Effectiveness of Low-Salinity and CO2 Flooding Hybrid Approaches in Low-Permeability Sandstone Reservoirs

Abstract

Low-salinity waterflooding (LSW) has been an emerging Enhanced Oil Recovery (EOR) technique. LSW-alternating-CO2 injection has recently proven to be a particularly strong EOR stimulation technique. Limited simulation studies have been performed to properly exhibit the interpretation of experimental studies in this area. This study aims to expand the interpretation and application of LSW in sandstone formations.

Low-permeability sandstones of 18% clay content, and medium west Texas crude oil with 29 °API, were selected for evaluating the potentials of LSW and CO2 flooding. Coreflood experiments were conducted and interpreted by reservoir simulation, which enabled a better understanding of the underlying science of the studied EOR techniques. Five secondary waterflooding experiments and two LSW-CO2 hybrid processes were performed using aged 3 in. outcrop Grey Bandera sandstone cores. The wettability of the rock was tested using axisymmetric contact angle measurements to study the effect of injected brine salinity. The interfacial tension values were measured for the different brines and medium crude oil systems. The corefloods were interpreted using reservoir simulation by history-matching the experimental oil RF (Recovery Factor) and the pressure drop across the core. A detailed focus was laid on interpretation of SCAL (Special Core Analyses) relative permeability curves for the various waterflooding operations, by comparing curves generated from published correlations, history-matched curves by simulation, and that by Corey's correlations.

Fines migration and viscosity override of low-salinity waters over medium oil influenced the oil recovery profile during LSW. Wettability alteration proved to be effective during LSW for longer injection and production periods. Corey's exponents were found to be premature indications of rock wettability during LSW, proving the importance and reliability of experimental wettability characterization. Low-salinity waterflooding proved to be marginally effective over conventional waterflooding for sandstones with high clay contents and low permeabilities. Low-permeability sandstone reservoirs proved to be suitable targets for hybrid EOR techniques using CO2 and LSW.