Speaker
Description
Hydrogen (H$_2$) containment in the subsurface is of growing importance for underground energy storage and is also relevant to nuclear waste disposal, where H$_2$ may be generated as a by-product, e.g. from radiolysis. Underground hydrogen storage will be done in reservoir formations sealed by low-permeability rocks, while engineered barrier systems for nuclear waste disposal are hosted within similarly low-permeability geological formations, including claystones. Gas accumulation may lead to elevated pore pressures that can trigger capillary failure and compromise barrier integrity, making the capillary sealing capacity of such formations critical. This capacity can be quantified by the capillary breakthrough pressure. Experimental data on hydrogen breakthrough in claystones are scarce, and most existing measurements do not explicitly account for the influence of stress. As breakthrough pressure is controlled by the smallest available pore throats, it is expected to depend strongly on confining pressure and on rock properties related to burial history, such as thermal maturity.
In this study, laboratory H$_2$ gas breakthrough experiments were conducted on fully water-saturated claystone samples to investigate the influence of confining pressure, thermal maturity and bedding anisotropy, on capillary sealing behaviour. Core plugs were prepared from intact Amaltheen Claystone cores obtained from boreholes in the Hils and Sack Synclines of the southern Lower Saxony Basin (northern Germany), a region characterized by a south–north increase in thermal maturity, with samples drilled both parallel and perpendicular to bedding to assess the influence of burial-related compaction and anisotropy on gas breakthrough behaviour.
Experiments were performed using a stepwise gas pressurization method, in which gas pressure was incrementally increased on the upstream side of the sample while monitoring the downstream pressure response. Gas breakthrough was identified by a distinct and sustained increase in downstream pressure, indicating the formation of a continuous gas pathway through the sample. These measurements were complemented by determinations of the effective gas permeability.
Preliminary results show a clear dependence of as breakthrough pressure on confining pressure, with progressively higher gas pressures required to initiate breakthrough as stress increased. Values increased from 0.75 to 3 MPa over a stress range of 5 to 20 MPa (relatively low mature sample; parallel to bedding). This behaviour is attributed to stress-induced pore compaction leading to increased capillary entry pressures. Effective permeabilities increased by up to one order of magnitude post-breakthrough.
Breakthrough pressure was also found to increase systematically with thermal maturity. As thermal maturity reflects the maximum burial depth experienced by the rock, this trend is interpreted to result from the development of tighter pore structures in more mature samples. Values increased from 3 MPa to 5.5 MPa for samples with vitrinite reflectance between 0.48 to 0.70 %VRr. In addition, breakthrough pressure differed between samples drilled parallel and perpendicular to bedding, demonstrating slight anisotropy in transport behaviour.
Overall, the results demonstrate that gas breakthrough in these mudstones is controlled by stress, burial-related compaction, and bedding anisotropy. These findings provide experimentally constrained bounds on gas pressures that claystone host rocks can sustain and contribute to risk assessment of sealing integrity.
| Country | Germany |
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