The Response Of Pore Structure Eigenvalue Of Low-Permeability Sandstones To Effective Stress Changes

The components of low-permeability sandstone mineral are usually complex with great difference of framework grains sorting and great diversity of pore types and their combinations, which shows various shapes of capillary pressure curve with great variation range of capillary pressure curve characteristics parameter and pore structure eigenvalue. Existing researches discovered that low-permeability sandstone shows the obvious stress sensitivity in the process of oil and gas exploration. These researches focused mainly on the changes in the relationship between permeability and effective stress, and the evaluation of permeability damages, which however lack in-depth reason analysis via micro-pore structure eigenvalue. This paper mainly researches the changes in the relationship between pore structure eigenvalue and effective stress by taking low-permeability rock as objective and by adapting methods of laboratory experiment, theoretical model analysis, and pore network model simulation.The main research works are as follows:(1) Experiments of rock casting thin sections analysis, electron microscope (SEM) observation and mercury injection capillary pressure (MICP) curve are conducted, and the microscopic pore structure of low-permeability sandstone is analyzed; permeability stress sensitivity test is carried out, and the macroscopic permeability stress sensitivity of low-permeability sandstone is evaluated based on calculation and analysis of its sensitivity.(2) The pore structure eigenvalue stress change instrument is improved based on the semipermeable plate device. Protection device of high-pressure semi-permeable plate and test method of pore structure eigenvalue under effective stress are proposed. Contrast experiment between routine mercury injection and unidirectional mercury injection are carried out as well as the unidirectional mercury experiment under different effective stress. The capillary pressure curve under different effective stress and cumulative pore size distribution curve are acquired and analyzed.(3) Typical mathematical models of capillary pressure curve are studied and compared; the power function of capillary pressure curve is derived and verified based on capillary pressure curve types and inverse function expression.(4) Calculation methods of rock pore structure eigenvalue are compared, and integral method is chosen to calculate rock pore structure eigenvalue; changes in relationship between the pore structure eigenvalue including average value, sorting coefficient and skewness and the effective stress are compared and analyzed.(5) Theoretical basis of random pore network model is analyzed; characteristic parameters of trilateral, quadrangle, circle, ellipse and cone are summarized as well as flow equation, conductivity equation, and non-dimensional radius equation that fluid flows in these network models. The rock effective stress equation of partial saturated wetting-phase fluid is derived. The solution methods of coefficient matrix are compared Gauss-Seidel iterative is used to decompose the coefficient matrix.(6) Pore network models are built and solved. The mixed random pore network model is programmed and simulated on the Qt Creator platform that reflects the change between pore structure eigenvalue and effective stress. Based on the mixed random pore network model, the change of the non-dimensional radius accompanied by effective stress under single-shape pore throat is analyzed. The influence of proportion and aspect ratio value on calculation results is studied. The change between non-dimensional radius and effective stress is analyzed under different pore throats, aspect ratio values and proportion combinations. The explanation for experimental data is realized via these combination schemes.(7) The function between rock permeability and pore structure eigenvalue is built, and the reasons for permeability stress sensitivity of low-permeability sandstone are revealed on the base of mix random pore network model that reflects the change between pore structure eigenvalue and effective stress.(8) J function is derived on the base of rock capillary buddle model; a novel method that can be used to obtain average capillary pressure curve is proposed and verified; the influence of effective stress on average capillary pressure curve J function is studied.A systematic research on change in relationship between pore structure eigenvalue and effective stress is conducted through experimental device improvement and test, theoretical model build and analysis, and random pore network model simulation; the accurate obtainment of pore structure eigenvalue is thus realized which provides fundamental data for formation evaluation and oil & gas exploration. Meanwhile, the reasons for permeability stress sensitivity are explained via the change in relationship between pore structure eigenvalue and effective stress, which enriches the study methods of rock stress sensitivity.