1)

[0054] The pore pressure is calculated based on the formation pressure coefficient:

[0055]

[0056] Where P0 is the overburden pressure; ρ r is the rock density; g is the gravitational acceleration; D is the target layer depth; P p is the pore pressure of the target layer; χ is the formation pressure coefficient.

[0057] In S2, the effective stress of the rock skeleton in the initial state is determined according to the overburden pressure and pore pressure of the target layer, which is:

[0058] σ i =P0-P p (3)

[0059] Where P0 is the overburden pressure; ρ r is the rock density; σ i is the effective stress on the rock skeleton in the initial state.

[0060] In S2, the step lengths of the confining pressure and back pressure increase in the experiment are determined based on the calculated overburden pressure and pore pressure, specifically:

[0061]

[0062]

[0063] Where ΔP c is the step length of the confining pressure increase; ΔPb is the step length of back pressure rise; N is the number of steps; P0 is the overburden pressure; ρ r is the rock density.

[0064] As a preferred solution, in S3, the overburden pressure and the initial pore pressure rise to the original formation state, and the overburden pressure and the pore pressure are increased, and the gas is injected at a constant pressure at the injection end, and the inlet pressure is 1-2 MPa higher than the back pressure; until the overburden pressure and the initial pore pressure rise to the original formation state, and stand for 45-50 hours.

[0065] [Example]

[0066] First, the core required for the experiment was prepared. In order to simulate the influence of partial stress on the reservoir borne by the salt-gypsum layer above the gas reservoir, a steel sleeve was wrapped on the outer layer of the standard core to replace the salt-gypsum layer. In order to simulate the influence of stress on the reservoir borne by the salt-gypsum layer and the local impermeable area above the gas reservoir, a 1mm steel sleeve was added to replace the salt-gypsum layer, and an embedded steel column was used to replace the local impermeable area. Then, the overburden pressure of the test target layer was calculated based on the rock density, and the pore pressure of the test target layer was calculated based on the formation pressure coefficient measured during the drilling process, so as to determine the initial state of in-situ stress sensitivity; then, the gas permeability test process was connected, and the confining pressure and back pressure were increased step by step until the overburden pressure (confining pressure) and the initial pore pressure (back pressure) were restored to the original state of the reservoir, and then left to stand for 48 hours; then, the confining pressure was kept constant, and the back pressure at the outlet was gradually reduced to simulate the exhaustion production process, and the inlet pressure was always kept 2-3MPa higher than the back pressure, and the gas velocity at the outlet was measured to calculate the permeability. Until the back pressure drops to the abandoned pressure. The above method is used to solve the problems existing in the current stress sensitivity measurement method, and a set of testing methods for the in-situ stress sensitivity characteristics during the depletion production process of deep gas reservoirs with different reservoir characteristics is established, which can more accurately reflect the permeability changes during the exploitation of deep gas reservoirs and be better applied to work system formulation and production capacity prediction.

[0067] See also Figure 1 , the technical solution adopted is:

[0068] 1) Core preparation

[0069] A natural standard core with a diameter of 2.5 cm was selected, and after the permeability test, its diameter was polished to 2.3 cm. A steel sleeve with a diameter of 2.5 cm and a thickness of 1 mm was wrapped around the core, and glue was filled between the steel sleeve and the core to simulate the influence of a thin salt gypsum layer on the stress of the reservoir; a natural standard core with a diameter of 2.5 cm was selected, and after the permeability test, its diameter was polished to 2.1 cm. A steel sleeve with a diameter of 2.5 cm and a thickness of 2 mm was wrapped around the core, and glue was filled between the steel sleeve and the core to simulate the influence of a thick salt gypsum layer on the stress of the reservoir; a natural standard core with a diameter of 2.5 cm was selected, and after the permeability test, its diameter was polished to 2.3 cm, and three through holes with a diameter of 1 mm were drilled in the cross section of the core. A 1 mm diameter steel column was inlaid in the core hole, and the steel column and the core were filled with glue. The core is wrapped with a steel casing with a diameter of 2.5 cm and a thickness of 1 mm, and the space between the steel casing and the core is filled with glue to simulate the influence of thin salt-gypsum layers and local impermeable areas on the stress of the reservoir.

