Applicability of solvent based Huff-and-Puff Method to enhance heavy oil recovery

Over and above solvent based processes, specifically, the cyclic solvent injection well known as "Huff-and-Puff', has demonstrated a significant potential to enhance heavy oil recovery. Solvent and CO2 Huff-and-Puff are analogies to cyclic steam stimulation; however, in this method, steam is replaced with CO2, hydrocarbon solvent or mixture of the two. This study attempts to validate the feasibility of the Solvent Based Huff-and-Puff Method with respect to enhancing heavy oil recovery and to investigate the effect of fluid, operation, and reservoir parameters on its' performance. Thus, both experimental and reservoir simulation approaches were applied and, the impact of the aforementioned parameters on the performance of the process was investigated. All experiments were conducted in a Berea core with the dimensions of 30.48 cm by 5.07 cm. The core has a permeability of 1800 md and a porosity of 24% which was mounted in a high pressure, stainless steel core holder. Before conducting each Huff-and-Puff Test, the core was saturated with an oil sample representative of Saskatchewan heavy oil reservoirs and exhibited a viscosity of 952 mPa.s, at a temperature of 28°C. Prior to the tests, a complete phase behavior (PVT) analysis of the oil sample and solvents mixture was conducted using CMG- WinProp(TM) software. Over 12 sets of Huff-and-Puff Experiments, utilizing the pure solvent of carbon dioxide, methane, and mixtures of CO2 and propane, were performed at different operating pressures. A soaking time period of 24 hrs and a cut-off pressure of 276 kPa were considered for all cycles. In addition, all Huff-and-Puff Cycles were continued for each operating pressure until production dropped below one percent of the original oil in place. The production trend and recovery factor for each experiment were determined. The final oil recoveries, at the highest operating pressure of 7239 kPa for pure CO2 and, at 6895 kPa for pure methane, were 71 and 50 % OOIP, respectively. As for solvent gas mixtures with different propane fractions of %28 and %19 by mole, the final oil recoveries obtained were 56 and 54 % OOIP, respectively. The measured viscosities of the produced oil (generally for higher operating pressures) indicated the produced oil is lighter, especially during the first few cycles. During the tests, it was observed that most produced oil occurred during the first five cycles. In addition, it was found that the viscosity of produced oil during the first few cycles, at the operating pressure of 7239kPa, was reduced from an initial value of 952 to approximately 62 mPa.s when pure CO2 was injected. Also, as far as the capacity of this study was able to observe, implementing a longer soaking time improved the incremental recovery of the first cycle. However, the lengthier soaking time did not noticeably change the ultimate oil recovery. Since pilot and field studies are time consuming and relatively expensive, numerical simulation was initially utilized to history match the results obtained in the laboratory experiments and later to investigate the effect of the fluid and reservoir key parameters on the performance of this technique. Based on this fact, CMG-STARS(TM) was used and the discrepancy between recovery factors obtained from the experiments and those of the numerical simulation model were found to be in the range of 4 to 10% for CO2 and Methane Huff-and-Puff Tests at different operating pressures. Also, an attempt was made to investigate the potential of this method when applied to a field scale model. Based on the results of a parametric study, a longer soaking time improved the incremental recovery. There appeared to be no significant impact of very high injection rate on the performance of the Huff-and-Puff Method when it was studied through numerical simulation.