Magnetic Resonance Imaging of Chemical EOR in Core to Complement Field Pilot Studies

Before possible multi-well pilots and full field-scale implementation, chemical enhanced oil recovery (EOR) projects usually follow an incremental screening procedure from the laboratory through single-well pilots. Laboratory-scale core floods at reservoir conditions validate surfactant performance before committing to field trials. However, in single short plugs, volumetric averages do not correctly quantify remaining oil due to capillary end effects, oil banks, and other flow heterogeneities. Magnetic resonance imaging (MRI) protocols extend traditional core floods and allow quantitative resolution of non-uniform saturation on millimetre length scales. At low magnetic field strengths, MRI data correspond directly to the measurement physics of nuclear magnetic resonance (NMR) logging tools. NMR and dielectric dispersion tools are ideal for quantifying remaining oil in single-well field trials, being independent of Archie parameters which are uncertain under complex salinity and wettability distributions. We report a laboratory study on short core plugs from a North Oman carbonate field, confirming the efficacy of an alkaline surfactant (AS) formulation at reservoir conditions. Good injectivity of surfactant was seen in selected reservoir material, suggesting that surfactant formulation was not responsible for inconsistent injectivity seen in single well field tests using a logdrill-inject-log pilot protocol. The low-field MRI results corroborate oil saturation changes seen by wireline logs on a decimetre length scale in the single-well field test. Remaining oil determinations can thus be compared at various length scales ranging from millimetres (MRI), to decimetres (log-drill-inject-log pilots), and finally to field scale trials using single well chemical tracer tests.