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

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.

Field Method for Determination of Bicarbonate Alkalinity

“…Recently, our group has developed an automatic titration method, which has been proved to provide both alkalinity and total organic acid accurately in different salinity and compositions brine samples . Kan et al 2013) This automatic titration method has also been round-robin tested by four production and service companies and the accuracy of the automatic titration method are further confirmed Tomson et al 2004). Therefore, this automatic titration method was chosen to measure the alkalinity and other parameters of the field samples in this study and the results obtained from the head-space method were compared to those from automatic titration to verify the accuracy.…”

“…The bicarbonate alkalinity was measured by pressure readings and total organic acids were measured by ion chromatography and converted to equivalent acetic acid concentration. Since the alkalinity of water samples is unknown, the Rice titration method Kan et al 2013), is used to give the actual alkalinity of each component for the produced water samples. To further demonstrate the new method, the results getting from the head-space method are compared to those obtained with the Rice titration method.…”

“…The parameters of the reservoir fluid system, such as pH, saturation indices of different minerals are vitally important to evaluate the potential corrosion or scale risks during the production process. Since in-situ pH is very expensive and difficult and pH will change with different temperature, pressure and other environmental conditions (Raghuraman et al 2007;Mathews et al 2009), pH of the fluid from reservoir to surface is usually estimated by the composition of brines, CO 2 partial pressure in gas phase and the flow rate (Kan et al 2013). The pH and calcite SI of each produced water sample at formation and wellhead were calculated by the ScaleSoftPitzer (SSP), the software developed by the author's group, to verify the accuracy of this new method (Kan and Tomson 2012).…”

“…The traditional method for alkalinity measurement is acidimetric titration (Andrew 2005). This method can be used to determine the alkalinity only when bicarbonate predominantly contributes to alkalinity (Apha et al 2012), but the end point becomes unclear or exaggerated in the presence of organic acids (Kan et al 2013) (shown in Fig. 1 and Fig.…”

Interrelationship of Weak Organic Acids, Bases, pH and HPLC Analysis Method in Produced Waters

“…Therefore their detection in produced waters is of particular interest. (Kan 2013) Subsurface waters associated with petroleum accumulations have been shown to have organic acid concentrations as high as 5000 mg/l (Carothers and Kharaka 1978). Saturated aliphatic carboxylic acids with two to five carbon atoms are the most commonly observed organic acids in subsurface waters (Fisher 1987).…”