The research goals are to improve the effectiveness of polymer gels to increase volumetric sweep efficiency of fluid displacement processes and to reduce water production in production wells. Improvements in these areas have the potential to slow the rate of decline in oil production from existing wells and increase the ultimate oil recovery from existing reservoirs.

University of Kansas Center for Research
Lawrence, KS

Polymer gels are being used to improve volumetric sweep efficiency in oil recovery displacement processes and to control water flow in oil production wells. However, the potential exists for significant improvement in these processes with corresponding increases in oil recovery and large reductions in operating costs due to decreased water handling.

The application of gels to improve volumetric sweep efficiency is limited by the lack of gel systems that can be used for in-depth treatment, i.e., by an inability to have the gels penetrate large distances into the reservoir away from the treated well. Researchers have demonstrated that the formation and growth of pre-gel aggregates is a major factor that inhibits deep penetration of gels into reservoir rock. One task of this project was to develop a mathematical model of the aggregate growth. A second problem during the placement of gelants occurs in carbonate reservoirs where fluid-rock interactions interfere with the gelation chemistry. Researchers studied the role of carbonate dissolution on the in-depth propagation of gelants.

Water production in production wells can be reduced by using gel systems that exhibit DPR. The mechanism producing this beneficial phenomenon has been elusive for several investigators. The project performer has developed a conceptual model of DPR. Development of this model allows the opportunity to develop systems that can be used to control water flow without significantly affecting oil flow into a production well.

Project Results
Project researchers have accomplished the following:

• Development of a mathematical model that simulates polymer crosslinking processes.
• Kinetic study of the reaction between chromium acetate and polyacrylamide.
• Development of a mathematical model that simulates the transport of chromium acetate through carbonate rocks.
• Experimental study of the effect of gelant composition and oil/water pressure gradients on disproportionate permeability reduction (DPR).

Benefits
The mathematical model and accompanied data describing the formation and growth of pre-gel aggregates is a major advancement in the fundamental understanding of the placement of gelants in oil reservoirs. Additionally, the role of carbonate dissolution on the in-depth propagation of gelants is more clearly known, and a mathematical model was developed to simulate the chemistry. These tools allow for the improvement of current gel systems and the development of new systems that can be applied in the field. Reduced water production from improved systems reduces operating costs for the oil producer and mitigates environmental concerns of large volumes of produced water. Lower operating costs extend the life of reservoirs, resulting in increased oil recovery.

Project Summary
DPR is the phenomenon whereby a gel treatment in a porous medium reduces the water permeability by a greater factor than the oil permeability is reduced. A conceptual model that describes mechanisms responsible for DPR was developed from flow experiments. The model's main features are a) flow channels through the gels are formed during the injection of oil that removes a significant portion of the gel volume by partial displacement and dehydration of the gel; b) high oil saturations in the channels during water flow causes the water permeabilities to be reduced by significant factors; and c) during oil flow, oil saturates most of the channel volume, resulting in a much smaller reduction of the oil permeabilities. Experimental results of the effects of gelant composition and flow rates of water and oil on DPR are presented.

A mathematical model was developed that simulates the chemical reactions where polymer molecules are crosslinked to form a three-dimensional network or gel. The model was based on statistical probabilities of reactions and yields information on various molecular-weight quantities as functions of conversion of reactive sites and time. Incorporated in this model are results of a kinetic study of the reaction of chromium acetate and polyacrylamide, currently the most commonly used gel system. A second mathematical model was developed that describes the fluid-rock interactions that occur during flow of gelant components through carbonate rocks.

(August 2005)
A six-month, no-cost extension to a three-year project extended the termination date to December 31, 2005.

$691,443

$499,913 (42% of total)

NETL - Betty Felber (betty.felber@netl.doe.gov or 918-699-2031)
U. of Kansas - G. Paul Willhite (Willhite@ku.edu or 785-864-2906)

Publications
Two annual reports submitted to DOE for the periods July 1, 2002, to June 30, 2003 (November 2003) and July 1, 2003, to June 30, 2004 (December 2004).

Ganguly, S., Willhite, G.P., Green, D.W., and McCool, C.S., Effect of Flow Rate on Disproportionate Permeability Reduction, SPE 80205, SPE International Symposium on Oilfield Chemistry, Houston, TX (February 5-7, 2003).

Jin, H., McCool, C.S., Willhite, G.P., Green, D.W., and Michnick, M.J., Propagaton of Chromium(III) Acetate Solutions Through Dolomite Rock, SPE Journal, 8, June 2003, pp. 107-113.

Jain, R., McCool, C.S., Green, D.W., Willhite, G.P., and Michnick, M.J., Reaction Kinetics of the Uptake of Chromium(III) Acetate by Polyacrylamide, scheduled for publication in SPE Journal, December 2005.

Nguyen, T., Green, D.W., Willhite, G.P., and McCool, C.S., Effect of Composition of a Polyacrylamide-Chromium Acetate Gel on the Magnitude of Gel Dehydration and Disproportionate Permeability Reduction, SPE 89404, SPE/DOE Fourteenth Symposium on Improved Recovery, Tulsa, OK, April 17-21, 2004.

Cheng, M., Wang, C., McCool, C.S., Green, D.W., and Willhite, G.P., Modeling of Pre-Aggregate Growth During the Gelation of a Polyacrylamide-Chromium(III) Acetate Gel System Using the Theory of Branching Processes, SPE 93354, SPE International Symposium on Oilfield Chemistry, Houston, TX (February 2-4, 2005).