The permanent mineral storage of carbon using brine is a critical component of geologic carbon sequestration in saline aquifers. It is suggested that pH has a significant effect on the precipitation of mineral carbonates. The design of an ex situ process to efficiently drive this otherwise geologic reaction requires an understanding of the evolution of brine pH from the initial step of pH adjustment, through to the final stages of the conversion. The effects of temperature, pressure, and pH on the formation of carbonates, such as calcite, during the reaction Of CO2 with natural gas well brine were investigated. A high pressure/high temperature reactor equipped with a liquid sampling valve allows for the monitoring of brine pH and composition throughout 6-h reactions at temperatures of 75 and 150 degrees C and pressures of 600 and 1500 psi. Results show that temperature has a greater control on changes in pH than pressure. However, initial brine pH is the main factor controlling the formation of carbonates. ICP-AES analysis of brine samples to identify changes in brine composition during reaction suggests that a much longer reaction time is needed to approach steady state and to establish a clear correlation between variations in solution chemistry and the precipitation of carbonates. XRD has confirmed the precipitation of calcite in experiments with an adjusted initial pH to approximately 9. (c) 2005 Elsevier B.V. All rights reserved.