Study On The Gas Diffusion Of Organic-rich Shale In Reservoir Conditions

China is a major consumer of natural gas.The natural gas resources stored in the shallower strata have been proven and industrialized.However,natural gas as a non-renewable resource has limited reserves.Today,as demand continues to increase,conventional natural gas resources have gradually dried up,and the import of natural gas is not an expedient measure.Therefore,it is urgent to find a substitute for shallow natural gas resources.In recent years,shale gas has gradually entered people's field of vision as a green and clean energy source.After a period of development,shale gas mining has been commercialized in some developed countries.China's proven shale gas reserves rank first in the world and are widely distributed.Although China's shale gas reserves are abundant,the commercial gas production of shale gas is still at a lower stage.Currently,continuous gas production is only possible in parts of the Sichuan Basin.Shale gas is a kind of ?self-generated and self-storing? natural gas energy,mainly in free and adsorbed state,which is present in shale matrix and organic pores,and is partially dissolved in shale kerogen.Unlike traditional oil and gas resources,shale gas reservoirs have complex pore structures and pore sizes up to the nanometer level.Conventional mining methods cannot be applied to the actual exploitation of shale gas reservoirs.The pre-production of shale gas wells mainly comes from the free gas in the reservoir,while the gas production in the middle and late stages mainly comes from the shale matrix of methane desorption-diffusion effect.Therefore,the diffusion effect of shale gas plays an important role in the sustainable exploitation of shale.This paper has carried out the following research work on the diffusion and migration process of gas in the reservoir:(1)Due to the existence of large pores(pore radius > 50 nm),mesopores(2 nm < pore radius < 50 nm)and micropores(pore radius < 2 nm)in shale reservoirs,and multi-scale pore structures with joint fissures.Therefore,the pore structure characteristics of shale are explored by means of high pressure mercury intrusion technology and low temperature nitrogen adsorption technology.At the same time,the content of organic carbon and clay minerals in shale was analyzed by experiments,and the distribution of pore structure in shale was further understood.(2)In order to analyze the transport operation of shale gas in the reservoir pores,the shale fracture coefficient measuring device was used to measure the shale fracture without loading any ground stress and changing the gas pressure continuously.The diffusion coefficient of the shale fractured rock sample was measured.The test results show that the diffusion coefficient of shale changes with the change of gas pressure.(3)For the adsorption and diffusion phenomena existing in the shale matrix,the conventional three-axis rock test equipment is used to simulate the original state of the shale in the reservoir,using different fluid media,including the adsorbent gas methane and carbon dioxide.A series of experimental studies have been carried out on adsorbed gases helium and nitrogen.It is found that the adsorption in the shale matrix has a certain influence on the diffusion behavior of the gas in the reservoir.The shale diffusion coefficient is calculated by the bidisperse diffusion model,At the same time,the variation law of shale diffusion coefficient under the condition of presence or absence of geostress is compared.(4)Finally,considering the distribution law of shale pore structure,a numerical model of shale diffusion coefficient was established by using " bidisperse diffusion model " combined with numerical simulation software.The influence of deformation caused by shale under the stress of the original rock on the diffusion behavior is further studied.The correctness of the above test is verified by comparison,and the significance of gas diffusion effect on shale gas exploitation is clarified.In this paper,the gas diffusion behavior of one of the key steps in shale gas mining,combined with the macroscopic and microscopic angles,was carried out under the condition of presence or absence of geostress,and finally the shale pore structure characteristic parameters were obtained.Study the diffusion process of shale gas in complex pore structure and the effect of different effective stress on shale gas diffusion performance.A numerical model corresponding to the experimental conditions was established using numerical calculation software.The final results of the experiment further enrich the theoretical study of gas diffusion behavior in shale gas reservoirs,and provide a strong reference for the prediction of shale gas reservoirs in China and the development of mining plans.