Experimental investigations of the effects of acid gas (H_2S/CO_2) exposure under geological sequestration conditions

摘要

Acid gas (mixed CO_2 and H_2S) injection into geological formations is increasingly used as a disposal option. For example, more than 40 acid gas injection projects are currently operating in Alberta, Canada [1]. In contrast to pure CO_2 injection, there is little understanding of the possible effects of acid gases under geological sequestration conditions on exposed materials ranging from reactions with reservoir minerals to the stability of proppants injected to improve oil recovery to the possible failure of wellbore cements. The number of laboratory studies investigating effects of acid gas has been limited by safety concerns and the difficulty in preparing and maintaining single-phase H_2S/CO_2 mixtures under the experimental pressures and temperatures required. In an effort to address the lack of experimental data addressing the potential effects of acid gas injection, the Plains CO_2 Reduction Partnership (PCOR) in the United States has developed approaches using conventional syringe pumps (ISCO 260D pumps) and reactor vessels (Parr Instruments) to prepare and maintain H_2S/CO_2 mixtures under relevant sequestration conditions of temperature, pressure, and exposure to water and dissolved salts. Exposures up to several months can be conducted at temperatures and pressures up to 350°C and 414 bar (6000 psi) using any desired H_2S/CO_2 mole ratio. Up to 16 individual samples racked in separate glass vials can be exposed at one time, and the use of separate glass vessels allows different salt brine concentrations to be evaluated. In addition to performing static exposure experiments, the pumps and vessels are sufficiently leakfree that reaction rates can be determined by monitoring mass flow at the pumps. Interestingly, this is much easier to perform for reactions with H_2S than with CO_2, because H_2S is much less compressible and has lower Joule–Thompson heating/cooling effects than CO_2. Thus, obtaining accurate values for the mass of CO_2 used during an experiment based on pump volume is much more difficult than for H_2S, although controlling the pump cylinder temperature with a water jacket allows reasonable measurements to be made. These systems have been used to determine reaction rates of model systems, such as the formation of magnesium carbonate from magnesium silicate and the formation of pyrite from iron oxide (Fe_3O_4). For example, the use of H_2S (as measured at the pump) was steady at ca. 0.5 grams per day (for 18.6 grams of Fe_3O_4) until the reaction was complete. The half-life of the reaction was 20 days, and the mass balance (0.54 moles H_2S actual compared to 0.48 moles theoretical) was reasonable.