Abstract

Cyclic solvent injection (CSI)—a non-thermal process option for development of post-CHOPS (cold heavy oil production with sand) heavy oil reservoirs—works by dissolving a gaseous solvent, mainly Methane (CH4) or Carbon Dioxide (CO2), and sometimes along with Propane or Butane into the in-situ heavy oil. A foamy oil is generated in-situ and produced during the production cycle due to pressure depletion. The generation and stability of foamy oil are considered important contributing factors to increased recovery from both solution gas drive and viscosity reduction mechanisms. This study used an innovative automatic flow loop apparatus to study questions about the pressure and gas type/concentration window for foamy oil generation and stability. The flow loop was applied to accurately monitor these phenomena through a number of property measurements at continuous flow conditions. Fluids inside the apparatus were automatically flowed back and forth between two upright Isco syringe pumps controlled by a LabVIEW data aquistion and control interface. The live oil in the system was depressurized either instantaneously or at a certain pressure drawdown rate to initiate foamy oil. Then, constant-volume pressure rebound (CVPR) tests and constant-pressure volume expansion (CPVE) tests were conducted, while density, viscosity, volume, and quality of the foamy oil phase were automatically measured and logged. The test results obtained with two separate gases, CH4 or CO2, revealed several fundamental mechanisms of foamy oil behaviour. Foamy oil stability was shown to be a complex function of live oil properties, solution/injection gas type, pressure drawdown rate, and shear rate. In the CVPR tests, when crude oil was saturated with CH4 or CO2 at the same bubblepoint pressure, the same pressure depletion decrement resulted in higher rebound pressure and longer time reaching stabilization for CH4 than CO2, due to slower CH4 bubble nucleation and release from the foamy oil phase. By contrast, the CPVE tests showed that the volume and viscosity increase ratio of the foamy oil phase compared to the original live oil were strongly affected by depletion pressure and drawdown rate. This indicates that, during a CSI process, there is an optimized window of pressure depletion rate and decrement that creates stable foamy oil at non-equilibrium conditions.

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