This paper evaluates fundamental differences in depletion performance of wells producing from ultra-low permeability1 ("shale") reservoirs and wells producing from conventional reservoirs, with the ultimate goal of defining what differentiates "conventional" from "shale" performance. It attempts to answer the question – what permeability range defines "ultra-low" depletion performance and what permeability range defines "conventional" depletion performance – with the intermediate permeability range being a transition from ultra-low to conventional behavior. We "map" the transition from conventional depletion performance to "shale" depletion performance in terms of formation permeability.
In this study we compare depletion performance for reservoir permeabilities ranging from 10 nD to 100 md, with PVT, relative permeability functions, and other rock properties being the same for all simulation cases. Four reservoir fluid systems, ranging from rich gas condensate, near-critical oil to light oil, were used in this study. A one-section drainage area was used even though we are aware that shale resources are often developed with smaller well spacing (typically 80- or 160-acre strips, ~ one mile long).
Depletion performance of conventional gas condensate and oil reservoirs - where oil recovery and producing gas-oil ratio (GOR) are independent of permeability and flowing BHP, is valid for k > 0.1 md. At permeability levels of ~ 0.001 md (1000 nD) or less, the depletion performance of shale and ultra-tight reservoirs is characterized by that producing GOR is a strong function of flowing bottomhole pressure (BHP) and degree of undersaturation (Whitson and Sunjerga, 2012).
Our results from this study show that conventional reservoir performance, depending somewhat on the reservoir fluid system, is observed for k > 0.5 md, while ultra-tight "shale" performance is found for k < 1000 nD (0.001 md), with a gradual transition between these permeability values and shale-like depletion performance appearing already at 0.01md.