Bulk and Surface Aqueous Speciation of Calcite: Implications for Low-Salinity Waterflooding of Carbonate Reservoirs

Abstract

SummaryLow-salinity waterflooding (LSW) is ineffective when reservoir rock is strongly water-wet or when crude oil is not asphaltenic. Success of LSW relies heavily on the ability of injected brine to alter surface chemistry of reservoir crude-oil brine/rock (COBR) interfaces. Implementation of LSW in carbonate reservoirs is especially challenging because of high reservoir-brine salinity and, more importantly, because of high reactivity of the rock minerals. Both features complicate understanding of the COBR surface chemistries pertinent to successful LSW. Here, we tackle the complex physicochemical processes in chemically active carbonates flooded with diluted brine that is saturated with atmospheric carbon dioxide (CO 2 ) and possibly supplemented with additional ionic species, such as sulfates or phosphates.When waterflooding carbonate reservoirs, rock equilibrates with the injected brine over short distances. Injected-brine ion speciation is shifted substantially in the presence of reactive carbonate rock. Our new calculations demonstrate that rock-equilibrated aqueous pH is slightly alkaline quite independent of injected-brine pH. We establish, for the first time, that CO 2 content of a carbonate reservoir, originating from CO 2 -rich crude oil and gas, plays a dominant role in setting aqueous pH and rock-surface speciation.A simple ion-complexing model predicts the calcite-surface charge as a function of composition of reservoir brine. The surface charge of calcite may be positive or negative, depending on speciation of reservoir brine in contact with the calcite. There is no single point of zero charge; all dissolved aqueous species are charge determining. Rock-equilibrated aqueous composition controls the calcitesurface ion-exchange behavior, not the injected-brine composition. At high ionic strength, the electrical double layer collapses and is no longer diffuse. All surface charges are located directly in the inner and outer Helmholtz planes.Our evaluation of calcite bulk and surface equilibria draws several important inferences about the proposed LSW oil-recovery mechanisms. Diffuse double-layer expansion (DLE) is impossible for brine ionic strength greater than 0.1 molar. Because of rapid rock/brine equilibration, the dissolution mechanism for releasing adhered oil is eliminated. Also, fines mobilization and concomitant oil release cannot occur because there are few loose fines and clays in a majority of carbonates. LSW cannot be a low-interfacial-tension alkaline flood because carbonate dissolution exhausts all injected base near the wellbore and lowers pH to that set by the rock and by formation CO 2 . In spite of diffuse double-layer collapse in carbonate reservoirs, surface ion-exchange oil release remains feasible, but unproved.Introduction LSW refers to improved oil recovery achieved by waterflooding a reservoir with brine of lower salinity than that of the field brine. The process is alluring because of simplicity, favorable economics, and large potential. In some applications, seawater is inj...

Adsorption of Charged Surfactants onto Calcium Carbonate

“…Bulk aqueous ions can specifically interact with calcium carbonate surface binding sites and bring in charge. Therefore, charge arises in the β-plane due to ions bound via specific interactions (Yutkin et al, 2016). The diffuse layer starts at the d-plane to neutralize charge in the β-plane caused by specific interactions and/or excess of charge in the β-plane caused by neutralization of some sites at the surface (Yutkin et al, 2016, Fig.…”

“…Those that invoke irreversible change (i.e., asphaltenes) are of special interest. Lager et al (2006) proposed a realistic mechanism of crude-oil deposition and wettability change involving carboxylate groups (Yutkin et al, 2016). The carboxylate would form from carboxylic at crude oil brine interface at high pH.…”

“…The carboxylate would form from carboxylic at crude oil brine interface at high pH. This multicomponent ion-exchange mechanism was widely accepted in the literature, used, and discussed by several groups (Hu et al, 2019;Eftekhari et al, 2017;Srisuriyachai and Meekangwal, 2017;Yutkin et al, 2016;Lager et al, 2011). Yet, recent direct confirmation for silica does not provide strong evidence (Hu et al, 2019).…”

Wettability Alteration Induced by Surface Roughening During Low Salinity Waterflooding

“…Many attempts to increase oil recovery have been done, such as chemical-based enhanced (surfactant [1][2][3], polymers [4,5], and alkaline solution [6][7][8]), heatbased (electromagnetic heating) oil recovery [9][10][11][12][13][14][15][16][17][18][19][20][21] and steam injection [12][13][14]), and microbial-based oil recovery to modify reservoir and crude oil properties [15,16]. However, environmental issues and high operational cost of the aforementioned enhanced oil recovery methods hamper their application in the field.…”

“…However, double layer expansion as the proposed mechanism has some limitations. It requires the presence of a water layer with a thickness of ~1 nm [20]. In addition, a unique characteristic of sandstone reservoirs should be noted, which is the presence of a high content of clay [21,22].…”