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Abstract
The volume of CO2 that can be stored in the Captain Sandstone saline aquifer in
the North Sea was investigated by building a geological model and performing
numerical simulations. These simulations were also used to calculate the best
position for the injection wells, and the migration and ultimate fate of the
CO2.
The overall migration of CO2 and the pressure response over the entire saline
aquifer was studied by the calculated injection of 15 million tonnes CO2 per
year. The injection rate was restricted to a maximum of 2.5 million tonnes CO2
per year for each of a possible 12 wells considered. An important objective was
to predict how to avoid flow of the injected CO2 toward potential leakage
points, such as the sandstone boundaries and faults. The migration of injected
CO2 towards existing oil and gas fields was also a determining factor.
The summary conclusions are:
- The Captain Sandstone saline aquifer has significant potential CO2 storage
capacity. Even with all boundaries closed to flow, the probable storage
capacity is calculated to be about 358 million tonnes, giving a storage
efficiency of 0.6% of pore volume, with an expected operating life-span of
15-25 years.
- The possible storage capacity of the formation may be at least four times
greater if the aquifer boundaries are open. This increase would be a result of
displacement of salt water, and not CO2.
- The storage capacity if the sandstone is closed to flow may be increase from
358 to 1668 million tonnes of CO2 by significant additional investment in 15 to
20 water production wells.
- Injection of up to 2.5 million tonnes CO2 per year in one well has an
impact on the pressure throughout the entire formation, and thus interference
between different injection locations must be considered.
Keywords: CO2, CCS, Storage Capacity, Saline Aquifer
Introduction
Carbon Capture and Storage (CCS) is considered to be an important means of
reducing greenhouse gas emissions. CO2 may be stored in depleted oil and gas
fields, deep saline aquifers or unmineable coal seams. Of these options, deep
saline aquifers have the greatest storage potential both world-wide (IPCC,
2005) and in the UK(Scottish Carbon Capture and Storage, 2009). However, there
is much uncertainty in the size and structure of aquifers compared to
hydrocarbon reservoirs and evaluation of CO2 storage capacity is therefore a
key step in the appraisal of CO2 storage sites. The objective of study was to
assess the CO2 storage capacity of the Captain Saline Aquifer using the dynamic
method (Jin et al., 2010, Smith et al., 2012). Throughout this paper the
Captain Sandstone saline aquifer is referred to as Captain, which is
differentiated from the Captain hydrocarbon field, currently operated by
Chevron, by referring to the latter as the Captain Field.
The process was to build a 3D statistical geological model of Captain in Petrel
(Schlumberger, 2009), using data supplied by the British Geological Survey
(BGS), other project partners and from the open literature, and then use this
geological model to build a dynamic reservoir simulation model which would run
using ECLIPSE (Schlumberger, 2010). The ECLIPSE model was then used to
calculate storage capacity, taking into account factors such as injectivity,
well placement, and uncertainty in the geological model and conditions at the
system boundaries.
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