Study On Collapse Mechanism Of Weak Surface Controlled Shale Reservoir

In the process of developing shale gas,borehole instability is one of the important factors restricting the development of shale gas.According to the field data,wellbore collapse drilling accidents often occur during the development of shale gas in the Sichuan and Chongqing,causing huge economic losses.In this paper,the shale rock mechanical properties of the Longmaxi Formation shale outcrop were tested,and the weak surface controlled shale reservoir collapse model was established.Based on the field drilling data,the influencing factors of shale gas well collapse pressure were analyzed.The main research contents and research results are as follows:(1)The uniaxial and conventional triaxial compressive strength tests were carried out under consideration of factors such as immersion of drilling fluids,core angles,and confining pressure.The experimental results show that the drilling fluid soaking will reduce the shale strength;the core perpendicular to the bedding plane and parallel to the bedding plane will have the highest strength;the confining pressure will increase the strength and plasticity of the shale.(2)The circumferential stress distribution of shale gas wells is analyzed,and the prediction model of collapse pressure considering weak surface is established.(3)The structural characteristics,reservoir characteristics,in-situ stress characteristics and formation pressure characteristics of Ning 201 well area are analyzed.(4)Based on the data of Changning H25-9 well,the mud density window prediction is based on the established collapse pressure prediction model,and the prediction results are consistent with the actual situation.(5)Based on the on-site logging data of a well in Changning area,the different parameters were changed to calculate the collapse pressure,and the influence of different factors on the collapse pressure was analyzed in detail according to the calculation results.The results show that with the increase of the deviation of the well,the increase of the maximum horizontal principal stress,the increase of the pore pressure,the decrease of the foliation dip,the decrease of the sliding friction coefficient of the bedding plane and the decrease of the cohesion of the bedding plane,the collapse pressure will increase.In addition,the azimuth of well and the maximum principal horizontal principal stress orientation have a certain impact on the collapse pressure.The research results are expected to provide drilling guidance for the development of shale gas in the Sichuan and Chongqing,reduce drilling accidents such as collapse of the wellbore,improve drilling speed and shorten the drilling cycle.