Often, a key factor in the successful hydraulic fracture stimulation of unconventional reservoirs is the opening or shearing (and later extension) of natural fractures or weakness planes around a created hydraulic fracture. The behavior of natural fractures, or weakness planes, in response to hydraulic fracture stimulation can be complicated. Furthermore, the stimulation of these fractures and weakness planes is dependent on several critical, in-situ conditions that can increase (or decrease) the contribution of natural fractures and weakness planes to well production. The optimal economic completion, then, requires considering these factors during both stimulation design and post-stimulation evaluations.
The simplistic, and traditional, assumption that hydraulic fractures are bi-wing, planar and symmetric around the wellbore has tended to bias the interpretation of different aspects of the stimulation process. However, hydraulic fracture monitoring methods, such as microseismicity, pressure evaluations, and the coring through of hydraulic fractures, have confirmed the complex nature of fracture propagation in unconventional plays, often due to the presence of natural fractures and weakness planes. Therefore, an improved consideration of natural fracture and weakness plane behavior during hydraulic fracturing will result in a better understanding of fluid treating pressures and hydraulic fracture geometry, which will help lead to more accurate estimations of production for unconventional plays.
In this paper, the results of an extensive parametric study of in-situ stress conditions, in-situ pressure, natural fracture mechanical properties (cohesion and friction angle) and characteristics (joint orientation and initial aperture), and different operating conditions (single stage, simultaneous hydraulic fracture stages, and sequential hydraulic fracture stages) on injection (net) pressure behavior is presented. The results were generated using a 2-D distinct element model and capture the important role that, for example, initial natural fracture aperture and in-situ pressure play in the development of hydraulic fracture injection pressures in unconventional reservoirs.