Study On Mechanism Of Wellbore Collapse And Instability In Horizontal Well Of Layered Shale

The wellbore instability of shale has been a worldwide drilling engineering technical problem,and it is also one of the core issues of safe and efficient drilling.The wellbore instability can cause great difficulties to the drilling engineering,mainly manifested in problems such as hole shrinkage.The research on shale wellbore stability is a very large and complex system.Although scholars at home and abroad have done a lot of research on shale wellbore stability for a long time,the research results are not consistent due to the complex problems of shale oil reservoir matrix density,rich clay minerals,bedding plane development,easy hydration and expansion,and significant anisotropy.At present,there are many problems,which restrict the efficient development of shale oil.Therefore,in combination with the actual situation of the bedded continental shale oil reservoir in Block G,and in view of the problem of wellbore collapse and instability in horizontal wells,this paper systematically and deeply carried out the physical and chemical properties of shale,rock mechanics test and the test of the effect of drilling fluid on shale hydration.The anisotropic characteristics and hydration mechanism of shale are studied,and the failure strength criterion and strength criterion evaluation index of bedded shale considering the effects of anisotropy and hydration are established.Based on this strength criterion,a wellbore stability model of mechanical-chemical coupling for horizontal wells of bedded shale oil is established.The finite element numerical simulation research was carried out under the conditions of different in-situ stress mechanism,different water content,different well trajectory,different number of weak planes,and different dip angles of weak planes.The influence of the above parameters on the stability of shale wellbore is analyzed.Through field example verification,this model is used to correct the drilling fluid density window of two shale oil wells in Block G.The results show that when the bedding direction of shale is different from the loading direction,the shale failure mode changes significantly;When loading along the foliation direction,the triaxial strength of rock decreases by 18.8%and the cohesion decreases by 51.5%after 10 days of immersion in oil-based drilling fluid;When loading in the direction of vertical foliation,the triaxial strength decreased by 17%,the cohesion did not change in 4days,and the overall strength was high,indicating that the shale has certain anisotropy;When the shale is not weakened,the collapse density changes from 1.02 to 1.73 g/cm~3;When the water content of shale reservoir increases,the strength value of shale will deteriorate,and the equivalent density of wellbore collapse pressure will rise rapidly.When the water content is10%and 15%,the rise range of collapse density is 0.15～0.22 g/cm~3 and 0.31～0.45 g/cm~3respectively;Under any kind of in-situ stress mechanism,the wellbore in the vertical section is the most stable.When the well deviation angle is around 45°,the wellbore is most vulnerable to instability;The equivalent density of collapse pressure corresponding to different well inclination angles and azimuth angles increases with the increase of water content,because the weakening degree of hydration stress and hydration on rock mechanical parameters increases with the increase of water content,resulting in the equivalent density of collapse pressure increasing with the increase of water content;The existence of structural plane will increase the equivalent density of overall collapse pressure.The research results of the paper play an important role in systematically and deeply understanding the wellbore instability mechanism of the bedded continental shale in Block G,reducing the wellbore collapse accidents of horizontal wells,providing basic theory for safe,high-quality and efficient horizontal well drilling,and finally realizing the efficient and economic development of shale oil.