Structural geology and well planning in the Clair Field

Abstract

The Clair Field is a giant oilfield located approximately 70 km west of the Shetland Isles, UK. It was discovered in 1977 and brought on stream some 28 years later. Key to unlocking its economic potential was a series of appraisal wells drilled in the early 1990s that identified fractures as the primary production mechanism. Structural geology contributed in several ways to the detailed planning of the development and appraisal wells. In the sandy (Tertiary) tophole section, outcrop analogues and offset wells were used to establish an appropriate standoff from major faults. This was to mitigate the risk of wellbore instability in what is otherwise a relatively benign sequence to drill. The mid-section, Upper Cretaceous mudstone is prone to wellbore instability, believed to be caused by strength anisotropy with respect to bedding. Here, polygonal faulting may contribute directly to wellbore instability. The associated bed rotation also influences anisotropic failure, which depends in part on the wellbore-to-bedding intersection angle. An example is given of how an understanding of the structural evolution of the overburden section impacts well casing placement. Finally, judgement on the nature of the faulted contact between two fault blocks was required for the pressure prognosis of a planned well.

Overpressure transmission through interconnected igneous intrusions

“…Mafic intrusions are identified throughout the FSB, where they are collectively termed the Faroe-Shetland sill complex (FSSC), and extending northward into the Møre Basin and south into the Rockall Trough (Schofield et al, 2017). Critically, the FSSC, and the sills in other Atlantic margin basins, are observed to preferentially intrude the Cretaceous and lower Paleocene sedimentary succession, which is predominantly composed of marine shales (Stoker, 2016) and represents a significant lowpermeability sealing unit (Ogilvie et al, 2015).…”

Integrated Fault Seal Analysis: An Introduction

“…Faults that were once a hindrance to reaching (risk of wellbore instability etc., Ogilvie et al 2015) deeper reservoir targets, could, in some geological settings, themselves, become targets for production (fractures around faults) and be an integral part of a field depletion plan. This will likely coincide with an increasing focus upon unconventional reservoirs (e.g., the faulted, oil-bearing mudstones in the overburden section of the Valhall Field, Central North Sea studied by Bradley et al 2019).…”

Overpressure Transfer Through Interconnected Igneous Intrusions

“…Mafic intrusions are identified throughout Final peer reviewed accepted copy of manuscript awaiting publication in AAPG Bulletin the FSB where they are collectively termed the Faroe-Shetland Sill Complex (FSSC), and extending northwards into the Møre basin and south into the Rockall Trough . Critically the FSSC, and the sills in other Atlantic Margin basins, are observed to preferentially intrude the Cretaceous and lower Paleocene sedimentary succession, which is predominantly composed of marine shales (Stoker et al, 2016) and represents a significant low-permeability sealing unit (Ogilvie et al, 2015).…”