Study On The Evolution Mechanism Of Shale Permeability Under The Coupling Effect Of Gas Transport And Stress

With the increasing demand for energy,unconventional oil and gas resources represented by shale gas are receiving much attention.Vigorously developing and utilizing shale gas resources can further optimize the energy supply structure.Accurately predicting the flow law of alkane gas in the rock formation and the prediction of its production have attracted much attention,and the permeability is generally regarded as one of the key control factors for shale gas transport.However,due to the complex microstructure and volatile reservoir conditions in the shale matrix,factors such as stress and slippage effect will cause different changes in the permeability characteristics of shale,it is still a challenge to accurately describe the gas flow behavior in the shale matrix and predict changes in permeability.At the same time,the gas transport in the shale matrix does not always conform to the viscous flow state described by Darcy's law,On the contrary,when the average molecular free path of the gas is equivalent to the shale pore size,the viscous flow state is Sporadic and irregular.The entire gas extraction process is a non-linear process under the alternate influence of flow components such as gas adsorption,viscous flow,slip flow and Knudsen diffusion.Therefore,studying the evolution mechanism of shale permeability under the coupling effect of gas transport and stress has certain positive significance for the production and yield of shale gas.The test samples in this paper are selected from the shale of Niutitang Formation from Well Fengcan 1 in Fenggang area,northern Guizhou Province,the physical property tests and seepage tests with variable effective stress under different pressure difference conditions were carried out by using the adsorption device,the mercury porosimeter,and the triaxial seepage device.Based on this,a permeability model with the linkage effect of pore structure and compression characteristics was established;Based on the adsorption experiment,an adsorption model considering the effects of temperature and excess adsorption capacity was established,and the adsorption deformation equation at different temperatures was further established;On this basis,a permeability model considering the correction of the slip boundary conditions and a permeability model based on the influence of different weight coefficients coupling stress/strain and gas transport are established.The main research results are:1)Carry out liquid nitrogen adsorption test,high pressure mercury intrusion test and seepage test.Through physical property tests,analyze the relationship between pore volume and compressibility and the distribution of pores and cracks in shale;Shale permeability is controlled by the structural characteristics of pores and cracks.Based on the description of pore structure and permeability,it is necessary to establish a linkage relationship between the two.This paper intends to conduct low-temperature liquid nitrogen adsorption tests and high-pressure mercury intrusion tests to obtain the relationship between initial permeability and pore structure parameters,and use the initial permeability as the intermediate bridge,based on the shale seepage test under constant external stress,a permeability model considering the linkage effects of pore structure and compression characteristics is established,and its rationality is verified through tests.It reflects the correlation between shale pore structure and its permeability characteristics,and also provides important theoretical support for hydraulic fracturing and other shale gas mining methods.2)Relevant scholars have carried out experimental studies on shale adsorption and seepage.Most of the adsorption models use simplified Langmuir models,which do not consider the influence of excess adsorption and temperature,which will affect the prediction effect of the model.At the same time,most studies on the influence of gas transport on the apparent permeability of shale,there are few models of apparent permeability under constant external stress conditions,and there are few reports on studies considering the coupling of gas transport and stress.In view of this,an adsorption model considering the excess adsorption capacity was established,combined with the stress/strain relationship,and a shale apparent permeability model considering the coupling of gas transport and stress was established,and its rationality was verified through experiments,and it provides theoretical support for shale gas permeability enhancement and exploitation.3)Relevant scholars mainly carry out research on shale seepage mechanism and experiment,and most of them do not consider the influence of dynamic pore structure in seepage models.At the same time,the experimental research is mainly the seepage test under different confining pressure conditions,and its theories are mostly embodied by the empirical equations of permeability and effective stress,there are few reports on theoretical studies based on the capillary model,coupled with dynamic pore structure and slippage effects.In view of this,this paper first establishes a capillary model,combining the adsorption deformation and stress/strain relationship,and considering the slippage effect,further establishes the shale apparent permeability model under different confining pressure conditions,verifies its rationality,and examines the characteristics of multi-scale mass transfer,analyze the evolution law of shale apparent permeability.