Thickeners for Natural Gas Liquids to Improve the Performance in Enhanced Oil Recovery and Dry Hydraulic Fracking

摘要

Natural gas liquid (NGL), a mixture consisting primarily of ethane, propane, and butane, is an excellent enhanced oil recovery (EOR) solvent. However, NGL is typically about ten times less viscous than the crude oil within the carbonate or sandstone porous media, which causes the NGL to finger through the rock toward production wells resulting in low volumetric sweep efficiency of the NGL solvent. In this work, targeted thickeners are broadly classified into two categories, polymeric thickener and small associative molecule thickener. In either case, the resultant thickened ethane, propane or butane solution is expected to be thermodynamically stable, transparent, and capable of flowing through the pore throats (~1 micron) of sandstone or carbonate rock.udIn the category of polymeric thickeners, a dilute concentration of a drag-reducing agent (DRA) poly(α-olefin) that has an average molecular weight greater than 20,000,000 was proposed as a thickener for liquid butane, liquid propane and liquid or supercritical ethane. High molecular weight polydimethyl siloxane polymer (molecular weight of ~1,000,000) and polyisobutylene (PIB) (molecular weight ~10,000,000) were also assessed as potential thickeners for NGLs. Phase behavior data (cloud points) and viscosity induced by these polymeric thickeners were obtained as a function of temperature, pressure and concentration. Results indicate that butane is the most effective NGL component at both dissolving the polymer and expanding the polymer coils. In general, viscosity enhancement increases with decreasing temperature and increasing pressure, reflective of increased NGL solvent strength at low temperature and high pressure. Only the DRA induced significant viscosity changes for NGL at dilute polymer concentrations. To the best of my knowledge, the DRA-alkane mixture data presented in this thesis represent the most significant polymer-induced increases in viscosity reported to date for butane and propane and the first report of thickening ethane.udThree types of small associating molecule thickeners were considered; trialkyltin fluoride, aluminum di-soaps, and crosslinked phosphate esters. Phase behavior (cloud point) and viscosity data were obtained as a function of temperature, pressure and concentration. The crosslinked phosphate ester mixture was difficult to dissolve completely in NGL and induced very modest viscosity changes, especially for ethane. Hydroxyaluminum di(2-ethyl hexanoate) was insoluble in ethane, but was the best thickener for propane and butane at temperatures above 40oC. However, the hydroxyaluminum di(2-ethyl hexanoate) mixtures required heating to 100oC to attain dissolution prior to cooling to the temperature of interest. Tributyltin fluoride was a remarkable thickener for ethane, propane and butane that did not require heating for dissolution. To the best of my knowledge, these tributyltin fluoride-ethane mixture results represent the first report of thickening ethane with a small associating molecule.