TRANSPORT OF SURFACTANTS IN FRACTURED CHALK ROCK – LABORATORY EXPERIMENTS AND NUMERICAL SIMULATIONS

摘要

Surfactants can be used in different types of enhanced oil recovery(EOR) processes,such as the stabilization of foam to improve the macroscopic sweep efficiency in gas floods, reduction of the interfacial tension to reduce the residual oil saturation,and wettability alteration to improve the spontaneous imbibition of water in fractured reservoirs.In fractured reservoirs,the retention of the surfactants will depend on how much of the matrix is contacted by the surfactant during the project period.In presented study,knowledge about transport of surfactants in fractured chalk has been established in laboratory experiments and simulations. Chalk plugs at 100% water-saturation and residual oil saturation after water flooding were used in the static experiments.Fractured models were created by placing core plugs in cells with an annulus space around the plugs.The fracture volume was filled with the surfactant solution.In addition to fractured models with all surface area of the plugs exposed to the surfactant solution,different fractured models were created by blocking some of the surface area for surfactant exposure.Water samples were regularly taken out from the fracture and analyzed for the surfactant concentration until the concentrations reached a plateau. It was observed that increasing the exposed surface area or decreasing the maximum transport length increased the rate of retention.The presence of an immobile oil phase did not have large effects on the total retention of the surfactant.The experiments were also modeled with the commercial reservoir simulator STARS.Excellent history-matching of the surfactant concentration profiles was obtained for the plugs with all surface area exposed.The same dataset was used to predict the surfactant concentration profiles for the fractured models with different surfaces blocked.Simulation studies at larger scale show that the whole matrix blocks will not be saturated with the surfactant during typical time for EOR-processes.The retention of surfactants will therefore be much lower in fractured reservoirs with diffusion dominated transport of surfactants from fractures to matrix than estimated based on flow through experiments.The amount of surfactant required for EORprocesses mainly affecting the flow in the fractures and/or close to the fractures,will for this reason be less than required for saturation of the whole matrix.For EOR-processes where transport of the surfactant into most of the matrix is required,the potential will be low in these reservoirs because the transport of surfactant in the matrix will be too slow.