Forskning ved Københavns Universitet - Københavns Universitet


A Combined Thermodynamic and Kinetic Model for Barite Prediction at Oil Reservoir Conditions

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

  • Bi Yun Zhen Wu
In marine environments, barite (BaSO4) is a key proxy that has been used for understanding the biological and chemical evolution of oceans and for tracking the origin of fluids. In the oil industry, barite scale can clog pipelines and pores in the reservoirs, reducing oil yield. The goal of this research was to develop a model, based on thermodynamics and kinetics, for predicting barite precipitation rates in saline waters at the pressures and temperatures of oil bearing reservoirs, using the geochemical modelling code PHREEQC. This task is complicated by the conditions where traditional methods to calculate ion activity fail and for which barite reaction rates have not been determined. The model development consisted of three subprojects: first, to identify a consistent set of parameters to describe the thermodynamics of the Ba2+-SO42--H2O system at standard state conditions, based on detailed review of the literature (PhD Study 1). The reviewed dataset was used as starting point for geochemical speciation modelling and applied to predict the stability of sulphate minerals in North Sea oil field brines. Second, for modelling of high salinity solutions using the Pitzer ion interaction approach, the temperature dependence of Pitzer parameters for NaCl, Na2SO4 and BaCl2 were derived from published osmotic coefficient data (PhD Study 2). Furthermore, barite solubility was determined experimentally at 90 °C and pressures of 150 and 250 bar. Comparison of barite solubilities calculated with the Pitzer model with the experimental values reported in this study and in the literature shows that these values agree within 20% at 0 < I < 4 mol kg-1, 25 < T < 250 C, 1 < P < 500 bar. Third, to determine barite reaction kinetics via experiments performed in batch reactors in NaCl concentration ranging from 0 to 1.5 mol kg-1 and at temperatures from 25 to 90 C (PhD Study 3). Results show that barite dissolution is significantly affected by the ionic strength of the solution. The derived equations for barite reaction rates predict the amount of barite that dissolves or precipitates in batch experiments to within 20% at temperature up to 90 C at 1 bar of pressure. Resulting thermodynamic and kinetic parameters were combined and coupled with PHREEQC to predict precipitation scaling rates in three oil production wells, where barite has been observed. Average linear growth rates of 3, 2.5 and 2 mm of barite per year were estimated. Calculations for a well with seawater breakthrough results in an overestimation compared to scale samples, suggesting that significant amounts of barite precipitate in the reservoirs prior to entering to the wells. The combined model allows estimates of barite scaling rates that can be compared with field observations. This information can help planning mitigation and optimise costs in oil production.
ForlagDepartment of Chemistry, Faculty of Science, University of Copenhagen
Antal sider247
StatusUdgivet - 2016

ID: 162953958