Investigation Of Plugging Pore And Fracture With Micro&Nano-Particles

There are rich shale gas resources in our state,but wellbore instability issue frequently occur during drilling,which seriously restricts the exploitation of shale gas.Nano or micro particles have been used to solve this issue,but the size of throat and fracture is different,the effect of particle plugging is not satisfactory.The nano and micro particles is different about the force.It is difficult to configure the plugging particles according to the conventional particle plugging rules to meet the needs of efficient and fast plugging of shale.Although lab experiment are used to study shale plugging,there are problems such as inability to reveal microscopic mechanisms,non-repeatability and high cost.Simulation methods can avoid the limitations of experiments.Therefore,this paper analyzes the characteristics of shale pores and fractures statistically,analyzes the force of nano and micro particles,establishes nano and micro particle plugging simulation method,carries out nano-micro particle plugging simulation,and studies nano and micro particle plugging pattern.The main works are as follows:(1)On the basis of investigation about the characteristics of representative shale,the material composition,mechanical properties,physical characteristics,shape,type,geometric shape and other characteristics are summarized,and the main characterization parameters and characteristics of shale pore and fracture are clarified.(2)Starting from the force of a single particle,the force acting on a single particle is analyzed and the force levels of nano-and micro-scale particles are compared.It is found that the main interphase forces between nano and micro particles are fluid drag and pressure gradient.The effect of van der Waals adhesion between nano particles is stronger than that of micro particles,and the adhesion can be ignored if the particle size is greater than 10μm or the distance is greater than 100 nm.On the basis of single particle force analysis,the discrete element method is introduced: MD model is useed to calculate inter-particle adhesion force,N-S equation is used to calculate fluid force.Finally,a simulation method for particle plugging pores and cracks with plugging efficiency and plugging quality as evaluation indicators is established.(3)Considering the difference in shape and size of pore throats and fractures,the two were considered separately in the simulation,and the influence of particle size and initial concentration on the plugging effect was investigated.The results show that nanoparticles can stabilize the pore throats and bridge the plugging.The particle size is greater than or equal to1/3 of the pore diameter,and the particle size that micron particles can stably plug the crack is greater than or equal to 1/2 the width of the fracture.In the particle size matching range,the greater the initial concentration,the faster the plugging speed,but There is a critical value,after the critical value is exceeded,the blocking speed is almost unchanged.(4)Considering the elastoplasticity of particles,aiming at the two forms of particle plugging failure,according to the principle of force balance,a particle micro-pressure bearing capacity model is established and the plugging stability of calcium carbonate and elastic microspheres is calculated and analyzed.This study provides a theoretical basis for the optimization of nano-micron plugging particles of shale drilling fluids and improving the plugging effect.It provides a theoretical basis for water intrusion and instability prevention during drilling of shale intervals,completes safe drilling of long horizontal sections of shale,and realize the efficient development of shale oil and gas resources.