• Energy Research

In the heating sector, borehole heat exchangers have become popular for supplying renewable energy. They tap into the subsurface to extract geothermal energy for heating purposes. For advanced applications, borehole heat exchangers require insulation in the upper part of the borehole either to meet legal requirements or to improve their performance. A priori numerical heat transport models of the subsurface are imperative for the systems’ planning and design. Only fully discretized models can account for depth-dependent borehole properties like insulated sections, but the model setup is cumbersome and the simulations come at high computational cost. Hence, these models are often not suitable for the simulation of larger installations. This study presents an analytical solution for the simulation of the thermal interactions of partly insulated borehole heat exchangers. A benchmark with a fully discretized OpenGeoSys model confirms sufficient accuracy of the analytical solution. In an application example, the functionality of the tool is demonstrated by finding the ideal length of a borehole insulation using mathematical optimization and by quantifying the effect of the insulation on the borehole heat exchanger performance. The presented method allows for accommodation of future advancements in borehole heat exchangers in numerical simulations at comparatively low computational cost.

Hydrogen storage in porous geological formations is a potential option to mitigate offsets between power demand and generation in an energy system largely based on renewables. Incorporating hydrogen storage into the energy network requires the consideration of multiple scenarios for storage settings and potential loading cycles, causing a high computational effort. Therefore, homogenous replacement models are constructed by applying different spatial averaging methods for permeability and linearized relative permeability to an ensemble of heterogeneous reservoir representations of a potential hydrogen storage site. The applicability of these replacement models for approximating storage characteristics, such as well flow rates, pressure changes and power rates, is investigated by comparing their results to the results of the full heterogeneous ensemble. It is found that using the arithmetic mean to estimate the lateral and the harmonic mean for the vertical permeability in the homogeneous replacement models provides an approximation to the median of the heterogeneous ensemble for pressure changes, storage flow rate, gas in place and power output. Basic time-dependent effects of reducing well flow, and thus the power rates, during an extraction cycle can also be represented by these homogeneous replacement models. Using geometric means is found not to yield a valid representation of the storage behaviour.

AbstractThis paper presents a code comparison of coupled multiphase flow and geomechanical processes resulting from CO2 injection into deep saline formations. The coupled simulator OpenGeoSys-Eclipse as well as Eclipse-Visage, GEM and OpenGeoSys are used for this purpose. Comparison of the results of the different simulators shows a strong dependence of the results on the grid discretization and the numerical methods applied. Further investigations of the one-way coupled geomechanical simulations shows that even if the multiphase flow results are nearly identical for the simulators used, the geomechanical response on the pressure build up can be different.

Abstract In this work, an innovative modular heat storage system is investigated experimentally and by numerical modeling. A single storage module consists of a helical heat exchanger in a water saturated porous cement matrix. The experiment comprises a 5 day thermal loading stage, followed by 16.5 days of passive cooling, and was especially designed to quantify the thermal insulation efficiency. An inverse modeling approach was applied to successfully match temperature measurements within the storage by numerical simulation. The thus determined heat loss rates amount to 130 W for the fully loaded storage and to 50 W on average during passive cooling.

Abstract Large-scale pressure build-up due to a CO 2 injection is investigated in the vertical and horizontal direction to determine locations and strategies suitable for pressure monitoring of the injection operation, cap rock integrity and large-scale geological setting. A realistic site-scale multi-layered model within the North German Basin is used, which explicitly accounts for a heterogeneous vertical structure including semi-permeable layers above the storage formation and below the cap rock. Results show that characteristic trends of the pressure signal are found for specific monitoring locations, depending on the horizontal and vertical distance to the injection well. Pressure signals and thus maximum pressures may be strongly delayed in the vertical direction, with a time shift larger than the injection period. This demonstrates that the maximum leakage risk in the cap rock may occur significantly after the injection period, not at the end of the injection. Vertical permeability contrasts between the cap rock and the storage formation as well as spatial heterogeneities cause variations in the shape and time evolution of the pressure signals and could thus be used to evaluate the vertical connectivity and the leakage risk. Boundary conditions and formation compressibilities control the pressure signal far from the injection well. It is shown that buoyant rise of CO 2 into overlying formations cannot be detected by pressure monitoring.

