Interrelationship of CO2, Weak Acids, Bases, and pH in Scale Prediction and Control

Abstract

Abstract pH is one of the most important parameters for evaluating the scale and corrosion potential of the water during oil and gas production. The effectiveness of chemical treatment can also be influenced by pH in the production tubing and reservoir. Unfortunately, pH is not a conservative parameter that can be determined independently. pH changes from reservoir to surface facilities due to changes in temperature and pressure of the production system and the corresponding G/O/W phase changes. Measured pH value of a produced water sample is often unreliable and influenced by temperature and ionic strength of the solution, degree of degassing and sample preservation. Measuring pH in line or with downhole probe at real field condition can be expensive and difficult. pH can be predicted theoretically from charge balance equations more economically and reliably by assuming a good knowledge of thermodynamic equilibrium constants, activity coefficients, G/O/W flow rates, temperature, pressure, and reliable produced water composition. The author's research group recently developed a new automatic titration method to simultaneously measure total alkalinity and weak organic acids concentrations of brine. The carbonate thermodynamics were evaluated with calcite solubility studies at ultra-high temperature and pressure. The newly developed thermodynamic data and method enable accurate prediction of produced water pH at temperature and pressure typically encountered in deep water production. In this paper, the influences of water chemistry parameters on pH are reviewed. The newly developed automatic alkalinity titration method is discussed. pH calculated by the recently validated thermodynamic constants and activity coefficients are compared with live water pH measurements.