The very low viscosity of high pressure natural gas liquids (NGLs; a mixture of ethane, propane, butane and small fraction of C5+) at reservoir condition is the primary cause of the viscous fingers that cause low sweep efficiency during miscible gas enhanced oil recovery (EOR). A compound capable of thickening ethane, propane and butane could significantly improve the mobility ratio, inhibit viscous fingering, and eliminate the need for the water-alternating-gas (WAG) method for improving mobility control via reducing the relative permeability of these high pressure light hydrocarbons. The ideal thickener should work at a very dilute concentration (~0.1wt%), dissolve without the need for heating, increase the viscosity of NGL to a viscosity value comparable to light oil (0.5-5 cP), and form a transparent solution capable of flowing through the micron-scale interconnected pores of oil-bearing sandstone or carbonate rock.
Extremely viscous NGLs or NGL gels could also find application in the “dry” hydraulic fracturing of water-sensitive formations. The same thickener used for EOR could conceivably, at higher concentrations, gel NGLs. The high viscosity fluid would form larger fractures than pure NGLs and would be able to transport larger amounts of larger sand particles into the fractures. Because the size of the wellbore and fracture are on the order of 1-10 cm, either a transparent NGL gel or a cloudy NGL gel composed on interlocking fibrous network of self-assembled molecules that retain the liquid hydrocarbon would be acceptable.
In this study we report on the design, synthesize of novel compounds, and the identification of commercially available compounds, that can thicken high pressure liquid or supercritical ethane, propane and butane. Both high molecular weight polymers and low molecular weight associating compounds are considered. It was found that ethane was more difficult to thicken than propane, which was more difficult to thicken than butane. At least three ultra-high molecular weight polymers were found to be effective propane and butane thickeners, but some require heating to elevated temperature and pressure to attain dissolution. One of these polymers was found to be soluble in ethane and capable of inducing modest changes in viscosity at dilute concentration; to the best of our knowledge this is the first report of ethane being thickened with a high molecular weight polymer. The viscosity change attained by these polymers was generally much less than that realized with the same mass concentration of low molecular weight associating compounds described below.
Two distinct types of small molecule thickeners were identified for NGLs. For the first time, a small associative thickener (composed of a single component) for ethane has been identified that is capable of thickening liquid ethane by a factor of ~100 at a concentration of 1wt%. The resultant solution is not only remarkably viscous, but also colorless and transparent. The same associative thickener was very effective in propane and butane. Another classification of small molecule associative thickeners, composed of a two-component reacting mixture, was found to thicken propane and butane. The solutions were translucent or opaque, however, indicative that a fine, fibrous network of the thickener molecules may be present in the mixture.
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