A large quantity of world's oil reserves is contained in carbonate reservoirs. Most of them are naturally fractured and mixed-wet to preferentially oil-wet which are unfavourable conditions for spontaneous water imbibition. This research proposes a combination of experimental methods for the evaluation of the efficiency of chemicals to recover oil from mixed-wet to oil-wet cores. The study is oriented towards the acquisition of data required for modelling chemically-assisted imbibition mechanisms.
Both spontaneous and forced imbibition experiments were performed on outcrop carbonate cores, following specific procedures for establishing given wettability indices. Alkalis were used with and without surfactants as chemical additives of the imbibing aqueous phase. Spontaneous imbibition tests were carried out with reactive and non-reactive oils, in order to dissociate/investigate the effects of alkali and surfactants on the water-oil InterFacial Tension and on the rock wettability. The alkalis exhibited a striking imbibition enhancement with a reactive oil. With a non-reactive oil, imbibition was moderate in the presence of alkali without surfactant. Formulations including a surfactant and an alkaline agent were evaluated and the results are discussed in relation with the InterFacial Tension. Moreover, the wettability index determined through centrifugation and relative permeability data were found to be consistent with the results of spontaneous imbibition tests. Finally, the generic experimental methodology proposed herein delivers a consistent set of relevant parameters for assessing and modelling the recovery efficiency of alkali-surfactant solutions in many fractured carbonate reservoirs with unfavourable recovery prognosis.