Hewett: A Promising Carbon Storage Site?

Increasing anthropogenic emissions of carbon dioxide have been directly linked to global warming. The UK government aims to reduce these carbon dioxide emissions by at least 50% by 2050. Carbon capture and storage is a promising new technology to mitigate some of the effects of carbon dioxide on climate.

The Hewett Unit, Southern North Sea, has been identified as a potential carbon storage site. It is a depleted gas field, therefore has a proven hydrocarbon seal, although was never filled to spill. It is situated on the UK continental shelf, 16 miles NE of Bacton, Norfolk. E.On has planned to capture CO2 at its Kingsnorth, Tilbury and other nearby power stations within the Thames and Medway estuaries. The CO2 will be transported via pipeline, some 270 miles to the Hewett Gas Field.

3D seismic and borehole data have been used to assess the suitability of the site for carbon storage and identify any potential high risk zones. Badley Geoscience's Traptester was used during interpretation of the seismic data to investigate the structural features in more detail. In particular, the fault sealing capabilities of the system were examined.

The Hewett Field, one of the main fields within the Hewett Unit identified for carbon storage, is a trap within the Triassic Bunter Sandstone Formation. The field is predominantly dip-closed on all sides, except on its most north-eastern flank where it is closed by the North Hewett Fault. This fault is considered to be the most important to analyse in order to understand the intricacies of fluid flow within the system. Production data has confirmed that pressure communication exists between the Hewett Field and the neighbouring Little Dotty Field, probably across the North Hewett Fault. Fault seal analysis has identified potential leak points across the North Hewett Fault that could allow for fluid flow, a finding that is consistent with the observed pressure communication. A key aim of the study is therefore to understand the implications of across-fault fluid flow for effective storage of carbon dioxide.

The Dowsing Fault Zone is a structurally complex area consisting of two major opposing listric faults bounding a heavily faulted graben. The interaction of all these faults, both major and minor, must be analysed in detail to assess the suitability of the site for possible expansion of carbon storage within the six nearby “D” fields which make up the remainder of the Hewett Unit.

Traptester will once again be used to assess the risk of fault reactivation, due to carbon dioxide injection, by analysing the stresses present in the system. This includes a detailed investigation of the pressures that exist in the formations using data provided by IHS Global. Pressure data used is compiled from IHS Global's historical records and wireline log data.

Preliminary results from the North Hewett Fault show the Hewett Field to be a promising carbon storage site. Further detailed analysis of the Dowsing Fault Zone will reveal if the storage site can be expanded into the six surrounding “D” fields to host a greater capacity of carbon dioxide.