Efficient Simulation for Low Salinity Waterflooding in Porous and Fractured Reservoirs

Low-salinity brine injection has emerged as a promising, cost-effective improved oil recovery (IOR) method for waterflooding reservoirs. Laboratory tests and field applications show that low-salinity waterflooding could lead to significant reduction of residual oil saturation. There has been a growing interest with an increasing number of low-salinity waterflooding studies. However, there are few quantitative studies on flow and transport behavior of low-salinity IOR processes. This paper presents a general mathematic model (1) to incorporate known IOR mechanisms and (2) to quantify low-salinity waterflooding processes. In our mathematical conceptual model, salt is treated as an additional “component” to the aqueous phase, based on the following physical considerations: salt is transported only within the aqueous phase by advection and diffusion, and also subject to adsorption onto rock solids; relative permeability, capillary pressure, and residual oil saturation depend on salinity. Interaction of salt between mobile and immobile water zones is handled rigorously using a multi-domain approach. Fractured rock is handled using the multiple-continuum model or a discrete-fracture modeling approach. The conceptual model is implemented into a general-purpose reservoir simulator for modeling low-salinity IOR processes, using unstructured, regular, and irregular grids, applicable to 1-D, 2-D, and 3-D simulation of low-salinity water injection into porous media and fractured reservoirs. As demonstrated, the model provides a general capability for quantitative evaluation of low-salinity waterflooding in site-specific investigations. Introduction Waterflooding has been widely used as a secondary method to improve oil recovery for most oil reservoirs. Apart from formation damage, water floods are traditionally designed without considering the composition of the injected brine. However recent laboratory coreflood studies and field tests have showed that low-salinity waterflooding could result in a substantial oil recovery increase (2-40%) over traditional water flooding in many cases, depending on the reservoir formation minerals and brine composition (McGuire, et al, 2005, Lager, et al, 2008). The possible mechanisms for low-salinity waterflooding to improve oil recovery could be attributed to: (1) the wettability change towards water wet as a result of clay migration (Tang and Morrow, 1999); (2) the pH increase as a result of CaCO3 dissolution, which increase oil recovery by several mechanisms including wettability alteration, generation of surfactants, and reduction in IFT (McGuire, et al, 2005,); and (3) multiple-component ion exchange (MIE) between clay mineral surfaces and the injected brine (Larger et al, 2006). In general, the oil recovery improvement during low-salinity water flooding is recognized to depend on MIE, clay content, formation water composition (Ca, Mg), and oil composition. In the petroleum industry, there has been a growing interest with an increasing number in low-salinity waterflooding studies. However, most of the work has focused on the extent of low-salinity water effect on improved oil recovery and the mechanisms of wettability alteration. In comparison, there are few quantitative studies on flow and transport behavior of lowsalinity IOR processes. Jerauld et al (2006) modeled low-salinity waterflooding as a secondary and tertiary recovery processes in one dimensional model using salinity dependent oil/water relative permeability functions, resulting from wettability. Tripathi et al (2008) studied the flow instability associated with wettability alteration using a Buckley-Leveret type, analytical model in one dimension. In this paper, we present a general numerical model for low-salinity water flooding in multidimensional, porous or fractured reservoirs. The model formulation incorporates known IOR mechanisms by low-salinity flooding for simulating low-salinity waterflooding processes. Two models, one homogenous model and one fracture model, were run to demonstrate the use of the proposed modeling approach in simulation of low-salinity water flooding.