Evidence, mechanisms and improved understanding of controlled salinity waterflooding part 1: Sandstones

Abstract

It is widely accepted that oil recovery during waterflooding can be improved by modifying the composition of the injected brine, typically by lowering the total salinity to less than 5,000ppm. Numerous laboratory experiments and field tests, in both clastic and carbonate rock samples and reservoirs, have demonstrated this 'low salinity effect' (LSE). However, despite a plethora of studies and data, the LSE remains poorly understood. Evidence to support the widely held view that improved recovery is conditional on the presence of clay minerals in sandstones, multivalent ions in the formation brine, and significant dilution of the injection brine, is surprisingly scarce. Moreover, there is no method to determine the optimum injection brine composition for a given crude-oil-brine-rock (COBR) system. Many studies have reported the successful application of controlled salinity water injection. However, many others (and more unpublished) observed no benefit, and the available data are often inconsistent and contradictory. This review collects and summarizes the available data for the first time and discusses the poreto mineral-surface-scale mechanisms that have been proposed to explain the LSE. Based on this, it outlines an integrated experimental programme that could be used to identify the optimal injection brine composition for a given COBR system. The available evidence suggests that the LSE is real, and is caused by one or more pore-to mineral-surface-scale mechanism(s) which facilitate improved oil recovery at the core-to reservoir-scale. These mechanisms occur at COBR interfaces, and are multi-ion exchange (MIE), local increase in pH (∆pH) and double layer expansion (DLE). However, the available evidence is not sufficient to unambiguously identify which, if any, of these mechanisms are essential. Other proposed mechanisms, such as clay swelling and fines migration, formation of natural surfactants at elevated pH, reduction in oil/brine interfacial tension, and increased solubility of polar oil compounds in brine, may occur in some cases but do not appear to be necessary to observe improved oil recovery. Understanding is hampered by a lack of common experimental conditions across length-scales. Core-scale measurements are often obtained at reservoir conditions of pressure, temperature, brine salinity and crude oil composition. In contrast, pore-and mineral-surface-scale measurements such as atomic force or scanning electron microscopy, contact angle and wetting surface, adsorption and adhesion, are often obtained at laboratory temperature and pressure, lower brine salinity and simplified crude composition. These contrasting experimental conditions may explain the contradictory data obtained to date. A common feature of all three proposed mechanisms for the LSE is that they lead to changes in zeta potential at mineral surfaces, either through changes in mineral surface charge (MIE, ∆pH) or changes in the thickness of the double layer (DLE). Thus they change the magnitude of the electrostatic forces acting between ...

Carbon Storage and Enhanced Oil Recovery in Pennsylvanian Morrow Formation Clastic Reservoirs: Controls on Oil–Brine and Oil–CO2 Relative Permeability from Diagenetic Heterogeneity and Evolving Wettability

“…However, with CO 2 WAG EOR, the presence of CO 2 both changes the character of remaining oil (as in Figure 14) and lowers pH of the low salinity injectate water. This would be expected to drive the system to be water-wet over time during water flooding [30,53,54]. This change in wettability invalidates the relative permeability functions used in the models, as the functions would become more like the non-aged rather than the aged relative permeability measurements (Figure 11).…”

Chemical Force Microscopy Study on the Interactions of COOH Functional Groups with Kaolinite Surfaces: Implications for Enhanced Oil Recovery

“…In the case of the experiments shown here, the pH was kept constant throughout its duration, and only the concentration was changed. These conditions are also ideal to test the double layer expansion mechanism, which generally predicts an increase in the width of the double layer extending away from the surface of the mineral, when the ionic strength of the solution is reduced [3,4]. This expansion then is linked to an increase in the range of repulsion forces, and therefore to a decrease in the overall adhesion of the organic molecules to the mineral surfaces.…”

“…Enhanced oil recovery methods include steam and CO 2 injection, chemical flooding, pH alteration, inter alia. One method that is currently highlighted is low salinity enhanced oil recovery (LSEOR) owing to its use of a low-cost, environment-friendly substance, its sustainability, and its effectiveness [3,4]. For this reason, a multitude of studies have been conducted to understand the fundamental geochemical processes driving LSEOR; nevertheless, a debate still exists on the exact nature and importance of these processes and hence whether LSEOR can be applied to any given field [3,4].…”

“…One method that is currently highlighted is low salinity enhanced oil recovery (LSEOR) owing to its use of a low-cost, environment-friendly substance, its sustainability, and its effectiveness [3,4]. For this reason, a multitude of studies have been conducted to understand the fundamental geochemical processes driving LSEOR; nevertheless, a debate still exists on the exact nature and importance of these processes and hence whether LSEOR can be applied to any given field [3,4]. Most authors agree, however, on a series of factors or conditions that need to be met for low salinity EOR to be effective, these include: the presence of clay minerals, the presence of a saline connate water (containing multivalent ions), exposure of the rock to acidic and basic oil components, and a significant reduction of salinity in the flooding water [3].…”

A Surface Complexation Model of Alkaline-SmartWater Electrokinetic Interactions in Carbonates

“…Some of these plausible mechanisms include electric double layer [4], in-situ soap generation (saponification effect) [8], and multi-ion exchange [9]. The surface charges of carbonate/brine and crude oil/brine are altered in such pore-scale processes, which affects the zeta-potential measurements used in understanding the rock wettability [10,11].…”