Theoretical Study Of In-Situ Combustion In Thick Inclined Oil Reservoirs
Authors: Acharya U.K. , Somerton W.H. 
Published: December 2007

Abstract
A two-dimensional mathematical model has been developed to simulate in-situ combustion in oil reservoirs. The model represents a vertical cross-section of the reservoir and considers the multiphase flow of fluids, heat transfer mechanisms, oxygen transport, combustion processes and temperature dependence of fluid properties. The model also includes gravity and capillary pressure effects.
The model has been numerically solved using SIP. Parametric study for reservoir dip, anisotropy and injection rate have given results that are qualitatively reasonable.

Introduction
A number of papers have appeared in the literature describing mathematical models of the in-situ combustion process of recovering oil from subsurface reservoirs. Of these, the work of Gottfriedis probably best known. The combustion recovery process, being extremely complex in nature and not being completely understood, is difficult to model numerically. Simplifying assumptions have been made which usually limit the application of the models to special cases.
One of the more difficult problems in California is the usually great thicknesses of producing formations. This coupled with the high viscosities of the oils leads to serious gravity override problems in many cases. Modeling this effect requires problems in many cases. Modeling this effect requires the use of three-dimensional models which properly account for gravity as well as viscous and capillary forces. Such models are difficult to formulate and require large computer storage capacities and long running times.
The model developed in the present work represents a vertical section through a reservoir in which gravity override may be studied. Parameter studies were made which included dip angle, permeability anisotropy, and injection rate. permeability anisotropy, and injection rate. Development of the mathematical model, the finite difference scheme and the solution techniques are reviewed and the results of the parameter studied are presented graphically and their significance discussed in the following.

THE IN-SITU COMBUSTION PROCESS
In developing the mathematical model, it was first necessary to describe the in-situ combustion process as it is currently understood. The process process as it is currently understood. The process is complex involving the multiphase flow of gas, oil and water, the transfer of heat by conduction and convection, oxygen transport by diffusion and convection, and the chemical kinetics involved in the exothermic reactions that generate heat. A significant quantity of heat is also transferred by phase changes taking place between water and steam. phase changes taking place between water and steam. Following is a brief description of the sequence of events which are believed to occur in in-situ combustion recovery of oil.
Before any combustion process is started air is initially flowed through the oil reservoir in order to establish some permeability (conductance) to the gas phase. Subsequently, heat is applied to the formation through the injection well either by means of a downhole heater or by some chemical reaction. Once the formation is heated to combustion temperatures the external heat supply is cut off; oxygen from the injected air sustains the exothermic reaction. A fraction of the oil in place burns to produce heat. The heat generated is transported by produce heat. The heat generated is transported by means of the following mechanisms:static stress redistribution around an excavation is analyzed in FLAC;data for coupling analysis, such as non-uniform velocity field (based on the new developed velocity model), mesh, absorbing boundaries, etc., are generated by FISH functions in FLAC and passed to SPECFEM2D; andby the phase changes taking place between water and steam.
The amount of heat transferred by phase changes is perhaps the most significant of the three modes listed above.
The increases in temperature reduce the viscosity of oil and water and their mobilities are thus greatly improved. A water bank forms and moves forward in the reservoir followed by an oil bank.