Study of Heavy Oil Recovery from a Fractured Carbonate Reservoir Using in Situ Combustion

In spite of its strategic importance, the topic of recovery of heavy crude oils from fractured carbonate reservoir has not been extensively addressed. Thermal methods seem well suited for this kind of problems, particularly in situ combustion has shown promising results in laboratory experiments. Extensive work has been done on development of thermal process simulator but for the in-situ combustion applied specially in fracture reservoirs where one is dealing with multi-scale multi-process problem, many unknowns are still exist. The recovery mechanism, reservoir and operational conditions on which the combustion can propagate in fractured systems are not enough clear. Also due to safety issues, air injection required careful assessment of the reservoir displacement mechanisms in particular the magnitude and the kinetics of matrix-fracture transfers. To understand the mechanism of heavy oil recovery from a fractured reservoir we propose the development of a numerical simulation strategy, starting from existing simulation tools that are adapted to this particular problem. This will allow firstly understanding the role of each driving mechanism and physical as well as operational parameters in recovery process and secondly choosing the suitable up-scaling method. The study is based on the fine grid, single porosity, multi-phase and multi-component simulation of a core surrounded by two parallel air invaded fractures using the thermal simulator. Firstly the simulator is validated for different processes: one and two dimensional diffusion and one-dimensional combustion are compared with the corresponding analytical solutions. The two-dimensional combustion is validated using experimental results available in the literature. The simulation results predict the conditions on which the combustion is sustained in the fractured reservoir as a function of oxygen diffusion coefficient, injection rate, and the permeability of the matrix. Oil production via natural drainage, hot fluid injection and in-situ combustion are compared to address the importance of different driving mechanisms. At the block scale the effect of fracture spacing, heterogeneity in the matrix and the grid size on the propagation of combustion and the oil production are studied and then a suitable up-scaling procedure is proposed.