Experimental Studies of Improved Oil Production from High-Porosity Analog Media

Abstract:

To study processes affecting gas-based enhanced oil recovery (EOR), we performed a laboratory investigation using high-porosity (35%) analog media consisting of ceramic and thermoplastic (Teflon) materials. Our motivation was to study processes that affect EOR in natural tight rocks and help furnish a compendium of the best and worst oil production strategies for field application. This feasibility study has been found to be valuable as laboratory EOR testing of tight/shale rocks is inherently complicated, time consuming, and error-prone. We employed a depressurization oil production technique where the EOR was examined using multiple oil soluble gas species in a large set of water-moistened and light oil (n-dodecane) saturated sample composites containing a wide range of fine porosity (50 nanometer - 3.5 micrometer pore sizes) and tight permeability (0.003 - 10 millidarcy). Generally, in depressurization, the liquid phase fluid (n-dodecane) would expand upon the lowering of pressure and “spill” into fractures where it flows to wells. Thus, when a low density and low viscosity gas is injected into the oil saturated media, gas-oil mixing occurs causing the oil to flow more easily and is easier to produce. Subsequent depressurization with possible gas production from the introduced fluid drives more oil into fractures. In our laboratory tests, conducted at relatively low system pressure (10 MPa) and temperature (60^oC) conditions, the overall primary oil production (e.g., no gas injection) remains similar to the natural tight/shale field oil production. However, the incremental oil recovery caused by the injected miscible gas(es) is quite large, averaging at least one order of magnitude higher recovery—by helium, methane, nitrogen, and various methane-carbon dioxide mixtures—when compared to the published EOR results in tight/shale rocks. Both methane and carbon dioxide outperformed helium and nitrogen as single-component EOR gas species, and the methane-carbon dioxide gas mixtures with higher carbon dioxide concentrations (and at higher pressures) increased oil recovery. In addition, a higher EOR is observed in composites with dominant nanopores rather than micropores. The results suggest that the micro- and nanoscale synthetic matrix pores generated large surface areas, stored substantial oil mass, and greatly promoted advective-diffusive processes. We also conclude that an atypically high EOR in our analog media results from its highly-permeable flow networks that are well-connected and productive.