A method and system for testing shale condensate gas dew point based on effective permeability

Abstract

The present invention provides a method and system for testing the dew point of shale condensate gas based on effective permeability, and relates to the technical field of natural gas exploration. The method of the present invention comprises: configuring a condensate gas sample, processing a core, establishing core pressure, injecting a condensate gas sample into the core and depressurizing the core, displacing the condensate gas sample with nitrogen, calculating the effective permeability of the core based on nitrogen viscosity, and changing a target pressure value to gradually reduce the interval in which the dew point pressure is located. The present invention displaces the condensate gas sample with nitrogen, and then uses nitrogen viscosity to calculate the effective permeability of the core at the target pressure value, thereby reducing the difficulty of obtaining the effective permeability of the core and the burden on test personnel, and by repeatedly setting a new target pressure value between a previous target pressure value and a subsequent target pressure value, continuously reducing the length of the interval in which the dew point pressure is located, so that the obtained dew point pressure is more accurate and reliable.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas exploration, and in particular to a method and system for testing the dew point of shale condensate gas based on effective permeability. Background Art

[0002] The phase behavior of condensate gas in unconventional reservoirs is quite different from that of conventional condensate gas. The reason is that nanoscale pores account for a high proportion of the pore structure of unconventional reservoirs, and the condensate gas in the nanopores is in a "confined" state. There is a "flow wall effect" between the condensate gas molecules and the pore walls. At the same time, due to the confinement, the interaction between condensate gas molecules is also strengthened, so the phase behavior of condensate gas deviates, which is manifested as the dew point of condensate gas in shale is different from that of conventional condensate gas. For conventional condensate gas, the most common way to test the dew point of condensate gas on a laboratory scale is to use a PVT device to perform PVT testing on oil and gas samples. This process has been widely recognized in the industry and has formed a set of related processes and supporting calculation methods. However, the test method conducted in a large-volume PVT cylinder is not applicable to testing the dew point of "confined" fluids.

[0003] In the prior art, there is a method and device for testing the dew point pressure of condensate gas in a porous medium with a publication number of "CN117074242A", the method comprising: obtaining the sample length and sample cross-sectional area of ​​the sample to be tested; placing the sample to be tested in a dew point pressure testing device for condensate gas in a porous medium, applying a preset pressure and gradually reducing it, while passing the condensate gas to be tested into the dew point pressure testing device for condensate gas in a porous medium, and obtaining multiple groups of gas characteristic data of the condensate gas to be tested when passing through the sample to be tested; determining the system average pressure in the dew point pressure testing device for condensate gas in a porous medium corresponding to each group of gas characteristic data; determining the gas viscosity of the condensate gas to be tested at each system average pressure; determining the permeability of the condensate gas to be tested at each system average pressure; determining the actual dew point pressure of the condensate gas to be tested in the sample to be tested according to the gas characteristic data and permeability at different system average pressures, the dew point pressure obtained by the above method is more accurate.

[0004] However, the prior art still has major defects. For example, the above technical solution requires the parameter of condensate gas viscosity when calculating the permeability. The configured condensate gas has a complex composition and the gas viscosity at different pressures is unknown. The Dranchuk-Abou-Kassem method is required to determine the deviation factor of the condensate gas to be tested at each system average pressure, and then the deviation factor is substituted into the analytical formula corresponding to the Lee-Gonza lez-Eakin semi-empirical correlation method to determine the gas viscosity of the condensate gas to be tested at each system average pressure. This method is relatively cumbersome and greatly increases the workload of the test personnel. In addition, the prior art determines the actual dew point pressure through the inflection point, which is actually to obtain the permeability change curve through multi-point fitting, and then find the inflection point through the permeability change curve. This inflection point is a virtual inflection point, and there is an error between it and the actual inflection point, which is difficult to apply in occasions requiring precise dew point pressure. Summary of the invention

[0005] The object of the present invention is to provide a method and system for testing the dew point of shale condensate gas based on effective permeability, so as to solve the problems raised in the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for testing the dew point of shale condensate gas based on effective permeability comprises the following steps:

[0008] S1, prepare a condensate gas sample and obtain the normal dew point pressure of the condensate gas sample;

[0009] S2, obtaining a core and injecting methane into the core to clean the core, and then depressurizing the core to remove the adsorbed phase and condensate oil inside it;

[0010] S3, clamping the core in the core holder, injecting methane into the core to pressurize it, until the pressure inside the core exceeds the normal dew point pressure by a first preset threshold value, and the pressure difference between the inlet and outlet of the core holder is stable, and the pressure building of the core is completed;

