|Small Molecular Associative Carbon Dioxide (CO2) Thickeners for Improved Mobility Control
||Last Reviewed November 2016
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) recently 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.
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 Berkeley National Laboratory, an Industry Technology Facilitator Conference in Kuwait City, an OMICS Oil & Gas Conference in Dubai, and at Society of Petroleum Engineers (SPE), American Institute of Chemical Engineers, and American Chemical Society 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. A presentation summarizing the CO2 and NGL thickeners was presented at the 20th SPE IOR Conference in Tulsa, OK, in April 2016. Two papers related to the ARPA-E-funded design of small molecule NGL thickeners were published in 2016, and three papers related to advances in small molecule CO2 thickeners have been submitted with projection publication dates later in 2016.
Contact was made with 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 visited the University of Pittsburgh research group four times during 2014 and 2015 and have recently supplied Dr. Enick with samples of polyfluoroacrylates and polyFAST co-polymers based on their new monomer..
Phase 2 began in January 2016. Phase 2 requires the identification of a CO2 thickener—present at 1wt.% or less—capable of tripling the viscosity of CO2 flowing through a sandstone or carbonate 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 conservative. 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 SCAL, 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.
Although several highly effective small molecule CO2 thickeners were developed under ARPA-E funding, each required the addition of a significant amount of co-solvent (e.g. hexane or toluene) to the CO2, which was deemed undesirable for the core testing.
Therefore they have begun the assessment of high molecular weight polyfluoroacrlate-based polymers made using a synthetic technique that could be scaled up to large volumes. The initial tests involved flowing pure CO2 through a Berea sandstone core at a constant flow rate, and then displacing (at the same rate) the pure CO2 with several pore volumes of CO2 thickened with 1wt.% of the polymer; pressure drop across the core length was continuously measured. Results indicated that very high increases in pressure drop (up to 100-fold) were realized upon injection of thickened CO2. However, it also became apparent that the pressure drop increase was due not only to viscosity enhancement, but also due to polymer adsorption, the presence of CO2-insoluble impurities in the polymer sample that blocked pore throats, and possibly polymer-induced changes to the clay in the core.
As a result, the decision was made to add an examination of using PolyFAST as a conformal agent, used to divert CO2 into areas that were previously unswept.
Current Status (November 2016)
The objective of the next round of tests will be to determine the best way to use a CO2+PFA solution as a conformance control agent. To better represent a thief zone, the brine-satured high permemability zone will first be displaced by many pore volumes of pure CO2, while the lower permemability zone will be kept in a brine saturated state. The team will then consider injecting the CO2+PFA solution in both the high and low permeability zones and then injecting pure CO2, and compare the results with having the CO2+PFA solution be injected only into an isolated thief zone followed by CO2 injection into both zones. The team will also conduct tests of CO2 and thickened CO2 displacing SACROC crude oil from cores to determine if improved oil recovery occurs, and if dramatic increases in pressure drop also occur.
Project Start: October 1, 2012
Project End: September 30, 2017
DOE Contribution: $747,997
Performer Contribution: $284,139
NETL – Adam Tew (firstname.lastname@example.org or 412-386-5389)
University of Pittsburgh – Robert Enick (email@example.com or 412-277-0154)
Quarterly Research Progress Report [PDF-437KB]
Quarterly Research Progress Report [PDF-501KB]