Abstract: Characterization of Natural Fractures in the Frontier Formation, Green River Basin, Wyoming; Application to Reservoir Simulation of Horizontal Wells

Hugo Harstad, Lawrence W. Teufel, John C. Lorenz

Significant gas reserves are present in the low-permeability sandstones of the Frontier Formation in the Green River basin, Wyoming. Successful exploitation of these reservoirs through horizontal drilling requires understanding the orientations and distributions of natural fractures. Outcrop studies, core analyses, and welltest data have been integrated to characterize these reservoirs and implement a realistic simulation of horizontal well productivity. Detailed maps of fracture systems have been constructed from outcrop pavements of the Frontier sandstones at the edge of the basin. They show relatively uniform distributions of vertical regional extension fractures and that bed thickness is the dominant geologic parameter controlling fracture behavior. Spatial frequency istribution of fractures has been derived from statistical analysis of fracture orientation, spacings, lengths, and interconnections and correlated to bed thickness. Fracture spacing and length decrease and fracture interconnectivity increases with decreasing bed thickness. A natural fractured reservoir model has been developed using this fracture relationship. This model incorporates fracture and matrix permeability determined in laboratory tests conducted at in-situ reservoir conditions. The model shows that the relationship between bed thickness and fracture frequency distribution and the contrast between matrix and fracture permeability determines the reservoir's effective transmissibility. Optimum productivity of horizontal wells has been calculated as a function of wellbore orienta ion and length relative to bed thickness to guide field development. Wellbore and welltest data provide validations for the geologic predictions of fracture characteristics and for simulation predictions of reservoir response.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France