Pore scale modelling of wettability effects and their influence on oil recovery

The wettability of a crude oil/brine/rock system influences both the form of petrophysical parameters (e.g. Pc and krw/kro) and the structure and distribution of remaining oil after secondary recovery. This latter issue is of central importance for improved oil recovery since it represents the "target" oil for any IOR process. In the present study, a three dimensional (3D) network model has been developed to derive capillary pressure curves from non-uniformly wetted (mixed and fractionally-wet) systems. The model initially considers primary drainage and the ageing process leading to wettability alterations. This is then followed by simulations of spontaneous water imbibition, forced water drive, spontaneous oil imbibition and forced oil drive - i.e. a complete flooding sequence characteristic of wettability experiments is considered. The model takes into account many pore level flow phenomena such as film flow along wetting phase clusters, trapping of wetting and non-wetting phases by snap-off and by-passing. Realistic variations in advancing and receding contact angles are also considered.The effects of additional parameters such as the fraction of oil-wet pores. mean co-ordination number and pore size distribution upon fractionally- and mixed-wet capillary pressure curves are discussed. Moreover, Amott oil and water indices are calculated using the simulated curves. Results indicate that oil recovery via water imbibition in weakly water-wet cores can often exceed that obtained from strongly water-wet samples. Such an effect has been observed experimentally in the past. The basic physics governing this enhancement in spontaneous water imbibition can be explained using the concept of a capillarity surface.Furthermore, based on these theoretical calculations, a general "Regime based" theory of wettability classification and analysis is proposed. A range of experimentally observed and apparently inconsistent waterflood recovery trends are classified into various Regimes, depending upon the structure of the underlying oil- and water-wet pore clusters and the distribution of contact angles. Using this approach, numerous published experimental Amott indices and waterflood data from a variety of core/crude oil/ brine systems have been analysed.