Lower Cretaceous Rodby and Palaeocene Lista Shales: characterization and comparison of top-seal mudstones at two planned CCS sites, offshore UK

Worden, Richard H ORCID: 0000-0002-4686-9428, Allen, michael ORCID: 0000-0001-5068-3661, Faulkner, daniel, Utley, James, Bond, Clare E, Alcalde, Juan, Heinemann, Niklas, Haszledine, R Stuart, Mackay, Eric and Ghanbari, Saeed (2020) Lower Cretaceous Rodby and Palaeocene Lista Shales: characterization and comparison of top-seal mudstones at two planned CCS sites, offshore UK. Minerals.

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Abstract

Petroleum-rich basins at a mature stage of exploration and production offer many opportunities for large scale Carbon Capture and Storage (CCS) since oil and gas were demonstrably contained by low permeability top-sealing rocks, such as shales. For CCS to work, there must be effectively no leakage from the injection site so that the nature of the top-seal is an important aspect for consideration when appraising prospective CCS opportunities. The Lower Cretaceous Rodby Shale and the Palaeocene Lista Shale have acted as seals to oil and gas accumulations (e.g. the Atlantic and Balmoral fields) and may now play a critical role in sealing the Acorn and East Mey subsurface carbon storage sites. The characteristics of these important shales have been little addressed in the hydrocarbon extraction phase, with an understandable focus on reservoir properties and their influence on resource recovery rates. Here we assess the characteristics of the Rodby and Lista shales using wireline logs, geomechanical tests, special core analysis (mercury intrusion) and mineralogical and petrographic techniques with the aim of highlighting key properties that identify them as suitable top-seals. The two shales, defined using the relative gamma log values (or Vshale), have similar mean pore throat radius (approximately 18 nm), splitting tensile strength (approximately 2.5 MPa) and anisotropic values of splitting tensile strength, but they display significant differences in terms of wireline log character, porosity and mineralogy. The Lower Cretaceous Rodby Shale has a mean porosity of about 14 %, a mean permeability of 263 nD (2.58 x 10-19 m2), is calcite-rich and has clay minerals that are relatively rich in non-radioactive phases such as kaolinite. The Palaeocene Lista Shale has a mean porosity of about 16 % a mean permeability of 225 nD (2.21 x 10-19 m2), and is calcite-free, but contains abundant quartz silt and is dominated by smectite. The 2% difference in porosity does not seem to equate to a significant difference in permeability. Elastic properties derived from wireline log data shows that Young’s modulus, material stiffness, is very low (5 GPa) for the most shale (clay mineral)-rich Rodby intervals, with Young’s modulus increasing as shale content decreases and as cementation (e.g. calcite) increases. Our work has shown that Young’s modulus, which can be used to inform the likeliness of tensile failure, may be predictable based on routine gamma, density and compressive sonic logs in the majority of wells where the less common shear logs were not collected. The predictability of Young’s modulus from routine well log data could form a valuable element of CCS site top seal appraisals. This study has shown that the Rodby and Lista Shales represent good top-seals to the Acorn and East Mey CCS sites and they can hold CO2 column heights of about 380 m. The calcite-rich Rodby Shale may be susceptible to localised carbonate dissolution and increasing porosity and permeability but decreasing tendency to develop fracture permeability in the presence of injected CO2, as brittle calcite dissolves. In contrast the calcite-free, locally quartz-rich, Lista Shale will be geochemically inert to injected CO2 and but retain its innate tendency to develop fracture permeability (where quartz rich) in the presence of injected CO2.

Item Type:	Article
Uncontrolled Keywords:	Rodby Shale Lista Shale Carbon Capture and Storage North Sea wireline logs splitting tensile stress mercury intrusion porosimetry SEM-EDS quantitative mineralogy geomechanical properties Young’s modulus CO2 column height
Depositing User:	Symplectic Admin
Date Deposited:	30 Jul 2020 15:33
Last Modified:	18 Jan 2023 23:38
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3095814