Efficiency and Oil Recovery Mechanisms of Steam Injection into Low Permeability, Hydraulically Fractured Reservoirs

Steam injection into heavy oils has been well characterized over the last 40 years, and while steam has been injected into light oils almost as long, the mechanisms and effectiveness of this process are much less understood. When this lack of understanding is coupled with the complexities of flow in low permeability fractured reservoirs, even less is known. This study examines thermal recovery in low permeability, fractured reservoirs using thermal compositional simulation. A diatomaceous reservoir provided input for the rock model. The oil phase is represented by three pseudo-components that characterize a relatively light crude oil. Both the areal and vertical recovery efficiencies are improved for steam injection compared to water injection. The incremental recovery depends on the distribution of permeability and is greatest for homogeneous distributions. In regard to recovery mechanisms, thermal expansion of the hydrocarbon fluids accounts for over half of the incremental recovery early in the steam drive; after roughly 0.4 pore volume of steam injection (cold water basis), the incremental recovery is split equally among thermal expansion, vaporization, and oil viscosity reduction. Late time behavior is dominated by vaporization as the distillate bank breaks through to the producer. As a consequence of steam injection three separate fluid banks form: a cold water bank, a combined hot water and distillate bank, and the steam front. Hence, displacement mechanisms are substantially different from the heavy-oil situation where oil viscosity reduction and, frequently, gravity drainage are dominant. Although the study uses a diatomite rock and fluid model, this work clearly extends to include all low permeability fractured reservoirs that have low primary and waterflood recoveries. For such reservoirs, significant additional recovery is obtained by implementing a steam drive.