• Energy Research

Abstract During injection of CO2, monitoring of the subsurface saturation changes is required. For well logging in cased boreholes only a limited number of techniques such as radiometric pulsed neutron-gamma (PNG) logging are applicable. The conventional PNG saturation model mainly considers a displacement process. But during CO2 injection additional processes such as evaporation and salt precipitation are expected to occur as a result of the mutual solubility between brine and CO2. For this purpose an extended PNG saturation model for NaCl-brines is developed and applied to a time-lapse PNG monitoring data set from the Ketzin site. The results show that for the observation well further away from the injection well, the conventional displacement saturation model is valid, with average CO2 saturations below 60%. In contrast, the data from the injection well shows that both evaporation and salt precipitation have occurred. Here, the largest CO2 saturations with values up to 100% are determined locally. The results of the extended saturation model indicate that dry-out regions, where only CO2 and halite with saturations up to 1.4% exist, and maximum halite saturations up to 14.1% occur in the vicinity of the brine levels. The halite saturation distribution in the injection well seems to be controlled by changes in the injection regime associated with changing brine levels, lithological heterogeneities, and capillary effects. PNG monitoring in combination with the extended saturation model is suited to determine displacement and evaporation/precipitation processes for CO2 storage operations.

Abstract The aim of this work was to quantify long-term mineral trapping at a reservoir scale for the Ketzin pilot site for CO2 storage. An integrative approach coupling geochemical to reservoir simulations was therefore used. The main advantage compared to fully coupled reactive transport simulations is the reduction of computational time. Reactive transport simulations have not yet been performed to validate the approach. Data needed for the numerical simulations is supplied by geophysical and geological investigations at the site as well as by core and fluid sample analysis. The selection of elements of the 3D grid was carried out distinguishing between those exposed to gaseous CO2 and those with only dissolved CO2. Porosity and water saturation were used as coupling parameters between geochemical and reservoir simulations. An analytical approximation to scale geochemical simulations for heterogeneous porosity and water saturation was developed to assign each selected element of the reservoir model a dynamic and unique mineral trapping scenario. This was crucial to ensure the effectiveness and precision of the approach. Geochemical simulations show that mineral trapping is consecutively taken over by siderite, dolomite and magnesite, whereby the effective rate of mineral reactions strongly depends on the porosity. Results at reservoir scale after 10,000 years yield continuous growth of mineral trapping with an amount of 17,000 tonnes, which is about 25% of the totally injected CO2.

AbstractGeochemical simulation models are the computational bottleneck for coupled reactive transport simulations. We investigate the use of a data-driven surrogate model in place of a geochemical simulation model to speed up the run-times of reactive transport simulations. This is a challenge because the surrogate model needs to use results of its predictions as inputs at each subsequent simulation step. We test the suitability of surrogate model approach on a popular reactive transport benchmark problem, often used for evaluating simulation models. We show that the concept is feasible and can make the simulations many times faster, however several open areas for future work remain.

AbstractA benchmark study is presented comparing the outcome of two different simulators, the established TOUGH and the modern, more efficient MUFITS, with the purpose to evaluate the coupling of geochemical processes to multiphase flow in the context of subsurface CO2 storage. The results of the two simulators are in excellent agreement. The implemented simplified one-way coupling is a computationally efficient alternative to fully coupled reactive transport simulations. Every coupled process simulation framework benefits from the inclusion of different tools, which is the only way to enable validation and control of numerical results.

Abstract We showcase a flexible, extensible yet efficient framework for reactive transport modelling, including the ability to replace “full physics” geochemical simulations with surrogate models for speedup. Surrogates are data-driven models trained on a set of pre-calculated simulations by means of machine-learning methods. We offer also an input-output-error visualization component for interactive assessment and tuning of their accuracy. Our framework, based on open source or freely available software, makes possible complex reactive transport simulations and ease further research on optimized algorithms to tackle many geoscientific problems.

Barite scalings are a common cause of permanent formation damage to deep geothermal reservoirs. Well injectivity can be impaired because the ooling of saline fluids reduces the solubility of barite, and the continuous re-injection of supersaturated fluids forces barite to precipitate in the host rock. Stimulated reservoirs in the Upper Rhine Graben often have multiple relevant flow paths in the porous matrix and fracture zones, sometimes spanning multiple stratigraphical units to achieve the economically necessary injectivity. While the influence of barite scaling on injectivity has been investigated for purely porous media, the role of fractures within reservoirs consisting of both fractured and porous sections is still not well understood. Here, we present hydro-chemical simulations of a dual-layer geothermal reservoir to study the long-term impact of barite scale formation on well injectivity. Our results show that, compared to purely porous reservoirs, fractured porous reservoirs have a significantly reduced scaling risk by up to 50%, depending on the flow rate ratio of fractures. Injectivity loss is doubled, however, if the amount of active fractures is increased by one order of magnitude, while the mean fracture aperture is decreased, provided the fractured aquifer dictates the injection rate. We conclude that fractured, and especially hydraulically stimulated, reservoirs are generally less affected by barite scaling and that large, but few, fractures are favourable. We present a scaling score for fractured-porous reservoirs, which is composed of easily derivable quantities such as the radial equilibrium length and precipitation potential. This score is suggested for use approximating the scaling potential and its impact on injectivity of a fractured-porous reservoir for geothermal exploitation.