Simplified Calculation of CACO3 Saturation at High Temperatures and Pressures in Brine Solutions
Authors: Oddo John E. , Tomson Mason B. 
Published: December 2007

A simplified method to calculate CaCO3 saturation is developed using only commonly measured field parameters. The calculated saturation index, Is, and pH values are accurate at high temperatures and pressures in brines and are compared with less sophisticated and more complex calculations. The final forms of Is and pH calculations are derived using conditional equilibrium constants dependent on temperature, pressure, and ionic strength. which eliminate the need for activity coefficients. The Is equation is presented in forms for calculation with known or derived pH and where the pH of the solution is unknown. Practical application of Is is shown by calculating the scaling tendency of several geopressured energy wells of the U.S. gulf coast region.

Introduction

Calcium carbonate (CaCO2) precipitation has been and continues to be a problem in aqueous systems. Calcium carbonate scale, although present at all temperature/pressure regimes, is most prevalent at high temperatures and pressures, where CaCO3 solubility is decreased with the increased temperature. Less sophisticated methods for determining scaling tendency, such as the Langelier and Stiff-Davis Is's, have built-in constraints when considering closed aqueous systems. With both methods the solution pH must be known to begin the calculations. There is no technique for reliable pH measurement at high temperatures and pressures. Neither method can account for pressure changes in the system or the changing solubility of CO2(g) with temperature or pressure. The Stiff and Davis constant K is not known above 194 degrees F. The method presented in this paper follows the Stiff and Davis method very closely at temperatures in range of the Stiff and Davis calculation and at known pH. Table 1 is a comparison of the Is presented here and the Stiff and Davis Is for four oilfield brines. 4 The method enables calculations of pH, if not known, and considers total pressure as well as varying CO2(g) partial pressures from commonly measured variables in the field-i.e., total calcium, bicarbonate alkalinity, ionic strength, temperature, pressure, and the mole fraction of CO2 in the gas phase. An effort has been made throughout the paper to maintain the resulting algorithm so it is simple enough to perform easily in the field with a handheld calculator. More complex computer codes exist for the calculation of CaCO3 scaling tendencies in aqueous systems at high temperatures and pressures, but these are constrained by the need for mainframe computers, complex codes, and large data bases. A sophisticated code, EQUILIB, developed by Shannon et al. requires computer facilities and takes pressure into account. This offers an opportunity to check the calculations presented here. Shannon et al. present two examples of results, an unflashed and a flashed brine, in their paper. The results and a comparison with Is presented in this paper are presented in Table 2, and good agreement with the more complex calculations is demonstrated.

Calculation of Is

The calculation of Is in brines involves changing chemical equilibria in solutions of interest with temperature, pressure, and ionic strength. The changing equilibria in the solutions can be dealt with by allowing the equilibrium constants in the governing chemical equations to vary with the changing conditions of the brines. The equilibrium constants then become conditional constants whose values depend on temperature. pressure, and ionic strength.

JPT

P. 1583^