Abstract
Due to significant temperature differences between the injected medium and in situ formation, injection of CO2 (as with water or other cold fluids) at depth induces thermal changes that must be accounted for a complete understanding of the mechanical integrity of the injection/storage system. Based on evaluations for the Northern Lights Carbon Capture and Storage (CCS) project, we focus on thermal effects induced on the caprock via conduction from cooling in the storage sands below. We investigate, using both analytical and numerical approaches, how undrained effects within the low permeability caprock can lead to volumetric contraction differences between the rock framework and the pore fluid which induce both stress and pore pressure changes that must be properly quantified. We show that such undrained effects, while inducing a more complicated response in the stress changes in the caprock, do not necessarily lead to unfavourable tensile conditions, and may, in fact, lead to increases in effective stress. These observations build confidence in the integrity of the caprock/seal system. We also show, through conservative assumptions, that pressure communication between the caprock and storage sands may lead to a localised negative effective stress condition, challenging stability of the base caprock, which will be mitigated for in field development planning.
Highlights
The Northern Lights full-scale demonstration project [1] represents a significant step forward in developing an offshore Carbon Capture and Storage (CCS) industry offshoreNorway
This cooling has resulted in a contraction of the pore fluid, but because of the extremely low permeability in the caprock, the contraction results in a suction, or negative pore pressure change in the cooled parts, up to 80 bar at 10–15 m into the caprock (Figure 9, bottom)
Updated numerical simulations are ongoing to both consider variations in mechanical/thermal properties as observed in the Eos well as well as coupled fracture growth processes to better understand the true effects of non-elastic process development. This investigation shows that pronounced thermal changes within the low permeability caprock may occur during CCS projects
Summary
The Northern Lights full-scale demonstration project [1] represents a significant step forward in developing an offshore Carbon Capture and Storage (CCS) industry offshore. The project consists of shipping, temporary storage, pipeline transport and permanent geological storage via dedicated injection wells and is part of the larger Longship project [2], which includes onshore capture at one or two industrial facilities. The aim of the flexible ship transport solution is to eventually allow for low-cost, industrial decarbonisation at scale. The NO 31/5–7 (Eos) well was drilled in 2019–2020 to prove (A) committable storage and (B) storage integrity. As such extensive logging, coring, production testing and in situ stress testing (extended leak-off (XLOT)) were employed. The storage complex is defined as the Lower Jurassic Dunlin Group Johansen and Cook
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