oa Dilute surfactant flooding studies in a low-permeability oil-wet carbonate reservoir

The vast majority of surfactant flood applications to date have been targeted to high permeability sandstone reservoirs containing formation brine with moderate salinity and hardness. Surfactant flooding in oil-wet, low-permeability carbonate rock is only reported sparsely throughout the literature. In the present work we aimed at identifying an efficient enhanced oil recovery strategy using a dilute surfactant solution. The dilution is highly beneficial in order to reduce not only chemical costs but in particular the environmental footprint. The work contains results from laboratory core flood tests performed on a highly oil-wet, low permeable carbonate reservoir rock material. The rock material investigated had approximately 30% porosity and 5 mD permeability, whereas the formation brine had a high salinity of about 120,000 ppm with a hardness of about 10,000 ppm. Initial combinations of several commercial surfactants, co-surfactant and alkalis were tested for brine compatibility and from these tests two cost-effective surfactant systems were identified. The first system, denoted ITR, was capable of reducing the interfacial tension below 0.001 mN/m at reservoir salinity and hardness. The second system, referred to as WA, altered the wettability from strongly oil-wet to intermediate-wet without altering the interfacial tension. Adsorption of the surfactants was measured to be low for both systems. No mobility control agents, such as polymers, were used in any of the systems because of low rock permeability. Both surfactant systems yielded significant incremental oil, when injected in tertiary as well as in secondary mode. The ITR system recovered almost 95% of OIIP but required many pore volumes since the cores remained oil-wet. Strikingly the WA system recovered approx. 85% of OIIP in secondary mode but achieved this with much fewer pore volumes using a very low concentration of surfactant. The promising WA system was subjected to extensive analysis involving relative permeability function estimation and flooding simulations using UTCHEM. In summary, we have designed an efficient surfactant system capable of recovering significant amounts of oil from low-permeability carbonate reservoir rock material. Wettability alteration of highly oil-wet rock using dilute surfactant solutions may assist in unlocking significant volumes of additional oil from a variety of carbonate reservoirs.