Large-scale energy storage such as porous media hydrogen storage will be required to mitigate shortages originating from fluctuating power production if renewables dominate the total supply. In order to assess the applicability of this storage option, a possible usage scenario is defined for an existing anticlinal structure in the North German Basin and the storage operation is numerically simulated. A heterogeneous and realistic parameter distribution is generated by a facies modelling approach. The storage operation, which is performed using five wells, consists of an initial filling of the storage with nitrogen used as cushion gas and hydrogen as well as several week-long withdrawal periods each followed by a refill and a shut-in period. Storage performance increases with the number of storage cycles and a total of 29 million m(3) of hydrogen gas at surface conditions can be produced in the long term, equating to 186,000 GJ of energy when assuming a re-electrification efficiency of 60 %. In addition to downhole pressure monitoring geophysical techniques such as seismic full waveform inversion (FWI), electrical resistivity tomography (ERT) and gravity methods can be used for site monitoring, if their individual detecting capabilities are sufficient. Investigation of the storage scenario by virtual application of these methods shows that FWI and ERT can be used to map the thin gas phase distribution in this heterogeneous formation with the individual methods conforming each other. However, a high spatial density of receivers in a crosswell geometry with less than 500 m distance between the observation wells is required for this. Gravity mapping also shows anomalies indicating mass changes caused by the storage operation. However, monitoring the filling state of this hydrogen storage site is not possible.

AbstractThe geological subsurface offers large potential renewable energy storage sites through cavern or porous media storage systems. This work presents a methodology for assessing the size of the storage systems required, for modelling the storage operation and for predicting the induced effects and impacts on the environment by numerical simulations. The methodology is demonstrated for a hypothetical porous medium hydrogen storage and for geothermal heat storage. It is found that induced pressure effects may range over kilometers for gas storage, while temperature effects are limited to a few tens of meters for heat storage.

AbstractThe storage of CO2 in deep saline aquifers is due to the large available capacities and the common occurrence of these formations one of the major options for carbon dioxide sequestration. Besides the multiphase flow aspects geochemical, thermal and mechanical processes may alter the conditions within the reservoir as well as in the cap rock. Whereas single aspects of these processes can be investigated with experiments a multi-process simulator allows evaluating their combined consequences for the storage system over short and long time scales. In this paper the newly coupled software ECLIPSE-OpenGeoSys is presented that allows a combined simulation of multiphase flow, transport and geochemical reactions. ECLIPSE provides a fast and efficient solution for the multiphase flow whereas the open-source scientific software OpenGeoSys is used for calculating transport and geochemical reactions. This paper presents the code structure of the interface. Furthermore the coupled software is successfully applied to benchmarks

Abstract The transition to renewable energy sources to mitigate climate change will require large-scale energy storage to dampen the fluctuating availability of renewable sources and to ensure a stable energy supply. Energy storage in the geological subsurface can provide capacity and support the cycle times required. This study investigates hydrogen storage, methane storage and compressed air energy storage in subsurface porous formations and quantifies potential storage capacities as well as storage rates on a site-specific basis. For part of the North German Basin, used as the study area, potential storage sites are identified, employing a newly developed structural geological model. Energy storage capacities estimated from a volume-based approach are 6510 TWh and 24,544 TWh for hydrogen and methane, respectively. For a consistent comparison of storage capacities including compressed air energy storage, the stored exergy is calculated as 6735 TWh, 25,795 TWh and 358 TWh for hydrogen, methane and compressed air energy storage, respectively. Evaluation of storage deliverability indicates that high deliverability rates are found mainly in two of the three storage formations considered. Even accounting for the uncertainty in geological parameters, the storage potential for the three considered storage technologies is significantly larger than the predicted demand, and suitable storage rates are achievable in all storage formations.