Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK

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 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 approximately 14 %, a mean permeability of 263 nD (2.58 × 10−19 m2), and 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 approximately 16% a mean permeability of 225 nD (2.21 × 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 show 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 approximately 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 but retain its innate tendency to develop fracture permeability (where quartz rich) in the presence of injected CO2.

Pore Structure and Seal Capacity of Ilaro Formation Shales in the Eastern Dahomey Basin: Implications from Mercury Injection Capillary Pressure and Spontaneous Imbibition Analyses

“…9 Studies have reported shales with column heights greater than 300 m as having good CO 2 seal capacity. 13,14 In this study, the CO 2 column height (Figure 10) reveals the samples as being characterized by good seal capacity due to their ability to prevent the flow in a CO 2 saturated column above 290 m on average. Furthermore, apart from the ankerite deficient Ilaro-1, CO 2 heights (Figure 10) obtained from mesoporedominated samples greater than 300 m. Recently, the carbonate-rich Rodby shale with an average column 396 m was deduced as being characterized by a decreasing tendency to fracture during CO 2 injection due to carbonate dissolution.…”

“…Furthermore, apart from the ankerite deficient Ilaro-1, CO 2 heights (Figure 10) obtained from mesoporedominated samples greater than 300 m. Recently, the carbonate-rich Rodby shale with an average column 396 m was deduced as being characterized by a decreasing tendency to fracture during CO 2 injection due to carbonate dissolution. 14 is because 2 trapping via interactions with carbonate or feldspar is due to the dissolution of Ca or Mg that results in the of secondary carbonates, which trap the injected CO 2 . 4 Carbonates have a higher affinity to this compared to feldspar, with quartz having the lowest.…”

“…Techniques such as mercury injection capillary pressure (MICP) are suitable for seal capacity evaluation due to the quick and stable determination of the capillary entry pressure. , This suitability, along with interfacial tension and wettability influence on the column height of CO 2 within a rock, has led to seal capacity evaluation from the conversion of MICP data to CO 2 column height in several studies. , However, in order to overcome the shortcomings of MICP data, , a multidisciplinary approach to seal capacity analysis via integration of other laboratory fluid flow techniques has been encouraged . Since seal capacity is influenced by the pore throat size and pore size distribution (PSD), the evaluation of shale PSD via integration of spontaneous imbibition and MICP analyses implies that a similar combination can also be used to evaluate seal capacity.…”

Editorial for Special Issue “Characterisation of Mudrocks: Textures and Mineralogy”

“…This Special Issue on "Characteristics of Mudrocks: Textures and Mineralogy" contains seven papers, representing the works of 32 researchers in aspects related to the field of mudrock research. Areas covered include petroleum exploration [1], carbon capture and storage [2], diagenesis [3], environmental interpretation [4,5] and new techniques [6]. These papers should be of interest to all those interested in the field of mudrocks.…”

“…Worden, Allen, Faulkner, Utley, Bond, Alcalde, Heinemann, Haszeldine, Mackay and Ghanbari describe and discuss the characteristics of two important North Sea mudstone units that will act as top seals for the underlying storage of CO 2 in planned carbon capture and storage (CCS) schemes (Acorn and East Mey CCS) [2]. Wireline logs, geomechanical tests, special core analysis and mineralogical and petrographic techniques are used to demonstrate the potential of wireline logs as a predictor of Young's modulus and thus the susceptibility of top seals to brittle fracture and the development of fracture leakage pathways.…”