Experimental investigation of hydraulic fracture propagation in fractured blocks

Abstract

Natural fractures in reservoirs can be the cause of many adverse effects during hydraulic fracturing treatment. In the present paper, hydraulic fracturing tests are used to investigate the interaction of a propagating hydraulic fracture with a natural fracture in the fractured blocks. Systematic experiments were designed and performed on the cement blocks with different pre-existing fracture strikes and dips (30°, 60° and 90°). The effect of horizontal stress difference on the propagation of hydraulic fractures was also determined through a series of experiments with different values for Δσ, which were 5 and 10 MPa, respectively. Propagation arrest of the hydraulic fracture and crossing the pre-existing fracture were two dominating fracture behaviors at horizontal stress differences of 5 and 10 MPa, respectively. It was observed that both the magnitude of differential stress and the pre-existing fracture geometry can magnify the effect of a pre-existing fracture on hydraulic fracture propagation. When the horizontal differential stress is low (5 MPa), the hydraulic fracture crosses the pre-existing fracture at a high pre-existing fracture dip (90°) and at an intermediate to high pre-existing fracture strike (60°–90°), while hydraulic fracture is arrested by the opening of the pre-existing fracture at a pre-existing fracture strike (30°). Meanwhile, when the pre-existing fracture dip is low to intermediate (30°–60°) and its strike is low to high (30°–90°), hydraulic fracture is arrested by the opening and shear slippage of the pre-existing fracture in this situation. However, at a high horizontal differential stress (10 MPa), when the pre-existing fracture dip is low to high (30°–90°), the hydraulic fracture crosses the pre-existing fracture at the strike of the pre-existing fracture of 60°–90°, and when it is decreased to 30°, the hydraulic fracture is arrested by th eopening and shear slippage of the pre-existing fracture. Therefore, the pre-existing fracture’s strike and dip play a significant role in the propagation of hydraulic fracture at a low horizontal stress difference, while the role of the pre-existing fracture dip at a high horizontal stress difference is less than the pre-existing fracture strike on the propagation of hydraulic fracture.