[0011] S4, taking a pressure value higher than the conventional dew point pressure to reach a second preset threshold as the injection pressure condition, injecting a condensate gas sample into the core to displace methane and saturate the core, leaving the core still to stabilize the pressure of the condensate gas sample in the core, maintaining the inlet and outlet pressure difference of the core holder lower than a third preset threshold and keeping the inlet and outlet pressure difference stable;

[0012] S5, with the depressurization rate being lower than a fourth preset threshold value and the net stress of the core during the depressurization process being a preset net stress value as a constraint condition, depressurizing the core until the core pressure drops to a target pressure value, and leaving the core still to stabilize the pressure of the condensate gas sample in the core;

[0013] S6, with the core holder outlet pressure being kept constant at the target pressure value and the injection flow rate being no higher than the fifth preset threshold as the constraint conditions, nitrogen is injected into the core to displace the condensate gas sample, and the difference between the inlet and outlet pressures of the core holder is used as the displacement pressure difference. When the displacement pressure difference fluctuation value is less than the sixth preset threshold, it indicates that the core is in a seepage equilibrium state and proceeds to step S7;

[0014] S7, obtaining the inlet pressure, outlet pressure, outlet gas flow rate, nitrogen viscosity at the target pressure value, cross-sectional area and length of the core of the core holder, and calculating the effective permeability of the core at the target pressure value based on Darcy's formula;

[0015] S8, subtracting the seventh preset threshold from the target pressure value to reset the target pressure value, and repeating steps S2-S7 to obtain the effective permeability of the core after the target pressure value is reset. When the difference between the effective permeability of the core before and after the target pressure value is reset is less than the eighth preset threshold, step S8 is repeated. Otherwise, the two target pressure values ​​before and after the target pressure value is reset are defined as the front target pressure value and the rear target pressure value, respectively, and the process proceeds to step S9;

[0016] S9, obtain a new target pressure value between the previous target pressure value and the subsequent target pressure value, and repeat steps S2-S7 to obtain the effective permeability of the core at the new target pressure value, when the difference in effective permeability of the core between the previous target pressure value and the new target pressure value is not less than an eighth preset threshold, use the new target pressure value as the subsequent target pressure value and repeat step S9, and when the difference in effective permeability of the core between the subsequent target pressure value and the new target pressure value is not less than the eighth preset threshold, use the new target pressure value as the previous target pressure value and repeat step S9, until the difference between the previous target pressure value and the subsequent target pressure value is less than a ninth preset threshold, and determine that the dew point pressure of the condensate gas in the core is between the previous target pressure value and the subsequent target pressure value.

[0017] Furthermore, the specific logic of obtaining the core and injecting methane into the core to clean the core, and then depressurizing the core to remove the adsorbed phase and condensate oil inside it is as follows:

[0018] After obtaining the core of the real reservoir from the field or the oil field core library, the core is cleaned with methane. When injecting methane into the core for core cleaning, the injection pressure is higher than the conventional dew point pressure by a certain pressure value, and the volume of methane at least five times the pore volume of the core is injected into the core. Then a vacuum pump is used to reduce the pressure of the core until the pressure inside the core is reduced to 0, so as to remove the adsorbed phase and condensate oil inside the core.

[0019] Furthermore, the first preset threshold is 500psi or 600psi.

[0020] Furthermore, in step S4, the volume of the injected condensate gas sample is not less than 1.5 times the core pore volume, the second preset threshold value is 500 psi or 600 psi, and the third preset threshold value is 50 psi or 60 psi.

[0021] Furthermore, the fourth preset threshold value is 1psi / min or 1.5psi / min, the core net stress is the difference between the core confining pressure and the core pore pressure, the net stress preset value is 500psi or 600psi, and the initial value of the target pressure value is 500psi higher than the conventional dew point pressure.

[0022] Furthermore, the fifth preset threshold value is 1.5 ml / h or 2 ml / h or 2.5 ml / h, and the sixth preset threshold value is 0.15 psi / h or 0.2 psi / h or 0.25 psi / h.

[0023] Furthermore, the calculation formula of the effective permeability is as follows:

[0024]

[0025] Where K represents the effective permeability, P in Indicates the inlet pressure of the core holder, P out represents the outlet pressure of the core holder, ΔP represents the displacement pressure difference, μ represents the viscosity of nitrogen at the target pressure value, Q represents the gas flow rate at the outlet of the core holder, A represents the core cross-sectional area, and Δx represents the core length.

[0026] Furthermore, the seventh preset threshold is set to be between 100 and 550 psi, and the eighth preset threshold is set to be 0.15 μD, 0.16 μD or 0.17 μD.

[0027] Furthermore, the ninth preset threshold value is 7psi, 10psi or 13psi.

