Computer Simulation Study Of Drilling Fluid Loss And Plugging In Fractured Leaky Layers

Well leakage is a common downhole complex during drilling,and most of the well leaks are related to cracks.The well leakage will not only prolong the drilling cycle,loss of working fluid,damage to the reservoir and economic loss,but once the well leak is not well controlled,it may also cause serious accidents such as blowout and well collapse.Therefore,solving the problem of leakage in fractured formations saves time and cost for the site,which is very important in the current low oil price form.In this paper,the fractured stratum is taken as the entry point.For the leakage of drilling fluid and the problem of plugging,the combination of theoretical analysis,mathematical modeling and computer simulation is used to study the leakage mechanism of drilling fluid and improve the bearing capacity of the formation.According to the basic theory of non-Newtonian fluid,two kinds of drilling fluid leakage models of two-dimensional crack H-B mode and power law mode are established by combining fluid mechanics,continuity equation,rheological equation and momentum equation,and the natural fluid drilling fluid leakage model is improved.Taking the well-drainage of high-density oil-based drilling fluid as an example,the two models of drilling fluid leakage model were verified,and the relationship between the bottomhole pressure difference and the leakage rate was simulated.The results were the same as the actual field data,and the error was small.From the perspective of the drilling fluid rheological model,the rheological model of the drilling fluid is determined by the rheological curve comparison analysis method,and the optimal rheological model is selected,and the corresponding drilling fluid leakage model can be used to improve the prediction accuracy.Based on the H-B model drilling fluid loss model,the influence of each parameter on the loss is analyzed.The analysis results show that the crack width,bottomhole pressure difference,dynamic shear force and drilling fluid performance have a great impact on the leakage of drilling fluid.The crack inclination has little effect on the leakage of drilling fluid,and the roughness of the crack characterized by fractal dimension has little effect on the leakage.Based on the H-B model drilling fluid loss model,the leakage law of high-density drilling fluid in cracks was studied by numerical simulation.The results showed that initial crack width,crack inclination angle,bottom hole pressure difference,dynamic shear force and drilling fluid consistency coefficient for high-density drilling.The liquid leakage loss has a greater impact,while the drilling fluid flow index has less impact on the leakage.Aiming at the problem of low pressure hearing capacity and high pressure bearing plugging in fractured strata,rock mechanics,fracture mechanics and elastoplastic mechanics are used to establish a pressure-bearing plugging model that comprehensively considers the instability of the plugging layer and crack propagation.In the two aspects of pressure bearing capacity and fracture stability of the plugging layer,the theoretical method of improving the bearing capacity of the stratum is proposed,and the influencing factors of the compressive capacity of the plugging layer and the crack propagation pressure are analyzed.The results show that the strength,stiffness,elastic deformation and surface friction coefficient of the bridging material are larger.The lower the porosity of the sealing layer,the higher the pressure bearing capacity of the sealing layer;the greater the width of the crack sealing layer and the penetration of the sealing layer The lower the rate,the longer the crack tip length,the greater the crack propagation pressure and the more stable the crack.According to the pressure-bearing plugging model,the crack width prediction model is obtained,and the influence of different parameters on the crack width variation is simulated.The simulation results show that as the depth of the plugging material increases,the crack width also increases,the plugging efficiency is higher,and the crack width The smaller the variation range is;the closer the bridging material is to the crack opening,the smaller the crack width is,and the more stable the crack is.After the plugging layer is formed,the crack width increment is almost zero,and the crack width is no longer increased.