Polyfluoroacrylate (PFA) is a hydrophobic and oleophobic polymer that is soluble in high pressure carbon dioxide (CO2). In this study, the ability of PFA-CO2 solutions to greatly reduce the apparent permeability of split or cracked Portland cement cylindrical samples is assessed. The apparent permeability values of confined samples were determined before and after treatment with PFA-CO2 solutions. In four tests, PFA-CO2 solutions were continuously displacing pure CO2 from the cracked cement and the decrease in apparent permeability due to PFA adsorption and wettability alteration was monitored. The lowest apparent permeability cracked cement sample (81 nD) was completely sealed with a very small amount of solution. Samples with initial apparent permeabilities of 89 & mu;D and 29.4 mD exhibited 92% and 99% reductions in permeability, respectively, before the experiments had to be stopped because of the excessively large increase in pressure drop. A 50% reduction in apparent permeability was observed with a 3.80 mD sample. Four other split cement samples (bound together with tape) with an initial apparent permeability in the 9.0-70 mD range were removed from the core holder and immersed in a PFA-CO2 solution for 24 h to allow for PFA adsorption. Then the PFA-CO2 solution was depressurized, allowing for the deposition of additional PFA from the solution within the crack as the pressure fell below the cloud point pressure of the PFA-CO2 solution. These four samples were then confined again in a core holder and apparent permeability reductions of 29-93% were observed. Results from these eight experiments indicates that the more substantial reductions in the nD - mD apparent permeability of the cracked cement correlated to lower initial crack permeability, higher PFA concentration, and slower injection rate of the PFA-CO2 solution into the crack.

High molecular weight polyfluoroacrylate (PFA) is an amorphous, sticky, hydrophobic, oil-phobic, CO2-soluble homopolymer. 1 wt% PFA-in-CO2 solutions are four times as viscous as pure CO2. Although these solutions provided modest mobility control, unexpectedly large increases in pressure drop occurred because a portion of the PFA adsorbed onto the pore surfaces, altering wettability and significantly permeability. Further, PFA exhibited diminished solubility in CO2 in the presence of light extracted hydrocarbons. Although these effects rendered CO2-PFA solutions inappropriate for mobility control, excellent conformance control for dual parallel brine-saturated cores was attained when a CO2-PFA solution was first injected into the isolated high permeability sandstone core. This core was then placed in parallel with a low permeability sandstone core, and all of the subsequently injected CO2 was diverted to the low permeability core. Conformance control was not as effective in limestone, possibly due the low pH-induced erosion of the limestone that mitigated PFA adsorption.

Abstract In this study, we propose a CO2-polymer solution for conformance control agent in order to divert the subsequently injected CO2 away from thief zones and toward lower permeability oil-rich zones. A novel CO2-soluble polyfluoroacrylate (PFA) was synthesized. PFA is an amorphous, sticky, transparent, homopolymer that dissolves readily in CO2 at temperatures and pressures commensurate with CO2 EOR. PFA is based on a monomer that contains six (rather than eight) fluorinated carbons, thereby eliminating the environmental concerns associated with possible degradation products. Because PFA has high molecular weight, the addition of ~1wt% PFA to CO2 thickened CO2 by a factor of about four. Nnumerous core floods were then conducted to determine if the adsorption of PFA onto the rock surfaces could provide conformance control. When a CO2-PFA solution is injected into porous media, a portion of the dissolved PFA strongly adsorbs onto the mineral surfaces, regardless of what fluid was originally present in the pores. Because PFA is highly hydrophobic and oil-phobic, the thin PFA film deposited on the rock surfaces changes the wettability and dramatically reduces the permeability of the rock (especially sandstone) for subsequently injected fluids. This strong adsorption and change in wettability significantly reduces the permeability of the rock to subsequently injected brine or CO2. Dual parallel core floods were conducted to demonstrate the efficacy of PFA-CO2 solutions for conformance control. Excellent results were obtained when a CO2-PFA solution was injected solely into an isolated high permeability (80 mD) Berea sandstone core (the thief zone) that was previously flooded with brine and CO2. After this treatment, the Berea core was then placed in parallel with a 20 mD Carbon Tan sandstone core. All of the subsequently injected CO2 was diverted to the Carbon Tan core. Similar results were obtained with dual parallel limestone cores. To the best of our knowledge, PFA is the first known example of a CO2-soluble polymeric conformance control agent.