Natural Fracture Stratigraphy of Gas Shales in the Horn River Basin, NEBC, Canada: Relation to Lithostratigraphy and Implications for Hydraulic Fracture Growth
The Horn River Basin contains a 170-190m section of gas shales in the Muskwa, Otter Park and Evie Formations. One of the key decisions to be made in development of the resource is the optimal placement of horizontal wells within the stratigraphy to adequately stimulate and drain reserves from the entire section. Understanding the geomechanical properties of the reservoir is one of the keys to informing this decision. Construction of a calibrated in-situ stress vertical profile from elastic properties shows the section can be divided into two mechanical/reservoir packages - the Muskwa/Otter Park Zone and the Evie Zone. However detailed analysis of microseismic data acquired in stages completed through the Muskwa/Otter Park zone point to a more complex mechanical stratigraphy within this zone not identified by the vertical profiles. To better understand the mechanical stratigraphy we investigated if differences in the natural fracture character between the Muskwa and Otter Park exist that may explain some of the variation in microseismic geometry observed. Observations were gathered from drill cuttings, thin section, core, image logs & outcrop to achieve this.
Multiple fracture orientations and fill types were observed suggesting a complex history. Variation in fracture intensity and height between the Muskwa and Otter Park were also observed. An attempt to correlate these variations to log-derived mechanical properties and mineralogy had moderate success but proved unsatisfactory in places, possibly due to sub-log scale influences on fracture stratigraphy. A comparison between fracture stratigraphy and centimeter scale lithostratigraphy proved more correlative. DFN-based 2D hydraulic fracture models demonstrate the impact of variation in fracture properties, such as intensity and size, on hydraulic fracture geometry. Therefore it is proposed that the vertical stacking of units with different fracture stratigraphy, as is the case for the Muskwa and Otter Park, maybe a mechanism to explain the differing vertical microseismic patterns observed from completions in either unit. Furthermore given the common presence of fractures in the Horn River and the evidence of their reactivation seen in geomechanical models matched to microseismic, it is suggested that Stimulated Surface Area (SSA) may be a more appropriate attribute to extract from microseismic data for evaluating hydraulic fracture stimulations than half-length and Stimulated Rock Volume (SRV).
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California