Degradation of cement at the reservoir/cement interface from exposure to carbonated brine

As geologic sequestration of carbon dioxide moves from the laboratory to the field, it will become increasingly important to understand how cements react when exposed to carbonated brines. To gather relevant data, a year-long study was conducted that exposed Class H cement pastes embedded in sandstones and limestones (both likely host rocks for CO2 sequestration) to brines, so that the reaction proceeded by diffusion of reactants and products through the pores of the stone. The experiments were conducted with pH values between 3 and 7 at 20 and 50 degrees C, conditions typical of sequestration at a depth of 1 km. The exposed samples and the reactor fluids were analyzed at 1, 2, 3, 6, and 12 months using numerical modeling and multiple laboratory techniques including optical microscopy, X-ray diffraction, EPMA, ICP-OES, and permeametry. The results indicate that cements in sandstone are more vulnerable to degradation (alteration that damages the cements' strength of zonal isolation properties, e.g. permeability) than cements surrounded by limestone. The sandstone samples showed changes in the physical appearance and flow properties after one month, with an order of magnitude change in the permeability within 3 months of exposure. The samples exposed to the lowest pH conditions showed the greatest degradation throughout the experiment. The cements embedded in limestone remained undegraded throughout the experiment. (C) 2011 Published by Elsevier B.V.