Final Research Performance Report - Small Molecular Associative Carbon Dioxide (CO2) Thickeners for Improved Mobility Control
- Univ. of Pittsburgh, PA (United States)
The initial objective of this project was to promote the application of a CO2 thickener for improved mobility control during CO2 EOR based on solubility tests, viscosity tests, and core floods. Ultimately, it was demonstrated that the CO2-soluble polymeric thickeners are much better suited for use a CO2-soluble conformance control agents for diverting the flow of CO2 away from thief zones. Our team generated several effective small molecule CO2 thickeners with ARPA-e funding. Unfortunately, none of these small molecule thickeners could dissolve in CO2 without the addition of unacceptably large amounts of hexane or toluene as a co-solvent Therefore none were viable candidates for the core flooding studies associated with NETL award. Therefore during the entire core flood testing program associated with this NETL award, our team used only the most promising polymeric CO2 thickener, a polyfluoroacrylate (PFA). In order to produce an environmentally benign polymer, the monomer used to make the new polymers used in this study was a fluoroacrylate that contains only six fluorinated carbons. We verified CO2 solubility with a phase behavior cell. The thickening potential of all polymer samples was substantiated with a falling ball viscometer and a falling cylinder viscometer at Pitt. Two different viscometers were used to determine the increase in CO2 viscosity that could be achieved via the dissolution of PFA. Praxair, which has an interest in thickening CO2 for pilot EOR projects and for waterless hydraulic fracturing, agreed to measure the viscosity of CO2-PFA solutions at no cost to the project. Falling cylinder viscometery was conducted at Pitt in our windowed high pressure phase behavior cell. Both apparatuses indicated that at very low shear rates the CO2 viscosity increased by a factor of roughly 3.5 when 1wt% PFA was dissolved in the CO22. Our team also planned thickener concentrations and compositions at Pitt for the core tests that were conducted at Special Core Analysis Laboratories, Inc., (SCAL) in Midland, TX, where the ability for PFA to reduce CO2 mobility in a core was then tested. During the beginning of these tests, the PFA polymer was then shown to impart reasonable improvements in mobility control during the SCAL core tests; as the CO2-PFA solution displaced CO2 from the core at a constant volumetric flow rate, the pressure drop increased as expected. However, as the test progressed, there was clear and surprising evidence of dramatic reductions in core permeability due to PFA adsorption, especially for sandstones. For example, as the CO2-PFA solution displaced pure CO2 from sandstone and limestone cores, the pressure drop increased by factors of multiple hundreds to over a thousand. It was subsequently demonstrated that the PFA injected into the core either (a) adsorbed strongly and irreversibly onto the rock surfaces, (b) deposited/precipitated within the rock, thereby blocking pores in a manner that could be dislodged by large changes in flow rate or flow direction, or (c) remained in solution and passed completely through the core. The loss of PFA to the porous media and the unacceptably large increases in pressure drop both indicated that PFA was inappropriate for CO2 EOR mobility control, where thickener adsorption must be minimized and mobility reductions of only 10-100-fold are typically required. However, we realized that because the CO2-PFA solution could greatly reduce the permeability of porous media, it could serve as a near wellbore conformance control agent for blocking “thief zones”, where adsorption is acceptable and dramatic increases in pressure drop are desirable. These effects were more dramatic for sandstone than for limestone. Therefore, these PFA fluoroacrylate polymers can serve as a CO2-soluble conformance control agent for CO2-EOR, especially in sandstone formations. This injection of a single phase solution of CO2-PFA for permeability reduction is (to the best of our knowledge) the first report of a CO2-soluble conformance control additive. We also demonstrated that the optimal strategy for using CO2-PFA solutions for conformance control is analogous to the application of water-based polymeric gels; the CO2-PFA solution should first be injected only in an isolated thief zone to induce dramatic reductions in permeability only in that thief zone, and then CO2 should be injected into all of the zones. Finally, it was noted that given the propensity of PFA to adsorb onto sandstone, the adsorption of PFA from CO2-PFA solutions onto cement surfaces promote the sealing of extremely fine cracks in casing cement.
- Research Organization:
- Univ. of Pittsburgh, PA (United States); National Energy Technology Laboratory (NETL), Pittsburgh, PA, Morgantown, WV (United States)
- Sponsoring Organization:
- USDOE Office of Fossil Energy (FE)
- DOE Contract Number:
- FE0010799
- OSTI ID:
- 1414575
- Report Number(s):
- DOE-Pitt-0010799
- Country of Publication:
- United States
- Language:
- English
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