Liquid-Rich Shale Versus Conventional Depletion Performance

Abstract

This paper evaluates fundamental differences in depletion performance of wells producing from ultra-low permeability 1 ("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 performancewith 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.

Experimental Study of Permeability Decline in Tight Formations During Long-Term Depletion

“…In conventional reservoir wells, the depletion behavior can be correlated with volumetric average reservoir pressure, with little or no dependence on the reservoir permeability and flowing bottom-hole pressure (BHP). For tight reservoirs, however, the concept of volumetric average reservoir pressure and hydrocarbon recovery factors present less value (Whitson et al, 2014) and hence the characterization of depletion performance is notably complicated. Although the mechanisms of different types of tight reservoirs depletion are not fully understood, the field production data in recent years suggest somewhat unique behavior, as noted by various authors (Baihly et al, 2010, Whitson et al, 2014.…”

“…For tight reservoirs, however, the concept of volumetric average reservoir pressure and hydrocarbon recovery factors present less value (Whitson et al, 2014) and hence the characterization of depletion performance is notably complicated. Although the mechanisms of different types of tight reservoirs depletion are not fully understood, the field production data in recent years suggest somewhat unique behavior, as noted by various authors (Baihly et al, 2010, Whitson et al, 2014. Figure 1 shows the overlay of gas production rate of different tight reservoirs in the first 5 years (Baihly et al, 2010).…”

Three-Phase Flow Simulation in Ultra-Low Permeability Organic Shale via a Multiple Permeability Approach

“…Understanding the complex mechanism of multiphase flow plays a key role in improving the recovery of such reservoirs. Modeling studies and field performance are still key factors to estimate and forecast the production of liquid-rich shale reservoirs (Whitson et al 2014).…”