Research On Shale Hydration Damage Law And Wellbore Stability

The instability of shaft lining in drilling process is one of the main technical bottlenecks restricting the efficient development of shale gas.Because of the existence of clay minerals in shale,shale shows strong hydration characteristics.On the one hand,hydration weakens the supporting capacity of surrounding rock,on the other hand,it increases the difficulty of increases the risk of wellbore collapse.In this paper,the variation law of shale physical properties is analyzed under different dydration conditions,and the evaluation model of Wellbore Stability is modified by considering the influence of hydration.The wellbore stability analysis is carried out according to different in-situ stress distribution.The main research contents of this paper are as follows:(1)The mineral composition and content of shale in the study area were determined by X-ray diffraction test,and the objective conditions of shale hydration were analyzed by clay mineral characteristics and hydration mechanism.The results show that the brittle mineral content of shale is the highest in the study area,followed by clay mineral content,and illite is the main clay mineral,which is found that pengshui shale is more sensitive to water.(2)The effects of solution KCl concentration and action time on shale hydration are analyzed through the tests of expansion rate,moisture content and uniaxial compression of different shale samples under different test conditions.The expansion rate and moisture content of shale increase after hydration,and often accompanied by deterioration of strength and mechanical properties.At the same time,with the decrease of KCl concentration in the solution or the prolongation of hydration damage time,the degree of increase and deterioration increases accordingly.The law is greatly influenced by bedding plane.(3)The evaluation criterion of shaft lining stability considering intermediate principal stress is selected and modified considering the influence of hydration,then the calculation of collapse pressure is realized by programming language.According to the different in-situ stress distribution,the influence of hydration time on the stability of shale shaft lining is analyzed by using this model.The results show that the collapse pressure of shaft lining increases obviously at the beginning of hydration,and the increasing trend of collapse pressure slows down with the prolongation of hydration time.The research of this paper providis a new idea for the study of shale hydration damage and a new method for optimizing and enriching the influence of hydration on shale borehole stability.