Chemical Compositions in Modified Salinity Waterflooding of Carbonate Reservoirs -- Experiment

Abstract

<br>Modified or low-salinity waterflooding of carbonate oil reservoirs is of considerable economic interest because of potentially inexpensive incremental oil<br>production. The injected modified brine changes the surface chemistry of the carbonate rock and crude oil interfaces and detaches some adhered crude oil.<br>Composition design of the modified brine to enhance oil recovery is determined by labor-intensive trial-and-error laboratory corefloods. Unfortunately, limestone,<br>which predominantly consists of aqueous-reactive calcium carbonate, alters injected brine composition by mineral dissolution/precipitation. Accordingly, the rock reactivity<br>hinders rational design of the tailored brine to improve oil recovery. <br>Previously, we presented a theoretical analysis of 1D, single-phase brine injection into calcium carbonate-rock that accounts for mineral dissolution, ion<br>exchange, and dispersion (Yutkin et. al 2021). Here we present the results of single-phase waterflood-brine experiments that verify the theoretical framework. We show that concentration histories eluted from Indiana limestone cores possess features characteristic of fast calcium<br>carbonate dissolution, 2:1 ion exchange, and high dispersion. The injected brine reaches chemical equilibrium inside the porous rock even at<br>injection rates higher than 1000 ft/day. Ion exchange results in salinity waves observed experimentally, while high dispersion is responsible for long<br>concentration history tails. <br>Using the verified theoretical framework, we briefly explore how these processes modify aqueous-phase composition during the injection of designer brines into a calcium-carbonate reservoir. Because of high salinity of the initial and injected brines, ion exchange affects injected concentrations only in<br>high surface area carbonates/limestones, such as chalks. Calcium-carbonate dissolution only affects aqueous solution pH. The rock surface composition is affected by all processes.<br><br>

Adsorption of Polar Species at Crude Oil–Water Interfaces: the Chemoelastic Behavior

“… 5 − 9 However, despite intense research in this field, the underlying mechanisms that lead to improved oil recovery are still debated. 10 − 21 Moreover, the complex rheology of the interfacial films dictates the stability of oil/water emulsions, with implications in oil transport, processing, and final quality. 22 , 23 …”

“…In particular, the interfacial properties of crude oil have received significant attention over the past decades because of their relevance to oil recovery, transport, and processing. − For example, most oil extraction processes are based on injecting water-based liquids, called brines, into the porous oil reservoir. The efficiency of this process, known as waterflooding, is generally understood as an interplay between viscous and interfacial forces, and it has been shown that it can be enhanced by tuning the chemical composition of the brine, for example, by reducing its salinity or by selectively increasing the concentration of certain divalent ionic species. − However, despite intense research in this field, the underlying mechanisms that lead to improved oil recovery are still debated. − Moreover, the complex rheology of the interfacial films dictates the stability of oil/water emulsions, with implications in oil transport, processing, and final quality. , …”

Surfactant Adsorption and Ion Exchange on Calcite Surfaces

“…The system becomes even more complex when chemicals such as surfactant and polymer are included, which interact with other aqueous species and can adsorb to the calcite surface. A one-dimensional (1D) reactive transport model that can describe compositional changes of brines containing Na + and Cl – during flow through calcite was recently developed by Yutkin et al and validated with core flooding experiments. , In their model, both calcite and clay mineral phases are included to model surface complexation and ion exchange, respectively. The anionic surface sites are neutralized through 2:1 Ca 2+ /Na + mass-action ion exchange, while a diffuse layer is absent due to the collapse of the EDL on the outer Helmholtz plane.…”