Investigation of Oil Adhesion to Shale Rocks for EOR

Abstract

Significant work is ongoing within the industry to determine a best practice for maximizing oil recovery from fractured oil-wet shale reservoirs. Rapid decline curves are often observed and water flooding can be largely ineffective because of negative capillary pressure. The goal of this work is to identify a chemical solution that can maximize oil and gas recovery in unconventional reservoirs by reduction of hydrocarbon adhesion to shale rocks. In order to evaluate an optimal solution, numerous formulations were developed and tested for their impact on adsorption and adhesion on rock and/or sand, changes in interfacial tension, and how the formulation affects the wettability of the formation. Shale rocks were characterized for their surface energy, as this governs the adsorption and adhesion tension of crude oil, water, and chemicals to the solid surface. Formulations were selected that minimized the adsorption on rock and sand surfaces since such adsorption may lead to an increase in the surface tension of fluid pumped into the well and the interfacial tension between the crude oil and fluid. Contact angle measurements were used to determine the Van der Waals and Lewis acid-base components of surface energies for Barnett, Eagle Ford, Niobrara, and Bakken shales. In addition, contact angle measurements and interfacial tension were used to determine the adhesion of crude oil to the rock. Numerous chemical formulations were evaluated to identify products that can decrease the work of adhesion, making oil recovery more efficient (i.e. less work is required to remove oil drops from the rock surface and mobilize them). Competitive adsorption of formulations at the oil-water and rock-water interface was evaluated. The amount of natural surfactants in the oil and their adsorption on the rocks (reversible vs. irreversible) affect whether the rock is oil-wet or water-wet. If the adsorption is reversible, the rock would be more water-wet, resulting in higher oil recovery. Formulations which altered the wettability to water wet rapidly, but reduced the interfacial tension slightly, exhibited the highest oil recoveries. Based on wettability alteration, interfacial tension, and work of adhesion, a novel product was developed that is salt tolerant (in 30% TDS), thermally stable (115°C), and produces high oil recovery (i.e., 60% OOIP). Kinetics were also improved compared to conventional treatments and brine alone. In addition, this product showed a low static adsorption on the Bakken shale (0.20 mg product active/gram rock) and no emulsion tendency.

A Literature Review of CO2, Natural Gas, and Water-Based Fluids for Enhanced Oil Recovery in Unconventional Reservoirs

“…In a study related to oil adhesion on Bakken samples, it was shown that an anionic surfactant solution was salt tolerant (30% TDS brine) and thermally stable at 115 °C (Table 6, entry 11). 322 LSW (4% TDS) was better than high salinity brine (22% TDS) in oil recovery via imbibition, and the addition of 1000 ppm of anionic surfactant increased oil recovery for both brines. Similar (but less dramatic) improvements were attained with an amphoteric surfactant.…”

“…No clear correlation between IFT and oil recovery was observed, and the authors suggest that wettability alteration is the dominant mechanism of recovery. In a study related to oil adhesion on Bakken samples, it was shown that an anionic surfactant solution was salt tolerant (30% TDS brine) and thermally stable at 115 °C (Table , entry 11) . LSW (4% TDS) was better than high salinity brine (22% TDS) in oil recovery via imbibition, and the addition of 1000 ppm of anionic surfactant increased oil recovery for both brines.…”

“…Anionic surfactants appear to be most effective, ,,,,, but some researchers recommend the use of nonionic or cationic surfactants. Due to the predominance of capillary effects during long soak times, wettability alteration is likely more important than IFT reduction for surfactants used in ULRs. ,,,,,,,, Still, some researchers maintain that achieving ultralow IFT reduction is more important than wettability alteration oil recovery. , Excessive adsorption remains a common concern for surfactants intended to alter wettability. , Operators can further enhance the recovery of oil with surfactant solutions with optimal, usually low (∼4%), salinity. ,,,, Despite the preponderance of these research efforts, there appears to be a consensus that the extremely low permeability of the most significant ULR targets renders aqueous solutions less likely to yield economically viable rates of oil production than high-pressure gas-based EOR strategies such as natural gas or CO 2 huff-n-puff. This consensus is also reflected in the field trials, which are currently dominated by high pressure gas-based EOR efforts.…”

“…324,325 Operators can further enhance the recovery of oil with surfactant solutions with optimal, usually low (∼4%), salinity. 245,248,309,322,370 Despite the preponderance of these research efforts, there appears to be a consensus that the extremely low permeability of the most significant ULR targets renders aqueous solutions less likely to yield economically viable rates of oil production than highpressure gas-based EOR strategies such as natural gas or CO 2 huff-n-puff. This consensus is also reflected in the field trials, which are currently dominated by high pressure gas-based EOR efforts.…”

Surfactant-Assisted Spontaneous Imbibition to Improve Oil Recovery on the Eagle Ford and Wolfcamp Shale Oil Reservoir: Laboratory to Field Analysis

“…It was found that anionic surfactant works the best on quartz-rich rock, while on carbonate-rich Wolfcamp, the cationic surfactant performed dominantly . Production improvement by the addition of surfactant was also observed on the Bakken ,− and Eagle Ford − systems.…”