All the natural gas accumulations in the Irish Sea are held within sandstone reservoir rocks. Sandstone reservoirs are known as “conventional” reservoirs and have been the mainstay of worldwide oil and gas since the inception of the hydrocarbons industry.
A conventional reservoir, such as those in the North and Irish Sea, are contained in permeable rocks and are very different to unconventional reservoirs such as those found in shale deposits.
The natural gas province of the South North Sea, which has produced 34,000Bcf of gas to the UK and significant additional volumes to the Netherlands since the early 1980s, has been produced from conventional sandstone reservoirs of the Triassic, Permian and Carboniferous.
Sandstone rocks are formed in environments where sand is prevalent, namely; shallow seas, lakes, rivers and deserts. The sand-grains pack together as the loose sand is compacted and becomes sandstone, but inter-grain spaces remain in which natural gas can accumulate. When buried deep in the ground, the natural high-pressure forces the gas to flow freely from the inter-grain spaces and into gas production wells.
There have been questions asked regarding the possibility of “shale-fracking” in the 112/25 Isle of Man gas accumulation. Shale is a rock composed of mud or clay particles that when compacted into shale rock have negligible inter-particle space to hold free flowing natural gas. These shale rocks are one of a number of reservoirs types known as “unconventional” and they require a specific production technology to access and produce the gas.
The gas accumulation in 112/25 is not held in “unconventional” shale rock, it is held in “conventional” sandstone rock. The well data acquired by BP in 1982 is quite clear that the gas reservoir is sandstone and detailed laboratory work by CROGGA has provided a quantified description and detailed imaging of that sandstone.
One of the key questions with respect to the 1982 gas discovery well drilled by BP is; “if there is a gas accumulation, why did it not flow to surface when the well was tested”. A forensic analysis of the drilling reports gives clear information that the methods employed and materials used were not conducive to drilling a clean and low-invasive well. This would result in the damage of the reservoir immediately adjacent to the wellbore, which will drastically reduce the porosity and permeability character of the Collyhurst reservoir sandstone and so inhibiting the flow of gas into the well.
Through our analysis of sandstone obtained from the Crogga field, and reviewing data from earlier studies, we are confident that our appraisal of the area will demonstrate the feasibility of gas extraction through conventional techniques.