Wellbore-Instability Predictions in Cretaceous Mudstones: Clair Field, West of Shetlands
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 124464, ’Wellbore-In stability Predictions Within the Cretaceous Mudstones of Clair Field, West of Shetlands,’ by R. Narayanasamy, SPE, D. Barr, SPE, and A. Milne, BP plc, prepared for the 2009 SPE Offshore Europe Oil & Gas Conference & Exhibition, Aberdeen, 8-11 September.
Wellbore instability in Cretaceous mudstones overlying the Clair oil field has prevented successful drilling of high inclination wells in the past. Rock-failure analysis, based on a planes-of-weakness theory, fit previous drilling observations. Minimum-required-mud-weight predictions were made for a range of well configurations. The results of this study were used to plan an extended-reach-drilling (ERD) development well targeting a previously undeveloped part of the Clair field.
Introduction
The Clair oil field is on the UK Continental Shelf, 75 km west of the Shetland Islands in 140-m water depth. Discovered in 1977, first production was in February 2005 from a fixed steel platform with pipeline export to the Sullom Voe terminal on Shetland. The Clair field covers more than 200 sq km and contains more than 5 billion BO equivalent (BOE), of which 1.5 billion BOE is targeted by the Phase-1 development.
The currently producing Clair Phase-1 development comprises fractured Devonian sandstone reservoirs and is produced by waterflooding with a mix of downflank and pattern injection. The Devonian reservoir is more than 500 m thick, but only the upper zones (V and VI—250- to 300-m total thick-ness) are producing, principally through high-inclination or subhorizontal wells designed to target a combination of permeable natural fractures and the best-quality matrix. The reservoir is highly compacted and cemented and does not present wellbore-stability issues while drilling. However, the combination of shallow targets [gas/oil contact at 1550 m true vertical depth subsea (TVDSS) and oil/water contact at 2100 m TVDSS] and moderate water depth requires inclination to be built rapidly in shallow unconsolidated overburden formations. Core from Well A09 was obtained to characterize rock failure through various tests. The tests revealed anisotropic strength behavior. Analyzing several existing wells with the test results enabled predictions of minimum mud weight required for stability in future wells. The full-length paper details the mud-weight predictions for an ERD well and observations made while drilling it.
Drilling History
Conventional wellbore-stability analysis based on a Mohr-Coulomb failure criterion was consistent with drilling observations in most of the early wells. Subsequently, however, borehole collapse was experienced while drilling through Upper Cretaceous mudstones in Wells A06 and A08. Static mud weights of up to 1.42 SG were used in Well A06 and its two unsuccessful sidetracks at an inclination of 76° (measured from vertical). Similar problems were experienced while drilling Well A08, at a lower inclination of 57° and with 1.36-SG mud weight. An increase in mud weight to 1.54 SG enabled successful drilling of a side-track (Well A08Z) and another well with a similar inclination (Well A11). An intermediate-casing shoe was set in the Cretaceous mudstones in Well A11, and a 65° section then was drilled successfully with 1.58-SG mud weight. A challenging ERD well with 76° inclination (Well A15) was drilled successfully with 1.61-SG mud weight.