CO2 degassing in CaCO3 precipitation in the presence of oil: Implications, modeling, numerical simulation, validation, prototype development, and experimental results

Abstract

Precipitation and scaling are recurring problems in many water-handling processes. Particularly in oil production, such occurrences are critical due to the infeasibility or extreme difficulty of remediating precipitation or removing scale. This is due to adverse operating conditions: high pressures and temperatures, the action of corrosive gases such as CO2 and H2S, high salinity, and a confined system in a rocky medium. Brazilian Pre-Salt carbonate reservoirs are notable for the high CO2 content in their fluids, in which this acid gas becomes a variable that affects several stages of the exploratory process. The relationship between CO2 and calcium carbonate (CaCO3) is evident in the reaction Ca(aq)2++2HCO3 (aq)−⇄H2O+CaCO3 (s)+CO2 (aq). The pressure drop inherent to the production degasses the CO2 (CO2(aq) → CO2(g)), which shifts the equilibrium to the right and implies the formation of CaCO3(s). This paper presents some developments carried out by our research team to theoretically understand the phenomena related to CaCO3 precipitation and scaling in the severe conditions of the Pre-Salt oilfields. Exhaustive bibliographic research, thermodynamic projections, numerical simulations with real data from geothermal fields (without oil) and a Pre-Salt well (with oil), in addition to laboratory experiments in atmospheric conditions were the resources employed to reach the point where the research practically stagnated. This evidenced the need to build an experimental prototype capable of investigating the phenomena related to carbonate precipitation and scaling under conditions as close to the real as possible. We will present the main resources of equipment designed to explore several flow assurance issues, especially those related to calcium carbonate scaling. Some qualitative experiments showed the device's capacity with surprising preliminary results. In such trials, we use real Pre-Salt dead oil, inject CO2 and methane, apply a brine with high salinity, and operate up to 95 °C and 104 bar. We confirmed that CO2 greatly influences the CaCO3 precipitation, but we also observed unexpected phenomena, such as the large size of the aragonite crystals (a CaCO3 polymorph) and the high wettability of the Pre-Salt oil to CaCO3, which significantly altered the rheology of the oily phase.