1 CNPC Engineering Technology R&D Company Limited, Beijing 102206, China 2 Key Laboratory of Geotechnical & Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China 3 Department of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, China
Since 2009, large-scale shale gas development has begun in Changning-Weiyuan blocks, Sichuan Province. The main driven technologies are horizontal drilling and multi-stage hydraulic fracturing. But more than 30% of horizontal wells in this area encountered casing deformation in the process of hydraulic fracturing, which not only reduces the number of fracturing stages, but also causes problems in terms of well integrity, thus shortening the life cycle of wells and seriously restricting the efficient development of shale gas in this area. In this paper, the mechanisms of fault slippage and casing deformation caused by hydraulic fracturing in Changning-Weiyuan blocks are studied, and some prevention and control strategies of casing deformation are proposed. Firstly, the geometric characteristics of casing deformation in Changning-Weiyuan blocks are analyzed. At the same time, the correlation between casing deformation and fault slippage induced by hydraulic fracturing is summarized by analyzing the locations of faults given by the Ant-tracking seismic volume and the temporal and spatial characteristics of microseismic signals. Then the characteristics of in-situ stress in Changning-Weiyuan blocks is described. Through the sliding analysis of the reactivated natural fault in the field, it is found that the fault slip may take precedence over the occurrence of hydraulic fracturing during the pumping. The influence of fault stability and unstable sliding on casing deformation is discussed. The main controlling factors of controlling fault slip are analyzed and two different fault slip modes, triggered and induced, are proposed. The influence of mineral composition of fault gouge on the friction properties and stability of faults is analyzed. It is pointed out that when the clay content of fault gouge in Changning-Weiyuan blocks is less than a certain critical value, velocity weakening behaviors of fault gouge and unstable slip of fault can be promoted. Finally, some prevention and control strategies of casing deformation are proposed. (1) Based on the casing deformation caused by fault slippage, the influence of casing and cement ring materials and design on casing deformation is summarized. (2) Based on induced fault slippage, the influence of injection parameters on fault slip and casing deformation is studied. (3) On the basis of the induced fault slippage, a conceptual method for fault slip risk prediction is proposed. (4) Finally, a set of prevention and control scheme based on the integration of geological engineering is put forward.
