Increases in production from unconventional reservoirs can be attributed, in some instances, to the effect of well spacing and induced fracture placement. A firm understanding of the abundance and orientation of natural fractures is often limited, yet it is important to the characterization of unexpected levels of recovery. The uncertainty in the characterization of natural fracture density and orientation is typically high; consequently, value exists in understanding the mechanics and distribution of natural fractures, given plausible subsurface structural control mechanisms.
Post-faulting stress rotation is considered here to be the mechanism that enables non-parallel orientation of natural fractures coincident to the fault damage zone. Disparate fracture orientations occur in the fault process zone exclusively as a function of energy release rate. A fault likelihood attribute is used to spatially characterize natural fractures, considering the existence of a correlation between faulting and the formation of natural fractures. Subsurface consistency between the flow simulation model and the seismic attributes is then achieved by way of a petro-elastic model embedded in the reservoir simulator. After computing time-dependent elastic properties, their dimensionless representation is used to characterize matrix fracture interaction.
Primary fracture plane parallelism with respect to the fault damage zone is anticipated when the existing stress field simultaneously dictates the formation of faults and fractures. Fracture plane orthogonality or any off-axis orientation relative to the primary axis of faulting represents fracture propagation away from the strongly non-proportional plastic loading zone post-stress field re-orientation. This fracture plane re-orientation is potentially considered as a function of the stress field orientation after the development of faults; therefore, it can demonstrate variability in its azimuthal orientation. An observed correlation between the pressure profile and the computed dimensionless elastic property suggests the potential for pre- and post-production seismic surveys to better facilitate overall asset management and predictive capabilities.
Contemporaneous efforts to extend the profitability of unconventional reservoirs have extensively focused on the applicability of enhancements to permeability by way of induced hydraulic fractures in extremely low permeability environments. These induced fractures interact in-situ with natural fracture systems of varying degrees of conductivity to form complex conduits for flow characterized by increased flow capacity. Given limited understanding of natural fracture systems and the need for reduced interpretation and expedited execution cycles, the natural fracture systems are often mis-characterized or negated at the behest of operating schedules, limited technology, and budget restrictions. This, however, is contradictory to established value statements attributed to natural fractures systems. This value is evident in the work of Aguilera (2008) in which the value of systematic characterization of the location, orientation, and density of natural fractures, in addition to the stress field, is averred in the context of developing completion and well programs to maximize reservoir productivity. In its absence, the end result is typically an inadequate ad-hoc model description of production, comprised of data omission or some esoteric representation of loosely understood phenomena, leading to increased prediction uncertainty.
