Study On The Gas Breakthrough Pressure And Permeability Of Shale Fractures With Micro-nano Scale Of Different Matric Potentials

The gas escaping phenomenon in shale after overcoming the breakthrough pressure is becoming the intersection point of many resource and environment frontier issues.At present,the fracture is rarely considered in the process to determine the breakthrough pressure,and the gas flow characteristics after gas overcoming the breakthrough pressure and escaping from the fracture are still unclear.The fractures in shale are always partially water-saturated,and at the nano-to micro scale under the formation stress.A deeper study on the gas breakthrough pressures and gas flow characteristics of moisturized fractures with the nano-micro scale is of greatly theoretical and practical significance for the development of shale gas accumulation and exploitation,gas escaping after hydraulic fracturing,underground gas storage,nuclear waste geological storage and greenhouse gas geological storage.In this paper,the shale cores containing a single fracture obtained from the Carboniferous formation from eastern Qaidam Basin were used to analyze the gas breakthrough pressure and gas flow characteristics in the partially saturated fractures.The geochemical characteristics of the shale,the pore size and distribution of the matrix,and the surface morphology of fracture were analyzed.The fractures were moistened to various matric potentials under the environment of confining pressure limits and in the presence of both water and gas,and then CH₄ gas breakthrough pressure experiments and gas seepage experiments on the fractures of different matric potentials were conducted.Based on the experimental results,we apply a conceptual model of the breakthrough pressure in the fracture to qualitatively describe the relationship among the matric potential,water distribution in the fracture,and breakthrough pressure.At a low matric potential,water forms very thin water films on the fracture surfaces by both adsorption and capillarity.The breakthrough pressure is low,and the effect of water on the breakthrough pressure is not notable.As the matric potential increases,the thickness of adsorbed and capillary water film in the fracture increases,thereby resulting in an increase in the breakthrough pressure.We establish a mathematical model to quantitatively describe the relationship between the breakthrough pressure and matric potential.The experimental data show that the increase in breakthrough pressure with the matric potential is a process that proceeds from slow to fast,and there is a critical value of matric potential in this process.When the critical value is reached,the breakthrough pressure begins to increase rapidly,and water starts to act to prevent gas from escaping from the fracture.The breakthrough pressure decreases with the increase of the effective aperture in the form of a power function,and the increase magnitude of breakthrough pressure with the matric potential has a negative correlation with the increasing effective aperture.The experimental data show that gas permeabilities of the moisturized fractures are 1-3 orders of magnitude lower than that of the dry fractures.The phenomenon indicates that the presence of water significantly hinders the gas migration in the fractures.Poor connectivity of the fractures caused by the expansion of clay minerals and capillary condensation is the primary mechanism underlying the observed drastic decrease.The gas permeability decreases with the increase of gas pressure,and this phenomenon indicates that gas flow in the fracture manifests notable slippage.The gas slippage phenomenon is enhanced with increasing matric potential,and the relationship can be expressed with an exponential function.Under the assumption that rough fractures contain two sinusoidal surfaces,an analytical solution describing gas flow in unsaturated fractures considering the coupled effects of gas slippage and heterogeneous fracture structure is derived.The relative errors between the theoretical permeabilities predicted with the analytical model and the experimental permeabilities are within 10%,and the model-predicted results agree well with the experimental data.The gas permeability declines with the increase of matric potential,and a mathematical model between the gas permeability and matric potential is established to quantitatively describe the gas escaping rate from the fracture.