**3. Hydrocarbon gas thickeners**

## **3.1 Polymeric thickeners**

As discussed so far, most of the research regarding gas thickening agents have focused solely on CO2 because it is the most common injected fluid for MGI projects in the United States, Canada, and elsewhere [76, 77]. In addition, CO2 is a slightly more powerful polymer solvent than short-chain alkane gases. The structural symmetry of CO2 results in a substantial quadruple moment (Qi) at low pressure and temperature, which can magnify the quadrupole interaction by scale inversely with the molar volume to the 5/6 power (Qi \* = Qi. Vi −5/6) [78]. Despite these characteristics, CO2 is a weak solvent when compared to most organic solvents. However, there have been a few attempts at identifying polymeric thickeners for pure light hydrocarbon gases [79–81]. In the late 1960s, several patents reported initial attempts at thickening light alkane gases. Henderson et al. [81] made the first attempt to thicken a hydrocarbon by using three polymers including poly methyl laurylate, polybutadiene, and poly(alkyl styrene). These polymers (at a concentration of 0.25 vol%) are capable of improving the viscosity of light hydrocarbon gases by about 0.1%. Subsequently, Durben and co-workers examined polyisobutylene polymer (PIB, Mw: 130,000 g.mol<sup>−</sup><sup>1</sup> ) in a rich condensate mixture containing 75 vol% propane and 25 vol% heptane. They claimed to achieve a 2–5-fold viscosity increase at a concentration of 0.25 wt% of PIB [80]. However, none of the patented work reported the details of the method used to measure the viscosity of the solutions examined.

Subsequent attempts by Heller et al. to identify polymeric thickeners for LPG and CO2 [9] found that various poly-α-olefin polymers (PAO) based on n-pentene, n-hexene, and n-decene could be used. These polymers were found to be quite soluble in n-butane at a temperature of 298 K and pressure of 8.2 MPa; however, their solubility in CO2 was much more limited at a temperature of 305 K and pressure of 17.2 MPa. The addition of these polymers at concentrations ranging from 1 to 2.2 wt% to n-butane enhanced the viscosity by fivefold (**Table 2**). In a recent publication, Dhuwe et al. assessed the solubility and viscosity-enhancing property of high and ultrahigh molecular weight polymers in NGL (i.e. a mixture of ethane, propane, and butane) [82, 83]. Polymers evaluated included ultrahigh molecular weight drag-reducing agent (DRA) poly-α-olefin (Mw: 20,000,000 g.mol<sup>−</sup><sup>1</sup> ), high molecular weight PDMS (Mw: 980,000 g.mol<sup>−</sup><sup>1</sup> ), and PIB (Mw: 130,000 g.mol<sup>−</sup><sup>1</sup> ). Ultrahigh molecular weight DRA poly-α-olefin is commonly used in oil pipelines to supress the energy dissipation near the pipe wall that results from the turbulent flow at high flow rates. This polymer does not change the fluid properties (e.g. viscosity) at the dilute concentrations used for this application. Dhuwe et al. [83] found it to be sufficiently soluble in NGL if a significant amount of hexane is added as a co-solvent. For example, at 0.5 wt% of DRA polymer and 24.5 wt% hexane in propane or butane, the cloud point pressures at temperatures of 333 K were found to be equal to 3.07 and 0.77 MPa, respectively. However, it requires very high pressure to attain solubility in ethane (46.95 MPa) at the same concentrations. At 0.5 wt% of DRA polymer and 24.5 wt% of hexane, the viscosity of ethane and propane could be improved by 3–9 fold, while 23–30-fold enhancement was obtained in butane (**Table 2**). The reason for the greater increase in the viscosity of butane is explained by the larger butane solubility compared to the solubility of propane and ethane that aids the expansion of the polymer backbone (i.e. coil) that swells the DRA polymer [82, 83].

Furthermore, they have also tested the solubility of high molecular weight PIB and PDMS in NGL components. PIB was found to be insoluble in ethane, propane, and butane at temperatures ranging from 298 to 353 K and high pressure, while PDMS was soluble in all NGL constituents without the aid of a co-solvent [82, 83].

