Study And Application Of Fluid-solid-chemical Coupling Model For Wellbore Stability Of Hard Brittle Shale

Drilling technology in shale formation often encounters the problem of wellbore instability,which seriously restricts the progress and quality of the project.In order to explore the internal causes of wellbore instability and find a feasible solution,the mechanism of wellbore instability of shale is discussed through a series of research and analysis on shale in Shahejie formation.The overall can be summarized as follows:First,deep rocks are often under complex stress environments,and excavation disturbances will break the original stress equilibrium state and make it enter a more complex stress state;Second,although the shale is dense,there are many micro-cracks and micro-holes in the rock,which not only reduce the rock strength,but also provide a seepage channel for the invasion of groundwater,and the immersion of drilling fluid has produced the hydration effect of weakening shale strength;Third,excavation disturbances generate more secondary micro-cracks and micro-holes,which exacerbate the hydration effect of shale.Based on the theory of rock mechanics,physical chemistry and the finite element software,the theoretical system of multi field coupling study on the period of wellbore collapse in shale formation is systematically constructed by the method of theoretical analysis and numerical calculation.It provides a scientific basis for the safe drilling of shale formation and the optimization of drilling fluid.The research ideas of this paper are summarized as follows:1.The strain softening and permeability evolution of shale are analyzed,and a mathematical model considering the strain softening and permeability evolution of shale is established.And based on the finite element software ABAQUS,the subprogram interface USDFLD is used for secondary development to achieve strain softening and permeability evolution of the model.The established mathematical model is applied to drilling engineering,and the effects of strain softening and permeability evolution on wellbore stability are analyzed and discussed.The analysis results show that the effect of strain softening on the damage of the wellbore is large,and the effect of permeability evolution on the damage of the wellbore is small,which can be basically ignored.If the coupling effect of the two is considered at the same time,its influence on the stability of the wellbore is between the above two conditions,which is more in line with the site conditions.2.Through the analysis of the physical and chemical properties,microstructure and mechanical properties of the hard brittle shale,the instability mechanism is explained.Based on comprehensive consideration of the coupling effect between theactual unloading process of the rock during the drilling process,the weakening of the strength,and the plastic deformation,a coupling model for the progressive failure of shale wellbore was established,and the model was applied to drilling engineering.This paper discusses whether to consider the influence of hydration effect on the wellbore failure,improves the understanding of maintaining the wellbore stability of hard brittle shale from the mechanism,and provides a reference for the optimization of drilling fluid.3.Taking W structure as the research background,through numerical simulation,the wellbore stability of overbalance and underbalance drilling is analyzed.The analysis results show that rocks of different formation depths have different mechanical properties,and the wellbore failure laws presented are also different.The parameter sensitivity of underbalanced drilling is discussed.In terms of parameter sensitivity,water absorption diffusion coefficient and drilling fluid activity have a greater impact on wellbore damage,followed by membrane efficiency,while shale permeability has the least impact.The internal causes of wellbore instability under the two drilling methods are analyzed,and countermeasures against wellbore stability under different drilling methods are proposed,which provides a scientific basis for the study of shale wellbore stability problems in drilling engineering.