The overall research goal is to test the effectiveness of various CO2 thickener compounds that can induce very large changes in CO2 viscosity at typical injection and reservoir conditions associated with Carbon Dioxide Enhanced Oil Recovery (CO2-EOR).
The study consists of two phases. Phase 1 objectives are (1) to obtain commitment letters from CO2-EOR operators to provide oil, brine, and core field samples and information on field operating conditions for active or planned CO2-EOR floods, and (2) to develop laboratory testing plans for Phase 2. The Phase 2 objective is to assess the performance of compounds that demonstrate the ability to both dissolve into and thicken CO2 in order to reduce CO2 mobility and increase oil recovery over a wide range of operational and field conditions in core tests
University of Pittsburgh, Pittsburgh, PA 15260
Although large-scale CO2-EOR is practiced domestically, the potential for expansion is enormous. The single greatest obstacle to fully realizing that potential is the inherently poor volumetric sweep efficiency of the process. The very low viscosity of high-pressure CO2 is problematic for EOR projects because it exacerbates CO2 gravity override and induces viscous fingering, early breakthrough, poor sweep efficiency, and high CO2 injected to oil recovered ratios, especially in formations containing relatively uniform layers of rock.
Most of the CO2-EOR projects in the U.S. are in carbonate reservoirs, which tend to have high-permeability layers or networks of very high permeability fractures intermixed with low permeability layers or zones. Low viscosity CO2 causes conformance control issues; a significant portion of the injected CO2 flows into the higher permeability, watered‐out zones while a much smaller fraction of the CO2 enters the lower permeability, oil-bearing zones of interest. The result is very low sweep efficiencies in low permeability zones during EOR operations.
The U.S. Department of Energy (DOE) sponsored an extensive literature review of strategies for improved mobility and conformance control during CO2 floods. The findings indicated that the state‐of‐the‐art technique for mitigating unfavorable mobility ratios remains the water-alternating‐gas (WAG) process. Rather than implementing WAG or surfactant-alternating-gas (SAG) processes that require substantial amounts of water in an attempt to lower gas permeability, the University of Pittsburgh intends to dissolve a dilute (<1wt.%) amount of a “thickener” or “viscosifier” into the CO2, thereby yielding a transparent, thermodynamically stable, high-pressure CO2‐rich phase that is significantly more viscous than pure CO2. This research is being funded through a DOE ARPA-E project which, if successful, will provide the compounds for testing under NETL’s FE0010799 project.
The focus of the project is to design, synthesize, and characterize a CO2 thickener that costs less than $10/lb. and can be manufactured on a large scale. By dissolving a dilute amount of a “thickener” or “viscosifier” into the CO2, a transparent, thermodynamically stable, high-pressure CO2‐rich phase is created that significantly increases viscosity over pure CO2. An increase in CO2 viscosity should reduce problems with CO2 gravity override, viscous fingering, production well early CO2 breakthrough, poor sweep efficiency, and high injected CO2 to oil recovered ratios.
More than 90 percent of CO2-EOR floods employ water-intensive WAG processes for mobility control, creating a wide market for a CO2 thickener. A CO2 thickener has long been recognized as a game‐changing, transformative technology because of its potential to eliminate water injection for mobility control. Some of the remaining 10 percent of CO2-EOR projects that do not employ WAG are still plagued by mobility control issues. Therefore, the design of an economic CO2 thickener remains an extremely relevant research aspiration. These factors contribute to increased oil recovery, better recovery economics, and fewer environmental impacts.
During Phase 1,the initial phase of this project,various CO2 EOR operators were contacted by email, phone, and during in-person meetings. Written commitments for field samples (cleaned cores, oil, brine) and details of reservoir conditions were received from four companies. The Univesity of Pittsburgh (Pitt) had established relationships (especially with Kinder Morgan and Denbury Resources) that would have facilitated field trials, if the thickener was successfully developed.
A one-year no-cost extension on Phase 1 of this project allowed the researchers to continue developing the best thickener to the fullest extent under ARPA-e funding, prior to conducting the Phase 2 core testing associated with this NETL award. The design, synthesis, and purification of CO2 thickeners and initial assessments of their CO2 solubility and ability to thicken CO2 were conducted until April 30, 2016, under separate ARPA-e funding.
Phase 2 began on January 1, 2016, and extended to September 30, 2017. The ARPA-e-sponsored thickeners were designed to be small associating molecules that aggregate in solution to induce large increases in viscosity at low concentration. Pitt generated several effective small molecule CO2 thickeners with ARPA-e funding. Unfortunately, none of these small molecule thickeners could dissolve in CO2 without the addition of unacceptably large amounts of hexane or toluene as a co-solvent (e.g. 20wt% hexane, 80wt% CO2). Therefore, no small molecule thickeners were viable candidates for Phase 2 of this award.
During Phase 2, extensive core testing of the most promising polymeric CO2 thickener was completed. Pitt researchers verified CO2 solubility with a phase behavior cell and the thickening potential of all polymer samples with a falling ball viscometer and a falling cylinder viscometer. The researchers planned the core tests that were conducted at Special Core Analysis Laboratories, Inc., (SCAL) in Midland, TX., and interpreted the core flooding results. The researchers also generated several fluoroacrylate homopolymers (PFA) and polyFAST polymers via bulk polymerization (mixing monomer and initiator and heating, then separating the polymer from small amounts of un-reacted monomer).
The PFA polymer was shown to impact reasonable improvements in mobility control during the SCAL core tests; for example two different viscometers were used to show that the CO2 viscosity increased by a factor of about 3.5 when 1wt% PFA was dissolved in the CO2. However, there was clear and surprising evidence of dramatic reductions in core permeability due to PFA adsorption, especially for sandstones. In turn, researchers realized that the CO2-PFA solution could greatly reduce the permeability of an isolated “thief zone.” These effects were more dramatic for sandstone than for limestone. Therefore, these fluoroacrylate polymers can serve as a CO2-soluble conformance control agent for CO2-EOR, especially in sandstone formations. This injection of a single phase solution of CO2 + PFA for permeability reduction is likely the first report of a CO2-soluble conformance control additive. The researchers also demonstrated that the optimal strategy for using CO2+PFA solutions for conformance control is analogous to the application of water-based polymeric gels; the CO2+PFA solution should first be injected only in an isolated thief zone to induce dramatic reductions in permeability, only in that thief zone, and then CO2 should be injected into all of the zones.
Finally, it was noted that given the propensity of PFA to adsorb onto sandstone, the adsorption of PFA from CO2-PFA solutions onto cement surfaces may be capable of sealing cracks in casing cement that other remediation fluids (wet cement, solids-free resin, viscous aqueous emulsions) may have trouble accessing. Therefore, at the end of the project the research team reported on two proof-of-concept experiments for sealing cracked cement samples; the cracked samples had permeability of 81 nanoDarcies and 89 microDarcies. The results indicated that these small cracks could be completely sealed as CO2+PFA solutions flowed through them via the adsorption of PFA.
The project ended on September 30, 2017. The final report will be available under "Additional Information" when it is completed.