Numerical Simulation And Experimental Study On Enhancing Shale Wellbore Stability With Nanoparticles

Shale gas,as one of the significant unconventional natural gas,is abundant in reserves.And shale gas currently has entered a stage of rapid development in China,which will effectively alleviate the contradiction between China's natural gas supply and demand.It can also facilitate the adjustment of the national energy structure.However,shale is a typical low-permeability sedimentary rock.The wellbore instability has always been a difficult spot in the exploration and development of shale gas?oil?at home and abroad.More than 75%of the strata encountered are shale formations,and it causes more than90%of wellbore instability.Inhibiting shale hydration and blocking shale nanopores are the basis for the stability of shale gas horizontal wells and the sustainable exploitation of shale gas.Based on environmental and economic pressures,water-based drilling fluids will be more promising than oil-based drilling fluids if the interaction between drilling fluid and shale can be minimized.The best approach to use water-based drilling fluid without polluting the environment should be brine-based drilling fluid.Therefore,the influence of salt solution on shale permeability,membrane efficiency and wettability need to be clarified.Nanoparticles can block shale cracks and pores,that have been recognized by the academic community and industry.However,the motion of nanoparticles in shale pores has not been recognized.It is also unclear which parameters can enhance the clogging effect of nanoparticles on the micro or nano scale.Therefore,the thesis used the combination of experimental test,microscopic observation,theoretical analysis and numerical simulation to study the effect of brine on the physico-chemical seepage performance and surface properties of shale under pressure difference.The effect of brine-based solutions on shale permeability,membrane efficiency,and wettability provides the basis for formulating brine-based drilling fluids that maintain wellbore stability in shale gas horizontal wells.Afterwards,a numerical simulation model was provided to simulate the effects of particle size,concentration,velocity,particle ratio,gravity,rotation,density,shape,roughness,and channel tortuosity on shale pore plugging efficiency.The simulation results and experimental results were compared to verify the reliability of the model.Meanwhile,the microscopic changes of shale before and after particle plugging were observed by SEM,which provided theoretical and experimental basis for the clogging of shale pores by nanoparticles to prevent shale hydration.Based on the above research ideas,the thesis includes five chapters and the contents of each chapter are as follows.The first chapter introduced the shale gas status,shale gas horizontal well drilling,nanoparticles plugging experiments and particle plugging simulations.Afterwards,the research purpose and significance were illustrated.We summarized the basic information of shale gas horizontal wells and masterd domestic and foreign drilling fluid formula applied in horizontal well,and it can provide basis and reference for the development of drilling fluid formulations.The second chapter:Chongqing Xiushan,Shizhu and Pengshui shale were on-site collected,then shale mineral composition and morphological characteristics were analyzed.According to the nanoparticles screened experiments,the nanoparticles plugging experiments were carried out.The pore mechanism of nanoparticles blocking shale was studied from the aspects of pressure transmission experiment and microstructure analysis.Meanwhile,the effects of the type,existence state,particle size and concentration of nanoparticles on the performance of brine drilling fluid at different temperatures were studied.Chapter 3:Pressure transmission experiments on different concentrations and types of salt solutions were tested.The salt types were divided into five kinds,namely NaCl,KCl,CaCl₂,HCOONa and HCOOK.The concentrations were divided into three concentrations,respectively 5 wt%.10 wt%and 20 wt%.Then,the effects of different salt solutions on the permeability,membrane efficiency and wettability of Longmaxi shale were compared to obtain the optimal salt and its concentration to maintain the wellbore stability of shale.Chapter 4:The effects of particle size,concentration,velocity,particle ratio,gravity,rotation,density,shape,roughness,and channel tortuosity on the shale pore plugging efficiency were simulated.The simulation results and experimental results were compared to verify the reliability of the model.Chapter 5:The optimal formula of drilling fluid system was proposed through the experiment.And the optimized formula for shale gas horizontal well composed of nanoparticles,salts,fluid loss additives,lubricants and tackifiers was studied.Rheological properties and fluid loss performance remain stable at high temperatures.Simultaneously,the drilling fluid system was guaranteed to inhibit shale hydration and pass high standards of environmental testing.According to the above research,the following conclusions are drawn:?1?Classification of various shale pore pressure transmission properties in the Longmaxi shale with the same concentration of salt solution was clarified.5 wt%regularity:HCOONa>KCl>NaCl>CaCl₂>HCOOK.10 wt%regularity:HCOONa>HCOOK>NaCl>KCl>CaCl₂.20 wt%regularity:HCOONa>KCl>HCOOK>NaCl>CaCl₂.?2?The best five salt solutions with pore pressure transmission resistance were 20 wt%HCOONa,5 wt%HCOONa,20 wt%KCl,20 wt%HCOOK and 5 wt%NaCl.HCOONa exhibited better performance than other salts.The first three salt solutions with higher membrane efficiency were 5 wt%NaCl??