The Longyearbyen CO 2 Lab: Fluid communication in reservoir and caprock

Abstract

The Longyearbyen CO2 Lab of Svalbard, Norway was established to estimate the potential for geological carbon sequestration at Spitsbergen. Several monitoring wells were drilled in and around the planned CO2 injection site. These revealed a Triassic to Cretaceous stratigraphy consisting of (from top to bottom) a zone of permafrost, the aquifer, the caprock shale, and the upper, middle and lower reservoir. This paper uses two tools to investigate fluid communication within and between these entities: 87 Sr/ 86 Sr of formation waters extracted from cores using the residual salt analysis (RSA) method, and the δ 13 C of gases, principally methane and CO2, degassed from core samples. The Sr RSA data reveal that the upper reservoir rocks have very constant formation water 87 Sr/ 86 Sr in wells several kilometres apart, suggesting good lateral communication on a geological timescale. However, there is a distinct barrier to vertical communication within the middle reservoir, indicated by a step change in 87 Sr/ 86 Sr, corresponding to thin but presumably laterally extensive (>1.5 km) lagoonal mudrocks. The aquifer, which shows a gradient in 87 Sr/ 86 Sr, is also interpreted to have some degree of vertical internal communication on a geological time scale. The caprock shale shows large-scale (over 350 m) smooth vertical gradient in 87 Sr/ 86 Sr. This is indicative of an ongoing mixing process between high-87 Sr/ 86 Sr waters within the caprock and lower-87 Sr/ 86 Sr waters in the underlying reservoir. Diffusion and flow modelling of the Sr data suggest that at some time in the past shale, fluid transport properties were enhanced by the formation of temporary pressure escape features (fractures or chimneys) during deep burial and uplift or cycles of glaciation. Nevertheless, the smooth compositional gradient in the caprock indicates that fluid mixing has subsequently taken place slowly, dominated by diffusion. This 3 interpretation is supported by the gas isotope data, where systematic variations in gas δ 13 C values also indicate slow and incomplete diffusional fluid mixing. These are positive indicators for caprock effectiveness during a CO2 injection project.

Numerical modelling of permafrost spring discharge and open-system pingo formation induced by basal permafrost aggradation

“…;Braathen et al, 2012;Wangen et al, 2016). The low pressure indicates hydrogeological separation from upper groundwaters immediately beneath the permafrost, in line with Sr-isotope analyses from the drill cores (Huq et al, 2017;Ohm et al, 2019).…”

“…Pingo spring waters and sub-permafrost groundwater in Adventdalen indeed contain both CH 4 and CO 2 (Hodson et al, In Review). The methane is dominated by a biogenic fingerprint (Hodson et al, 2019) and contemporary methanogenesis is probable (Huq et al, 2017). In order to explain pressure build-up, the essential question is whether any ongoing process produces or releases gas to the groundwater, thereby increasing pressure.…”

Fluid flow properties of the Wilhelmøya Subgroup, a potential unconventional CO2 storage unit in central Spitsbergen

“…In the analysis, the flow barrier has been modelled as fully sealing, but the DH7A data are also consistent with some minor leakage across the boundary, e.g., with a multiplier lower than 0.01. A multiplier in this range is also consistent with the lack of observable interference in DH5R, especially with unstable and noisy data in DH5R after well operations with falling water level coupled with variable gas influx in DH5R (Huq et al, 2017). Poor reference data, as in this case, require a strong response to be clearly identifiable as interference, as with no barriers or barriers with only moderate flow restrictions.…”

“…Natural gas was encountered at several stratigraphic intervals during the drilling campaign (Senger et al, 2016;Huq et al, 2017;Ohm et al, 2017). Thermogenic gas and oil-stained sandstones were encountered in the De Geerdalen Formation (DH4; Fig.…”

“…250 m below DH5R and DH7A. Numerous thin intrusions are also encountered within the Wilhelmøya Subgroup in DH4 but are likely sills (Ogata et al, 2014 Huq et al (2017) showed that the strontium isotope composition of formation water within the reservoir is uniform, suggesting these faults will not compartmentalise the reservoir on a geological time scale.…”