Study On Seepage Characteristics Of Shale Deterioration Under Dry-wet Cycle

With the development of pumped storage power station in China,the national investment in water conservancy project construction is constantly increasing,and the construction of reservoir inevitably involves shale bank slope,which is prone to deterioration due to the dry-wet cycle in the periodic rise and fall of reservoir water level in the water-leveling zone,which brings adverse effects on the stable operation of reservoir.At present,there are relatively few studies on the instability of shale bank slope caused by reservoir water level rise and fall.In this paper,gas permeability tests and uniaxial compressive tests were carried out to evaluate the permeability and mechanical properties of the Silurian Wufeng-Longmaxi shale in Changning area of Sichuan Province under the action of wetting and drying cycles.Microscopic analysis techniques such as SEM,EDS and XRD were used to study the response characteristics of shale meso-structure,composition and other indexes to the dry-wet cycle,and the mechanism of shale seepage degradation induced by the dry-wet cycle was revealed from the mesoscopic perspective.The research results provide theoretical support for the evaluation of the deterioration behavior of shale bank slope under the influence of reservoir water level rise and fall and the operation protection of reservoir bank.The main research contents and achievements of this paper are as follows:(1)The influences of confining pressure,osmotic pressure and the number of dry-wet cycles on the permeability characteristics of shale are analyzed by conducting gas permeability tests of underlayed shale with different confining pressure and osmal pressure differences.The results show that the permeability increases with the number of dry-wet cycles,but the increasing range is affected by confining pressure and osmotic pressure.With low confining pressure and high osmotic pressure difference,shale gas permeability changes greatly with the increase of dry and wet cycles.With the increase of confining pressure and the decrease of osmotic pressure difference,the change range of shale gas permeability decreases with the increase of dry and wet cycles.(2)Based on the mechanical properties test of shale samples treated with dry and wet cycles,the influences of the number of dry and wet cycles,bedding dip Angle and other factors on the compressive strength and elastic modulus of shale are obtained.The results show that the more dry-wet cycles there are,the weaker the mechanical properties of shale.The number of dry-wet cycles is approximately exponentially correlated with the elastic modulus and peak strength.The compressive mechanical properties of shale are related to bedding dip Angle,and the distribution of peak strength is approximately in accordance with JPW model.The weak joint plane model is suitable for estimating peak strength of rock mass with equal bedding structure of shale.(3)Using SEM,EDS,XRD,optical profilometer scanning and other testing methods,the micro-evolution characteristics of the shale after the dry-wet cycle treatment,such as microstructure,element composition,surface morphology,etc.The results show that the effects of dry and wet cycling on the microstructure of shale are mainly reflected in the aspects of block thinning,block hydration,pore minimization,sharp edges smoothing,etc.The proportion of hydrophilic minerals is higher than that of non-hydrophilic minerals and organic matter.The roughness of flat surface of shale sample increases slightly with the increase of dry-wet cycles.(4)Combined with the macro and micro characteristics of shale under the dry-wet cycle,the mechanism of shale seepage degradation under the dry-wet cycle is explained.Combined with the changes of macroscopic permeability and mechanical properties of shale under the action of dry-wet cycle,the macroscopic failure mode of matrix and joint surface,and the changes of shale microstructure and mineral composition,the promoting effects of dry-wet cycle on the internal joint surface penetration,the change of seepage channel,the development of microdefects and mineral hydration of shale are analyzed,and the deterioration mechanism of shale under the action of dry-wet cycle is systematically explained.