Top-down in-situ combustion in heavy oil reservoirs have strong bottom aquifer support
Al Manahali, Mohammed Omar Salim
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The underlying sustained demand for oil despite fluctuations in the oil price, and the requirement to replace dwindling reserves, both encourage oil companies to consider developing heavy oil reservoirs through implementation of EOR methods. Injection of air into the reservoir and initiation of a fire front causes the reservoir temperature to increase with a resulting decrease in the viscosity of the oil; this results in higher production rates and a better recovery factor. The main objective of this study is to investigate numerically the potential for applying the combustion process using a combination of real field data (from the Nimr field) and data from the literature, and to evaluate the overall process performance. This entails using a 2D cross-sectional model, which is constructed based on available field properties, to enable a detailed investigation of the fire front behaviour. The optimum operating conditions for the in-situ combustion process are determined by conducting a suite of sensitivity calculations. These sensitivity calculations are divided into two groupings, classified as well configurations and reservoir heterogeneities. Under both groupings, the modelling of the combustion process also considered the presence of the strong bottom water aquifer support. The results of this study suggest that the application of in-situ combustion in the heavy oil reservoir with strong bottom water aquifer is a technically viable proposition. The appropriate choice of well configurations is considered to be a vital component in the successful implementation of the combustion process, and leads to better process performance in terms of increasing the recovery factor. The presence of aquifer support should be regarded as a challenge to the initiation and sustaining of the fire front, and hence a carefully selected well placement plan (e.g. top-down) could make the difference between success and failure of the process. Depending on the well configurations selected, the impact of reservoir heterogeneities on the combustion process varied significantly. The combustion process recovery factor decreased as the fire front velocity changed, which is due to the large volume of coke been produced and deposited. This modelling study demonstrated the main approaches to optimise the combustion process performance, and while some data is field specific, the modelling results are generic.