A holistic understanding of the physicochemical processes induced by CO2 injection and storage in a reservoir is based on a geoscientific characterisation of the overall geological system consisting of reservoir rocks and cap rocks. It requires in a first step a comprehensive baseline characterisation (sedimentological, mineralogical, geochemical, mechanical, etc.) of pertinent parameters and conditions. To properly handle the large amount of different geoscientific information a Data Management System (DMS) was developed, which proved indispensable to conduct such a multi-disciplinary project. The DMS provides a tool for scientific process management, data analysis, integration and visualisation, data transfer and scheduling through specialised database systems and retrieval techniques, storage technology, and efficient data access. Sedimentological (facies), mineralogical and petrophysical data classify the Altmark Rotliegend sandstones of high quality with best porosity and permeability in altered/bleached aeolian sandstones. Those properties are strongly controlled by sediment deposition (facies-type) and by early and late diagenesis, namely by early pore-filling cementation and late fluid-rock interaction. The fluids involved probably originated from Carboniferous formations and ascended along (re-) activated faults through the Rotliegend volcanic rocks during Triassic-Jurassic times. Major features controlling the extent of fluid-rock reactions are grain size and grain sorting, respectively pore size, pore shape and pore connectivity. Especially grain coating chlorite and pore-filling carbonate and anhydrite will most probably affect reactivity during CO2 injection. Laboratory batch experiments with core samples reveal that CO2 saturated brines cause dissolution of (pore-filling) minerals and alteration of fluid and flow properties of the reservoir rocks. This also was emphasised by a parameter study based on numerical modelling, which indicate a tendency of increasing anhydrite dissolution and calcite precipitation with increasing CO2 partial pressure. Increasing temperature and salinity counteracts this effect. Changes of rock properties observed comprise an increase in porosity, permeability, water binding capacity, rock wettability and a slight decrease in residual gas saturation. The recent stress field was determined by direct measurement of the effective primary principal stresses. The total primary stresses were calculated from the effective primary stresses, pore-pressure effectiveness and pore pressure. The orientations of the primary horizontal stresses coincide with the directions of the fault zones in the Salzwedel area. Stress ratios for the anhydrite and claystone strata, calculated from the obtained effective primary stresses, are expected to be modified with a pilot injection into the reservoir. Methods and numerical tools were developed, which are dedicated to the numerical characterisation of the nearly depleted gas reservoir as well as to the simulation of the processes during CO2 injection and migration storage. The only practical option for predicting the long-term behaviour of CO2 in reservoirs is numerical analysis, supported by the understanding gained from the relatively short-term laboratory and field-scale experiments. Corresponding to the real site conditions, compositional gas flow has to be considered including mutual interactions with thermal, mechanical and geochemical (reactive) processes taking into account high pressures and temperatures. Process- as well as site-related benchmarks were jointly developed and implemented. It is observed that the high accuracy of complex equations of state (EoS) do not justify the higher computing costs compared to simple, cubic equations of state. The Peng-Robinson equation of state is the most suitable EoS with regard to pure CO2. At no stage of the process there is any evidence of plastic deformation. Both reservoir and cap rock behave elastically. In general, CO2-EGR operation might cause only local changes in the pore pressure of the reservoir, while CO2 storage will increase the reservoir pressure dependent on the injection rate if no gas is produced at the same time. Simulations revealed that an initial tensile stress regime is the safest precondition while the compressive stress state is problematic with regard to the reservoir integrity. The calculated, virtual tracer test, applying krypton, indicates that concentrations at the monitoring wells would be too low to be detected. Hence a test under the considered constraints is not feasible.
Clean: Co2 Large-scale Enhanced Gas Recovery in the Altmark Natural Gas Field, 2013, p. 53-98