[0028] A shale condensate gas dew point testing system based on effective permeability, used to perform the above-mentioned shale condensate gas dew point testing method based on effective permeability, comprising:

[0029] A core holder 5, wherein the core holder 5 is used to hold the core, and a high-temperature oven 6 for heating the core is disposed outside the core holder 5, and a confining pressure pump 204 for establishing confining pressure for the core is disposed outside the core holder 5;

[0030] The gas injection assembly includes a first intermediate container 101 for storing methane, a second intermediate container 102 for storing condensate gas samples, and a third intermediate container 103 for storing nitrogen. The first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 are connected to a first injection pump 201, a second injection pump 202, and a third injection pump 203 at one end away from the core holder 5, respectively. The first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 are connected to a first injection pump 201, a second injection pump 202, and a third injection pump 203 at one end close to the core holder 5. The first valve 301, the second valve 302, and the third valve 303 are respectively connected, and the first valve 301, the second valve 302, and the third valve 303 are connected to the inlet end of the core holder 5 through the same fourth valve 304, so that the methane, condensate gas, and nitrogen in the first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 can be injected into the core in the core holder 5, and a first pressure gauge 401 for obtaining the pressure of the inlet end of the core holder 5 is connected between the inlet end of the core holder 5 and the fourth valve 304;

[0031] An exhaust component, the exhaust component includes a gas-liquid separator 9, a waste gas bottle 10 connected to the exhaust end of the gas-liquid separator 9, and a back-pressure valve 7 connected to the back-pressure pump 205. A fifth valve 305 is connected between the back-pressure valve 7 and the outlet end of the core clamp 5, and a second pressure gauge 402 for obtaining the outlet pressure of the core clamp 5 is connected between the fifth valve 305 and the outlet end of the core clamp 5. A gas flow meter 8 is connected between the back-pressure valve 7 and the gas-liquid separator 9.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The shale condensate gas dew point testing method and system based on effective permeability of the present invention uses nitrogen to displace the condensate gas sample, and then uses the nitrogen viscosity to calculate the effective permeability of the core at the target pressure value. Nitrogen is a common gas, and its gas viscosity has been widely mentioned and cited in previous test data or literature materials. No additional test is required to obtain the viscosity of nitrogen at the target pressure value. Compared with the method in the prior art that requires the condensate gas viscosity to be obtained through experiments, the difficulty of obtaining the effective permeability of the core is greatly reduced, and the burden on the test personnel is reduced. By repeatedly setting a new target pressure value between the previous target pressure value and the subsequent target pressure value, the length of the interval where the dew point pressure of the condensate gas in the core is located is continuously reduced. Through multiple tests, the interval value of the dew point pressure of the condensate gas in the core can be accurately locked. Compared with the prior art that determines the virtual inflection point through curve simulation, the dew point pressure of the condensate gas in the core obtained is more accurate and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1This is a flow chart of a method for testing the dew point of shale condensate gas based on effective permeability in the present invention;

[0035] Figure 2 It is a schematic diagram of a testing system for shale condensate gas dew point based on effective permeability in the present invention;

[0036] Figure 3 is a curve diagram showing the change of gas phase effective permeability of the core with the pressure inside the core in the present invention;

[0037] Figure numerals: 101 first intermediate container, 102 second intermediate container, 103 third intermediate container, 201 first injection pump, 202 second injection pump, 203 third injection pump, 204 confining pressure pump, 205 back pressure pump, 301 first valve, 302 second valve, 303 third valve, 304 fourth valve, 305 fifth valve, 401 first pressure gauge, 402 second pressure gauge, 5 core clamp, 6 high temperature oven, 7 back pressure valve, 8 gas flow meter, 9 gas-liquid separator, 10 waste gas bottle. DETAILED DESCRIPTION

[0038] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

[0039] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0040] Embodiment 1:

[0041] See also Figure 2 The present invention provides a shale condensate gas dew point testing system based on effective permeability, which is used to perform the testing method in the following embodiment 2. The testing system includes:

[0042] A core holder 5, the core holder 5 is used to hold the core, and a high-temperature oven 6 for heating the core is disposed outside the core holder 5, and a confining pressure pump 204 for establishing confining pressure for the core is disposed outside the core holder 5;

[0043] The gas injection assembly includes a first intermediate container 101 for storing methane, a second intermediate container 102 for storing condensate gas samples, and a third intermediate container 103 for storing nitrogen. The first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 are connected to a first injection pump 201, a second injection pump 202, and a third injection pump 203 at one end away from the core holder 5, respectively. The first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 are connected to a first injection pump 201, a second injection pump 202, and a third injection pump 203 at one end close to the core holder 5, respectively. A first valve 301, a second valve 302, and a third valve 303 are connected, and the first valve 301, the second valve 302, and the third valve 303 are connected to the inlet end of the core holder 5 through the same fourth valve 304, so that the methane, condensate gas, and nitrogen in the first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 can be injected into the core in the core holder 5, and a first pressure gauge 401 for obtaining the pressure of the inlet end of the core holder 5 is connected between the inlet end of the core holder 5 and the fourth valve 304;

