Modeling Low-Salinity Waterflooding
- Gary R. Jerauld (BP) | Kevin J. Webb (BP Exploration) | Cheng-Yuan Lin (BP plc) | James C. Seccombe (BP)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2008
- Document Type
- Journal Paper
- 1,000 - 1,012
- 2008. Society of Petroleum Engineers
- 5.3.1 Flow in Porous Media, 5.6.4 Drillstem/Well Testing, 2.2.2 Perforating, 5.1 Reservoir Characterisation, 1.8 Formation Damage, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 5.5 Reservoir Simulation, 5.5.8 History Matching, 5.3.2 Multiphase Flow, 5.4.2 Gas Injection Methods, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 2.5.2 Fracturing Materials (Fluids, Proppant), 4.1.5 Processing Equipment, 5.6.5 Tracers, 4.3.4 Scale, 5.4.1 Waterflooding, 6.5.2 Water use, produced water discharge and disposal, 1.6.9 Coring, Fishing, 5.8.7 Carbonate Reservoir, 5.3.4 Reduction of Residual Oil Saturation
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Low-salinity waterflooding is an emerging enhanced-oil-recovery (EOR) technique in which the salinity of the injected water is controlled to improve oil recovery vs. conventional, higher-salinity waterflooding. Corefloods and single-well chemical-tracer tests have shown that low-salinity waterflooding can improve basic waterflood performance by 5 to 38%. This paper describes a model of low-salinity flooding that can be used to evaluate projects; shows the implications of that model and demonstrates its use to represent corefloods, single-well tests, and field-scale simulations; and gives insight into the reservoir engineering of low-salinity floods.
The model represents low-salinity flooding using salinity-dependent oil/water relative permeability functions resulting from wettability change. This is similar to other EOR modeling, and conventional fractional-flow theory can be adapted to describe the process in 1D for secondary and tertiary low-salinity waterflooding. This simple analysis shows that while some degree of connate-water banking occurs, it need not hinder the process.
Mixing of injected water with in-situ water delays the attainment of low salinity, potentially preventing attainment of low salinity all together if very small slugs of low-salinity water are used. This paper demonstrates the importance of mixing to modeling of low-salinity flooding and suggests addressing it in engineering calculations. Care must be taken in representing mixing appropriately in interpreting data and in constructing models. The use of numerical dispersion to represent physical dispersion in 1D, radial, and pattern simulations of this process is demonstrated (i.e., coarse-grid simulations are shown to give the same result as fine-grid simulations with an appropriately large physical dispersion). In many applications, the fine-grid simulation necessary to represent appropriate levels of dispersion is not practical, and pseudoization is necessary. We demonstrate that this can be achieved by changing the salinity dependence and shapes of relative permeability curves.
Waterflooding is widely used to improve recovery from oil reservoirs but, except to avoid formation damage, is largely designed without regard to the composition of the brine injected. Yildiz and Morrow (1996) showed that changes in injection-brine composition can improve recovery, thereby introducing the idea that the composition of the brine could be varied to optimize waterflood recovery. Tang and Morrow (1997) (Tang and Morrow 1999; Morrow et al. 1998; McGuire et al. 2005) built on this idea by demonstrating the benefit lowering brine salinity has on oil recovery. There has been a substantial amount of research on low-salinity injection, which has included more than 20 reservoir-conditions corefloods on a range of sandstone reservoirs both in secondary and tertiary mode, more than 10 single-well chemical-tracer tests (SWCTTs), and a log/inject/log test (McGuire et al. 2005;Webb et al. 2004; Webb et al. 2005; Lager et al. 2006). These tests have shown improvements of waterflood-process efficiency by 5 to 38% by using low-salinity water or by corresponding reductions in residual-oil saturation of 3 to 17% pore volumes (PV). The purpose of this work is to present a simple extension to waterflood simulators that can be used to translate corefloods or SWCTTs into field-scale estimates of low-salinity waterflood (LSWF) oil recovery and demonstate this with examples from a sandstone reservoir.
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