The Study Of Seepage Characteristics In Shale Under Multi-field Coupling Conditions

With the rapid development of national economy, the demand for energy surges day by day. Energy dependence on the import has become bigger and bigger, which will threat our energy security. The exploration and development of the world's non-renewable energy show that shale gas is the most promising replacer of conventional oil and gas resources. Therefore, developing shale gas efficiently is significant to retard the contradiction of energy requirement and ensure the energy security of China. Supercritical carbon dioxide(SC-CO2)-based shale gas reservoir fracturing has been considered a promising way to develop shale gas efficiently. However, there still exist some uncertainties in the seepage mechanisms of shale matrix after CO2 injection.This paper investigated the mineral compositions, pore structure and mechanical properties of shale from Sichuan Basin of China, and conducted the study of seepage characteristics in the shale under multi-field coupling conditions(such as pore pressure, effective stress, temperature, Klinkenberg effect and swelling/shrinkage effects) using the self-developed seepage apparatus. The major findings and conclusions can be drawn as follows:(1) The mineral compositions of Sichuan Basin Longmaxi formation shale are variable. The contents of brittle minerals are higher than clay minerals, which indicate a better fracturing effect. Moreover, the used shale is rich in organic matters, which is beneficial to gas generation and growing the storage space of gas.(2) The results of pore structure analysis show that the Sichuan Basin Longmaxi formation shale has large gas capacity due to the wide pore size distribution and the development of micropores. However, the poor development of mesopores and macropores are not conducive to the gas flow.(3) The results of mechanical experiments indicate that the mechanical properties of Longmaxi formation shale have high brittleness, fracability and extensive anisotropy which are good for fracturing. Furthermore, the triaxial compressive strength is larger than the axial compressive strength. In addition, the plasticity is increased with increasing the confining pressure, which is bad for fracturing.(4) The permeability of shale can be influenced by some other factors(such as pore pressure, effective stress, temperature, Klinkenberg effect and swell/shrinkage effect). Experimental results indicated: 1) The permeability of shale decreases as negative power function with increasing the effective stress, while the mould samples are more sensitive to the effective stress than fractured sample; 2) Under the same condition of effective stress, the decrease of the permeability may be related to the adsorption-induced swelling, and the tested permeability relationship of shale under different gases is He?N2?CO2; 3) The shale samples fractured by SC-CO2 have no remarkable Klinkenberg effect of gas flow in low pore pressure. The permeability increase with increasing the pore pressure. However, Klinkenberg effect is remarkable in mould sample. 4) The temperature can affect the percolation mechanisms in shale by influencing the pore structure, mechanical properties and adsorption behaviors.The experimental results can provide scientific basis of dynamic prediction of the mechanisms of seepage in the CO2-injected reservoirs and estimate the long-term production of gas.