[0070] 2) Calculate the overburden pressure and pore pressure of the target layer

[0071] The overburden pressure of the target layer is calculated based on the rock density, and the pore pressure of the target layer is calculated based on the formation pressure coefficient measured during the drilling process, so as to determine the initial state of in-situ stress sensitivity. First, the gravity of the overburden stratum borne by the skeleton is calculated using formula ①:

[0072] P0=ρ r gD (1)

[0073] The pore pressure is calculated based on the formation pressure coefficient:

[0074]

[0075] 3) Determine the effective stress on the rock skeleton in the initial state

[0076] According to equations (1) and (2), the effective stress of the rock skeleton in the initial state is calculated as follows:

[0077] σ i =P0-P p (3)

[0078] 4) Determine the rising step length of the overburden pressure (confining pressure) and initial pore pressure (back pressure)

[0079] According to the calculated overburden pressure P0 and pore pressure P p Determine the step length of the confining pressure and back pressure increase in the experiment:

[0080]

[0081]

[0082] 5) Connect the experimental process and restore the formation pressure

[0083] According to the gas permeability test method, the experimental process is connected, and then the confining pressure is increased, the back pressure is increased, and the injection end is injected at a constant pressure. The inlet pressure is generally 1-2MPa higher than the back pressure. Until the overburden pressure (confining pressure) and the initial pore pressure (back pressure) rise to the original formation state, and then stand for 48 hours.

[0084] (6) Calculate the original state permeability k0

[0085] According to the national standard of gas permeability, calculate the original state permeability k0.

[0086] (7) Depletion mining

[0087] Keep the confining pressure constant, and gradually reduce the back pressure at the outlet according to the pre-set step length to simulate the exhaustion production process, and make the injection pressure higher than 2-3MPa until the back pressure drops to the abandoned pressure of 20MPa. According to the national standard of gas permeability, calculate the permeability K (Pb) corresponding to different back pressures.

[0088]

Case 1

[0089] (1) Core cleaning and drying: The core was cleaned with a mixture of petroleum ether and anhydrous ethanol and then placed in a drying oven at 140 degrees Celsius for 48 hours to dry thoroughly.

[0090] (2) Connect the equipment and heating. Place the dried core into the core holder, turn on the core holder heating device, set the temperature to 80°C, and continue heating for 10 hours.

[0091] (3) Establish a pressure system. First, use a high-pressure displacement pump to establish confining pressure with high-purity nitrogen. The core confining pressure is increased in steps of 4 MPa and the back pressure is increased in steps of 3 MPa. The confining pressure and back pressure are increased step by step until they are stable and then the next level of pressure increase is carried out. The confining pressure and back pressure are increased alternately. After 30 steps of increase, the stress of the core skeleton is restored to the original formation state.

[0092] (4) Pressure reduction test. After the pressure system is stable, keep the surrounding pressure unchanged, take 5 MPa as a test point, first reduce the back pressure, then reduce the internal pressure. The internal pressure is about 2 MPa higher than the back pressure. After the pressure stabilizes, measure the gas flow at the outlet, and record the inlet and outlet pressures at the same time until the back pressure drops to the waste pressure of 20 MPa.

[0093] Table 1 shows the permeability k0 in the original state and the permeability K(Pb) data corresponding to different back pressures tested and calculated using the technical solution of the present invention, indicating that the application of the present invention can reflect the permeability change law during actual reservoir depletion exploitation and determine the in-situ stress sensitivity characteristics.

[0094] Table 1 In-situ stress sensitivity test results of core 11# in deep gas reservoir

[0095]

[0096]

[0097] [Case 2] After selecting a standard core of 1mm# (length 5.02cm, diameter 2.49cm) from a deep gas reservoir to test the permeability, its diameter was polished to 2.3cm. The core is wrapped with a steel sleeve with a diameter of 2.5cm and a thickness of 1mm, and the space between the steel sleeve and the core is filled with glue to simulate the influence of a thin salt-gypsum layer on the stress of the reservoir. The overburden pressure is calculated to be 123MPa, the initial back pressure is 89.7MPa, and the waste pressure is set to 20MPa. The experimental gas is 99% high-purity industrial nitrogen, and the experimental temperature is 140°C. The steps of the in-situ stress sensitivity experiment are as follows:

[0098] (1) Core cleaning and drying: The core was cleaned with a mixture of petroleum ether and anhydrous ethanol and then placed in a drying oven at 140 degrees Celsius for 48 hours to dry thoroughly.

