Understanding the Oil Recovery Mechanism in Mixed-Wet Unconventional Reservoirs: Uniqueness and Challenges of Developing Chemical Formulations

Abstract

Several surfactant formulations that had been tested successfully in oil-wet unconventional reservoirs were tested in mixed-wet to oil-wet unconventional reservoir cores and did not generate the expected results. To study the mechanisms of oil recovery and understand the uniqueness of these shale reservoirs, a series of studies were performed utilizing Eagle Ford (EF) and Canadian Bakken shale rocks and fluids. In this study customized chemical formulations for improving production from the EF and the Canadian Bakken were developed. Previously related formulation development for the Bakken and Permian basins relied upon wettability alteration as the oil recovery mechanism; however, no significant oil recovery compared to brine was seen from wettability-altering formulations using EF and Canadian Bakken shale rock and fluids. Several imbibition tests showed that baseline oil recovery by brine was 20-30% of original oil in place (OOIP) for both formations. High oil recovery by brine was attributed to the mixed to water-wet nature of the pore surface. A well-connected fracture system may have also contributed. Further, there was no correlation between oil recovery and contact angle measurements. Failure of wettability alteration as an oil recovery mechanism led to investigation of interfacial tension (IFT) reduction as an alternative mechanism. Testing this hypothesis, a change in the EF formulation reduced IFT to 0.03 dyne/cm and had oil recoveries above 60% OOIP. However, these formulations were not stable at 320 °F. Formulation KPIs were set as lowering IFT and being stable up to 320 °F. Out of many formulations tested, two containing multiple actives in a specific mixture of solvents passed the KPIs and were tested for imbibition oil recovery. A synergistic mixture had a final oil recovery above 56% OOIP as compared to 20-25% OOIP for brine alone. The imbibition oil recovery results indicate that although the ultimate oil recovery by brine alone is significant, the early oil production is significantly slower than by surfactant solutions. Upscaling the laboratory time to the field time emphasizes the value of implementing customized surfactant formulation in both early and late oil production. Similarly, there was no correlation between wettability contact angle measurements and oil recovery for the Canadian Bakken shale. Surfactant formulations which exhibited low IFT (~0.01 dyne/cm) significantly accelerated the oil production and recovered an additional 30-45% OOIP in the tertiary mode from the imbibition tests. Further laboratory studies via the Washburn method, imbibition tests, and zeta potential measurements validated lowering IFT, not altering the wettability, as a primary oil recovery mechanism in the mixed-wet EF and Canadian Bakken. Optimal formulations for EF and Canadian Bakken will be tested in the field by mid-2018.

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

“…A recent study has shown that oil recovery with cationic IFT-reducing surfactant can give higher oil recovery than wettability-altering surfactants in mixed wettability media (Table 6, entry 6). 319 In experiments using Eagle Ford cores, low salinity brine (7000 ppm TDS) yielded greater oil recovery than any of the 0.1 wt % wettability-altering surfactant formulations after 50 h of immersion. The LSW was even more effective than an anionic surfactant solution that reduced the water−shale contact angle from 170°to 44°at a concentration of 0.1 wt %.…”

“…Although most reports focused on surfactants for EOR in ULRs agree that wettability alteration is the more important oil recovery mechanism during spontaneous imbibition of tight rocks, a few reports support the use of IFT-reducing surfactants. A recent study has shown that oil recovery with cationic IFT-reducing surfactant can give higher oil recovery than wettability-altering surfactants in mixed wettability media (Table , entry 6) . In experiments using Eagle Ford cores, low salinity brine (7000 ppm TDS) yielded greater oil recovery than any of the 0.1 wt % wettability-altering surfactant formulations after 50 h of immersion.…”

“…Oil recovery via imbibition increased dramatically, with recovery of OOIP as high as 68% being attained when the cores were immersed in the surfactant solutions. The cationic IFT-reducing surfactant also exhibited stability at formation temperature in the absence of emulsion formation …”

“…The injected fluids can be formulated for specific formations via the changes in the number, type, and concentration of surfactants, selection of brine salinity, and introduction of other additives . 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.…”

“…The cationic IFT-reducing surfactant also exhibited stability at formation temperature in the absence of emulsion formation. 319 Another group used NMR monitoring of fluid saturations during imbibition tests as a rapid way to compare the performance of surfactant formulations (Table 6, entry 12). 323 An ultralow IFT surfactant (which also behaved as a wettability-altering surfactant) was compared to a benchmark wettability-altering surfactant (a nonionic alkyl ethoxylate).…”