Raiders of the Lost Mud: the geology behind drilling incidents within the Balder Formation around the Corona Ridge, West of Shetland

Abstract

The Faroe-Shetland Basin, NE Atlantic continental margin, hosts a number of important hydrocarbon fields, though deep water and narrow weather windows mean drilling costs are considerably higher than other parts of the UK Continental Shelf. Any additional drilling complications are therefore important to predict and negate as such issues can result in avoidable multi-million pound cost implications. This study focuses on the Corona Ridge, an intra-basinal high which contains the Rosebank Field, where a plethora of drilling issues, of enigmatic origin, are common within a key stratigraphic marker known as the Balder Formation. Drilling fluid loss, bit balling, wellbore breakouts, and wellbore "ballooning", where lost drilling fluid returns to the wellbore, are all recognised within the Balder Formation along the Corona Ridge. We find that many of the drilling incidents can be traced back to both the lithological character of the Balder Formation, and the mid-Miocene tectonic inversion of the Corona Ridge. Moreover, we find that this geological explanation has wider implications for exploration in the region, including mitigation of drilling incidents in future wells through drill bit selection.The Faroe-Shetland Basin, located on the NE Atlantic continental margin, represents one of the last remaining exploration frontiers of the UK Continental Shelf, with arguably the greatest remaining potential for significant new discoveries (Ellis & Stoker 2014;Austin et al. 2014). One particularly prospective area of the Faroe-Shetland Basin is the Corona Ridge (Fig. 1), an intra-basinal high which hosts a number of oil and gas discoveries, notably the ~240 million barrels of oil equivalent Rosebank Field (Austin et al. 2014). A challenging aspect of exploring around the Corona Ridge area, though, is high drilling costs associated with deep water (up to 1.5 km in places) and extreme weather conditions (Austin et al. 2014), necessitating the use of either fifth generation semi-submersible drill rigs ($125,000/day) or dynamically positioned drillships ($145,000/day) (IHS Markit 2018) in order to drill exploration and appraisal wells. Further exploration costs result from a myriad of drilling complications, particularly through thick volcanic sequences. Previous research has focused on drilling efficiency through these volcanic sequences, emphasising how key lithological properties contribute to different drilling and well control issues (

Seismic, petrophysical and petrological constraints on the alteration of igneous rocks in the Northern Carnarvon Basin, Western Australia: implications for petroleum exploration and drilling operations

“…Enfield-3 has similar neutron density and sonic responses to Enfield-4 and Stybarrow-2; however, they are less apparent. This pattern of petrophysical log changes in altered ashfall deposits is similar to that seen in wells intersecting the volcanic ash of the Balder Clay in the Faroe Shetland Basin, offshore Scotland, UK (Watson et al 2020).…”

“…Nonetheless, swelling can cause wellbore instability through cave-ins and expansion of formation into the wellbore, which also causes problems when running casing (Anderson et al 2010). Hydrated clays can cause cuttings to clump together, affecting borehole cleaning, and can lead to balling of clay on the drillbit (Anderson et al 2010;Watson et al 2020). A more costly consequence of clay swelling would be loss of the drilling assembly, forcing a sidetrack, or in the worst case, complete well abandonment (Anderson et al 2010).…”

“…Understanding igneous lithologies and issues related to the intersection of igneous rocks is crucial for a comprehensive appreciation of magmatic impacts on both petroleum systems and drilling. For example, igneous intrusions can overmature source rocks at a local scale and compartmentalise hydrocarbon reservoirs (Holford et al 2013;Senger et al 2017); transgressive intrusions can act as conduits for overpressure, risking blowout if penetrated accidentally (Schofield et al 2020); whilst volcanic ash altered to bentonite clay can also present drilling difficulties (Watson et al 2020).…”

The Rosebank Field, NE Atlantic: Volcanic characterisation of an inter‐lava hydrocarbon discovery

“…Elements such as lithofacies‐linked drilling operational challenges, a common feature in some volcanic drilling scenarios (e.g. Mark et al., 2018; Millett et al., 2016; Watson et al., 2020), can be populated in either quantitative (rock physics templates) or subjective (lithofacies stability inferences), in the same way leading to an improved scenario testing work‐space for future drilling. A close replication of the real seismic data by synthetic modelling enables important model testing for the inter‐well regions while new well results will obviously enable model refinement.…”

New insights into the structure, geology and hydrocarbon prospectivity along the central-northern Corona Ridge, Faroe–Shetland Basin.

“…Potential and proven source rocks are from both Scotchman et al (2006) and . Play types are modified from numerous authors individually cited (Goodchild et al 1999;Herries et al 1999;Duncan et al 2009;Larsen et al 2010;Trice 2014;Ogilvie et al 2015;Watson et al 2019). Note: wells and discoveries located along the Corona Ridge are shown with an asterisk (*); Eriboll (213/23-1) contained only oil shows and is added for context.…”