Prediction Of CaCO3 Scale Under Downhole Conditions

Abstract

ABSTRACT CaCO3 is one of the most common scales in oilfield and geothermal operations. Various thermodynamic models have been reported in the literature to predict the precipitation of CaCO3. Each of these existing models has its limitation when applied to the real situations of the field operations. Some models (e.g., Stiff-Davis method) ignore many conditions leading to CaCO3 precipitation. In the present paper, we describe a model to predict the CaCO3 scale formation at any location within the field as a function of water composition, temperature and pressure. This model is based on equilibrium thermodynamics, but uses only the experimentally measured parameters. The input data for the model are (1) one water analysis, (2) one pH measurement at any location in the field, (3) the temperature at which the only field pH measurement is made, (4) the CO2 concentration of the gases (if a gas phase exists upstream or at the point where the liquid sample is taken and the pH is measured), and (5) the amount of flash (if a gas phase exists upstream or at the point of sample collection). The model then takes these measured data and calculates the equilibrium brine composition (including pH) in the reservoir. The calculated brine composition data can then be used either to predict the CaCO3 precipitation for any set of prescribed conditions or to predict the CaCO3 precipitation at various locations within an oilfield or geothermal operation. For a set of temperature and pressure, the model goes through a decision to determine whether a single phase or two phase condition exists for the brine. For the case of the existence of the single phase brine, the model calculates the amount of CaCO3 precipitate taking into account the pertinent chemical reactions involving the various carbonate species. For the case of the existence of two phase brine, the model first computes the CO3 emission and its effect on the pertinent reactions Involving the various carbonate species and, hence, calculates the amount of CaCO3 precipitate.