INCREASED OIL RECOVERY FROM MATURE OIL FIELDS USING GELLED POLYMER TREATMENTS
Gelled polymer treatments are applied to oil reservoirs to increase oil production and to reduce water production by altering the fluid movement within the reservoir. This report describes the results of a three-year research program aimed at reducing barriers to the widespread use of gelled polymer treatments by (1) developing methods to predict gel behavior during placement in matrix rock and fractures, (2) determining the persistence of permeability reduction after gel placement, and (3) developing methods to design production well treatments to control water production. The work focused on the gel system composed of polyacrylamide and chromium acetate. The molar mass of the polymer was about six million. Chromium(III) acetate reacted and formed crosslinks between polymer molecules. The crosslinked polymer molecules, or pre-gel aggregates, combine and grow to eventually form a 3-dimensional gel. A fundamental study to characterize the formation and growth of pre-gel aggregates was conducted. Two methods, flow field-flow fractionation (FFFF) and multi-angle laser light scattering (MALLS) were used. Studies using FFFF were inconclusive. Data taken using MALLS showed that at the gel time the average molar mass of gel aggregates increased by a factor of about three while the average size increase was approximately 50%. Increased acetate concentration in the gelant increases the gel time. The in situ performance of an added-acetate system was investigated to determine the applicability for in-depth treatments. Increased acetate concentrations delayed the development of increased flow resistance during gelant injection in short sandpacks. The development of increased flow resistance (in situ gelation) was extended from 2 to 34 days by increasing the acetate-to-chromium ratio from 38 to 153. In situ gelation occurred at a time that was approximately 22% of the bulk gelation time. When carbonate rocks are treated with gel, chromium retention in the rock may limit in-depth treatment. Chromium retention due to precipitation was investigated by flowing chromium acetate solutions through carbonate rock. Chromium precipitated faster in the rocks than in beaker experiments at similar conditions. A mathematical model previously developed fit the precipitation data reasonably well. The stability of gels when subjected to stress was investigated by experiments with gels placed in tubes and in laboratory-scale fractures. Rupture pressures for gels placed in small diameter tubes were correlated with the ratio of tube length to tube ID. In fractures, fluid leakoff from the fracture to adjacent matrix rock affected gel formation and gel stability in a positive way. Disproportionate permeability reduction (DPR) was studied in unconsolidated sandpacks and in Berea sandstone cores. A conceptual model was developed to explain the presence of DPR. The effect of a pressure gradient, imposed by injection of oil or brine, on the permeability of gel-treated cores was investigated. DPR increased significantly as the pressure gradient was decreased. The magnitude of the pressure gradient had a much larger effect on water permeability than on oil permeability.
- Research Organization:
- University of Kansas (US)
- Sponsoring Organization:
- (US)
- DOE Contract Number:
- AC26-99BC15209
- OSTI ID:
- 820714
- Resource Relation:
- Other Information: PBD: 1 May 2003
- Country of Publication:
- United States
- Language:
- English
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