Improved Oil Recovery With Chemicals in Fractured Carbonate Formations

Abstract

Abstract 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. Introduction 60% of worldwide oil reserves are located in carbonate reservoirs (Akbar et al., 2001). The total oil recovery does not exceed generally 10%. Processes currently used for improving the oil recovery from carbonate reservoirs include miscible gas injection, water alternate gas (WAG), simultaneous water alternate gas (SWAG), foam-assisted WAG, microbial EOR. Carbonate reservoirs are naturally fractured (Chillinger et al., 1983). Producing from such reservoirs is complicated (Roehl et al., 1985; Allan et al., 2003) and need a careful production management. Such reservoirs are often characterized by high-permeability fractures and a low-permeability matrix medium. Usually, the productivity index is high at the beginning, because of the fracture-driven production, but it can decrease quickly if matrix-to-fracture fluid transfer is slow or insufficient. This may lead to a premature water breakthrough if a water-drive recovery method is applied (Cosentino et al., 2002).The GOR and reservoir pressure have to be carefully monitored in order to keep the recovery potential from the matrix medium, especially if a gas drive recovery process is considered. Matrix production takes place by fluid expansion inside the matrix, imbibition in the matrix and buoyancy between oil and water. The first mechanism has a very limited contribution to the matrix oil recovery. For imbibition, wettability is the main parameter (Zhou et al., 2000; Morrow et al., 2001; Tong et al., 2002; Hirasaki et al., 2004) because capillary forces predominate in the matrix blocks that are most often bypassed by the injected water. The role of imbibition has been recognized in numerous types of recovery processes including waterflooding of heterogeneous reservoirs, alternate injection of water and gas, also steam injection through the imbibition of condensed water. The process of water sucking into a porous medium by the action of capillary forces is referred to as spontaneous imbibition (Morrow et al., 2001). The recovery of oil by spontaneous imbibition of brine into the reservoir rock is very important for fractured reservoirs where reserves are mainly held in very low-permeability matrix blocks. Under water injection or aquifer drive, subsequent recovery of oil from the rock matrix, if any, is mainly dependent on spontaneous imbibition of water, and is a relatively slow process. The capillary pressure is positive if the rock is water-wet and negative in the oil-wet case. For water-wet fractured reservoirs, the capillary forces are the main driving forces of spontaneous imbibition and contribute to the replacement of oil by water.