Experimental investigation of shale gas production impairment due to fracturing fluid migration during shut-in time

摘要

Hydraulic fracturing has been applied to exploit hydrocarbon resources for a number of decades. During the fracturing process, large amounts of pressurized fracturing fluid is injected to create and to propagate the fracture. In the exploitation of unconventional reservoirs, fracturing fluid recovery can be very low and even less than 10%. Any unrecovered fracturing fluid can be imbibed into the formation and block the rock pores, thus reducing the effective permeability of gas and causing gas production impairment. This study investigates gas production impairment due to spontaneous migration of fracturing fluid into a shale formation as a function of shut-in time. Core flooding experiments were designed to mimic initial leak-off volume, followed by shut-in time and flow back. Results are presented in terms of regained permeability ratios as a function of shut-in time. Findings from this work indicate that the regained permeability of shale slightly decreases with shut-in time, as the fluid front propagates within the rock. Results are also compared to previous experiments on tight sand cores. From this comparison, it was concluded that lithology also plays a determining factor in the relationship between shut-in time and regained permeability. The level of impairment caused by fracturing fluid migration was found to be significantly higher in shale cores than tight sands, which is attributed to the inherent lower permeability of shale formations. (C) 2015 Elsevier B.V. All rights reserved.