Estimation of Long-Term CO 2 and H 2 S Release During Operation of Geothermal Power Plants

ABSTRACT The concentrations of CO 2 and H 2 S in undisturbed liquiddominated high-temperature geothermal reservoir waters are generally controlled by temperature dependent equilibria with various mineral buffers. These equilibria cause the concentrations of these gases to increase with temperature. The presence of equilibrium steam in the reservoir (two phase reservoir) will cause the gaseous concentrations in the fluid to be higher than the aqueous equilibrium concentrations at any particular temperature. In the range of about 230-300°C, the CO 2 buffer is considered to be clinozoisite + prehnite + quartz + calcite. In hightemperature waters of low salt content, which are strongly reducing, the H 2 S buffer is considered to be pyrite + pyrrhotite + epidote + prehnite. In waters of higher salinity, the respective H 2 S mineral buffer may consist of pyrite + magnetite + hematite. The concentrations of CO 2 and H 2 S in steam of wet-steam wells producing from liquiddominated reservoirs are higher than those of the parent fluid, frequently in the range 50-300 and 2-20 mmoles/kg of steam, respectively. However, values as high as 1000 mmoles/kg for CO 2 and 50 mmoles/kg for H 2 S are not uncommon. The concentrations of these gases in steam from wet-steam wells depend on 1) their concentration in the parent geothermal water, 2) the steam fraction, which has formed by depressurization boiling, 3) the reservoir steam fraction, if present, 4) the steam separation pressure, and 5) the boiling processes, which lead to the steam formation. Long-term utilisation of geothermal reservoirs may lead to decline in the concentrations of CO 2 and H 2 S in the steam. The decline can be caused by recharge of cooler water into producing aquifers and/or progressive boiling of water retained in the aquifer rock by capillary forces. Further, enhanced boiling, which is a consequence of reservoir pressure draw down, and steam separation during lateral flow into production wells may cause the well discharge to become depleted in gas. The separated steam may form a steam cap over the liquid reservoir and/or enhance fumarolic activity. Although gas emissions from geothermal power plants may be enhanced much during the early years of production relative to natural discharge, in the long run, the integrated gas emission may not exceed that of the natural gas flux. A steady state may be reached between the flux of gases from the magma heat source into the geothermal system and from the geothermal system into producing wells and fumaroles. The source of noble gases, apart from He, in geothermal fluids is air saturated meteoric water. The relative abundance of noble gases in geothermal steam may aid assessment of which processes are responsible for changes in the concentrations of the environmentally important CO 2 and H 2 S

On-line monitoring of the gas composition in the Full-scale Emplacement experiment at Mont Terri (Switzerland)

An on-line gas monitoring has been conceived and implemented to study the evolution of the composition of the free gas phase in the Full-scale Emplacement (FE) experiment in the Underground Rock Laboratory at Mont Terri (Switzerland). The FE experiment is a trial run for a spent-fuel emplacement drift for a repository according to the Swiss concept for radioactive waste disposal. The monitoring of gas species such as He, Ar, Kr, Xe, N2, O2, H2, CH4, and CO2 was performed successfully over several months. The partial pressures of gases relevant for the operational safety such as H2 and CH4 have been found to be below the concentration threshold for ignition. The combination of the on-line monitoring data and conventional noble-gas isotope measurements reveals rapid gas exchange between the pore space of the compacted bentonite granulate material used as backfilling and both the access niche and the host rock surrounding the FE tunnel (Opalinus Clay). Such fast gas exchange partly explains the disappearance of oxygen from the bentonite pore space detected by O2 sensors even prior to sealing of the drift and the accumulation of a fraction of terrigenic gases such as 4He, 40Ar, CH4, and CO2

A natural cement analogue study to understand the long-term behaviour of cements in nuclear waste repositories: Maqarin (Jordan)

The geological storage of nuclear waste includes multibarrier engineered systems where a large amount of cement-based material is used. Predicting the long term behaviour of cement is approached by reactive transport modelling, where some of the boundary conditions can be defined through studying natural cement analogues (e.g. at the Maqarin natural analogue site). At Maqarin, pyrometamorphism of clay biomicrites and siliceous chalks, caused by the in-situ combustion of organic matter, produced various clinker minerals. The interaction of infiltrating groundwater with these clinker phases resulted in a portlandite-buffered hyperalkaline leachate plume, which migrated into the adjacent biomicrite host rock, resulting in the precipitation of hydrated cement minerals. In this study, rock samples with different degrees of interaction with the hyperalkaline plume were investigated by various methods (mostly SEM-EDS). The observations have identified a paragenetic sequence of hydrous cement minerals, and reveal how the fractures and porosity in the biomicrite have become sequentially filled. In the alkaline disturbed zone, C-A-S-H (an unstoichiometric gel of Ca, Al, Si and OH) is observed to fill the pores of the biomicrite wallrock, as a consequence of reaction with a high pH Ca-rich fluid circulating in fractures. Porosity profiles indicate that in some cases the pores of the rock adjacent to the fractures became tightly sealed, whereas in the veins some porosity is preserved. Later pulses of sulphate-rich groundwater precipitated ettringite and occasionally thaumasite in the veins, whereas downstream in the lower pH distal regions of the hyperalkaline plume, zeolite was precipitated. Comparing our observations with the reactive transport modelling results reveals two major discrepancies: firstly, the models predict that ettringite is precipitated before C-A-S-H, whereas the C-A-S-H is observed as the earlier phase in Maqarin; and, secondly, the models predict that ettringite acts as the principal pore-filling phase in contrast to the C-A-S-H observed in the natural system. These discrepancies are related to the fact that our data were not available at the time the modelling studies were performed. However, all models succeeded in reproducing the porosity reduction observed at the fracture–rock interface in the natural analogue system