Enick, Robert M. (Dept. of Chemical and Petroleum Engineering, University of Pittsburgh) | Lee, Jason J. (Dept. of Chemical and Petroleum Engineering, University of Pittsburgh) | Cummings, Stephen D. (Dept. of Chemical and Petroleum Engineering, University of Pittsburgh) | Zaberi, Husain A. (Dept. of Chemical and Petroleum Engineering, University of Pittsburgh) | Beckman, Eric J. (Dept. of Chemical and Petroleum Engineering, University of Pittsburgh) | Dailey, Chris (Special Core Analysis Laboratories, Inc.) | Vasilache, Mihai (Special Core Analysis Laboratories, Inc.)
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.
Lee, Jason (University of Pittsburgh) | Dhuwe, Aman (University of Pittsburgh) | Cummings, Stephen D. (University of Pittsburgh) | Beckman, Eric J. (University of Pittsburgh) | Enick, Robert M. (University of Pittsburgh) | Doherty, Mark (GE Global Research) | O'Brien, Michael (GE Global Research) | Perry, Robert (GE Global Research) | Soong, Yee (US DOE NETL) | Fazio, Jim (US DOE NETL) | McClendon, Thomas R. (US DOE NETL)
CO2 miscible and immiscible displacements and hydrocarbon miscible floods are commonly plagued by low volumetric sweep efficiency, early gas breakthrough, high gas utilization ratios, and significant gas re-compression and recycle. Rather than addressing these problems via the water-alternating-gas (WAG) injection sequence that reduces gas relative permeability or the generation of gas-in-brine foams for reduced mobility, we propose increasing the viscosity of high pressure CO2 or NGL via the dissolution of dilute concentrations of thickening agents.
There are two strategies for increasing the viscosity of high pressure fluids; the dissolution of ultrahigh molecular weight polymers or associating polymers, or the dissolution of small molecules that self-assemble in solution to form viscosity-enhancing linear or helical supramolecular structures. Ideally a very small amount of the thickener will be required (roughly 0.1wt%) to elevate the CO2 or NGL viscosity to the same value as the oil being displaced (typically a 10-100 fold increase). Further, the thickened CO2 or thickened NGL should be a stable, transparent solution that does not require a heating/cooling cycle for viscosity enhancement to occur.
Thickener solubility and viscosity were determined over a 25-100oC range. Each of the three major NGL constituents (ethane, propane and butane) was thickened with an ultrahigh molecular polymer (commercial drag reducing agent), resulting in a 2-30 fold increase in viscosity at polymer concentrations of 0.5wt% or less. The polymer dissolved at the lowest pressure in butane and was most effective as a thickener in butane.
Three small molecule thickeners were identified for the NGL constituents; tri-alkyl-tin fluoride, hydroxyaluminum disoap, and a phosphate ester-crosslinker mixture. Remarkable viscosity enhancements were attained for propane and butane with the tri-alkyl-tin fluoride and aluminum soap; the crosslinked phosphate ester solutions exhibited modest viscosity increases. Only tri-alkyl-tin fluoride thickened ethane.
CO2 thickeners were assessed with a falling ball viscometer and pressure drop associated with flow through Berea sandstone. 4-5 fold increases in viscosity were attained with 1wt% of a high molecular weight polyfluoroacrylate. 3-4 fold increases in viscosity were attained with 1wt% high molecular weight polydimethyl siloxane, but a very large amount of toluene co-solvent was required. Although a remarkably effective small molecule thickener was designed for CO2 (100-fold increase at 1.3wt%), it required a heating/cooling cycle and a very large amount of hexane co-solvent.
We have identified the first polymeric and small molecule thickeners ever reported for ethane. Further, this study presents the largest viscosity increases ever reported for propane and butane with polymers and small molecule thickeners. We have presented the most effective polymeric thickeners for CO2 reported to date. This paper also summarizes numerous molecular architectures that are not viable for CO2 and highlights the most promising compounds that continue to be refined.