Exploration and Production Technologies
Small Molecular Associative Carbon Dioxide (CO2) Thickeners for Improved Mobility Control Last Reviewed 12/4/2015

DE-FE0010799

Goal 
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

Performer
University of Pittsburgh, Pittsburgh, PA 15260

Background
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.

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, wateredout 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 permeabiltity zones during EOR operations.

 

DOE  recently sponsored an extensive literature review of strategies for improved mobility and conformance control during CO2 floods. The findings indicated that the stateoftheart technique for mitigating unfavorable mobility ratios remains the water-alternatinggas (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. percent) amount of a “thickener” or “viscosifier” into the CO2, thereby yielding a transparent, thermodynamically stable, high-pressure CO2rich 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.

Impact
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 CO2rich 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 gamechanging, 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.

Accomplishments
Letters of commitment have been obtained from Denbury Resources, Kinder Morgan, Tabula Rasa, and Conoco Phillips. Discussions are ongoing with Denbury Resources concerning the equipment requirements for a field trial because Denbury appears to be the company most interested in pursuing one. In fact, a small single-well test of high-pressure pumps and static mixers to introduce CO2-soluble additives was conducted at a Denbury field under Dr. Enick’s direction.  Project personnel have made presentations concerning this work at the University of Rhode Island, Rochester University, Lawrence Berkely National Laboratory, an Industry Technology Facilitator Conference in Kuwait City, an OMICS Oil & Gas Conference in Dubai, and at SPE, AIChE, and ACS conferences. Dr. Enick also received a request from Kinder Morgan, a major CO2 EOR operator, to consider recommending a natural gas liquid (NGL) (propane-heptane) thickener for a hydrocarbon miscible flood in the Yates field.  Propane is easier to thicken than CO2, and many of the CO2-thickener candidates should work more readily in propane. Plans are being made to deliver a recommendation for an NGL thickener to Kinder Morgan for several options, including a propane pilot, a natural gas liquid (NGL) flood, or the injection of CO2-NGL mixtures.  An abstract related to the thickening of CO2 and NGL has been accepted for presentation at the 20th SPE IOR Conference in Tulsa, OK, April 2016. At least three papers related to the ARPA-e funded design of small molecule thickeners are planned for submission in early 2016.

Contact was made with Daikin, the manufacturer of a new environmentally benign fluoroacrylate. Fluoroacrylate  is the main component of the polyFAST thickener that researchers are considering.The thickener was originally developed with a fluoroacrylate that was biopersistent and an environmental risk. Representatives of a fluoroacrylate manufacturer have visited the University of Pittsburgh research group four times during the last year and have recently supplied Dr. Enick with samples of polyfluoroacrylates and polyFAST co-polymers based on their new monomer. Injection pumps and static mixers for use in adding a thickener to a CO2 pipeline were tested in association with a major CO2 EOR operator. Detailed plans for simulating EOR flooding in the laboratory using the most promising CO2 thickeners developed under ARPA-E funded research are also being developed. Phase 1 of this NETL project has been completed. A one-year no-cost extension to determine the optimal thickener (designed using ongoing ARPA-E funding) for use during Phase 2 of the NETL coreflood study was requested and granted. 

Current Status (December 2015)
Phase 2 is scheduled to begin on January 1, 2016.  Phase 2 requires the identification of a CO2 thickener—present at 1wt. percent or less—capable of tripling the viscosity of CO2 flowing through a core. Based on falling ball viscometry, which is conducted at higher shear rates than those associated with flow through porous media, two compounds have been identified.  Falling ball viscometry is simple to perform and can rapidly screen candidates for CO2 thickening capability; but because these CO2-thickener solutions exhibit higher viscosity at lower shear rates, the falling ball results tend to be pessimistic. Therefore both compounds (one polymer and one small associating molecule) are also being assessed in a flow-through-porous apparatus that yields a more meaningful viscosity result at shear rates commensurate with those that occur in the field. The University of Pittsburgh and petroleum engineers at SCLA, Inc. in Midland Texas have collaborated on the design of a set of core flooding experiments to assess the efficacy of the CO2 thickener candidates.

ARPA-e funded synthesis of new, small molecule thickeners is also continuing until April 30, 2016.  For example, one promising candidate is capable of thickening CO2 100-fold at a concentration of 1wt%. However it requires extremely large concentrations of a co-solvent.  This small-molecule thickener structure was successfully modified  to reduce  the co-solvent requirement by 50% and efforts are continuing to completely eliminate the need for a co-solvent during Phase 2 testing.  A polymeric thickener that does not require co-solvent has been synthesized bya major chemical company is also ready for testing in Phase 2.

Project Start: October 1, 2012
Project End: September 30, 2017

DOE Contribution: $747,997 
Performer Contribution: $284,139

Contact Information:
NETL – Gary Covatch (gary.covatch@netl.doe.gov or 304-285-4589)
University of Pittsburgh – Robert Enick (rme@pitt.edu or 412-277-0154)

Additional Information

Quarterly Research Progress Report [PDF-437KB]

Quarterly Research Progress Report [PDF-501KB]

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