[0044] An exhaust component includes a gas-liquid separator 9, a waste gas bottle 10 connected to the exhaust end of the gas-liquid separator 9, and a back-pressure valve 7 connected to the back-pressure pump 205. A fifth valve 305 is connected between the back-pressure valve 7 and the outlet end of the core clamp 5, and a second pressure gauge 402 for obtaining the outlet pressure of the core clamp 5 is connected between the fifth valve 305 and the outlet end of the core clamp 5. A gas flow meter 8 is connected between the back-pressure valve 7 and the gas-liquid separator 9.

[0045] Embodiment 2:

[0046] See also Figure 1 , 3 The present invention provides a method for testing the dew point of shale condensate gas based on effective permeability, which is executed by the testing system in the first embodiment, and includes the following steps:

[0047] S1, prepare a condensate gas sample and obtain the normal dew point pressure of the condensate gas sample;

[0048] The specific method of preparing the condensate gas sample is: according to the formation pressure, the original gas-oil ratio of the formation fluid and the formation temperature, the degassed oil and separated gas obtained at the oil field are used to prepare the original condensate gas sample of the formation. This is the existing technology and will not be described in detail here.

[0049] The conventional dew point pressure of the condensate gas sample is obtained by performing a PVT test on the condensate gas. The use of the PVT test to obtain the conventional dew point pressure of the condensate gas sample is an existing technology and will not be described in detail here.

[0050] It should be noted that the conventional dew point pressure of the condensate gas sample obtained through the PVT test is the dew point pressure of the condensate gas under conventional conditions, not the dew point pressure of the condensate gas in unconventional reservoirs.

[0051] S2, obtaining the core and injecting methane into the core to clean the core, and then depressurizing the core to remove the adsorbed phase and condensate therein, as follows:

[0052] After obtaining the core of the real reservoir from the field or the oil field core library, the core is cleaned with methane. When injecting methane into the core for cleaning, the injection pressure is higher than the conventional dew point pressure by a certain pressure value, and the volume of methane at least five times the pore volume of the core is injected into the core to fully clean the core. Then, a vacuum pump is used to reduce the pressure of the core until the pressure inside the core is reduced to 0, so as to remove the adsorbed phase and condensate oil inside the core;

[0053] The injection pressure may be selected to be 600 psi, 700 psi, etc. higher than the conventional dew point pressure, and is not limited here;

[0054] It should be noted that the pore volume of the core can be obtained by testing the core, which is a prior art and will not be elaborated here.

[0055] S3, clamp the core in the core holder, inject methane into the core to pressurize it, until the pressure inside the core exceeds the normal dew point pressure and reaches a first preset threshold, and the pressure difference between the inlet and outlet of the core holder is stable, and the pressure building of the core is completed, as follows:

[0056] Open the first valve 301, the fourth valve 304, and the fifth valve 305, so as to inject the methane in the first intermediate container 101 into the core in the core holder 5 through the first injection pump 201, and after the outlet of the core holder 5 is vented (whether the outlet of the core holder 5 is vented can be obtained through the second pressure gauge 402), close the fifth valve 305 and continue to inject methane into the core until the pressure value of the core internal pressure higher than the normal dew point pressure reaches a first preset threshold, and adjust the back pressure pump 205 until the inlet and outlet pressure difference of the core holder 5 is stable, so as to complete the pressure building of the core, which is to prevent the precipitation of condensate oil due to a sudden pressure drop when the condensate gas sample is saturated;

[0057] The first preset threshold value may be 500psi, 600psi, etc., which is set by the staff according to the actual situation and is not limited here;

[0058] Among them, the fluctuation of the difference between the inlet pressure and the outlet pressure of the core clamp 5 is lower than the preset fluctuation threshold value, which is regarded as the inlet and outlet pressure difference of the core clamp 5 being stable. The preset fluctuation threshold value can be 1psi / h, 1.5psi / h, 2psi / h, etc., and is not limited here. The inlet pressure of the core clamp 5 is obtained through the first pressure gauge 401, and the outlet pressure of the core clamp 5 is obtained through the second pressure gauge 402.

