Real-time chemical stratigraphy in Shearwater Field North Sea

To safely and successfully drill and complete the infill wells and reinstate production, RPS applied an innovative approach to drilling. Our geologists employed near real-time chemical stratigraphy during the well-site operations to reduce subsurface uncertainty.

The Shearwater Field, 225 km off the coast of Aberdeen, is part of the Central North Sea high-pressure–high-temperature (HPHT) petroleum system and significant volumes of gas-condensate remain. Drilled initially between 1997 and 1999, and producing gas in 2000, these wells targeted the Virgin Fulmar reservoir, where initial pressures were above 15 000 psi. This early production caused the depletion of the primary reservoir and a decrease of in situ pore pressure. It also resulted in reservoir compaction, in turn causing mechanical failure of all original development wells, halting production prematurely.

Key details

Project name

Real-time chemical stratigraphy

Client

Shell

Location

Shearwater Field, North Sea, UK

 

Services provided

Real-time chemical stratigraphy

Operational geologists face many technical challenges when working on HPHT wells - extreme pressure, variable pore pressure profiles, greater drilling depths and subsurface uncertainty makes drilling a complex undertaking, often requiring specialised drilling programme support.

And in the Shearwater Field, these factors are compounded by the reservoir depletion and compaction which weakens the overburden. Specialised drilling techniques are used to mitigate these factors, including managed pressure drilling (MPD), drill-in-liner (DIL) technology and wellbore strengthening materials.

These drilling techniques make conventional Logging While Drilling (LWD) data unavailable, increasing stratigraphic uncertainty. And the high temperatures raise the chance of tool failure – the driller is left with the choice to continue with an increased risk or lose time replacing tools.

A novel approach to the challenges of drilling HPHT wells was required.

Daniel Atkin, Senior Reservoir Geologist & Chemical Stratigraphy Coordinator at RPS, and his team used chemical stratigraphy to provide valuable additional data, predominantly through critical ‘drill-in-liner’ sections.

Testing of archived drilling material showed that this approach would be beneficial in meeting well-site objectives, and the use of real-time chemical stratigraphy was approved.

During drilling operations, ditch cutting fragments were collected at regular intervals of 5’ to 10’. Samples were cleaned, dried, dusted and visually inspected under a microscope. RPS’ geologists used specially calibrated mobile spectrometers to carry out the geochemical analyses, which provided data on the chemical elements present. They used the data to refine and expand the previous zonal classification scheme. In addition to geochemical zonations, geologists used synthetic gamma-ray (Synth GR) profiles to help in the stratigraphic positioning and to place key casing points.

Drilling increasingly complex petroleum systems require innovative approaches that are effective and repeatable. In the Shearwater Field, adding geochemical data to well-site operations reduced subsurface uncertainly through the refinement of the zonation framework. The Synth GR profiles showed an extremely close relationship with conventional GR profiles and this technique effectively reduced subsurface uncertainty through critical sections of the wells.

The risk involved in ‘drilling blind’ through the DIL sections decreased, and when LWD drilling equipment malfunctioned during the drilling of two of the wells, the Synth GR approach served as a valuable backup. It saved significant operation timing and associated costs. 

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