Transport wells

The separated CO₂ is transported through flowlines 14 kilometres from the Krechba central processing facility to three injection wells, pumping the CO₂ in an isolated zone in the Krechba reservoir for permanent storage.

Injection wells

The eastern and northern perimeters of the Krechba reservoir are where the CO₂ horizontal injection wells are located; there are three wells in total to achieve good spatial coverage or contact across the rock formation as well as increasing gas output levels. Drilling them was a significant challenge for two reasons:

1. the local rock formations have very low permeability (only 10 – 20 millidarcies)
2. the porous saline rock formation comprising the target zone is only 20 metres thick and lies two kilometres below the surface, requiring horizontal drilling and a high degree of well drilling accuracy

Well design and construction

Each well was first drilled vertically into the target zone and then steered horizontally for a further 1.8 kilometres to find areas of high rock porosity to make subsequent injection of the CO₂ across the sand face easier.

State-of-the-art ‘measurement while drilling’ techniques were used. These enabled rock porosity to be logged at intervals by instruments behind the drill bit. The resulting 3D seismic images and drilling data were relayed live from Algeria to the UK, enabling the wells to be ‘geo-steered’ along the formation remotely from BP’s technology centre in Sunbury.

The vertical section of the wells – the metal tube or casing – is surrounded by cement completions to prevent CO₂ escaping up the side of the wellbore. In theory, cement can be dissolved by CO₂ but, in fact, the gas reacts chemically at the cement surface to form a toothpaste-like barrier. This then prevents further contact with the cement structure beneath and any degradation of the cement casing.

The horizontal well sections are completed along their length with slotted liners which have slots or perforations to assist the CO₂ to leave the well and so permeate the rock formation. Under these conditions of high pressure and temperature, CO₂ is a supercritical fluid, with the density of a liquid, but the viscosity of a gas.

A minor emission of CO₂ was detected and stopped in June 2007. Less than 0.1 tonne was involved and there were no health and safety implications. This occurred at a pre-existing well which had not been permanently decommissioned – and was being used as an observation well. The well has now been fully decommissioned and has full integrity.

Well location – a critical factor

The goal was to locate the wells to allow CO₂ to be pumped into the storage formation some five kilometres ‘down-dip’ – a position within a sloping, or ‘dipping’, strata situated below the top of a geological structure – from the gas-water contact. The natural gas is extracted from the ‘up-dip’ gas accumulation – located higher in the slope of the same structure.

Three injection wells are used to achieve good spatial coverage and enable high flow rates of CO₂. The CO₂ is a supercritical fluid at these conditions of pressure and temperature. It passes through the slotted liners but forcing it across the sand face and into the rock is harder, and its velocity is simultaneously reduced. But once in place, it is expected to remain permanently trapped.