|
Abstract:
|
Highly fractured carbonate reservoirs are a class of reservoirs characterized by high conductivity fractures surrounding low permeability matrix blocks . In these reservoirs , wettability alteration is a key method for recovering oil . Water imbibes into the matrix blocks upon water flooding if the reservoir rock is water -wet . However , many carbonate reservoirs are oil -wet . Surfactant solution was used to enhance spontaneous imbibition between the fractures and the matrix by both wettability alteration and ultra -low interfacial tensions . The first part of this study was devoted to determining the wettability of reservoir rocks using Amott -Harvey Index method , and also evaluating the performance of surfactants on wettability alteration , based on the contact angle measurement and spontaneous imbibition rate and ultimate oil recovery on oil -wet reservoir cores . The reservoir rocks have been found to be slightly oil -wet . One cationic surfactant BTC8358 , one anionic surfactant and one ultra -low IFT surfactant formulation AKL -207 are all found to alter the wettability towards more water -wet and promote oil recovery through spontaneous imbibition .
The second part of the study focused on the parameters that affect wettability alteration by surfactants . Some factors such as core dimension , permeability and heterogeneity of porous medium are evaluated in the spontaneous imbibition tests . Higher permeability leads to higher imbibition rate and higher ultimate oil recovery . Heterogeneity of core samples slows down the imbibition process if other properties are similar . Core dimension is critical in upscaling from laboratory conditions to field matrix blocks . The imbibition rate is slower in larger dimension of core . Further , we investigated the effects of EDTA in surfactant -mediated spontaneous imbibition . Since high concentration of cationic divalent ions in the aqueous solution markedly suppresses the surfactant -mediated wettability alteration , EDTA improved the performance of surfactant in the spontaneous imbibition tests . It is proposed in the thesis that surfactant /EDTA -enhanced imbibition may involve the dissolution mechanism . More experiments should be conducted to verify this mechanism . The benefits of using EDTA in the surfactant solution include but not limited to : altering the surface charge of carbonate to negative , producing the in -situ soap , reducing the brine hardness , decreasing the surfactant adsorption , and creating the water -wet area by dissolving the dolomite mineral . |