The Effect of Natural Fractures on Hydraulic Fracture Propagation

Abstract

Abstract Various fracture interaction criteria were reviewed and a systematic study is presented to analyze the effect of natural fractures on hydraulic fracture propagation. From the literature review, the approach of Warpinski and Teufel was adopted to evaluat the fracture propagation that would occur after a hydraulic fracture intersects an existing natural fracture. The study determined the conditions of differential stress, angle of intersection, and fracture toughness for which dilation of the natural fracture would occur. Criteria for the subsequent modes of fracture propagation are then presented. Introduction Hydraulic fracture propagation in the presence of natural fractures is substantially different from fracture propagation in reservoirs without natural fractures. Conventional hydraulic fracture design is based on the assumption that the rock is homogenous and the fracture propagates symmetrically in a plane perpendicular to the minimum stress. In naturally fractured reservoirs due to interaction with natural fractures, the fracture may propagate asymmetrically or in multiple strands or segments. The presence of natural fractures alters the way the induced fracture propagates through the rock. Experimental investigations[1–3] have shown that the propagating fracture crosses the natural fracture, turns into the natural fracture, or in some cases, turns into the natural fracture for a short distance, then breaks out again to propagate in a mechanically more favorable direction, depending primarily on the direction of the natural fracture relative to the stress field. Laboratory studies suggest that hydraulic fractures tend to cross the existing fractures at high differential stress and high angles of approach and the direction of induced fracture propagation is near perpendicular to the natural fracture. At low angles of approach and low differential stress, the natural fracture opens, diverting the fracturing fluid and preventing the induced fracture from crossing, at least temporarily. This paper reviews several experimental and theoretical studies and discusses different fracture interaction criteria developed in the past. A study is presented based on an idealized interaction model to investigate the effect of a natural fracture on hydraulic fracture geometry based primarily on the differential stress and the angle of interaction. Review of Theoretical and Experimental Studies Several field and lab experimental studies have been done in the past to investigate the effect of natural fractures on the propagation of an induced hydraulic fracture. This section reviews all the results and conclusions of those studies. In 1963, Lammont and Jessen[1] conducted a series of laboratory experiments on six different types of rocks. These experiments were run under triaxial compression up to 1142 psi and with angles of approach, ? (Fig. 1), between the hydraulic fracture and pre-existing fracture varying from 30º to 60º. They found that in all successful cases the induced fracture was able to cross the existing fracture and the orientation of the hydraulic fracture was such that it turned and intersected the existing fracture at right angles. A similar behavior was observed when the hydraulic fracture departed from the existing fracture. It has been observed that the location of the point of exit on the existing fracture was random and it was not controlled by the stress concentration at the end of the fracture but rather by some particular weakness in the rock matrix.