The majority of alkali-surfactant-polymer (ASP) applications to date have targeted medium-to-high permeability sandstone reservoirs containing reservoir brine with moderate salinity and hardness. Surfactant flooding experiments have been reported in carbonates, but applications targeting oil-wet, low-permeability limestone rock are still uncommon.
This paper contains results from laboratory core flood tests performed on an oil-wet limestone rock from the Al Shaheen field, offshore Qatar. The rock samples investigated had approximately 30% porosity and 5 mD permeability, whereas the reservoir brine had a salinity of about 120,000 ppm of which about 10,000 ppm were divalent cations.
The first screening step involved testing combinations of several commercial surfactants, co-surfactant and alkalis. Two cost-effective surfactant systems were identified. The first system, denoted ITR, was capable of reducing the interfacial tension below 0.001 mN/m over the required range of salinities. The second system, referred to as WA, effectively altered the wettability from strongly oil-wet to intermediate-wet. Static adsorption was measured to be low for both systems. No polymer was used because the permeability was very low. 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. The WA system, on the other hand, recovered some 85% of OIIP in secondary mode but achieved this with much fewer pore volumes.
The WA system was subjected to extensive analysis. Relative permeability curves from unsteady-state core flooding data were derived and experiments were simulated with UTCHEM.
The main conclusion from the extensive laboratory work is that surfactant systems can be tailored to recover a significant amount of oil from a low-permeability carbonate reservoir. Wettability alteration may assist in unlocking significant volumes of additional oil from this complex, offshore field.