Cold Heavy Oil Production with Sand (CHOPS) is widely used as a primary non-thermal production technique in thin heavy oil reservoirs in Western Canada and the Orinoco Heavy Oil Belt in Venezuela. Several solvent and hybrid steam/solvent schemes have been proposed to increase the recovery factor from these deposits. Development of the complex wormhole networks renders the scalability of these processes from laboratory measurements to field applications challenging. In this paper, numerical simulation is used to analyze how scaling of solvent transport and dispersion would vary with developed wormhole characteristics. It proposes a practical workflow to a scale up these mechanisms for field-scale simulation.
First, a series of mechanistic compositional simulation models at the lab scale is constructed to model a cyclic solvent injection scheme (CSI). These models are calibrated against experimental measurements of solvent diffusion measured in porous media. Next, a set of detailed high-resolution (fine-scale) simulation models, where both matrix and high-permeability wormholes (modeled as fractal networks) are represented explicitly in the computational domain, is constructed to model how the solvent propagates away from the wormholes and into the bypassed matrix. Flows of solvent and oil in the matrix and wormholes are directly simulated. Following this, a dual-permeability approach is adopted to facilitate the scale-up analysis, where wormhole intensity is correlated to shape factor and apparent dispersivity. Characteristics at different averaging scales (i.e. scale-up level) are examined. Field-scale simulation are constructed using average petrophysical and fluid properties extracted from several CHOPS reservoirs in Saskatchewan, which are, to some extent, similar to those found in the Orinoco Belt. The initial conditions in terms of fluid saturations, pressure distribution and wormhole development are representative of those commonly encountered at the end of CHOPS.
Solvent transport and mixing in the wormhole networks can be captured by parameters such as shape factor and apparent dispersivity in an equivalent coarse-scale dual-permeability system. Effective dispersivity increases with averaging scale and wormhole intensity. Considering identical surface solvent injection rate, effective dispersivity would enhance oil production and reduce gas production due to an increase in mixing between solvent and oil. Several solvent injection blends are evaluated to maximize recovery efficiency.
Field-scale simulations are typically performed with grid block sizes that are much larger than the wormhole scale, and numerical analysis is often performed by arbitrary adjustment of dispersivity. This work offers a practical way to scale up solvent transport mechanisms in post-CHOPS applications. It facilitates more efficient and accurate assessment of solvent transport from lab measurements to field applications. This work serves as a starting point for formulating a systematic workflow to simulate solvent processes in wormhole networks that span over multiple scales.