Many unconventional reservoirs contain natural fractures. These fractures may be non-conductive but open preferentially during hydraulic fracturing treatment, or they may be conductive prior to treatment and provide an enlarged tributary drainage volume with different lateral extents than those suggested by conventional models of unconventional reservoirs.
This paper presents a Discrete Fracture Network (DFN) study of gas production from an Eastern unconventional reservoir that contains pre-existing, conductive fractures. The natural fractures are known through a combination of innovative flow logging during drilling, image logging of the wells, and Tomographic Fracture Imaging™ (TFI). Chemical frac-tracer monitoring confirms that a natural fracture network accesses a considerably larger volume of rock than the microseismic data alone would indicate.
The results of these methods provide the basis for constructing a discrete fracture network model that honors the conventional microseismic data, the flow logs, and the TFI fractures. Simulations of gas production from this network model show that, although the major portion of production comes from the hydraulic fractures and nearby closely-spaced natural fractures, the tributary drainage volume of the well extends well beyond the footprint of the hydraulic fractures themselves.
The role of natural fractures in unconventional reservoirs has been a topic of considerable discussion and uncertainty. In some cases natural fractures may be non-conductive but may preferentially open and control the propagation of hydraulic fractures. In other cases the natural fractures may be conductive and provide some component of production in addition to hydraulically-stimulated artificial or natural fractures. The economic development of unconventional resources involves efficient and relatively rapid drilling and completion strategies that do not allow time for extensive characterization of the wells. Highly characterized wells that provide validation for conceptual models of unconventional production are relatively uncommon. The Mallory 145 well pad is one of these sites, and while the results should not be considered exemplary of unconventional reservoirs overall, they do provide significant insights into how an unconventional system may behave. Some aspects of the Mallory 145 experiment are described in Mulkern et al, 2010; Franquet et al, 2011; Moos et al, 2011; Geiser et al, 2012; Lacazette and Geiser, 2013; and Lacazette et al, 2013.