EVALUATION OF TOE-TO-HEEL AIR INJECTION (THAI) COMBINED WITH WET COMBUSTION

Abstract

Heavy oil cannot be easily produced by means of conventional oil recovery methods due to its extremely high viscosity. However, by utilizing a suitable Enhanced Oil Recovery (EOR) technique, heavy oil can be produced. In-situ Combustion (ISC) with modification of well configuration called Toe-to-heel Air Injection (THAI) is one of the solutions. The technique is performed by injecting air from horizontal or vertical well to create fire front and producing heated oil from horizontal wells. Further modification of THAI is performed by co-injecting water with air to generate in-situ steam to recover heat in burnt zone. In this study, combination of THAI and wet combustion is performed and parameters affecting oil recovery are studied. From the results, it is found that oxygen content is very important design parameter for in-situ combustion. As fire front must be generated, enrichment of oxygen is required and increasing oxygen through increment of oxygen concentration is more efficient than increase of air injection rate. For properties related to wet combustion, time to start injecting water plays more important role for wet combustion compared to amount of water injected as the early time to start wet combustion can speed up distribution of heat of combustion to entire reservoir. Moreover, optimal air injection rate that is obtained from dry combustion can be slightly increased in case of wet combustion as co-injection of water helps retarding air flow ability. So that chance of burning at production well is extended. Injecting air from vertical well obtained benefit from higher amount of air injected compared to horizontal well injector as reservoir pressure is low in shallow depth. By using the best conditions for wet combustion, oil recovery of about 55% is achieved which is higher than dry combustion case of about 17 %. From an increase of heat capacity and a reduction of heat conductivity, effect of thermal conductivity is more pronounced than heat capacity and high value of thermal conductivity is preferable for this combined technique. Lower irreducible water saturation is favorable as it would provide benefit in steam propagation. Good vertical permeability is desirable as it increases amount of air injected which consecutively speeds up time to create fire front. During the combustion process, combustion may appear at production well when operating parameters are improper or can be from uncertainty of reservoir parameters such as low and high irreducible water saturation, low vertical permeability and high value of heat capacity and this situation is extremely dangerous and must be avoided.