Control of pH in Produced Geothermal Water Using Buffer Solutions

Abstract

There is a frequent need to adjust the pH of produced geothermal liquid to avoid corrosion or scaling. The current practice of dosing with strong acids to control silica scaling is challenging because precise dose rate control is required to avoid overdosing. An alternative approach using pH buffer solutions has been investigated as a more practical means of stabilising pH in geothermal fluids. A pH buffer is a mixture of an acid and its conjugate base (e.g. acetic acid and acetate) which has been formulated to a desired pH – such as pH 5 for silica scale control. Addition of a buffer solution to a geothermal fluid will shift the pH towards to the pH of the buffer, from either a lower or higher pH. Using pH buffer solutions thus avoids the problem of overdosing. Experiments were conducted in the laboratory (at 25°C) and in the field (100°C) to test the efficacy of a pH 5 buffer prepared from acetic acid (the least expensive available). These experiments showed effective control of pH was possible. The thermodynamic data for acetic acid suggest it will be effective at higher temperatures, in the range of power station operating conditions. Because of the higher cost of acetic acid, compared with sulphuric acid, the use of buffers may be more appropriate for lower mass flow, pH-critical flows. However, in higher flows it could potentially be used to trim the pH after initial dosing with less expensive sulphuric acid. Further testing of pH buffers is required at higher temperatures and to investigate potential downstream adverse effects (e.g. degradation products and precipitates).