Posted by corrib man on April 08, 2002 at 23:03:18:

Anyone who might be interested in a little detail about the gas off Mayo, see below. Kind of technical but a few nuggets of interesting information like the average well cost is 8 million pounds and first gas is estimated around Q1 2004.
(found on the web and taken from a lecture abstract Enterprise are giving at the Geological Society in London on April 9th)
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The Corrib gas field is located offshore Republic of Ireland, about 70 km west of Co. Mayo, in the Slyne Basin. This is a narrow Triassic / Jurassic half graben and is one of a series of linked basins which are present along the west coast of Ireland and the British Isles. The Corrib field is a dry gas field, with the gas reservoired in Triassic Sandstones (Sherwood Sandstone Equivalent). These form a relatively simple anticlinal trap, with a complex faulted overburden which is structurally detached from the reservoir by the Mercia Halite, which also forms the top seal. The field was discovered in 1996 by well 18/20-1 (Enterprise Oil Operator, partners Statoil and Santa Fe (now Marathon)) and a further five appraisal wells have led to the field reaching development sanction at the end of 2001.

Since the field discovery three main technical issues have challenged the subsurface team. These are:

Seismic data Quality: The 2D data in the area is extremely poor in quality, due to the presence of near surface volcanic lava flows which cause severe attenuation, scattering and act as a strong multiple generator.
Drilling Costs: The discovery well cost £20.5mm pounds, of which a significant part was due to slow drilling rates through the Broadford Beds, a hard sand/shale/limestone formation. At these costs, field development would have been uneconomic.
Reservoir Quality: The appraisal wells have encountered significant lateral variation in the reservoir sandstone permeability and, to a lesser extent, porosity.
The seismic data has been dramatically improved through the use of 3D and Pre-Stack Depth Migration. Although scepticism existed at the time, tests on 2D data showed that enhancing the low frequency end of the spectrum improved reflector continuity sub-basalt. This low frequency bias, coupled with the expected improvement in multiple attenuation through improved velocity control (from 3D) resulted in a remarkable improvement in data quality. Subsequent reprocessing refined and improved the multiple attenuation process, resulting in a good quality Pre-Stack Depth migration, which has removed significant uncertainty in the reserves and well prognoses.

Drilling costs were particularly high due to the problems associated with drilling the Broadford Beds. This unit is approximately 600 metres thick and drilled at 1-1.5m/hour in 18/20-1. Due to better drill bit selection, removal of a casing string, synthetic muds (with skip and ship for the cuttings) and careful well planning to avoid the Broadford Beds, the average well cost is now around £8mm.

The reservoir sandstones were deposited as part of the extensive fluvially dominated Lower Triassic Sherwood Sandstone Formation depositional system that covered large parts of the UK and Irish continental areas. In the Corrib Field, these sandstones are approximately 400 meters thick, and consist dominantly of braided fluvial channel sandstones, with subordinate sand-flat and playa mudstone deposits. Significant lateral variability in reservoir quality across the field cannot be explained by a corresponding variation in the gross depositional setting or by inter-well scale diagenetic variability. However recent detailed analyses of the extensive petrographic database available from the field suggest that the reservoir quality variation is linked to subtle variations in the sandstone grain population across the field.

On the successful completion of 18/25-3 in the summer of 2001, the field was received Board sanction, and a Petroleum Lease was awarded by the Irish Government for the Corrib field on the 19th November 2001. The field, which is the first commercial gas discovery on the Irish / British Isles Atlantic margin, will be developed using the current appraisal wells (all suspended) linked to a sub-sea manifold, and the tied back to shore via a 70kms pipeline and umbilical. Currently it is estimated that first gas will be around the beginning of 2004.

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