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

摘要

Pilot tests of surfactant additives in completion fluid and gas huff n'puff in depleted wells have proventhe possibility of production enhancement in unconventional liquid reservoirs(ULR).However,numericalsimulation studies regarding EOR techniques neglect two important features of the ULR:extensive fracturediscontinuity and high fracture density.This work explores how these two features effect depletion forecastsand EOR evaluation in ULR by applying discrete fracture network(DFN)modeling and optimizedunstructured gridding.In this study,grid generation algorithms for Perpendicular Bisection(PEBI)gridding are improved tohandle reservoirs with complex fracture geometry and high fracture intensity.The depletion behavior of thedual-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 buildDFN models to represent realistic stimulated tight reservoirs.Dynamic fluid flow models are calibratedthrough history matching of depletion.To properly model EOR processes at the field scale,results frompublications of lab experiments regarding surfactant imbibition and CO2 huff n'puff are used to generatesimulation parameters.A series of surfactant spontaneous imbibition and gas huff n'puff simulations areperformed 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 commonin ULR.By tuning parameters within a reasonable range,DFN dynamic fluid flow models match theproduction 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 inproduction enhancement.Simulation results of gas huff n'puff indicate the main recovery mechanisms arere-pressurization and viscosity reduction characteristic of multicontact miscibility.And for reservoirs belowthe bubble-point,another recovery mechanism is the increase of heavy components'flux.However,eitherincreasing the soak period or increasing the portion of the production period in each cycle has a minor effecton recovery enhancement.This study reveals the significance of using DFN with the unstructured grid to study the EOR processes inULR.This approach can capture the rapid and extreme change in phase saturation and component fractionwithin the stimulated reservoir volume(SRV).Our results demonstrate the important factors that affect thefield-scale EOR performance in ULR.