Investigation On Cracking Process Of Shale Nanopores Under Hydraulic Fracturing And Gas-Liquid-Solid Flow Mechanisms

Shale gas development is of great significance for our country to achieve carbon peaking and carbon neutrality goals.Shale gas has low carbon emissions in the process of energy consumption,and huge domestic resources.However,shale reservoir has the characteristics of deep burial,extremely low porosity,extremely low permeability and poor automatic fluidity,so it must be artificially transformed by hydraulic fracturing to realize economic exploitation.In the process of hydraulic fracturing,a large amount of water-containing fracturing fluid is injected into the reservoir and the original pore morphology of the formation is changed.The main pores of shale reservoir are intergranular pores of clay minerals and organic pores.As the particle size of clay minerals is below micron,the pore size is mostly in the nanometer scale.The surface of the clay minerals was observed by electron microscopy as a rough multilayered lamellar structure.With hydraulic fracturing and complex fluid perturbation,clay particles are removed from the matrix surface,flow with the gas and fluid,and may damage the reservoir by blocking pore channels.Therefore,it is of great scientific significance to study the micro-conversion mechanism of clay nanopore hydraulic fracturing and its influence on subsequent shale gas seepage,and its research results and theories are important to support shale gas development.In this paper,molecular dynamics method is adopted.Kaolinite and illite are selected as clay mineral representatives.Water and methane are selected as fracturing fluid and shale gas components,respectively.Through gas-liquid-solid three-phase modeling and simulation calculation,the theory and mechanism of the above problems are studied from a micro perspective,and the research achievements and mechanism understanding are as follows:(1)The whole process of deformation and cracking of reservoir pore structure during hydraulic fracturing is simulated from a microscopic perspective.The hydraulic fracturing method is used to study the failure of clay structure from the perspective of fluid-structure coupling.Based on the slit pore structure of kaolinite,it is found that the silicon-oxygen tetrahedral layer of kaolinite minerals shows brittleness during failure under different hydraulic impact modes and fracturing intensities,while the aluminumoxygen octahedral layer is more ductile.The damaged silica tetrahedral layer cracks along three crystal directions because kaolinite crystals form three easy cracking surfaces during the growth of the bond chain.During the fracturing process,the clay formation is separated into particles and falls into the slit pore space.The clay layer interacts with the water molecules of the fracturing fluid and together affects the gaswater channel.The simulation results of different impact modes and different bearing surfaces show different pore subsidence depths.The size of the pore space affects the integrity of the fracturing process.A space that is too small does not allow all deformation processes to occur.The variation of the equivalent stress distribution with the simulation time is basically consistent with the observed structural deformation and deformation process.The forced deformation behavior of the clay mineral layer is well observed by the stress distribution.(2)The shale nanopore considering roughness and the gas-liquid-solid flow simulation under the influence of movable particles are realized.Real natural pores have certain rough surfaces.In order to match the actual formation conditions more closely,the clay particle model in the reservoir is constructed by periodic bond chain theory,and placed in the shale pores to form rough surfaces or participate in the flow as movable particles.The relative molecular dynamics simulation operates stably.This study breaks through the limitation of the traditional static pore skeleton,and obtains many complex influences of the rough surface of pores and movable particles on the fluid in pores.The process of clay particles moving forward or stagnating under gaswater driving is analyzed.It is also of great significance to the gas-water flow containing sand in fractures caused by hydraulic fracturing with sand addition.(3)The influence of illite pore surface roughness on gas-water two-phase flow state under different pressure gradients is studied.Illite pores with convex roughness surface were constituted by attaching additional bulk clay particles to the original surface of illite slit pores.The clay particles that constitute the roughness have three models,which are constructed based on three different crystallographic cuts of illite.It is found that all three kinds of surface roughness produced significant blocking effects on the two-phase flow,and the edge structure characteristics of the clay particles determined the degree of flow blocking.The method of increasing the pressure gradient to break the water bridge connection in past studies failed.The particle edges of the rough surfaces within the pore provide water molecules with a velocity perpendicular to the pore surface,connecting water molecules on the upper and lower surfaces of the pore,which facilitates the water bridge while reducing the mobility of the gas.(4)The influence of movable particles in shale pores on gas-water two-phase flow is analyzed.It is observed that when the pressure gradient exceeds a certain critical value,clay particles are disturbed by high pressure fluid and detach from the pore surface and participate in gas-water flow.The adsorption effect of pore surface on movable clay particles is reduced,which reduces the resistance to fluid flow and greatly improves the flow capacity of gas and water.At the separation points of the three clay particles,a significant increase in gas-water flux was observed.In clay-free flow,the fluid velocity in the center of the pore is usually a parabolic distribution of Poiseuille flow.The clay particles slow down the water molecules around them.The larger the size of clay particles,the closer the velocity of surrounding water molecules is to that of clay particles,and the more obvious the influence on the velocity of water cluster.At low pressure gradients,the decrease of water bridge velocity in the pore intensifies the obstruction of gas flow and reduces the flow flux of methane.The size of movable clay particles significantly affects the morphology of water molecules in pores.The larger the clay particle size,the more pronounced the diversion effect on the fluid within the pore,which reduces the critical pressure gradient required for the formation of anhydrous bridge flow state.(5)The effects of water saturation and particle size in the pore on the flow state are clarified.Under the conditions of high and low water saturation,the large clay particles are bound in the water film and the water bridge respectively,and no water bridges that do not completely close the pore channels appear.The water bridge morphology of small clay particles in pores is different from that of large clay particles under different gas and water saturation.When the water saturation is zero,clay particles smaller than the pore diameter reattach to the pore surface,forming a certain pore roughness.The hydroxyl group at the edge of the particle with a size greater than or equal to the pore diameter combined with the clay surface exist to block the pore channels.The clay particles move forward slowly or stagnate completely under the push of gas.Water molecules can wet the hydrophilic pore surface to avoid contact between the particle edge and the pore surface,effectively reducing the risk of pore blockage by clay particles.