CN-116735418-B - Test method for carbon dioxide/nitrogen mixed gas adsorption replacement shale methane

CN116735418BCN 116735418 BCN116735418 BCN 116735418BCN-116735418-B

Abstract

The invention provides a test method for carbon dioxide/nitrogen mixed gas adsorption displacement shale methane, which comprises the steps of injecting a first preset amount of methane into a reference tank to obtain first pressure change data in the reference tank, injecting a second preset amount of carbon dioxide/nitrogen mixed gas into the reference tank in a staged mode to obtain second pressure change data in the reference tank, and determining the adsorption amount of the shale to the carbon dioxide, the nitrogen and the methane at each pressure point and the desorption amount of the methane in the shale at each pressure point based on the first pressure change data, the second pressure change data, the volume of the reference tank and the residual volume of a sample tank. The potential influence of gas bidirectional diffusion on the competitive adsorption of the mixed gas in the process of adsorbing and replacing shale methane by the carbon dioxide/nitrogen mixed gas is fully considered, and a solid foundation is provided for improving the recovery efficiency of the adsorbed gas, so that the recovery ratio of the shale gas is effectively improved.

Inventors

QIN CHAO
CAO MENGYAO
HUANG YUJIE
CHEN SHIWAN
ZUO SHUANGYING

Assignees

贵州大学

Dates

Publication Date: 20260505
Application Date: 20230619

Claims (6)

1. The test method for the methane adsorption displacement shale of the carbon dioxide/nitrogen mixed gas is characterized by comprising the following steps of: placing a sample into a sample cell of a test system; calibrating the free space volume of the test system by helium to determine the volume of a reference cell in the system and the residual volume of the sample cell; Injecting a first preset amount of methane into the reference tank to obtain first pressure change data in the reference tank; Injecting a second preset amount of carbon dioxide/nitrogen mixed gas into the reference tank in stages to obtain second pressure change data in the reference tank; Determining an adsorption amount of the shale to the carbon dioxide, the nitrogen and the methane at each pressure point and a desorption amount of the methane in the shale at each pressure point based on the first pressure change data, the second pressure change data, the reference tank volume and the sample tank residual volume, the determining an adsorption amount of the shale to the carbon dioxide, the nitrogen and the methane at each pressure point and a desorption amount of the methane in the shale at each pressure point based on the first pressure change data, the second pressure change data, the reference tank volume and the sample tank residual volume, comprising: Calculating the amount of the substance of the gas using a true gas state equation, wherein the amount of the substance of the gas is used to represent the adsorption amount and the desorption amount; Acquiring initial pressure and injection pressure before and after methane injection; Determining an initial adsorption amount of the methane by the shale based on the initial pressure and the injection pressure, wherein the desorption amount of the methane in the shale at each pressure point is equal to the initial adsorption amount minus the adsorption amount of the methane by the shale after each injection of the carbon dioxide/nitrogen mixed gas for pressurization; The adsorption amount of the shale on the carbon dioxide, the nitrogen and the methane at each pressure point is equal to the gas amount in the reference cell at the pressure point plus the gas amount in the sample cell at the pressure point, and the total gas amount in the reference cell and the sample cell after the adsorption balance is subtracted.
2. The carbon dioxide/nitrogen mixed gas adsorption displacement shale methane testing method of claim 1, wherein prior to initiation of the test, the method further comprises: and starting a vacuum pump to clean at least one of the gas mixing tank, the booster pump, the reference tank, the sample tank and the pipeline.
3. The carbon dioxide/nitrogen mixed gas adsorption displacement shale methane testing method of claim 1, wherein prior to said free space volume calibration of said testing system with helium, said method further comprises: heating the oil bath box to a preset temperature; and starting a vacuum pump to perform vacuum treatment.
4. The carbon dioxide/nitrogen mixed gas adsorption displacement shale methane testing method of claim 1, wherein said staged injection of a second predetermined amount of carbon dioxide/nitrogen mixed gas into said reference cell comprises: opening a gas taking needle valve, and calibrating gas components in the reference pool by using a weather chromatograph; Closing the gas taking needle valve and recording the first pressure of the reference tank; Opening a first valve to enable the carbon dioxide/nitrogen mixed gas in the reference cell to enter the sample cell; Closing the first valve when the first pressure point reaches balance in the adsorption test process of the first pressure point, and recording the balance pressure; Opening the gas taking needle valve again, and calibrating the gas components in the adsorption tank after adsorption balance by using the meteorological chromatograph; Closing the gas taking needle valve and recording the second pressure of the reference pool.
5. The carbon dioxide/nitrogen mixed gas adsorption displacement shale methane testing method of claim 4, further comprising: And repeatedly performing gas component calibration and pressure recording operation until the injected carbon dioxide/nitrogen mixed gas reaches a target injection amount, wherein the injection amount of carbon dioxide, nitrogen and methane is kept to meet a preset proportion in the injection process.
6. The carbon dioxide/nitrogen mixed gas adsorption displacement shale methane testing method of any of claims 1-5, wherein said staged injection of a second predetermined amount of carbon dioxide/nitrogen mixed gas into said reference cell comprises: And injecting the carbon dioxide/nitrogen mixed gas into the reference tank by using a booster pump according to a preset injection speed and a preset mixing proportion.