[0099] (2) Connect the equipment and heating. Place the dried core into the core holder, turn on the core holder heating device, set the temperature to 80°C, and continue heating for 10 hours.

[0100] (3) Establish a pressure system. First, use a high-pressure displacement pump to establish confining pressure with high-purity nitrogen. The core confining pressure is increased in steps of 4 MPa and the back pressure is increased in steps of 3 MPa. The confining pressure and back pressure are increased step by step until they are stable and then the next level of pressure increase is carried out. The confining pressure and back pressure are increased alternately. After 30 steps of increase, the stress of the core skeleton is restored to the original formation state.

[0101] (4) Pressure reduction test. After the pressure system is stable, keep the surrounding pressure unchanged, take 5 MPa as a test point, first reduce the back pressure, then reduce the internal pressure. The internal pressure is about 2 MPa higher than the back pressure. After the pressure stabilizes, measure the gas flow at the outlet, and record the inlet and outlet pressures at the same time until the back pressure drops to the waste pressure of 20 MPa.

[0102] Table 2 shows the permeability k0 in the original state and the permeability K(Pb) data corresponding to different back pressures tested and calculated using the technical solution of the present invention, indicating that the application of the present invention can reflect the permeability change law during actual reservoir depletion exploitation and determine the in-situ stress sensitivity characteristics.

[0103] Table 2 In-situ stress sensitivity test results of 1mm# core and 1mm steel casing in deep gas reservoir

[0104]

[0105] [Case 3] After selecting a deep gas reservoir standard core 1mm + steel column # (length 5.067cm, diameter 2.49cm) to test the permeability, its diameter was polished to 2.3cm. Three through holes with a diameter of 1mm were drilled in the cross section of the core. A 1mm diameter steel column was inlaid in the core hole, and the steel column and the core were filled with glue. The core was wrapped with a steel sleeve with a diameter of 2.5cm and a thickness of 1mm, and the space between the steel sleeve and the core was filled with glue to simulate the influence of thin salt gypsum layers and local impermeable areas on the stress of the reservoir. The overburden pressure was calculated to be 123MPa, the initial back pressure was 89.7MPa, and the waste pressure was set to 20MPa. The experimental gas was 99% high-purity industrial nitrogen, and the experimental temperature was 140°C. The steps of the in-situ stress sensitivity experiment are as follows:

[0106] (1) Core cleaning and drying: The core was cleaned with a mixture of petroleum ether and anhydrous ethanol and then placed in a drying oven at 140 degrees Celsius for 48 hours to dry thoroughly.

[0107] (2) Connect the equipment and heating. Place the dried core into the core holder, turn on the core holder heating device, set the temperature to 80°C, and continue heating for 10 hours.

[0108] (3) Establish a pressure system. First, use a high-pressure displacement pump to establish confining pressure with high-purity nitrogen. The core confining pressure is increased in steps of 4 MPa and the back pressure is increased in steps of 3 MPa. The confining pressure and back pressure are increased step by step until they are stable and then the next level of pressure increase is carried out. The confining pressure and back pressure are increased alternately. After 30 steps of increase, the stress of the core skeleton is restored to the original formation state.

[0109] (4) Pressure reduction test. After the pressure system is stable, keep the surrounding pressure unchanged, take 5 MPa as a test point, first reduce the back pressure, then reduce the internal pressure. The internal pressure is about 2 MPa higher than the back pressure. After the pressure stabilizes, measure the gas flow at the outlet, and record the inlet and outlet pressures at the same time until the back pressure drops to the waste pressure of 20 MPa.

[0110] Table 3 shows the permeability k0 in the original state and the permeability k(Pb) corresponding to different back pressures tested and calculated using the technical solution of the present invention, indicating that the application of the present invention can reflect the permeability change law during actual reservoir depletion exploitation and determine the in-situ stress sensitivity characteristics.

[0111] Table 3 In-situ stress sensitivity test results of 1mm+steel column #core plus 1mm steel casing and steel column in deep gas reservoir

[0112]

[0113]

[0114] In summary, the in-situ stress sensitivity test method for deep gas reservoirs disclosed in this application prepares cores according to reservoir characteristics, which can reflect the stress sensitivity characteristics of different reservoir characteristics. The confining pressure and back pressure are restored to the original state of the reservoir, so that the overburden pressure and pore pressure on the rock skeleton of the deep gas reservoir are consistent with the in-situ state, and the back pressure is reduced step by step to simulate gas reservoir exhaustion production. This method can more accurately reflect the permeability change law during actual reservoir exhaustion production, better optimize the working system and predict oil and gas production capacity.

