Modeling and validation of the CO2 degassing effect on CaCO3 precipitation using oilfield data

Abstract

Calcium carbonate scaling can become a challenge for Brazilian Pre-Salt oil production. The high CO2 content that makes up the gas stream in carbonate reservoirs is remarkable. The relationship between CO2 and CaCO3 is evident in the chemical balance: Ca2+(aq) + 2 HCO3–(aq) ⇆ H2O + CaCO3(s) + CO2(aq). Aqueous CO2 present in carbonate reservoir acidifies the medium and keeps CaCO3 dissolved. The head loss provided by oil production moves CO2(aq) to the gaseous state (CO2(aq) → CO2(g)). It disfavors calcium carbonate to keep dissolved, which implies CaCO3 precipitation. The two mentioned chemical reactions are closely linked and are mutually dependent. A previous thermodynamic study carried out by our group, considering a pseudo-equilibrium state, estimated that CO2 degassing is responsible for 61% to 89% of calcium carbonate scaling. However, it is not possible to carry out laboratory experiments with a pseudo-equilibrium condition. Therefore, a methodology was developed based on experimental feasibility to overcome this issue, making possible its application with computational packages. The software includes not only the aqueous solution thermodynamics but also the interaction with the hydrocarbon phases. An applied review of the role of CO2 on precipitation, considering aqueous, oily, and gaseous phases, was carried out with real-field data of Pre-Salt to support the analysis. Likewise for aqueous CO2, oily CO2 degassing (CO2(o) → CO2(g)) also implies CaCO3 scaling, and the CO2 partitioning between the three phases is a phenomenon that critically governs the precipitation. Regarding the simulation results, the percentages that we previously found were confirmed. In runs with real-field data, the results fell within the theoretical percentage range but indicated a distinct behavior from the influence of CO2 with the change in BSW. These discoveries correspond to some operational problems found in oilfields and may help to explain their still unknown causes.