Modeling Development of a Thermal Gas-Oil Gravity Drainage Process in an Extra-Heavy Oil Fractured Reservoir

Abstract

Abstract Thermal Gas-Oil Gravity Drainage (T-GOGD) is an attractive Enhanced Oil Recovery (EOR) method applicable to naturally-fractured reservoirs (NFR). The process is successfully applied in Qarn Alam, a heavy oil field in Oman. This paper presents a dynamic modeling optimization and uncertainty analysis workflow for T-GOGD in a bitumen-bearing fractured reservoir based on realistic 3D fracture characterization. In T-GOGD, the fractures are displaced to steam to provide a (matrix) gravity drainage potential while heating the reservoir at the same time. One of the recovery mechanisms associated with T-GOGD is thermal expansion, which can result in high initial rates, but may cause plugging of the fracture system in the case of extra-heavy oil (bitumen) if the expanded oil cools down before it is being produced. This situation requires short-distance well configurations and/or steam stimulation cycles to establish communication. In a NFR, steam vapour occupies the fracture system while oil drains through the matrix, increasing the area for heat transfer with respect to the steam chamber case; the process therefore differs significantly from SAGD and a different production function applies. Shell's in-house reservoir simulator MoReS with advanced dual-permeability capability, is used to model development of T-GOGD in a bitumen reservoir employing 3D element-of-symmetry models. A realistic fracture characterization and modeling process is described. The geometrical well configuration and operating schedule and strategy are optimized on an economic function for a deterministic subsurface realization. Using an uncertainty analysis workflow, cumulative distribution functions of recovery and steam-oil ratio are generated. Finally, robust optimization is explored using an economic objective function. The study concludes that 1) relatively large well spacing is feasible while injector-producer horizontal well offset is necessary to avoid steam channeling to the producer well, 2) live steam production control is a robust operating strategy, 3) performance is most sensitive to matrix permeability and oil viscosity, and 4) vertical fracture connectivity plays an important role on the process performance. T-GOGD has significant potential to develop bitumen resource in naturally fractured carbonates.