Description

Test method for carbon dioxide/nitrogen mixed gas adsorption replacement shale methane Technical Field The invention relates to the technical field of shale gas recovery, in particular to a test method for replacing shale methane by carbon dioxide/nitrogen mixed gas adsorption. Background Shale gas is unconventional natural gas and mainly exists on the surface of shale organic matter-clay mineral and in a pore-crack structure in an adsorption state and a free state, wherein the adsorption gas accounts for 20% -85% of the total gas content. Hydraulic fracturing is the main technical means of shale gas exploitation, but the shale gas recovery rate is generally low (< 30%) at present, wherein the low recovery efficiency of the adsorption gas is one of the main reasons for the low shale gas recovery rate. Aiming at the engineering problem, students at home and abroad sequentially put forward a shale oil reservoir development method, the difference of adsorption characteristics of shale on different gases (CO 2、N2、CH4) is utilized, the method of injecting CO 2、N2 and mixed fluid thereof into the shale gas reservoir in the later stage of development is utilized, the injected fluid is forced to exchange molecules with residual adsorption state CH 4 in a competitive adsorption mode, the adsorption state CH 4 is promoted to be converted into a free state, the shale gas recovery ratio is improved, and meanwhile, CO 2 participating in molecular exchange can be captured by the shale reservoir, so that CO 2 geological sequestration is realized. After CO 2/N2 is injected into shale gas reservoirs, the molecular exchange process of CO 2/N2 and CH 4 is a key factor affecting residual shale gas recovery and CO 2 sequestration. However, in the process of adsorbing and displacing residual CH 4 by CO 2/N2 in an actual shale gas reservoir, a gas bi-directional diffusion process of CO 2/N2 adsorption and CH 4 desorption exists, but in a great deal of previous researches, a competition adsorption test is carried out by injecting CO 2/CH4、N2/CH4、CO2/N2/CH4 mixed gas to reflect the molecular exchange process of CO 2、N2 and the mixed gas thereof and CH 4, and potential influence of the bi-directional diffusion of the gas on the competition adsorption of the mixed gas is not considered. Thus, a new solution is needed to solve the above-mentioned technical problems. Disclosure of Invention In the summary, a series of concepts in a simplified form are introduced, which will be further described in detail in the detailed description. The summary of the invention is not intended to define the key features and essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. In a first aspect, the invention provides a test method for replacing shale methane by carbon dioxide/nitrogen mixed gas adsorption, which comprises the following steps: placing a sample into a sample cell of a test system; calibrating the free space volume of the test system by helium to determine the volume of a reference cell and the residual volume of a sample cell in the system; Injecting a first preset amount of methane into the reference tank to obtain first pressure change data in the reference tank; injecting a second preset amount of carbon dioxide/nitrogen mixed gas into the reference tank in stages to obtain second pressure change data in the reference tank; The adsorption amount of the shale to carbon dioxide, nitrogen and methane at each pressure point and the desorption amount of the methane in the shale at each pressure point are determined based on the first pressure variation data, the second pressure variation data, the volume of the reference cell and the residual volume of the sample cell. Optionally, before the start of the test, the method further comprises: And starting the vacuum pump to clean at least one of the gas mixing tank, the booster pump, the reference tank, the sample tank and the pipeline. Optionally, before the free space volume calibration of the test system with helium, the method further comprises: heating the oil bath box to a preset temperature; and starting a vacuum pump to perform vacuum treatment. Optionally, the method further comprises: opening a gas taking needle valve, and calibrating gas components in a reference pool by using a meteorological chromatograph; Closing the air taking needle valve and recording the first pressure of the reference tank; Opening a first valve to enable carbon dioxide/nitrogen mixed gas in the reference cell to enter the sample cell; In the adsorption test process of the first pressure point, closing the first valve when the first pressure point reaches balance, and recording the balance pressure; opening the gas taking needle valve again, and calibrating the gas components in the adsorption tank after adsorption balance by using a meteorological chromatograph; The take-off needle valve was closed and the referenc