Mobility Control by CO2-foam Injection for Integrated EOR

Abstract

This thesis investigated integrated enhanced oil recovery (IEOR) methods in fractured carbonate rocks. The objective was to study the oil recovery by miscible CO2 injection in fractured rocks using different rock types and compare oil recovery performance by CO2-foam injections. CO2 injections were also performed on reservoir shale cores to evaluate permeability. Routine analysis was performed on 48 outcrop (chalk and limestone) and 4 reservoir carbonate cores. Experiments by CO2 injection have been performed on five setups at three different locations; Department of Physics and Technology, Bergen; Haukeland University Hospital, Bergen and Texas A&M University, College Station, Texas. Experiments were conducted at varying pressure and temperature and fractured networks. The fractured permeability was several orders of magnitude larger than the matrix permeability. Pure supercritical CO2 was injected prior to supercritical CO2-foam injection in strongly water-wet outcrop cores, whole and fractured at pressure of 90 bar and temperature of 35°C. Oil recovery by pure supercritical CO2 injection was most efficient in whole cores, above 85% OOIP, whereas in fractured cores the oil recovery and oil production rate was significantly reduced and oil was only produced by diffusion. Pre-generated foam injection showed increased oil recovery compared to pure CO2 injection in limestone, but only minor increased oil recovery in chalk. Subsequent injection of CO2-foam reduced the gas mobility in fractures and diverted flow into the oil-saturated matrix. In situ foam generation during tertiary foam injection in fractured limestone network showed increased differential pressure due to generation of strong foam and improved oil recovery, additional 6-10 % OOIP produced. Waterfloods and tertiary CO2 injections in heterogeneous reservoir carbonate cores were performed above minimum miscibility pressure of CO2 and crude oil. Waterflood oil recovery ranged between 17 - 46% OOIP, whereas subsequent CO2 injections showed significant enhanced oil recovery, above 85% OOIP for all cores. A best practice" for permeability and re- saturation of unpreserved shale core plugs was established.