Summary

Natural fractures are present in all unconventional reservoirs in the Permian Basin and have the potential to impact completion operations and hydrocarbon production. We present an overview of natural fracture and in-situ maximum horizontal stress (SHmax) orientations in unconventional reservoirs in the Permian Basin and discuss the spatial and vertical heterogeneity in orientations across the Midland and Delaware Basins.

Fracture characterization using image logs and cores have been performed on ~60 wells to identify fracture orientations, height, fracture fill, aperture, and stress orientations. Orientations measured from the image logs are used to determine regional fracture trends, and calculated fracture densities highlight the difference in fracture occurrence along wellbores. SHmax orientation, determined from induced fractures seen in image logs, is compared to observed fracture trends and used to predict hydraulic fracture orientations.

Short, calcite-healed, strata-bound fractures are the dominant type within the basin, regardless of location or formation. Open fractures are far less common and have similar dimensions and orientations as the healed ones. In general, fracture densities increase with depth with the highest values in the Wolfcamp Formation.

Despite similarities between fracture character in the Delaware and Midland Basins, differences in fracture orientation exist. The Delaware Wolfcamp typically has two orthogonal fracture sets, and all fracture trends rotate throughout the basin. This rotation is systematic with a NE-SW striking trend in the northern portion of the basin, an E-W striking trend in the central part of the basin, and a NW-SE striking trend in the southern portion of the basin. Throughout the basin, the dominant fracture trend is parallel to present-day SHmax, which has a similar systematic rotation. In contrast, the Midland Basin hosts a single consistent NE-SW fracture strike with no observed rotations, while the present-day SHmax orientation trends E-W, oblique to the natural fracture trend.

A plausible cause for the different spatial character of both the natural fractures and in-situ stress trends between the Delaware and Midland Basins is the Central Basin Platform, likely acting as a mechanical barrier, separating the two basins' tectonic and burial histories.

This data set underpins a growing understanding of stress and fracture trends in the Permian Basin, and their relationships. These regional trends can be used to guide well planning and completion strategies as well as field development decisions. Because of the regional variations and heterogeneity of fracture and SHmax trends, Permian Basin wide generalities are not advised.

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