EVALUATION OF STEAM ASSISSTED GRAVITY DRAINAGE PROCESS COMBINED WITH HYDRAULIC FRACTURING IN SHALY-SAND RESERVOIR

Abstract

Thermal recovery is a technique usually implemented in reservoirs containing viscous oil. Nowadays, a technique called Steam Assisted Gravity Drainage (SAGD) is modified and implemented in most fields around the globe. Compared to waterflooding, SAGD improves both sweep and displacement efficiencies, leading to high oil recovery. However, SAGD requires good vertical connectivity that can be deteriorated by which the presence of shale acting as barrier to the flow path of fluids. In order to overcome this low connectivity between wells, hydraulic fracturing is generated to create vertical paths. In this study, two sandstone reservoir models are constructed to possess structural shale and laminated shale, respectively. Hydraulic fracturing is performed to evaluate the performance on these models by evaluating both controllable and uncontrollable parameters. The entire study employs STARS commercialized by Computer Modelling Group (CMG) as a tool. From reservoir simulation results, models combined with hydraulic fractures improve oil recovery by 3.6 and 15.2 % in structural and laminated shale base models respectively when the best steam injection rate is used. Steam-oil ratios are also reduced in both cases, indicating more favorable condition. Symmetrical distribution of hydraulic fractures optimizes volumetric sweep efficiency and high steam quality is desirable for this combined technique. In addition, steam trap is found to be an effective method to improve thermal efficiency. However, oil recovery factor is unavoidably reduced as amount of injected steam is limited. For reservoir parameters, high percent of shale volume requires better vertical communication through hydraulic fracturing due to their low heat conductivity. Benefit from hydraulic fracturing is lowered when laminated shale is discontinuous as channels for steam to penetrate to inaccessible zone are increased. Finally, reservoir with high vertical permeability results in high steam-oil ratio in the early period, which might lead to uneconomical condition. Steam trap technique must be applied together with hydraulic fracturing in order to prevent this situation.