Insights into Field Application of EOR Techniques from Modeling of Tight Reservoirs with Complex High-Density Fracture Network

Abstract

Abstract Pilot tests of surfactant additives in completion fluid and gas huff n' puff in depleted wells have proven the possibility of production enhancement in unconventional liquid reservoirs (ULR). However, numerical simulation studies regarding EOR techniques neglect two important features of the ULR: extensive fracture discontinuity and high fracture density. This work explores how these two features effect depletion forecasts and EOR evaluation in ULR by applying discrete fracture network (DFN) modeling and optimized unstructured gridding. In this study, grid generation algorithms for Perpendicular Bisection (PEBI) gridding are improved to handle reservoirs with complex fracture geometry and high fracture intensity. The depletion behavior of the dual-porosity methods and the DFN method are compared based on the "sugar-cube" conceptual model. Data including outcrop maps and FMI log are used to characterize fracture network geometry and build DFN models to represent realistic stimulated tight reservoirs. Dynamic fluid flow models are calibrated through history matching of depletion. To properly model EOR processes at the field scale, results from publications of lab experiments regarding surfactant imbibition and CO2 huff n' puff are used to generate simulation parameters. A series of surfactant spontaneous imbibition and gas huff n' puff simulations are performed on those calibrated DFN models to study the impact of fracture geometry on EOR performance. Simulation results indicate that dual-porosity methods are not correct if the transient period of fracture-matrix flow lasts for extaned periods or the continuity of fractures is poor, both of which are very common in ULR. By tuning parameters within a reasonable range, DFN dynamic fluid flow models match the production data and can represent the realistic stimulated ULR. Surfactant assisted spontaneous imbibition (SASI) in the matrix domain results in a marginal production increase compared to water imbibition. It is found that wettability alteration incurred in the fracture system may play a more important role in production enhancement. Simulation results of gas huff n' puff indicate the main recovery mechanisms are re-pressurization and viscosity reduction characteristic of multicontact miscibility. And for reservoirs below the bubble-point, another recovery mechanism is the increase of heavy components' flux. However, either increasing the soak period or increasing the portion of the production period in each cycle has a minor effect on recovery enhancement. This study reveals the significance of using DFN with the unstructured grid to study the EOR processes in ULR. This approach can capture the rapid and extreme change in phase saturation and component fraction within the stimulated reservoir volume (SRV). Our results demonstrate the important factors that affect the field-scale EOR performance in ULR.