Feasibility of Thermal Oil Recovery Processes in Offshore Heavy Oil Reservoirs: Simulations and Experiments

Abstract

Abstract The Bohai Bay Basin is located in the eastern part of North China. Within it lies one of the largest offshore heavy oil deposits in China, roughly 4 billion tons of oil in place. In order to recover sufficient heavy oil at economical rate within limited lifetime of the production platform, CNOOC launched cyclic steam stimulation (CSS) and multi-component thermal fluid (MCTF) huff and puff process on offshore oilfields in Bohai Bay. However, thes processes realize recovery factors less than 15%, which implies that >85% of the oil remains in the ground after the processes become uneconomic to continue operation. Therefore, the incentive is strong for the development of appropriate follow-up techniques that will maximize the recovery potential of and profitability from these offshore heavy oil reservoirs in Bohai Bay, China. A series of experiments were conducted to test thermal conductivity of unconsolidated sandstones, heavy oil-water and heavy oil-steam relative permeabilities used in thermal simulations. The effects on these parameters of temperature, pressure and heavy oil viscosity were also studied. Furthermore, reservoir simulations were conducted to guide the design of oil recovery processes for three heavy oil reservoirs with a range of in situ oil viscosities from 342 to 926 mPa·s by comparing different operating strategies, including steam flooding, hot water flooding, and MCTF (high pressure steam, CO2 and N2) flooding. The experimental results of the thermal conductivity varied from 0.385 W/ (m °C) to 0.488 W/ (m °C) under systems and temperatures used in this study. Higher temperature are beneficial to decreasing residual oil saturation and increasing water and oil relative permeabilities, which is the main mechanism for thermal recovery method to enhance oil recovery. The simulation results show that, for lower viscosity heavy oil, MCTF flooding as a follow-up process achieved much better performance compared to steam flooding and hot water flooding operation. However, for intermediate and higher viscosity heavy oil, steam flooding was found to be more effective.