Laboratory Simulation Of Mud Shale Microcracks And Evaluation Of Plugging

With the depletion of conventional natural gas resources,the status of shale gas in the energy structure of our country becomes more and more important.In the development of shale gas,ensuring wellbore stability is one of its key technologies.Good plugging ability of drilling fluid is the basis of wellbore stability,and plugging agent is the basic guarantee for good sealing ability of drilling fluid.The micro-crack micro-pores in mud shale formations are often developed at the nano-micro level,the permeability is extremely low,and the average particle size of solid particles in ordinary plugging agents is large,and it is difficult to enter the shale formation to form an effective plug.Nano-scale plugging agents can exhibit good sealing performance on shale formations.However,for the sealing evaluation of nano-micro cracks,there is no unified effective method and standard for the time being.Therefore,we need an efficient and feasible method for the simulation of shale fractures to meet the requirements for the sealing evaluation of Nano-scale plugging agents for shale.After investigating the evaluation methods for plugging agents both domestic and overseas,the slurry formulation was optimized to produce a low-permeability and thick cake.0.18mm to 0.40mm silver was used to make seams for the mudcake.The factors affecting the crack forming were studied.The lowest permeability of the mudcake made from the low permeability can reach 4.235×10-4mD.Through the research and control of the method of making joints,the equivalent crack width of the single crack of mudcake crack can be steadily controlled at about 50?m.At the same time,the influence of different fracture sizes,dip angles and arrangement methods on the overall permeability of the mud cake was studied.This paper is using the PFC2D software developed by ITASCA,the process of blocking the microfracture of mud shale by rigid particle plugging agent was simulated.The software was based on the geological data of the shale section of Longmaxi Formation in Weiyuan area,and the foundational environment including simulation was built.The establishment of wellbore rock mass,the generation of cracks in the wellbore rock,the formation of plugging agent particles,the setting of the motion contact rules,the setting of the particle carrier drilling fluid,and the output feedback of the simulation results.The influence of single plugging agent particles on the plugging effect,the influence of different plugging agent particle size pairing plugging effect,and the effect of different fracture inclination angles on the plugging effect were studied,and the plugging agent particles that met the crack width were obtained.The blocking mechanism of the plugging agent particles in micro-fractures was analyzed by observing the state of particle movement during the blocking simulation.In the end,the physical verification of computer simulation plugging results was carried out,and the results of simulation verification were further analyzed and processed.The advantages of nano-plugging agents for plugging of micro-crack formations in shales were proposed.Four types of nano-plugging agents commonly used in the market were selected,including Nano-Fe3O4 dispersions,Nano-LAT dispersions,Nano-silica dispersions and Nano NR-1.The dispersions were optimized for their dispersion in water,and then their plugging properties were simply evaluated.The plugging mechanism was briefly analyzed,and the most various nano-plugging agents were obtained.Based on the analysis of the effects of water-based drilling fluids on wellbore hydration performance,we used the triaxial stress test to compare and measure the maximum differential stress values of cores after immersion in the wellbore collapse stress.The effects of various types of plugging agents and other drilling fluid treatment agents on the tensile strength of shale.The results show that the nano-plugging agent can reduce the collapse of borehole wall to a certain extent.