A Pore-Scale Investigation of Fluid Displacement and Residual Trapping Under Intermediate-Wet Conditions

Residual trapping in which ganglia of fluid are isolated and immobilisedin porous media by capillary forces is innate to several subsurfaceengineering applications including carbon geo-sequestration. Residualtrapping is highly significant in carbon dioxide (CO2) sequestration, asentrapment of supercritical CO2 in rock pore spaces, limits upwardmigration of the buoyant CO2 plume and enhances long-term CO2 storagesecurity. It is estimated that residual trapping contributes up to 40%of overall trapping CO2 in the first century following injection (1).The amount of residual trapping depends largely on the wettability ofthe porous rock.Brine filled saline aquifers have been identified ashaving the largest potential for CO2 storage with an estimatedcumulative storage capacity of 104 Giga-tons of CO2 (2). Likewise, thefocus of many studies has been devoted to investigating residualtrapping in water-wet, brine filled sandstone reservoirs, and littleattention has been given to intermediate-wet and oil-wet carbonatereservoirs. However, until CO2 storage technology reaches maturity,initial CO2 sequestration projects will most likely be conducted indepleted and oil producing carbonate reservoirs due to economic benefitsassociated with CO2 enhanced oil recovery and the existence of installedinfrastructure which can be reassigned for CO2 injection purposes (3).Accordingly, in this work, the intrinsically water-wetting surfaces oflaser fabricated glass micromodels (4); which are two-dimensionalrepresentations of natural porous rock structures, were chemicallymodified to imitate intermediate-wet reservoir conditions through asilanization procedure. Imbibition experiments were conducted in themicromodels using two proxy, CO2-brine fluid pairs; deionized (DI) waterand n-decane as well as DI water and air.Fluid displacement underintermediate wettability was analysed and compared with water-wetconditions and residual fluid saturations were quantified for differentporous structures. The Volume of Fluid method was used to simulate theexperiments in OpenFOAM. Results from the micromodel experiments wereused to validate the simulations.This work has demonstrated that fluiddisplacement during the imbibition process occurs through a series ofcooperative pore-filling events under intermediate-wet conditions andthe presence of dead-end pores was found to enhance residual trapping ofthe non-wetting fluid. Coupling experimental and simulation studiesprovides a unique insight to multiphase flow under intermediate wetconditions.