(366ad) CO2-Soluble Wettability-Altering Surfactants for CO2 Fuff-and-Puff in Fractured Formations

Despite the prolific increase in oil production due to the use of horizontal fractured wells in formations such as the Bakken (NK) and Eagle Ford (TX), only 5% of the oil in these formations is produced during primary (natural pressure-induced) recovery. To recover some of the remaining 95% of the oil via CO₂ “huff-and-puff”, high pressure CO₂ is injected into the fractured shale. The CO₂ is allowed to diffuse into the oil retained within the very low permeability shale for a while, and then a CO₂-oil mixture flows back to the surface as the well is opened for production. CO₂-induced oil recovery is attributed to several mechanisms, including oil swelling, oil viscosity reduction, extraction of oil into CO₂, and dissolution of CO₂ into oil. However, significant wettability alteration is not a notable mechanism for CO₂ huff and puff enhanced oil recovery.

CO₂ huff-and-puff results in the permanent storage of a portion of the CO₂ in the shale, the production of oil to the surface, and the simultaneous production of some of the CO₂ that can be captured and re-used in the next huff and puff cycle.

The focus of this research project is related to improving CO₂ huff-and-puff via the dissolution of a dilute concentration (0.01-1.00 wt%) of an additive in CO₂ that changes the fractured shale wettability from oil-wet toward CO₂-wet. Shale wettability alteration from oil-wet toward CO₂-wet occurs due to surfactant adsorption and/or oil emulsification. (There are no commercially available CO₂-soluble cationic surfactant candidates that could alter wettability via removing naturally occurring anionic oil components via ion pairing.) This concept, if successful, will impart another mechanism to the CO₂ huff and puff process that enhances oil recovery.

Each wettability-altering candidate is a commercially available compound that contains a CO₂-phile (e.g. carbonyl, polyethylene oxide or propylene oxide) and an oil-phile (e.g. alkyl group). The solubility of surfactants in CO₂ was determined as a function of temperature and pressure. The surfactant candidates included an ethoxylated tridecyl alcohol, a propoxylated tridecyl alcohol, 3-pentanone, Span 20, and Tween 20. CO₂ cloud points curves (i.e. solubility curves) for these compounds were determined, and those surfactants that were soluble in CO₂ at typical CO₂ huff and puff conditions were tested for wettability alteration.

Wettability alteration measurements consisted of high pressure contact angle experiments in which flat oil-wet shale samples were immersed in high pressure CO₂-surfactant solutions, and a drop of oil was placed on the shale surface. In the absence of surfactant, the oil droplet spread on the oil-wet shale. However, when surfactants were present in the CO₂, shifts in wettability from strongly oil-wet toward intermediate oil-CO₂ wet or CO₂-wet were observed as the oil droplet beaded up on the shale.

CO₂-oil interfacial tension (IFT) at high temperature and pressure is affected by the surfactant in CO₂ As shown in several recent literature reviews, reduction of CO₂-oil IFT to ultralow values is not necessary for this process; oil recovery via wettability alteration is actually promoted by significant changes in wettability coupled with modest reductions in CO₂-oil IFT.

Finally, CO₂ huff-n-puff experiments were conducted with CO₂ and CO₂-surfactant solutions using small cylindrical shale cores. The shale core was first aged in a high-pressure hot oil for a week to ensure the shale is oil-wet and oil-saturated. The shale was weighed before it was immersed in a high-pressure solution of CO₂ or CO₂-surfactant solution for 24 hours. The vessel was then vented over a 10-minute period, during which time oil and CO₂ emerged from the shale. Oil recovery was determined via material balance. In-situ NMR measurements will also be used to determine the distribution of oil and CO₂ and oil within the fractured shale during and after the experiment.

A ranking of the wettability-altering agents indicates that some of the candidate surfactants have little discernible impact on oil recovery, however others induced an increase in the fraction of oil recovered during the huff and puff process that could justify the increased cost of adding surfactant to the CO₂.

Research Interests:
Transport Dynamics of CO₂ and surfactant: My research interest is investigating the transport phenomena and movement of CO₂ and surfactant in fractured formations, aiming to understand the mechanism in which they penetrate the rock displace the trapped oil.

Mixture Separation: I also aim to research how mixtures of can be separated using CO₂ and their solubilities at high temperatures and pressures. These separated mixtures can then easily be used for their desired purposes.

Maximizing Production; Minimizing Environmental Impact: My research interest is also aimed towards optimizing huff-and-puff process to maximize production and recovery of oil while also minimizing any environmental impact this process might cause. While CO₂ sequestration is an important aspect, my aim is study these concept further to optimize the whole process.