Study Area
The Lower Cretaceous (Aptian-Albian) Captain Sandstone is a target reservoir for C02 disposal in the Goldeneye Field, Outer Moray Firth, UK. This is the Acorn CCS project. I attended a core viewing (Fig. 1) during the 2023 EGS conference in Aberdeen (16 - 19th May) and made some observations on the core laid out from wells 14/29A-5 and 20/4B-7.
Fig. 1. Selection of geological structures from core in the Lower Cretaceous Captain Sandstone in the Goldeneye Field.
Do the fractures breach the seal ?
One of these observations is an open fracture (maybe 2) running through a small shale bed within the sandstone (Fig. 2). Could this be a breach to a shale barrier or seal ? We would need to integrate this information with other data such as log data and pressures. In my experience, in this area (including the giant Britannia field), the Lower Cretaceous sandstones are not naturally fractured to the point that they can be classified as fractured reservoirs. In fact, most fractures (particularly when they occur in large quantities) are induced by coring practices. Therefore, the risk is probably low.
https://www.ogilviegeoscience.co.uk/blog/2023/5/29/induced-fractures-in-sandstone-core
Anyway, this illustrates the importance of core observations and the need then to integrate those with other data in order to answer key questions such as the integrity of this barrier. One of these would be a comparison of of the capillary pressure of the shale versus the capillary pressure of the Hydrocarbon column height.
Fig. 2. Open fracture running through the shale that is encased by sandstones. High confidence pick in green, low confidence dashed. Note how the fracture tips out in the finer grained injection structure at the top.
Small faults are more common than open fractures
Small faults (also known as shear fractures) are more common in these cores (Fig. 3). The Lower Cretaceous in this region has a wide variety of facies including turbidites, debrites and slurry sands and the distribution of those is a key uncertainty with more relevance to depletion planning than the distribution of faults and fractures.
Fig. 3. Small faults (pencil point) are more common in the Lower Cretaceous sands but the sandstones are not heavily faulted or fractured.
Takeaway Points
-Natural fractured can create essential permeability in a reservoir for the recovery of hydrocarbons.
-On the other hand, they may breach top seals such as shale barriers, in which a core visit (if possible) would be beneficial to understand this risk.
-However, other factors need to be taken into account in order to risk the seal integrity such as capillary pressure of the shale versus the capillary pressure of the Hydrocarbon column height.