Feasibility Study of In-Situ Combustion in Naturally Fractured Heavy Oil Reservoirs

Abstract

Abstract In-situ combustion is a thermal recovery method used for enhanced heavy oil recovery. In this process air is injected to the reservoir in order to achieve ignition and to maintain the combustion front while pushing the heated oil toward producing wells. This study deals with the feasibility of in-situ combustion process in fractured heavy oil reservoirs. A one dimensional, three-phase in-situ combustion simulator with six components, two cracking and three oxidation reactions is used in this study. Primarily, a conventional simulation model based on experimental data available in the literature was constructed and sensitivity study tests were performed. In the second part of this project, the conventional model was modified to a fractured model and various parameters and mechanisms such as oil recovery factor, average temperature of the system, cumulative oil and water production, diffusion, and wet combustion process were investigated. Results indicate the importance of grid block size, injection rates, kinetic models, and equilibrium ratios of heavy and light oil components on simulation process. Simulation results indicate that the optimum water/oil ratio leads to an increase in the amount of oil recovery and a reduction in the amount of air to be injected. The study presented here with its promising outcome is a pre-requisite to justify laboratory experimental investigation of the in-situ combustion process in naturally fractured reservoirs. Introduction In-situ combustion is a complex Enhanced Oil Recovery (EOR) process normally suitable for medium to heavy crude oils. The process involves all the complexity associated to the multi-phase fluid flow through porous media with chemical and physical transition of the crude oil components under high temperature and high pressure conditions. The process becomes further complex when it is aimed for the heavy oil recovery from naturally fractured reservoirs. Because of the complexity involved, its field application has been limited and difficult to handle. With the inevitable peak oil in horizon and the rising global demand for crude oil, renewal interest has been generated toward heavy oil reserves that thermal recovery techniques are mostly suited for their recovery. Conventional production methods are not suitable for heavy oil reservoirs and technological advancements are needed to make heavy oil deposits a more viable resource. Advanced technologies are essential to enable production, transportation, and refining of heavy crude oils at a reasonable cost. This is essential to make the share of heavy oil production to levels greater than the current 10% of overall crude oil production. There are several giant heavy oil reserves in the world such as heavy oil deposits belt encountered in a 700 kilometers long by 60 kilometers wide along the Orinoco River in eastern Venezuela. The middle east region has 36% of the world's heavy oil deposits followed by the United States with 11% and Russia with 6%. Table 1 presents the major deposits of heavy oil and tar sands in the world1. In-Situ Combustion Process To appreciate the need for further investigation of the process, it seems useful to make a brief review of the process and the improvements achieved over a period of more than half a century. In-situ combustion is a thermal recovery technique in which a small fraction of the heavy end of the crude oil is burned to create the heat needed to raise the temperature of the reservoir and the crude. Since the viscosity of the crude oil reduces exponentially with temperature, the process helps the crude oil to flow more readily from the rock into the production well. Upgrading of crude oil because of thermal and catalytic cracking is another major phenomenon occurring during the process that facilitates the flow of crude oil through the rock. In-situ combustion has gone through a wide range of variations and improvements due to the needs encountered in its application to various types of reservoirs and crude oils involved.