Studies On Gas Migration Characteristics And Permeability Evolution Of Organic Rich Shale

With the worldwide growing demand for natural gas and decrease of conventional gas, as an important unconventional energy, shale gas exploration and development has become increasingly important and urgent. Organic-rich shale is compact and low permeable, which are both the source rock and the reservoir rocks of shale gas. The occurrence and transport mechanism of shale gas is also different from that of conventional natural gas. Up to now, shale gas exploration and development in China is still in its early stages. There are few theories for shale gas extraction mechanism, which constrain the development of shale gas in China. The study of basic theory provides the necessary theoretical guarantee for the rapid exploration of shale gas in China, thus, the study of gas migration mechanism in organic-rich shale has extremely important significance for the development of shale gas industry.The organic rich shale of Lower Cambrian Niutitang formation in western Hunan province was taken as research subject. Observation and description of fissure and pore structure characteristics of shale were carried out at different scales by a variety of test means. Experimental study on mechanical properties of shale with different angles of bedding was carried out. Then, experimental studies of methane adsorption, swelling and diffusion properties under triaxial compress conditions were caried out. The anisotropic property and change law of adsorption induced swelling varying with gas pressure adsorption quantity and were revealed. Considering pressure boundary conditions of shale gas reservoirs in real project, permeability tests under the condition of back pressure were carried out, thus, the evolution laws of shale permeability under different effective stress and deviatoric stress were revealed. Based on the above experimental studies and fundemental poroelasticity, shale permeability evolution model was established, and six factors impacting shale permeability changes during exploitation of shale gas were studied. The main conclusions are as follows:(1) Proportion of holes and large holes in Niutitang Shale is imbalance; the main contribution of pore volume and surface area comes from nano-pores smaller than 5.89nm in diameter.(2) Shale shows good brittleness, its strength and velocity has anisotropy properties. High brittleness mineralogy is the internal mineral composition reason of higher brittle deformation and brittle fracture; its internal layered sedimentary pyrite particles, hard particles and flakes alternately layered structure are the internal structure reasons of anisotropy characteristic.(3) Under given confining pressure, pore volume of shale still shows increasing trend with gas injection pressure increases. Free gas ratio of total gas content within Niutitang shale increases with the gas pressure increases. Shale adsorbed methane isotherms are more similar to the I-type adsorption curve and obey Langmuir equation. There are hysteresis between adsorption and desorption curve of methane. Under the given confining pressure, diffusion coefficient of helium and methane in macropores increases with increasing injection pressure. The trend of diffusivity coefficient of micropores is opposite to that of macropores.(4) The curve of shale swelling strain caused by the absolute adsorption amount of methane versus gas pressure is Langmuir curve type. Performance between swelling strain and inject amount of helium and methane is positively related to a linear relationship. Shale deformation exists hysteresis amount between methane adsorption and desorption experiments, and the amount of hysteresis strain increases with decreasing pressure. Shale deformation caused by injection of helium and methane has anisotropic characteristics, and shale anisotropy coefficient shows an increasing trend with the total gas pressure strain increases.(5) Shale permeability decreases with increasing confining pressure, and volume strain increases with increasing confining pressure. Shale permeability and deformation have anisotropy characteristics. Nano-pores dominant presence causes slippage phenomenon of gas migration in the shale under different pore pressure. Shale permeability decrease in the early deviatoric stress load, when approaching destruction, permeability began to gradually increase.(6) In the early stage of mining shale gas, effective stress plays a leading role on shale permeability; in the latter stage of the production process, desorption plays a dominant role. Shale gas permeability showed an increasing trend first and then decreasing trend in mining process of reservoir rock.