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.

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.

University of Pittsburgh
Pittsburgh, PA

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 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.

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.

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

$977,000

$260,000 (21% of Total)

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

Final Project Report [PDF-973KB]