| Understanding of shale gas percolation mechanism is the basis of the study of shale seepage and seepage test is main way to obtain different seepage mechanism of shale.Pulse attenuation method as a test of the important methods for low permeability core physical parameters,can effectively test the shale core porosity and permeability,and to evaluate the formation percolation ability to provide accurate data to support.At present,there are still some problems in the model and calculation solution established by the national standard for the test of pure matrix core.At the same time,there is no test method for multi-scale shale core pore permeability parameters with different fracture development conditions,and it is necessary to understand the shale seepage mechanism through experiments.Based on the idea of core pulse attenuation method,this paper first establishes the seepage differential equation in cores with different fracture development conditions,then designs a method to test the permeability and porosity of pure matrix cores,matrix natural fracture cores and matrix shear fracture cores,considering the multi-scale effect of shale,combining with the adsorption characteristics of shale,and then studies the seepage mechanism of shale.The main understandings obtained in this paper are as follows:(1)Aiming at the unsteady flow of gas in the pure matrix core,the differential equation of seepage in the matrix core was established by combining with the pulse attenuation method to describe the gas seepage in the one-dimensional core of the pure matrix.The analytical solution of the pressure response of the upper and lower bottles in Laplace space is obtained,and the pressure response curves of the upper and lower bottles in real space are obtained by using Stephest numerical inversion,since the solutions given in this paper are more precise than the Brace solutions.(2)Aiming at the unsteady flow problem of gas in the matrix natural fractured core,a one-dimensional two-hole and two-permeability unsteady flow model was established to describe the gas flow in the one-dimensional core with natural fractures.Similarly,Laplace transform and Stephest numerical inversion are used to solve the pressure response curve of upstream and downstream bottles.By fitting the experimental data,the permeability and porosity of core matrix and fractures can be obtained simultaneously.(3)Aiming at the problem of unsteady flow of gas in matrix shear fracture cores,a three-dimensional cylindrical dual porosity and dual permeability unsteady flow model was established to describe the gas flow in shear fracture cores.The model was solved by using the pull transform and cosine orthogonal transform,and the analytical solution of the model was obtained.The pressure response curves of the upstream and downstream bottles were obtained through the definition of cosine orthogonal change and Stephest numerical inversion.By fitting the experimental data,the permeability and porosity of core matrix and shear joints can be obtained simultaneously.(4)Considering shale adsorption,the proposed model is modified to more accurately describe the flow of gas in shale core.The apparent permeability model considering the multi-scale effect of shale was established,and the parameter interpretation method was established.Through a group of experimental data,relevant parameters such as shale core matrix,flow pattern distribution in fractures,porosity distribution,equivalent pore diameter,slip coefficient and diffusion coefficient were obtained by fitting at the same time.(5)The experimental device of pulse attenuation method was designed,and the measurement method of pulse attenuation permeability and porosity in the adsorption multi-scale seepage shale core was proposed.The pressure derivative was introduced into data fitting for the first time,which greatly improved the accuracy of data interpretation.Based on the experimental data,the gas percolation mechanism and flow pattern distribution of WY shale core are analyzed. |
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