Wettability Survey in Bakken Shale With Surfactant-Formulation Imbibition

Abstract

Summary For ultratight shale reservoirs, wettability strongly affects fluid flow behavior. However, wettability can be modified by numerous complex interactions and the ambient environment, such as pH, temperature, or surfactant access. This paper is a third-phase study of the use of surfactant imbibition to increase oil recovery from Bakken shale. The surfactant formulations that we used in this paper are the initial results that are based on our previous study, in which a group of surfactant formulations was examined—balancing the temperature, pH, salinity, and divalent-cation content of aqueous fluids to increase oil production from shale with ultralow porosity and permeability in the Middle Member of the Bakken formation in the Williston basin of North Dakota. In our previous study, through the use of spontaneous imbibition, brines and surfactant solutions with different water compositions were examined. With oil from the Bakken formation, significant differences in recoveries were observed, depending on compositions and conditions. Cases were observed in which brine and surfactant (0.05 to 0.2 wt% concentration) imbibition yielded recovery values of 1.55 to 76% original oil in place (OOIP) at high salinity (150 to 300 g/L; 15 to 30 wt%) and temperatures ranging from 23 to 120°C. To advance this work, this paper determines the wettability of different parts of the Bakken formation. One goal of this research is to identify whether the wettability can be altered by means of surfactant formulations. The ultimate objective of this research is to determine the potential of surfactant formulations to imbibe into and displace oil from shale and to examine the viability of a field application. In this paper, through the use of modified Amott-Harvey tests, the wettability was determined for cores and slices from three wells at different portions of the Bakken formation. The tests were performed under reservoir conditions (90 to 120°C, 150- to 300-g/L formation-water salinity), with the use of Bakken crude oil. Both cleaned cores (cleaned by toluene/methanol) and untreated cores (sealed, native state) were investigated. Bakken shale cores were generally oil-wet or intermediate-wet (before introduction of the surfactant formulation). The four surfactant formulations that we tested consistently altered the wetting state of Bakken cores toward water-wet. These surfactants consistently imbibed to displace significantly more oil than brine alone. Four of the surfactant imbibition tests provided enhanced-oil-recovery [(EOR) vs. brine water imbibition alone] values of 6.8 to 10.2% OOIP, incremental over brine imbibition. Ten surfactant imbibition tests provided EOR values of 15.6 to 25.4% OOIP. Thus, imbibition of surfactant formulations appears to have a substantial potential to improve oil recovery from the Bakken formation. Positive results were generally observed with all four surfactants: amphoteric dimethyl amine oxide, nonionic ethoxylated alcohol, anionic internal olefin sulfonate, and anionic linear α-olefin sulfonate. From our work to date, no definitive correlation is evident in surfactant effectiveness vs. temperature, core porosity, core source (i.e., Upper Shale or the Middle Member), or core preservation (sealed) or cleaning before use.

Oil adsorption in shale nanopores and its effect on recoverable oil-in-place

“…Although shales are composed of organic matter and inorganic minerals, a recent experimental study suggested that Bakken shale is oilwet (Wang et al, 2012). This confirms that the inner pore walls are most likely to be carbonaceous.…”

Water imbibition of shale and its potential influence on shale gas recovery—a comparative study of marine and continental shale formations

“…Practices show that additional wells shut-in after fracturing can improve productivity effectively, and a 4.4 fold improvement can be seen in some wells (Yaich et al, 2015). Adding surfactants into fracturing fluids can improve this effect which is approved in Bakken (Wang et al, 2012). The soak-back technique is to use the spontaneous imbibition of fracturing fluid as a drive force to improve the gas recovery, however; additional wells shut-in pose several bad effects to some wells.…”

Impact of Surfactants for Wettability Alteration in Stimulation Fluids and the Potential for Surfactant EOR in Unconventional Liquid Reservoirs

“…Wettability affects flow behavior and, when altered, imbibition mobilizes oil because of capillary pressure changes from negative to positive (Wang et al 2012). To alter wettability, surfactants solutions are added to frac fluids to shift rock wettability to water-wet, this enhance imbibition by overcoming capillary forces and letting the water phase to penetrates into the matrix displacing oil in place (Shuler et al 2011).…”

“…In previous studies, cationic surfactants have shown improve oil recovery by wettability alteration in oil-wet chalk rocks (Austad et al 1998;Sharma and Mohanty 2013;Zhang and Austad 2005); however, this type of surfactant requires high concentrations and is too expensive to economically be implemented on the field (Adibhatla and Mohanty 2008;Chen et al 2001). In addition, anionic and nonionic surfactants have also been studied in fracture carbonates and chalk reservoirs effectively shifting wettability and reducing IFT, improving oil imbibition (Adibhatla and Mohanty 2008;Austad et al 1998;Babadagli et al 1999;Chen et al 2001;Sharma and Mohanty 2013;Wang et al 2012;Zhang and Austad 2005).…”

“…Various experiments have been conducted on the study of wettability and IFT alteration using surfactants based on spontaneous and forced imbibition in carbonate and sandstone reservoirs (Adibhatla and Mohanty 2008;Austad et al 1998;Babadagli et al 1999;Chen et al 2001;Hirasaki and Zhang 2003;Sharma and Mohanty 2013;Shuler et al 2011;Wang et al 2012;Zhang and Austad 2005); however, these experiments have limited application on unconventional reservoirs due to their ultralow permeability and low porosity values. There is limited literature on the study of combined effect of wettability and the corresponding IFT alteration effect on imbibition process on core samples from unconventional plays.…”