Anthraquinone Siloxanes as Thickening Agents for Supercritical CO₂

Abstract

A series of aromatic amide-functionalized, low-molecular-weight polydimethylsiloxanes were synthesized, and their solubility and thickening ability in supercritical carbon dioxide (scCO 2 ) were evaluated. Amide-terminated polydimethylsiloxane (PDMS) oligomers with simple or electron-rich aromatic groups were found to be moderate viscosity oils, indicating that the end groups do not interact strongly enough to be thickeners. Attachment of electron-deficient aromatic groups, such as 4-nitrophenyl, biphenyl, or anthraquinone, onto these amides produced solid polysiloxanes, even at relatively higher degrees of polymerization on the PDMS core (x = 40−50). PDMS derivatives with anthraquinone-2-carboxamide (AQCA) end groups were clear, somewhat rubbery solids. These compounds were found to act as thickeners and gelators of hexanes. In addition, they were able to thicken and/or gel mixtures of hexane and scCO 2 . The best results were obtained with an AQCA-terminated branched PDMS. Branched fluorenone-2-carboxamide was found to be a waxy solid that was very soluble and, therefore, not a thickener for hexane, and fluorenone-4-carboxamide was a viscous oil, indicating that these amide groups did not interact sufficiently to be useful. 10,10-Dioxothioxanthone-2-carboxamides were more efficient gelators of hexane than anthraquinone-2-carboxamides. However, they were not sufficiently soluble in scCO 2 , even in the presence of hexane co-solvent, to be useful as thickeners. ■ INTRODUCTIONThere has been a steady increase in CO 2 flooding and steam flooding enhanced oil recovery (EOR) techniques in recent decades, with the injection of high-pressure CO 2 recently becoming the most prominent EOR method in the U.S. 1 High-pressure CO 2 can extract light components from crude oil because it reduces CO 2 −oil interfacial tension and forms a CO 2 -rich solution that is an excellent solvent for the crude oil ahead of it. Further, CO 2 swells the oil and reduces oil viscosity. Although not as strong of a solvent as propane, natural gas liquids (NGL), or liquefied petroleum gas (LPG), the U.S. has several prolific subterranean formations containing high-pressure, high-purity CO 2 that enable massive amounts of this fluid to be available at a relatively low cost. However, the viscosity of supercritical CO 2 at typical operating conditions is roughly 10−100 times less than that of the light oil being displaced. Therefore, this oil displacement process is plagued by viscous fingering of CO 2 through the sandstone or carbonate rock. This results in a high CO 2 use rate, a disappointingly low oil production rate, and the recompression and recycle of large volumes of CO 2 . About 90% of the CO 2 flooding projects in the U.S. mitigate the high CO 2 mobility by co-injecting water, which reduces the CO 2 saturation (i.e., volume percent of CO 2 in the pores) and thereby lowers the relative permeability of CO 2 . The presence of the increased water in the pores may make it more difficult for the CO 2 and oil phases to contact one another, 2 but ...

Effect of different factors on the yield of epoxy-terminated polydimethylsiloxane and evaluation of CO2 thickening

“…Silicone cannot be dissolved separately at high-pressure CO 2 , which has been shown by a high-pressure dissolution experiment. 11,[29][30][31] According to the literature 10,31 and a high boiling point (110.6 C), toluene is an excellent organic liquid to thicken CO 2 . Compared with hexane, a smaller amount is another important reason to choose toluene.…”

“…5,9,10 Nonetheless, it remains a principal challenge to increase the poor solubility and thickening capability in liquid CO 2 . 11 To increase the thickening property of silicone in liquid CO 2 , researchers have employed chemical modication, polymer graing and co-solvent assistance. 11 Excessive addition of a co-solvent resulted in excellent solubility in CO 2 , but was deemed unwise and expensive.…”

Direct Gas Thickener

“…They incorporated sulphonate groups onto the ends of polyisobutylene, and although this polymer is soluble in CO 2 at a concentration of 0.4 wt%, the sulphonated ionomer was only partially dissolved in CO 2 , which is a weak solvent for this ionic group. In a recent publication, O'Brien et.al [30] used a route similar to the approach proposed by the PRRC group. The difference between the two studies is that the PRRC group used functional groups in the polymer chain that experiences intermolecular interactions with CO 2 , while O'Brien proposes using functional groups that form self-assembly and intermolecular interactions via hydrogen bonding (donor-acceptor), π-π stacking, and dipole-dipole interactions.…”

Study on the optimization of silicone copolymer synthesis and the evaluation of its thickening performance

“…40 In contrast, siloxane polymer is advantageous because it is inexpensive and environmentally friendly, and it subsequently has attracted the interest of researchers around the world. [12][13][14] Moreover, comparisons between the water consumption of hydraulic fracturing with that of CO 2 fracturing technology have revealed some signicant differences during oileld development, and water pollution was increased. Furthermore, greenhouse gas emissions have also decreased based on CO 2 fracturing technology.…”

“…The viscosity of a mixture of silicone polymer, co-solvent, and Sc-CO 2 has been previously measured, and the data verify that a series of silicone polymers are effective in thickening Sc-CO 2 . 12,13 This has generated interest in the development of linear siloxane polymers for use as silicone thickeners. 14 Currently, uoropolymers have been recognized and deployed as good alternatives due to their enhanced solubility in CO 2 , and they have been applied in a wide range of CO 2 fracturing projects in various oilelds.…”