The majority of polymer applications to date have targeted medium-to-high permeability sandstone reservoirs containing reservoir brine with moderate salinity and hardness. Polymer flooding for enhanced oil recovery has been field-tested in carbonates, but applications tailored towards oil-wet, low-permeability limestone rocks are uncommon.

This paper contains results from laboratory core flood tests performed on four strongly oil-wet limestone rocks from the Al Shaheen field, off-shore Qatar. The rock samples investigated had 25–30% porosity and 0.3–28 mD permeability; the reservoir brine had a salinity of about 120,000 ppm of which 10,000 ppm were divalent cations. The bulk fluid testing involved fluid compatibility tests, long-term stability tests, as well as rheology measurements. Three commercial polymers were screened, one of which successfully passed all the screening criteria. Core flood experiments were subsequently performed to determine relative permeability, adsorption, inaccessible pore volume, as well as permeability and mobility reduction. The crude oil had a viscosity of 8 cP at reservoir conditions, yielding a slightly unfavourable mobility ratio for the baseline water flood.

At concentrations of 500–1,000 ppm, the selected HPAM polymer gave rise to some mobility and permeability reduction, as expected, but it did not seriously plug any of the four low-permeability cores. The dynamic adsorption was very low, around 10–20 μg/g rock, the inaccessible pore volume was 15–20%, which is in line with industry experience, and the injectivity reduction was in agreement with ECLIPSE and UTCHEM simulations, which suggest that significant polymer degradation did not take place. The low adsorption is attributed to the strong oil-wetness of the rock. Since the polymer flows in the water phase, presence of a strong oil film likely prevents the polymer from adsorbing to the rock. The results from a subsequent water flood showed that inaccessible pore volume introduced by the polymer persists during the subsequent water flood; such a feature has yet to be incorporated in UTCHEM and ECLIPSE. In fact, the data revealed many features, which could not be adequately captured by numerical simulation tools.

The main conclusion from the laboratory work is that polymer systems with good viscosifying power can be tailored to recover oil from low-permeability carbonate rocks, pushing the permeability limit down to around 1 mD.

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