[0059] S4, taking the pressure value higher than the conventional dew point pressure to reach the second preset threshold as the injection pressure condition, injecting the condensate gas sample into the core to displace the methane and saturate the core, leaving the core still to stabilize the pressure of the condensate gas sample in the core, maintaining the inlet and outlet pressure difference of the core holder lower than the third preset threshold and keeping the inlet and outlet pressure difference stable, as follows:

[0060] The first valve 301 is closed, and the second valve 302 is opened, so that the condensate gas sample in the second intermediate container 102 is injected into the core in the core holder 5 through the second injection pump 202. Specifically, the pressure value higher than the conventional dew point pressure reaches the second preset threshold as the injection pressure condition, and the condensate gas sample is injected into the core to replace the methane. The back pressure pump 205 is adjusted until the inlet and outlet pressure difference of the core holder 5 is stabilized again, until the gas-oil ratio of the condensate gas sample flowing out of the outlet of the core holder 5 is consistent with the gas-oil ratio of the condensate gas sample on the PVT test report, indicating that the condensate gas sample has saturated the core and completely replaced the methane in the core. Then, the second valve 302 and the fourth valve 304 are closed, and the core is left to stand for a period of time (such as 30 minutes) to stabilize the pressure of the condensate gas sample in the core, and the back pressure pump 205 is adjusted to make the inlet and outlet pressure difference of the core holder 5 lower than the third preset threshold, and the inlet and outlet pressure difference of the core holder 5 is kept stable;

[0061] Wherein, when injecting condensate gas sample into the core to displace methane and saturate the core, the volume of the injected condensate gas sample is at least 1.5 times the pore volume of the core;

[0062] Among them, when the pressure fluctuations at the inlet and outlet of the core holder 5 are both lower than the preset fluctuation threshold, the pressure of the condensate gas sample in the core is considered to be stable. The preset fluctuation threshold can be 1ps i / h, 1.5ps i / h, 2ps i / h, etc., which is not limited here;

[0063] The second preset threshold value may be 500psi, 600psi, etc., which is set by the staff according to the actual situation and is not limited here;

[0064] Among them, the third preset threshold can be 50psi, 60psi, etc., which is specifically set by the staff according to the actual situation and is not limited here.

[0065] S5, with the depressurization speed lower than the fourth preset threshold value and the net stress of the core during the depressurization process being the net stress preset value as a constraint condition, depressurize the core until the core pressure drops to the target pressure value, and let the core stand to stabilize the pressure of the condensate gas sample in the core, as follows:

[0066] With the depressurization speed lower than the fourth preset threshold and the net stress of the core during the depressurization process being the net stress preset value as the constraint condition, the back pressure pump 205 is adjusted to depressurize the core until the core pressure drops to the target pressure value, the fifth valve 305 is closed and the core is left to stand for 30 minutes to stabilize the pressure of the condensate gas sample in the core;

[0067] The fourth preset threshold value may be 1ps i / min, 1.5ps i / min, etc., which is set by the staff according to the actual situation and is not limited here;

[0068] The core net stress is the difference between the core confining pressure and the core pore pressure. The preset value of the net stress can be 500psi, 600psi, etc. It is set by the staff according to the actual situation and is not limited here.

[0069] The initial value of the target pressure value is at least 500 psi higher than the normal dew point pressure. Of course, it can also be 550 psi, 600 psi, etc. higher than the normal dew point pressure. The specific setting is made by the staff according to the actual situation and is not limited here.

[0070] S6, with the core holder outlet pressure kept constant at the target pressure value and the injection flow rate not higher than the fifth preset threshold as the constraint conditions, nitrogen is injected into the core to displace the condensate gas sample, and the difference between the inlet and outlet pressures of the core holder is used as the displacement pressure difference. When the displacement pressure difference fluctuation value is less than the sixth preset threshold, it indicates that the core is in a seepage equilibrium state and enters step S7, which is as follows:

[0071] Because nitrogen is stable and does not adsorb on the inner wall of the pores, and has no extraction effect on the condensate oil, and does not affect the saturation of the condensate oil in the core, nitrogen is used to displace the condensate gas sample. Specifically, the third valve 303, the fourth valve 304, and the fifth valve 305 are opened to inject the nitrogen in the third intermediate container 103 into the core in the core holder 5 through the third injection pump 203 to displace the condensate gas sample. In order to avoid the problem that the condensate oil is reversely evaporated into the gas phase due to the increase of local pressure due to the excessive pressure inside the core, the condensate gas sample is displaced by the third valve 303, the fourth valve 304, and the fifth valve 305. The core holder outlet pressure is a constraint condition for the target pressure value. At the same time, in order to minimize the amount of condensate oil displaced by nitrogen, the nitrogen injection flow rate is not higher than the fifth preset threshold value as a constraint condition so that nitrogen is slowly injected into the core to displace the condensate gas sample, and the volume of the injected nitrogen is at least 5 times the pore volume of the core, so as to ensure that nitrogen completely replaces the original gas phase in the core. When the displacement pressure difference is less than the sixth preset threshold value, it indicates that the core is in a seepage equilibrium state, and step S7 is entered to calculate the effective permeability of the core under the target pressure value;

[0072] The fifth preset threshold value may be 1.5 ml / h, 2 ml / h, 2.5 ml / h, etc., which may be set by the staff according to the actual situation and is not limited here;

[0073] Among them, the sixth preset threshold can be 0.15ps i / h, 0.2ps i / h, 0.25ps i / h, etc., which is specifically set by the staff according to the actual situation and is not limited here.

