Steam injection is the most common technique in heavy oil/bitumen recovery. However, the emission of greenhouse gases into the atmosphere, its water requirements and excessive operational cost and problems entail finding alternative solutions. One approach is combining steam and solvent injection by taking advantage of steam injection preheating the reservoir for more effective solvent recovery application. In this case, the performance of subsequent solvent injection strictly depends on the temperature and pressure in the reservoir. Recent experimental studies on superheated solvent injection showed that solvent in the gas formed near the saturation line yields an optimal recovery minimizing the asphaltene precipitation and maximizing the recovery. This paper investigates this process through a numerical modeling exercise and formulates the optimal pressure and temperature conditions for different reservoir conditions and hydrocarbon solvents.
We first report the results of numerical simulation of laboratory experiments, in which heavy oil was exposed to solvent vapour at high temperatures. To achieve these results, a radial 3D numerical model of 15x1x63 cells was constructed using a commercial numeric simulator. The injection of either propane or butane into sand packs or consolidated sandstones at elevated temperatures was simulated. A pressure-temperature sensitivity analysis was carried out for different core sizes to understand the dynamics of the gravity drainage process associated with asphaltene precipitation. Asphaltene pore plugging behaviour was modeled and diffusion of solvent into the heavy oil was analyzed to determine both ideal solvent type and optimal operating conditions for solvent injection at high temperatures.
Our preliminary results and observations showed that the solvent should be in the gas phase and its sensitivity to temperature and sample height (for effective gravity drainage) is more critical than the pressure. There also exists a critical temperature that yields amaximum recovery and this value was determined for the rock/reservoir types and solvents considered in this study. Solvents considered, i.e., propane and butane, behaved differently in terms of asphaltene precipitation and its effects on ultimate recovery. The history matching to the experimental data was achieved primarily by considering this effect.