Experimental investigation into simultaneous and sequential propagation of multiple closely spaced fractures in a horizontal well

Abstract

Stage- and cluster-spacings have been increasingly reduced to place multiple closely spaced hydraulic fractures (HFs) in horizontal wells. However, whether these tightly-spacing fractures can be effectively created under the condition of strong stress interference remains unclear, especially with the lack of relevant experimental investigations. In this study, a novel experimental process was proposed to model the simultaneous and sequential propagation of multiple closely spaced fractures. Rock splitting and 3D reconstruction technology were used to characterize HF geometries. The effects of the number of clusters per stage, stage spacing, and net pressure in the previously created fractures on the propagation geometries were investigated in detail. The results show that closely spaced perforation clusters in a stage tend to be unevenly and asymmetrically initiated. In the simultaneous propagation of two closely spaced HFs, one dominant fracture may occur, but the suppressed fracture will likely coalesce with the predominant fracture. Simultaneously propagating fractures tend to create multiple sub-fractures by communicating bedding planes (BPs), which is accompanied by a serious oscillation in the injection pressure. In sequential propagation, the observed phenomenon demonstrat that the net pressure in a previously created fracture can alter the stress field near the perforation cluster in the subsequent stage. Affected by the local stress conditions, the subsequent fracture would probably initiate and propagate at a nonorthogonal angle to the horizontal wellbore and intersect with the previously created fracture. The experimental findings provide both experimental proof of the interference mechanism of simultaneous and sequential fracture propagation and a basis for the optimum design of hydraulic fracturing.