Assessment of fault slip in shale formation during hydraulic fracturing and its influence factors

Key words:  shale gas  hydraulic fracturing  slip of fault  casing deformation  well integrity

Abstract:

More than 40% of the casing in production wells has been deformed in Weiyuan-Changning shale gas field, which has a great impact on the development of shale gas. The current field investigation indicates that fault slip induced by hydraulic fracturing could be the main reason for casing deformation. In this paper, the mechanisms of hydraulic fracturing induced fault slip were analyzed, combined with the theory of fracture mechanics, and a calculation model for the fault slip under hydraulic fracturing was established. The calculation results for well Wei202X based on new model show that the fault slip reaches to 46 mm in the activated point of fault. They are in line with the field observation of the fault slip occurred in the shale gas field. The sensitivity study results show that the displacement of fault slip increases with the fracturing fluid pressure and the length of hydraulic fractures. Adequate adjustments of the fracturing pump pressure and the fracture length can reduce the fault slippage, and then avoid serious casing deformation. The faults subjected with high difference of horizontal in-situ stresses and with in rock formation having high elastic modulus will slip with a large displacement once the slippage is activated. The cement sheath with compressible materials(ceramic balls, marble, etc.) can provide enough space for most of fault slip. The model established in this study can provide a calculation method for fault slip during hydraulic fracturing, which is useful for preventing casing deformation in shale gas wells.