[0115] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for in-situ stress sensitivity testing of deep gas reservoirs, characterized in that: include: S1: Prepare cores; obtain overburden pressure and pore pressure of the target layer; S2: Determine the effective stress of the rock skeleton in the initial state and the step length of the confining pressure increase and the step length of the back pressure increase according to the overburden pressure and pore pressure of the target layer; S3: Raise the overburden pressure and initial pore pressure of the core to the original formation state and calculate the original state permeability; S4: according to the confining pressure rise step length and pore pressure rise step length calculated in S2, the initial pore pressure value at the outlet is gradually reduced to simulate the depletion mining process, and the permeability corresponding to different pore pressures is calculated; S5: Compare the original permeability with the permeability corresponding to different pore pressures to obtain the loss rate and determine the stress sensitivity.

2. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 1, characterized in that: In S1, the cores include: cores simulating the stress effect of a thin salt-gypsum layer on a reservoir, cores simulating the stress effect of a thick salt-gypsum layer on a reservoir, and cores simulating the stress effect of a thin salt-gypsum layer and a local impermeable area on a reservoir.

3. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 2, characterized in that: The core for simulating the influence of the thin salt-gypsum layer on the stress of the reservoir is selected from a natural standard core, which is polished to a diameter of 2.2-2.3 cm; a steel sleeve is wrapped outside the core to form a core of natural standard size, and the space between the steel sleeve and the core is filled with glue.

4. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 2, characterized in that: The core used to simulate the stress effect of thick salt-gypsum layer on reservoir is selected from natural standard core, which is polished to a diameter of 2.1-2.2 cm; a steel sleeve is wrapped around the core to form a core of natural standard size, and the space between the steel sleeve and the core is filled with glue.

5. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 2, characterized in that: The core simulating the influence of thin salt gypsum layer and local impermeable area on the stress of reservoir is selected from natural standard core, which is polished to a diameter of 2.2-2.3 cm, and 2-4 through holes are drilled in the cross section of the core; and diameter steel columns are embedded in the core holes, and the steel columns and the core are filled with glue; the core is wrapped with a steel sleeve to form a core of natural standard size, and the space between the steel sleeve and the core is filled with glue.

6. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 1, characterized in that: In S1, the overburden pressure of the test target layer is calculated based on the rock density, and the pore pressure of the test target layer is calculated based on the formation pressure coefficient measured during the drilling process, so as to determine the initial state of in-situ stress sensitivity.

7. A deep gas reservoir in-situ stress sensitivity testing method according to claim 6, characterized in that: The overburden pressure of the target layer is calculated based on the rock density, and the pore pressure of the target layer is calculated based on the formation pressure coefficient measured during the drilling process, including: First, use formula (1) to calculate the gravity of the overlying rock strata borne by the skeleton: P0=ρ r gD (1) The pore pressure is calculated based on the formation pressure coefficient: Where P0 is the overburden pressure; ρ r is the rock density; g is the gravitational acceleration; D is the target layer depth; P p is the pore pressure of the target layer; χ is the formation pressure coefficient.

8. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 1, characterized in that: In S2, the effective stress of the rock skeleton in the initial state is determined according to the overburden pressure and pore pressure of the target layer, which is: s i =P0-P p (3) Where P0 is the overburden pressure; ρ r is the rock density; σ i is the effective stress on the rock skeleton in the initial state.

9. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 1, characterized in that: In S2, the confining pressure increase step length and the back pressure increase step length are determined according to the overburden pressure and pore pressure of the target layer, specifically: Among them, ΔP c is the step length of the confining pressure increase; ΔP b is the step length of back pressure rise; N is the number of steps; P0 is the overburden pressure; ρ r is the rock density.

10. The in-situ stress sensitivity testing method for deep gas reservoirs according to claim 1, characterized in that: In S4, the overburden pressure and the initial pore pressure rise to the original formation state, and the overburden pressure and the pore pressure are increased, and the gas is injected at a constant pressure at the injection end, and the inlet pressure is 1-2 MPa higher than the back pressure; until the overburden pressure and the initial pore pressure rise to the original formation state, and stand for 45-50 hours.

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