Modelling the Thermodynamic Behaviour of the Carbonate System: Application to the Prediction of Carbonate Scaling in Oil-field Brines

Abstract

Carbonate scaling in the wellbore and near-wellbore region of hydrocarbon reservoirs is a major production problem in the North Sea oil fields. Carbonate scale formation mainly results from changes in physicalchemical properties of fluids (i.e. pH, pCO2, T and P) during production or from chemical incompatility between injected water and formation water. Scale can impair productivity dramatically, thereby increasing hydrocarbon production costs. Prediction remains notoriously difficult, mainly due to difficulties for modelling the thermodynamic behaviour of the carbonate system in high salinity solutions at high temperature and pressure (HS/HT/HP). The DebyeHfickel approach, used for fluids of ionic strength (IS) lower than 0.1 molal, is not applicable to most oil-field brines (typical IS between 0.5 and 5 molal). True activities of dissolved species, accounting for all specific interactions between ions, must be known. The most comprehensive model to calculate activities in brines is the specific ion interaction model of Pitzer. A Pitzer-based computer model, e.g. SCALESIM, is thus being developed as part of an on-going ECfunded project (Qc-Scale; contract JOF3-CT95-0009) for prediction of carbonate scale in oil production operations.