This paper considers the mechanisms and characteristic flow patterns of low permeability reservoir systems. In this paper we focus on the issue of low permeability in conjunction with reservoir heterogeneity (as these often go hand in hand). Generally speaking, we focus on the single-phase gas flow case as this is most relevant — and we avoid concerns related to multiphase flow.
Low permeability reservoir systems exhibit unique flow behavior for the following reasons:
?? Low permeability (which yields poor utilization of reservoir pressure), this is caused in part by:
— Depositional issues: very small grains, mixed with detrital muds (clays).
— Diagenetic issues: clay precipitation, massive cementation, pressure compaction, etc.
?? Reservoir heterogeneity — dictated by deposition and post-deposition (diagenetic) events, including:
— Vertical heterogeneity: layering, laminae, etc.
— Lateral heterogeneity: medium to large scale geologic features (e.g., turbidite deposition, faults, etc.).
— Differential diagenesis, including hydrocarbon generation and migration.
These characteristics lead us to the relatively simple observation that low permeability reservoirs are simply poor conductors of fluids. As a matter of background, this work discusses the issues relevant to the origin of low (and ultra-low) permeability reservoirs, but our primary focus is flow at macro- and mega-scales (as would be observed at a well). An obvious comment at this point is that the reservoir permeability and the reservoir heterogeneity are fixed constants that we cannot change. While true, we can change our mechanism for accessing the reservoir (i.e., the well) and we can change our development strategy to ensure optimal performance and recovery of a particular reservoir.
As for changing our access to the reservoir, we can utilize hydraulic fracture stimulation techniques to create a conductive pathway into the reservoir from the well. This is and will be implicit in the continued development of low and ultra-low permeability reservoirs — regardless of the well type (vertical or horizontal). In this work, our emphasis is to consider the relatively simple case of a single vertical well with a hydraulic fracture and the resulting flow behavior that this type of well will experience. It is our contention that the elliptical flow regime dominates reservoir performance in low/ultra-low per-meability reservoirs, and we apply both analytical and numerical solutions to a typical field case to illustrate the validity of the elliptical flow regime.
Literature General/Reservoir Engineering:
As a general reference on reservoir engineering, the reader is directed to Dake (2001). In this reference Dake is prone to mix reservoir engineering with philosophy. In a typical scenario, Dake challenges the reservoir engineer to consider that the ability to model the reservoir system should not imply that the engineer truly understands the processes in the reservoir. Rather, such understanding is achieved through the interpretation of well/reservoir performance and material balance. This reference is particularly useful in terms of such fundamental flow behavior, but it is not a specialized text for well performance or geology/petrophysics.