[0074] S7, obtaining the inlet pressure, outlet pressure, outlet gas flow rate, nitrogen viscosity at the target pressure value, cross-sectional area and length of the core holder, and calculating the effective permeability of the core at the target pressure value based on Darcy's formula. The calculation formula of the effective permeability is as follows:

[0075]

[0076] It should be noted that K represents the effective permeability, P in Indicates the inlet pressure of the core holder, P out represents the outlet pressure of the core holder, ΔP represents the displacement pressure difference, μ represents the viscosity of nitrogen at the target pressure value, Q represents the gas flow rate at the outlet of the core holder, A represents the cross-sectional area of ​​the core, and Δx represents the length of the core;

[0077] The inlet pressure can be obtained through the first pressure gauge 401, the outlet pressure can be obtained through the second pressure gauge 402, the viscosity of nitrogen at the target pressure value can be obtained from existing test data or literature or nitrogen viscosity calculation software, the gas flow rate at the core holder outlet can be obtained through the gas flow meter 8, and the cross-sectional area and length of the core can be obtained through existing measurement tools;

[0078] As an implementation method, when the target pressure value is 2548 psi, the test data are specifically shown in Table 1 below:

[0079] Table 1

[0080]

[0081]

[0082] Substituting the data in Table 1 into Darcy's formula, we can obtain that when the target pressure is 2548 psi, the effective permeability of the core is 1.2449×10 -18 m 2 , after conversion to Darcy units, it is approximately 1.2449μD;

[0083] It should be noted that, by using nitrogen to displace the condensate gas sample through the method of step S6, and then using the nitrogen viscosity to calculate the effective permeability of the core at the target pressure value through the method of step S7, nitrogen is a common gas, and its gas viscosity has been widely mentioned and cited in previous test data or literature. There is no need to conduct additional tests to obtain the viscosity of nitrogen at the target pressure value, which greatly reduces the difficulty of obtaining the effective permeability of the core and reduces the burden on test personnel.

[0084] S8, subtracting the seventh preset threshold from the target pressure value to reset the target pressure value, and repeating steps S2-S7 to obtain the effective permeability of the core after the target pressure value is reset. When the difference between the effective permeability of the core before and after the target pressure value is reset is less than the eighth preset threshold, step S8 is repeated. Otherwise, the two target pressure values ​​before and after the target pressure value is reset are defined as the front target pressure value and the rear target pressure value, respectively, and the process proceeds to step S9;

[0085] It should be noted that when the difference in the effective permeability of the core before and after the target pressure value is reset is less than the eighth preset threshold, it means that the fluid in the core is still in a single phase and has not reached the dew point pressure of the condensate gas in the core. Then, the target pressure value should be further subtracted from the seventh preset threshold to reset the target pressure value. When the difference in the effective permeability of the core before and after the target pressure value is not less than the eighth preset threshold, it means that the fluid in the core has formed a gas-liquid two-phase flow. Then, the dew point pressure of the condensate gas in the core is between the two target pressure values ​​before and after the target pressure value is reset.

[0086] The seventh preset threshold value may be between 100 and 550 psi, which is set by the staff according to the actual situation and is not limited here;

[0087] Among them, the eighth preset threshold can be 0.15μD, 0.16μD, 0.17μD, etc., which is specifically set by the staff according to the actual situation and is not limited here.

[0088] S9, obtaining a new target pressure value between the front target pressure value and the rear target pressure value, and repeating steps S2-S7 to obtain the effective permeability of the core at the new target pressure value, when the difference in the effective permeability of the core between the front target pressure value and the new target pressure value is not less than an eighth preset threshold, taking the new target pressure value as the rear target pressure value and repeating step S9, and when the difference in the effective permeability of the core between the rear target pressure value and the new target pressure value is not less than the eighth preset threshold, taking the new target pressure value as the front target pressure value and repeating step S9, until the difference between the front target pressure value and the rear target pressure value is less than a ninth preset threshold, and determining that the dew point pressure of the condensate gas in the core is between the front target pressure value and the rear target pressure value;

[0089] Among them, the ninth preset threshold value can be 7psi, 10psi, 13psi, etc., which is set by the staff according to the actual situation and is not limited here. The smaller the ninth preset threshold value is, the more accurate the calculation of the dew point pressure of the condensate gas in the core is.

