Exploration and Production Technologies
Inexpensive CO2 Thickening Agents for Improving Mobility Control of CO2 Floods

DE-FC26-01BC15315

Program
This project was selected in response to DOE's solicitation DE-PS26-01NT41048 (December 1, 2000). The objective of this part of the solicitation was to access oil not recoverable by conventional methods by developing improved methods of gas, chemical, and microbial flooding for light oil recovery.

Project Goal
The goal of this project was to identify an inexpensive polymer capable of dissolving in carbon dioxide and increasing its viscosity, thereby enabling improved mobility control of CO2 floods.

Performer
University of Pittsburgh
Pittsburgh, PA

Project Results
During the final stages of this project, computational tools developed in the project were used to design CO2-soluble polymers. These tools allowed a qualitative explanation of why certain polymers were (or were not) CO2-soluble. At the end of the project, the design of a new polymer-poly(3-acetoxy oxetane), or PAO-that should be more CO2-soluble than PVAc was identified.

Benefits
An inexpensive, environmentally benign, safe, CO2 thickener composed of carbon, hydrogen, and oxygen (sulfur and nitrogen are also acceptable) added to CO2 as it is being injected will increase oil recovery from CO2 injection. The thickener prevents early breakthrough of CO2, reduces the amount of CO2 required to recover a barrel of oil, increases the rate of oil production, and ultimately increases the amount of oil that could be recovered from a formation. Use of effective thickeners would eliminate the additional cost of conducting a water-alternating-gas (WAG) process.

Background
About 1.5 billion standard cubic feet per day of CO2 is injected in U.S. oil reservoirs each day, accounting for production of 200,000 barrels per day of oil per. Despite the maturity of this technology, its performance still has much room for improvement. For example, about 8,000 standard cubic feet of CO2 is required for each barrel of oil recovered, which translates into the injection of 3.5 barrels of dense CO2 at reservoir conditions for each barrel of oil produced.

Further, large slugs of water must be injected alternately with slugs of CO2 (water-alternating-gas, or WAG) in order to make the CO2 flow more uniformly through the reservoir rather than "fingering" from the injection well to the production well and bypassing large regions of oil-bearing rock. Sweep efficiency problems are directly associated with the low viscosity of CO2 compared with that of the oil to be displaced.

The University of Pittsburgh research group, under previous funding from DOE, designed, synthesized, and evaluated the first CO2 thickener-poly(fluoroacrylate-styrene), or polyFAST-in the laboratory. PolyFAST remains the only CO2 thickener that has been reported. Although this result proved that a thickening agent could be designed for CO2, it was not a practical thickener for field application. Specifically, it was expensive, biologically and environmentally persistent, and not available in large quantity. All of these negative attributes were directly the result of the polymer having a high content of fluorine.

Project Summary
This project evaluated numerous polymers that contained no fluorine for this CO2 thickening application. Rather than performing a trial-and-error study of what dissolves in CO2, specific chemical groups that were known to have a strong and favorable interaction with CO2 were selected. The design of a thickener consists of two steps. First, a high molecular weight "base polymer" that is extremely soluble in CO2 must be designed. Second, the base polymer must be modified to become a thickener by adding a small amount of "CO2-phobic" groups that cause the dissolved thickener molecules to interact with one another. This type of interaction forms large, viscosity-increasing macro-molecules to form in the CO2. Unfortunately, this modification always will make the thickener less soluble in CO2 than the base polymer, so the base polymer must be soluble in CO2 at pressures less than the minimum miscibility pressure (MMP), or the pressure at which the CO2 flood is conducted.

This project focused on the first step-making the most CO2-soluble polymer possible. Oxygen-rich hydrocarbons (chemical groups composed of carbon, hydrogen, and oxygen) were determined to be the most promising "CO2-philic" groups. Ether, carbonyl, and acetate groups were determined to be particularly promising components for making a polymer that dissolves in CO2. Several low molecular weight polymers (a.k.a. oligomers) of comparable CO2 solubility were identified, including sugar acetates, polypropylene oxide, polymethyl acrylate, and polyvinyl acetate. The CO2-solubility of high molecular weight versions of these polymers varied dramatically. Poly(vinyl acetate), PVAc, was clearly the most CO2 soluble, inexpensive, high molecular weight, commodity polymer identified. For example, 5 wt % PVAc with a molecular weight of 600,000 could dissolve in CO2. Unfortunately, the pressure required to dissolve PVAc was much greater than the range of MMP values of CO2 floods. About 25 novel polymers were designed. Each was a candidate for being a CO2-soluble base polymer. However, none were more CO2-soluble than PVAc. After identifying PVAc, computing tools were developed to design inexpensive polymers that are even more soluble in CO2.

Current Status (August 2005)
This project has been completed and the final report is available below under "Additional Information".

Project Start: September 1, 2001
Project End: August 31, 2005

Anticipated DOE Contribution: $977,000
Performer Contribution: $260,000 (21% of Total)

Contact Information
NETL - Dan Ferguson (daniel.ferguson@netl.doe.gov or 918-699-2047)
U. of Pittsburgh - Robert Enick (rme@pitt.edu or 412-624-9649)

Additional Information

Final Project Report [PDF-973KB]

 
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