Laboratory studies of hydraulic fracturing by cyclic injection

Abstract

We assess the possibility of decreasing the breakdown pressure of rock and increasing the damage around hydraulic fracture by using pre-breakdown cyclic injection during hydraulic fracturing under triaxial stress conditions. Unlike the monotonous increase in pressure used in conventional hydraulic fracturing, the fluid is injected in cycles until breakdown. During cyclic injection, the peak pressure of each cycle is increased in an increment of 10% of the reference breakdown pressure. The reference breakdown pressure of the rock is the pressure at which the rocks fails during hydraulic fracturing by conventional injection. To obtain a reference breakdown pressures, specimens of dry and saturated Tennessee sandstone were hydraulically fractured by conventional injection. The decrease in breakdown pressure and increase in damage during cyclic injection is quantitatively compared with the case of conventional hydraulic fracturing. Acoustic emission (AE), fracture permeability, and Scanning Electron Microscope (SEM) images of the fracture surface were used to compare the damage around hydraulic fractures generated by conventional and cyclic injection hydraulic fracturing. Laboratory results indicate that the damage generated around hydraulic fracture by cyclic injection hydraulic fracturing of dry Tennessee sandstone is approximately twice that generated by conventional injection. Also, the breakdown pressure recorded during cyclic injection fracturing of dry Tennessee sandstone is lower and varies more than two standard deviations from that of conventional injection.