Dissolution Behavior of Carbonate Rocks from Brazilian Pre-Salt at Reservoir Conditions by LSWI

Abstract

The low salinity water injection (LSWI) is one of the most studied techniques applied to enhanced oil recovery (EOR) from carbonates reservoirs. Wettability alteration and dissolution reaction are mechanisms that support the understanding of this technique. However, the dissolution behavior of Brazilian pre-salt carbonates rocks when treated by LSWI is not yet known. Therefore, the purpose of this work is to evaluate the dissolution effects provided by low salinity water injection in carbonate rocks from a Brazilian pre-salt reservoir using two different injection flow rates in core flooding tests at reservoir conditions. Porosity variation along the length was determined by Computerized Tomography scan analysis. Permeability was determined through pressure drop data. Ions identification and concentration were determined by ions chromatography. The experiments consisted in carrying out two core flooding tests assembled in series at 8100 psi of injection pressure and 65°C. It was performed two different injection flow rates (1 mL/min and 0.1 mL/min) of 16 times diluted synthetic seawater. It was possible to observe three different porosity behaviors along the sample length in both flow rates tested and for both core holder samples. However, the porosity behavior intensity is different from each case. There are regions of predictable behavior, regions of random behavior and inert regions. According to the porosity plots, the sample from the second core holder of the series presented more interaction with the injection fluid. Probably, the mineralogical heterogeneity has influenced in the dissolution phenomenon. The permeability profile remained almost constant for both samples. The ion chromatography analysis revealed a huge variation of magnesium and calcium ions concentration in the first injected porous volume and, subsequently, a constant trend towards the base-line values.