Effects of Salinity on Oil Recovery (the “Dilution Effect”): Experimental and Theoretical Studies of Crude Oil/Brine/Carbonate Surface Restructuring and Associated Physicochemical Interactions

Abstract

The primary aim of this study was to investigate the "dilution effect", where dilution of the ionic concentration of the fluid injected into oil wells has been found to enhance oil recovery. We have measured crude oil/brine/carbonate surface (calcite) interactions using a variety of dynamic techniques including contact angles, surface forces apparatus, atomic force microscopy, interfacial tension, X-ray photoelectron spectroscopy, and other physical and chemical surface characterization techniques. The effects due to different brine (ionic electrolyte) solutions and temperatures, as well as the dynamics (timedependence) of these effects, were investigated. Ionic strengths varied from pure water to 350 000 ppm, and temperatures varied from 20 to 75 °C. We found that upon exchanging solutions (as occurs for waterflooding using dilute solutions), three different dynamic processes occur that have very different time scales: (1) the initial, rapid (seconds to minutes) physical ion exchange with the surfaces that locally changes the surface charge/potential and, hence, the double-layer and hydration forces, (2) the local electrochemical dissolution and restructuring of the surfaces (minutes to hours), which is also often accompanied by the desorption of preexisting organic−ionic layers on the mineral surface that come off as visible flakes with the oil, and (3) the largescale, diffusion-rate-controlled restructuring leading to macroscopic changes in rock morphology (months to years). We conclude that the "dilution effect" is in part due to the well-known colloidal interaction forces (electric double-layer, hydrophilic-hydration, and van der Waals). In addition, our experiments reveal (electro)chemical reactions involving dissolution, pitting, adsorption, and restructuring of the calcite surfaces, which increases their roughness (cf. the geological process of "pressure solution"). Both the colloidal forces and surface roughening and restructuring act to reduce the adhesion of the crude oil/brine interface to the calcite/brine interface (across the thin aqueous or "water" film), which in turn reduces the water-side contact angle (increasing the water-wettability and, presumably, oil recovery), with increasing dilution. These two contributionsreduced colloidal forces and surface rougheningappear to be essential for the "dilution effect" to be effective at all solution concentrations from formation water to pure water. We propose a semiquantitative model to explain the "dilution effect" based on a form of the wellestablished extended-Derjaguin−Landau−Verwey−Overbeek theory for the colloidal interactions between the crude oil and carbonate surface across brine of different concentrations and a modified Young−Dupréequation that accounts for the effects of surface roughness. We present the "dilution effect" in terms of "wettability maps" for the calculated (effective) adhesion energy of the crude oil/brine/carbonate system as a function of brine concentration (from formation water down to the infinite-dilution [i.e., pure ...

Chemical Compositions in Salinity Waterflooding of Carbonate Reservoirs: Theory

“…Because reaction Damköhler numbers are very large in reservoir applications, this result confirms our previous order-of-magnitude estimates that rock dissolution occurs at the injection wellbore(Yutkin et al 2018). Consequently, once past the reaction zone, any rock-dissolution-based mechanisms in low salinity waterflood are impossible(Chen et al 2017). From the corresponding concentration histories inFigure 4, rock reaction neither speeds nor slows species transport advance.…”

Wettability Alteration and Enhanced Oil Recovery Induced by Proximal Adsorption of Na+ , Cl− , Liu1, Wani2, Alhassan3 et al. 2018Phys. Rev. Applied485270View full textAdd to dashboardCite

“…Raising the temperature has been found to be of great benefit for EOR [12,14], while at high temperatures Mg 2+ can even substitute Ca 2+ from the chalk surface [12]. More recently, the injection of low-salinity brine in carbonate reservoirs was reported to enhance oil recovery [14][15][16][18][19][20]. It was further demonstrated that the low-salinity effect results from wettability alteration rather than mineral dissolution [19,21], but the specific mechanism responsible for the wettability alteration remains largely unknown.…”

Surface Forces Apparatus Measurements of Interactions between Rough and Reactive Calcite Surfaces

“…Any 4 substantial roughening of calcite led to a significant increase of the repulsive mechanical force contribution.Even though the AFM is an extremely powerful tool in force measurements, it usually provides no information about the instantaneous changes in contact surface topography. In this work, we used the Surface Forces Apparatus (SFA), a force measuring technique that enables in situ observations of surface alteration processes by multiple beam interferometry (MBI) 42,[48][49] . We follow how the growth, dissolution and related changes in surface roughness in µm-sized contact areas affect the magnitude and range of forces between dynamic and rough, polycrystalline calcite surfaces.…”