Fundamentals of wettability applied to Brazilian Pre-Salt reservoirs and wettability alteration evaluation in low salinity water injection

Abstract

Brazilian Pre-Salt carbonate reservoirs have been one of the greatest oil discoveries of the past decade. In fields such as Lula, it is expected to recover between five to eight billion barrels of oil equivalent. To fulfil this, knowledge of reservoir wettability is fundamental, given that it dictates how fluids (water, oil and, gas) are distributed within the porous media, and how the multiphase flow occurs in the pore-network. This information is important in the construction of relative permeability curves which are input for reservoir simulators. Few works have been published regarding initial wettability of Pre-Salt carbonate reservoirs in spite of their great oil recovery potential. In this way, to close the data gap in the literature, this study aimed to investigate parameters that influence initial wettability of pure rocks (calcite and quartz) and analogue rocks (coquinas) that partly represent the Pre-Salt reservoirs. Contact angle experiments were used to study the influence of parameters that are important to rock wettability, such as rock composition, brine salinity, temperature, rock ageing and, presence of carbon dioxide in brine. Initial wettability was analysed at core scale by spontaneous imbibition and core flood experiments, obtaining the Amott index to water. Through these experiments, it was possible to evaluate the performance of different brines respective to oil recovery. Furthermore, wettability alteration was investigated in low salinity water injection as a tertiary injection method. Contact angles experiments were also used to analyse wettability alteration in clay-less rocks, focusing on understanding the role of fluid-fluid interactions on low salinity water injection. Respective to initial wettability in pure rocks and rock analogues relevant to the Brazilian Pre-Salt, it was found that at reservoir temperature (60°C) calcite, and quartz were neutral and weakly oil-wet, respectively. Amott index to water indicated that at that temperature, coquina rocks were intermediate-wet (mixed-wet). Presence of carbon dioxide in brine indicated that both calcite and quartz rocks become more water-wet. Regarding the application of wettability to low salinity water injection, the results suggested that wettability alteration is not exclusive of clayey rocks, nor resulting from a change in the ratio of divalent to monovalent cations. They indicated that crude oil composition is crucial for wettability alteration to happen, and it is possible that crude-oil/brine interactions dictate the low salinity effect. In summary, this work generated important data regarding initial wettability of Brazilian Pre-Salt reservoirs which are scarce in the literature. These results also instigate further investigation respective to the effects of carbon dioxide in brine on the initial wettability of rocks considering complex brines (presence of divalent cations and anions) rather than just brines composed of sodium chloride. This work showed that crude-oil/brine interactions is important for the low salinity effect.