MODELING OF NON-THERMAL RECOVERY OF HEAVY OIL BY CYCLIC SOLVENT INJECTION

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There is a considerable push to develop heavy oil using low Green House Gases (GHG, non-steam) processes. This can be achieved using solvent, which may be particularly relevant in the heavy oil fields in Alberta and Saskatchewan that have previously undergone Cold Heavy Oil Production with Sand (CHOPS). Primary production from these reservoirs leads to recovery of 5 10% of Original Oil in Place (OOIP) so there are significant volumes of oil still present, and without the ability to inject steam in post-CHOPS fields this makes them excellent candidates to develop non-thermal solvent recovery processes. Cyclic Solvent Injection (CSI) is a single well process, whereby a vapour phase solvent is injected to pressurize the reservoir. At elevated pressures solvent dissolves into the heavy oil and reduces its viscosity. The well is then placed on production, so well pressure drops and provides a driving force for solvent-diluted oil to flow. Several field pilot studies of this process are already underway. The challenge to CSI is that as the production well pressure drops, solvent comes out of solution and oil viscosity will increase again. Production is a balancing act between achieving flow and keeping gas in solution. Intuition suggests the potential for significant incremental oil production, but what is not defined is whether this process can be commercial. By understanding the physics of CSI (i.e. what controls oil production), and being able to model the process properly, this can help to provide insights into the future viability of this process on a large scale. In our past experience in lab-scale core floods, CSI recovery is low after primary production and the pressure decline rate needs to be increased in each successive cycle. In the field, pressure drawdown rates are much slower than in the laboratory, so there are physics present that are not being captured yet in lab studies. The objective of this work is to determine what those physics are, and if there are operational parameters that can be controlled to improve recovery from CSI. The first part of this program focuses on understanding what mechanisms are important for oil recovery from repeated CSI cycles. Lab tests of CSI generally start after a much higher primary production than what is seen in the field. In addition, core floods generally do not consider the effects of gravity on CSI performance. Finally, PVT tests on oil-solvent systems do not properly represent the non-equilibrium response of solvents leaving solution in oil as a function of pressure and time. Different solvents may also have different non-equilibrium effects, so they may respond differently during depressurization (production) cycles. This program will run experiments to study all of these parameters, and model CSI production capturing the proper recovery mechanisms.