Modeling simultaneous growth of multiple hydraulic fractures and their interaction with natural fractures

Abstract

Abstract Hydraulic fracture diagnostics have highlighted the potentially complex natural of hydraulic fracture geometry and propagation. This has been particularly true in the cases of hydraulic fracture growth in naturally fractured reservoirs, where the induced fractures interact with pre-existing natural fractures. A simplified numerical model has been developed to account for mechanical interaction between pressurized fractures, and to examine the simultaneous propagation of multiple (>2) hydraulic fracture segments. Fracture intersection is presumed to communicate the hydraulic fracturing fluid to the natural fracture, which then takes up the continued propagation. Simulations for multi-stage horizontal well treatments and single stage vertical well treatments show that fracture pattern complexity is strongly controlled by the magnitude of the hydraulic fracture net pressure relative to the in situ horizontal differential stress as well as the geometry of the natural fractures. Analysis of the neartip stress field around a hydraulic fracture also indicates that induced stresses may be high enough to debond sealed natural fractures ahead of the arrival of the hydraulic fracture tip. Introduction Complex hydraulic fracture geometry has become more evident with the widespread application of improved fracture diagnostic technology.1 Multi-stranded fracture propagation from vertical wells has been confirmed by coring,2,3 while microseismic data in naturally fractured reservoirs such as the Barnett Shale suggests significant diversion of hydraulic fracture paths due to intersection with natural fractures.1 Apparent interaction between a propagating hydraulic fracture and pre-existing natural fractures seems to be the key component explaining why some reservoirs exhibit more complex behavior.4 There are several possibilities for the interaction between hydraulic and natural fractures. The likelihood of intersection between a hydraulic and natural fracture is partly a function of orientation. If the hydraulic and natural fracture directions are parallel, intersection is less likely, but there can still be interaction between close, en echelon overlaps of fractures, and the natural fractures may be reactivated by being within the process zone (region of altered stress) around the crack tip. If the natural fractures are orthogonal to the present-day hydraulic fracture direction, the propagating hydraulic fracture is likely to cross a large number of natural fractures as it propagates through the reservoir. For these cases of direct intersection, the hydraulic fracture could propagate across the natural fracture plane without deviation and without additional leak-off, a possible outcome for strongly cemented natural fracture planes. Even if the hydraulic fracture propagates across the natural fracture, the stress induced by the hydraulic fracture could open the natural fracture enough for it to divert fracturing fluid and increase the leak-off. If the fluid diverted into the natural fracture becomes significant, the natural fracture could start to propagate, creating a new strand of the fracture that could equal or eclipse the initial hydraulic fracture wing. A more extreme interaction would be where the main hydraulic fracture wing is arrested by its intersection with the natural fracture in a T-intersection. The preliminary model described here focuses on the consequence of direct T-type intersections between the hydraulic fracture.