Seal integrity for subsurface gas storage: Innovative assessment workflows

The long-term integrity of geological barriers in a storage complex is essential for the usage of the subsurface as carbon sink or energy storage system. Within the frame of this study, workflows have been adapted to predict seal quality for different settings, timescales, targets, fluids, and available data sources. The first workflow aims for the prediction of breakthrough pressure, which controls the seal capacity in absence of faults or other discontinuities. It is based on a basin wide compaction trend derived from theoretical models which are calibrated with measured capillary pressure curves. The approach is extended to wire line logging data and the quantification of dynamic migration processes. Furthermore, rock-fluid interactions of a representative caprock mineralogy of the Vienna Basin with carbon dioxide are quantified. Lastly, a petrophysical investigation workflow for the challenging engineering environment of the tectonically deformed and strongly heterogeneous salt bodies of the Haselgebirge Formation is established.
A broad sample and data set including Middle to Upper Miocene mudstone core samples from the Vienna Basin, wire line logs from 40 wells in the Vienna Basin and core samples from salt lithologies of the Haselgebirge Formation were investigated. The mudstone core samples cover a depth interval from 700 to 3400 m and were investigated by X-ray diffractometry, with an Eltra C/S analyzer, and by Rock-Eval pyrolysis for bulk mineralogy, total organic carbon, and free hydrocarbon contents. Broad ion beam - scanning electron microscopy, mercury intrusion capillary porosimetry, and helium pycnometry were applied to the mudstone samples to obtain pore structural properties. Wireline logs have been filtered for mudstone intervals and were utilized to calculate porosity and column height logs. Physical migration processes of CO2 were identified and quantified. Rock-fluid interactions were quantified in a 1D model. Clean red salt and anhydrite rich grey salt have been characterized regarding their mineral composition (X-ray diffraction), texture (computed tomography, scanning electron microscopy), porosity, permeability, and for their nitrogen gas breakthrough behaviour in multi and single stage triaxial tests.
The mudstone core samples revealed clear decreasing porosity depth trends and implied that mechanical compaction was rather uniform in the central Vienna Basin. Comparing the Vienna Basin trend to global mudstone compaction trends, regional uplift causing erosion of up to ~ 500 m Upper Miocene strata is inferred. The wireline log-derived porosity vs. depth trend matches the core petrophysical data (R² = 0.90) and confirms the established core petrophysical normal compaction trend. A trend of increasing Rock-Eval parameters S1 and production index (PI = S1/(S1+S2)) with decreasing capillary sealing capacity of the investigated mudstones possibly indicates vertical hydrocarbon migration through the low-permeable mudstone horizons. Both static and dynamic sealing scenarios, however, showed that seal capacity in the Vienna Basin is high, and storage risks associated with top seal integrity are likely negligible. The reactive transport model showed a drop in pH which results in the dissolution of carbonate and chlorite minerals and the formation of anorthite, siderite, and dolomite. In a closed system, however, the porosity changes are below 0.1% and therefore changes in pore throat radius distribution and permeability are considered negligible. The investigated salt rocks showed a gas breakthrough at low confinement pressure (1 MPa) but stayed gas tight at 25 MPa confinement pressure until a maximum differential stress of 30 MPa.
Overall the established seal quality prediction workflows, which are based on the combination of different data sources on different scales, contribute to a faster evaluation of storage targets in basin settings with limited access to exploration data. The identification and quantification of physical migration processes in combination with hydrochemical modelling increased the understanding of mudstone seal behavior in the Vienna Basin and points out the need for improved modelling tools. Furthermore the insights into migration processes and mineral reactions will increase public acceptance of CO2 storage technologies. Lastly, the petrophysical characterisation of heterogeneous salt rocks contributes to an application of salt cavern storage in more challenging engineering environments.