[0090] It should be noted that the difference between the previous target pressure value and the subsequent target pressure value is continuously reduced by the method of step S9, so as to continuously reduce the interval length of the dew point pressure of the condensate gas in the core. The interval value of the dew point pressure of the condensate gas in the core can be accurately locked through multiple tests. Compared with the method of determining the virtual inflection point by curve simulation in the prior art, the dew point pressure of the condensate gas in the core obtained by this method is more accurate and reliable. Under the conditions of sufficient experimental equipment accuracy and sufficient experimental cycle, this method can reduce the interval length of the dew point pressure range of the condensate gas in the core to within 10 psi, which is sufficient for application in occasions requiring precise dew point pressure.

[0091] As an implementation method, taking the core of a certain block as an example, the effective permeability of the core at different target pressure values ​​is shown in Table 2 below:

[0092] Table 2

[0093] Target pressure value / psi Effective permeability / μD 3560 1.2476 3041 1.2575 2548 1.2449 2499 1.0458 2384 0.9037

[0094] When the target pressure value is 2548psi, the measured gas phase effective permeability of the core is 1.2449μD, which is not significantly decreased compared with the previous target pressure value of 3041psi effective permeability of 1.2575μD. Therefore, in the next test of effective permeability, 2348psi is used as the target pressure value for testing. The measured effective permeability decreases significantly by 0.9037μD, indicating that the presence of oil and gas phases in the core in the pressure range (2384psi, 2548psi) leads to a decrease in gas phase permeability. Therefore, the next target pressure value is set to 2499psi, and the measured effective permeability is 1.0458. At this time, the pressure range of the dew point of the fluid in the core can be reduced to (2499psi, 2548psi).

[0095] Furthermore, the new target pressure value is an average value of the sum of the previous target pressure value and the subsequent target pressure value.

[0096] The above formulas are all dimensionless and numerical calculations. The formula is a formula for the most recent real situation obtained by collecting a large amount of data and performing software simulation. The preset parameters in the formula are set by technicians in this field according to actual conditions.

[0097] The above embodiments may be implemented in whole or in part by software, hardware, firmware or any other combination thereof. When implemented using software, the above embodiments may be implemented in whole or in part in the form of a computer program product. Those skilled in the art may appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein may be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed by hardware or software methods depends on the specific application and design constraints of the technical solution.

[0098] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, and may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0099] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be covered by the protection scope of the present application.

Claims

1. A method for testing the dew point of shale condensate gas based on effective permeability, characterized in that: The steps include: S1, prepare a condensate gas sample and obtain the normal dew point pressure of the condensate gas sample; S2, obtaining a core and injecting methane into the core to clean the core, and then depressurizing the core to remove the adsorbed phase and condensate oil inside it; S3, clamping the core in the core holder, injecting methane into the core to pressurize it, until the pressure inside the core exceeds the normal dew point pressure by a first preset threshold value, and the pressure difference between the inlet and outlet of the core holder is stable, and the pressure building of the core is completed; S4, taking a pressure value higher than the conventional dew point pressure to reach a second preset threshold as the injection pressure condition, injecting a condensate gas sample into the core to displace methane and saturate the core, leaving the core still to stabilize the pressure of the condensate gas sample in the core, maintaining the inlet and outlet pressure difference of the core holder lower than a third preset threshold and keeping the inlet and outlet pressure difference stable; S5, with the depressurization rate being lower than a fourth preset threshold value and the net stress of the core during the depressurization process being a preset net stress value as a constraint condition, depressurizing the core until the core pressure drops to a target pressure value, and leaving the core still to stabilize the pressure of the condensate gas sample in the core; S6, with the core holder outlet pressure being kept constant at the target pressure value and the injection flow rate being no higher than the fifth preset threshold as the constraint conditions, nitrogen is injected into the core to displace the condensate gas sample, and the difference between the inlet and outlet pressures of the core holder is used as the displacement pressure difference. When the displacement pressure difference fluctuation value is less than the sixth preset threshold, it indicates that the core is in a seepage equilibrium state and proceeds to step S7; S7, obtaining the inlet pressure, outlet pressure, outlet gas flow rate, nitrogen viscosity at the target pressure value, cross-sectional area and length of the core of the core holder, and calculating the effective permeability of the core at the target pressure value based on Darcy's formula; S8, subtracting the seventh preset threshold from the target pressure value to reset the target pressure value, and repeating steps S2-S7 to obtain the effective permeability of the core after the target pressure value is reset. When the difference between the effective permeability of the core before and after the target pressure value is reset is less than the eighth preset threshold, step S8 is repeated. Otherwise, the two target pressure values ​​before and after the target pressure value is reset are defined as the front target pressure value and the rear target pressure value, respectively, and the process proceeds to step S9; S9, obtain a new target pressure value between the previous target pressure value and the subsequent target pressure value, and repeat steps S2-S7 to obtain the effective permeability of the core at the new target pressure value, when the difference in effective permeability of the core between the previous target pressure value and the new target pressure value is not less than an eighth preset threshold, use the new target pressure value as the subsequent target pressure value and repeat step S9, and when the difference in effective permeability of the core between the subsequent target pressure value and the new target pressure value is not less than the eighth preset threshold, use the new target pressure value as the previous target pressure value and repeat step S9, until the difference between the previous target pressure value and the subsequent target pressure value is less than a ninth preset threshold, and determine that the dew point pressure of the condensate gas in the core is between the previous target pressure value and the subsequent target pressure value.

2. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: The specific logic of obtaining the core and injecting methane into the core to clean the core, and then depressurizing the core to remove the adsorbed phase and condensate inside it is: After obtaining the core of the real reservoir from the field or the oil field core library, the core is cleaned with methane. When injecting methane into the core for core cleaning, the injection pressure is higher than the conventional dew point pressure by a certain pressure value, and the volume of methane at least five times the pore volume of the core is injected into the core. Then a vacuum pump is used to reduce the pressure of the core until the pressure inside the core is reduced to 0, so as to remove the adsorbed phase and condensate oil inside the core.

3. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: The first preset threshold value is 500psi or 600psi.

4. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: In step S4, the volume of the injected condensate gas sample is not less than 1.5 times the core pore volume, the second preset threshold value is 500psi or 600psi, and the third preset threshold value is 50psi or 60psi.

5. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: The fourth preset threshold value is 1psi / min or 1.5psi / min, the core net stress is the difference between the core confining pressure and the core pore pressure, the net stress preset value is 500psi or 600psi, and the initial value of the target pressure value is 500psi higher than the conventional dew point pressure.

6. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: The fifth preset threshold value is 1.5 ml / h, 2 ml / h or 2.5 ml / h, and the sixth preset threshold value is 0.15 psi / h, 0.2 psi / h or 0.25 psi / h.

7. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: The calculation formula of the effective permeability is as follows: Where K represents the effective permeability, P in Indicates the inlet pressure of the core holder, P out represents the outlet pressure of the core holder, ΔP represents the displacement pressure difference, μ represents the viscosity of nitrogen at the target pressure value, Q represents the gas flow rate at the outlet of the core holder, A represents the core cross-sectional area, and Δx represents the core length.

8. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: The seventh preset threshold value is a value between 100-550 psi, and the eighth preset threshold value is 0.15 μD, 0.16 μD, or 0.17 μD.

9. The method for testing the dew point of shale condensate gas based on effective permeability according to claim 1, characterized in that: The ninth preset threshold value is 7psi, 10psi or 13psi.

10. A shale condensate gas dew point testing system based on effective permeability, used to perform the shale condensate gas dew point testing method based on effective permeability according to any one of claims 1 to 9, characterized in that: include: A core holder 5, wherein the core holder 5 is used to hold the core, and a high-temperature oven 6 for heating the core is disposed outside the core holder 5, and a confining pressure pump 204 for establishing confining pressure for the core is disposed outside the core holder 5; The gas injection assembly includes a first intermediate container 101 for storing methane, a second intermediate container 102 for storing condensate gas samples, and a third intermediate container 103 for storing nitrogen. The first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 are connected to a first injection pump 201, a second injection pump 202, and a third injection pump 203 at one end away from the core holder 5, respectively. The first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 are connected to a first injection pump 201, a second injection pump 202, and a third injection pump 203 at one end close to the core holder 5. The first valve 301, the second valve 302, and the third valve 303 are respectively connected, and the first valve 301, the second valve 302, and the third valve 303 are connected to the inlet end of the core holder 5 through the same fourth valve 304, so that the methane, condensate gas, and nitrogen in the first intermediate container 101, the second intermediate container 102, and the third intermediate container 103 can be injected into the core in the core holder 5, and a first pressure gauge 401 for obtaining the pressure of the inlet end of the core holder 5 is connected between the inlet end of the core holder 5 and the fourth valve 304; An exhaust component, the exhaust component includes a gas-liquid separator 9, a waste gas bottle 10 connected to the exhaust end of the gas-liquid separator 9, and a back-pressure valve 7 connected to the back-pressure pump 205. A fifth valve 305 is connected between the back-pressure valve 7 and the outlet end of the core clamp 5, and a second pressure gauge 402 for obtaining the outlet pressure of the core clamp 5 is connected between the fifth valve 305 and the outlet end of the core clamp 5. A gas flow meter 8 is connected between the back-pressure valve 7 and the gas-liquid separator 9.

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