This study examines the role of natural fractures on water and oil production from the Wolfcamp Shale in the West Texas Delaware Basin, one of Shell's largest unconventional developments. Individual wells have sustained production rates of 400-500 barrels of water per day and 100-200 barrels of oil per day, indicating the effective permeability of the stimulated and naturally fractured reservoir is several orders of magnitude greater than the matrix permeability of 20-35 nanodarcies. This study compares the fracture intensity in 6,500 feet of vertical whole core from 16 wells, with water-oil ratios from the nearby horizontal wells. Results show no correlation between natural fracture intensity and production, and that well and reservoir performance in this case are relatively independent of fracture intensity. Individual cores may not be representative of the reservoir, and/or other factors such as fluid pressure, saturation, aquifer drive mechanism and strength, and/or completion variables, are more important factors than fracture intensity in terms of production.
In hydraulically fractured reservoirs, the effective permeability of the reservoir can be orders of magnitude greater than the matrix permeability (Figure 1). Similarly, natural fractures are assumed to have a similar impact on performance, as they can be important controls on natural porosity and permeability at the scale of well drainage areas (Gross et al., 2009; Strickland et al., 2013; Ferrill et al., 2014a).
This increase in effective permeability from fractures can have a positive effect on oil production such as in the Niobrara Formation (e.g., Sonnenberg, 2011), or it can have a negative effect by increasing water production, such as in the Mississippi Lime and Barnett Shale (Loucks et al., 2017).
Characterizations of fracture intensity are thus considered critical to understanding and predicting well and reservoir in fractured reservoirs (Strickland et al., 2013; Ferrill et al., 2014b; Engelder et al., 2009; Bertotti et al., 2007; Gross et al., 2009). In cores and many outcrops, the variations in fracture intensity are quantified through detailed measurements of fracture geometries, spacings and apertures along scan lines of cores and outcrops (Gross et al., 2009; Strickland et al., 2013). In many cases, the resulting variations in fracture intensity correlate with sequence stratigraphic units, enabling the variations in fracture intensity observed in a well to be extrapolated over large areas based on the sequence stratigraphy. These variations in fracture intensity are assumed to relate to variations in well and reservoir performance (Gross et al., 2009; Strickland et al., 2013).