=0.014?,10 wt%KCl??=0.012?and 20 wt%HCOONa??=0.01?or 20 wt%NaCl??=0.01?.?3?The membrane efficiency contacted with Longmaxi shale was positively correlated with the water activity of NaCl,KCl and CaCl₂ solutions,but negatively correlated with the water activity of HCOONa and HCOOK solutions.The fitting of NaCl,KCl,CaCl₂and HCOONa was higher?R²=0.6969-1?,which was more valuable.The contact angle of shale with water became smaller after PTT experiment.On the contrary,the contact angle after PTT experiment with salt solution was larger than that of the original sample.In addition,the contact angle regularity of different concentrations with the same type of salt solution was 5 wt%>10 wt%>20 wt%after PTT experiment.?4?CFD and DEM methods were used to simulate particle suspensions on themicroscopic scale to plug shale pores.The trajectory of the nanoparticles and the final plugging effect could be obtained by CFD post-processing.The UDF program was programmed to ensure the rationality of the simulation results,and the sephere drag force could be modified the standard drag curve by using experimental data and empirical formulas.Meanwhile,the simulation results?particle size and concentration?were verified by theoretical formula and experimental data,increasing the credibility and applicability of the model.?5?Particle size and concentration were main factors affecting plugging efficiency,under the condition of particle size not exceeding the outlet size,the particle accumulation effect increased by 13%and 23%,as the particle size increased by 33%and 60%with5wt%particles concentration.In terms of particles concentration,the particle concentrations of 11 wt%and 5 wt%increased the plugging efficiency by 74.78%and50%in comparison to the 1 wt%particle concentration.Pores roughness and tortuosity had a major impact.However,particle velocity,ratio,gravity,rotation,density,shape and roughness had little effect on plugging efficiency.?6?A drilling fluid system for shale gas horizontal wells was developed,combined with experimental results of salt solution on permeability,membrane efficiency and contact angle,and the results of nanoparticle flow and plugging shale pore simulation.The system had good rheological properties,the apparent viscosity was 46 mPa·s,the shear stress was maintained at 14 Pa,the dynamic plastic ratio was maintained at 0.43 Pa/mPa·s at different temperatures.The filtration loss at normal temperature was 4 mL and reached a maximum of 7.5 mL at 120°C,and the pH of the filtration was maintained at 7 or 8,which was basically unchanged.The friction coefficient was maintained at about 0.2 at room temperature,and the fluctuation range was narrow.For a shale formation that required a lower friction coefficient,5%polyethylene glycol?PEG?could be added to reduce the friction coefficient to 0.13,and rheological and filtration properties could be kept constant.The water activity was between 0.889 and 0.897,indicating that the system has good high temperature resistance.?7?The optimal rheological model for drilling fluid systems was the Herschel-Bulkley model.The expansion test and rolling recovery test indicated that the drilling fluid system had excellent suppress shale hydration performance.Meanwhile,the bacterial biotoxicity test and heavy metal content analysis experiments showed that the brine-based drilling fluid formula had environmental protection characteristics.The pressure transmission experiments illustrated that the system containing nanomaterials could hinder the seepage of drilling fluid,reduced the shale permeability,and enhanced the stability of the borehole wall for the Longmaxi shale.The main innovations of the paper are as follows:?1?systematically explored the influence of different types and different concentrations of salt solution on the seepage process of the Longmaxi Formation shale,and provided the basis for formulating the brine-based drilling fluid for maintaining the stability of the shale gas horizontal wellbore;?2?Master the relationship between the water activity of the salt solution and the membrane efficiency of the Longmaxi Formation,and grasp the variation of the wettability of the shale surface under salt solution and pressure.?3?A numerical simulation model was established to program the mechanical model of the particle flow in the nanopore,simulating particle size,concentration,velocity,particle ratio,gravity,rotation,density,shape,roughness,and channel tortuosity.The effect of degree on shale pore sealing efficiency,and the reliability of the model is verified by comparison with theoretical results and experimental results of other labs.It provides a theoretical basis for the sealing of shale pores by nanoparticles to prevent shale hydration.