North Sea Core CIC specialises in collecting and distributing core from the UK Continental Shelf. Energy companies often obtain core samples when drilling wells for oil and gas exploration, appraisal and development and possibly for future geothermal and carbon capture and storage investigation. But how does the process of cutting core actually work?
The following information and images on drilling and coring operations has been kindly supplied by Andy Moffat. Please credit him and North Sea Core CIC for any material used.
North Sea Core CIC data is licensed under a CC-BY-4.0 License.
HOW IS CORE OBTAINED?
Core is a way of collecting undisturbed samples of the subsurface, that is the rocks meters to kilometers below the earth's surface. This is different to normal drilling operations, which is a destructive process that produces small bits of rock called ‘cuttings’. The stratigraphic sections typically of interest and that are usually cored are the reservoir sections, where hydrocarbons are thought to be present. However sometimes geologists also want core from other sections of interest.
Depending on the strength of the formation being drilled, not all core is recovered at the surface. For example, shales can become very broken up during drilling so the recovered core can be missing sections of the whole stratigraphy. It is therefore important to take wireline logs, especially Gamma Ray logs of the recovered core so the formation depths can be matched with the well log.
Once the core is extracted, small cylindrical core plugs are taken out of the cylinder of core. These plugs are cleaned, dried and used for various testing. This includes determining porosity, permeability, density to better assess how much oil is held within the rock and how "easy" is flows through the pores. Special Core Analysis (SCAL) will often be carried out, which include testing the relative permeability of the core and its capillary pressure.
Core also forms a great resource for geologists to understand the stratigraphy and architecture of the reservoir. This was especially important in the early days of North Sea exploration, where many kilometres of core was cut to gain knowledge of the depositional environments and thereby make predictions as to where similar reservoirs could be found.
Although most cores are cut to better understand reservoirs, sometimes a "Total Depth core" will be retrieved from a well. This is done to prove that the well has terminated in basement. Total depth cores are usually around 30 ft / 9 m long and are much cheaper to drill as they do not require the same aluminium or fibre glass sleeves needed for coring reservoir sections. These cores are often drilled in places like the West of Shetland.
Even more rarely, coring is used instead of drilling when trying to penetrate particularly hard formations which would otherwise cause the drill bit to wear down too quickly.
Nowadays, coring is an expensive and complex operation, commonly involving a team of 12 or more people:
Wellsite geologists tracks operations on an oil or gas well to provide advice about how to conduct drilling, collect samples and pick the coring points within the stratigraphy of interest.
Coring engineers oversee the coring equipment, cutting and retrieving the cores and laying down the cores ready for processing. Mud loggers assist as required.
Core technicians measure, cut and mark up the core, run core gamma ray logs and take small samples every meter along the core which is then used by the wellsite geologist to study the core and make decisions about further drilling. They cut shorter sections of core which are sealed in wax, ready for special core analysis. They are also responsible for photographing, boxing up and preparing the cores to be dispatched from the rig.
Core bits are attached to the end of the drill string to replace the drill bit when it is time for coring. In order to attach a core bit, drilling is stopped, and the drill string must be completely pulled out of the wellbore. This process takes time and is expensive as the drilling process is temporarily stopped. Therefore, core is usually only taken from reservoir sections where they are important for analysis. Core bits are similar to drill bits, with the exception that they have a hollow centre to allow for coring. Different core bits are used depending on the type of formation which is being cored and are described below;
CORING BACK IN THE DAY
The coring process in the 70s, 80s and 90s was quite different to today. Due to the lack of a protective sleeve, core recovery was lower and the rocks were prone to get damaged and broken up. The core was recovered on the drill floor and packaged into 3ft core recovery boxes. The core was then processed on the rig which involved writing the depths on the core, writing a core description and often taking a gamma ray log. Samples were taken for special core analysis and the core boxes were then shipped onshore. This core recovery process left core vulnerable to decontamination via rain etc. and any fluids held within the core could be evaporating.
Since the late 90s, core recovery operations have left core with a greater chance of good preservation. Core is now recovered in fibre glass or aluminium sleeves which protects the material and makes the recovery and handling safer for those on the rig floor.
The following links and references are on the regional offshore geology and terminology of the UK Continental Shelf (UKCS) and the North Sea, including those used to construct this website. Where possible, links to open access material have been listed.
Revision of the existing UK North Sea lithostratigraphical nomenclature and the establishment of a new standard scheme by the British Geological Survey (BGS) and the United Kingdom Offshore Operators Association (UKOOA).
BGS Report CR/15/124: Palaeozoic Petroleum Systems of the Central North Sea/Mid North Sea High (2016)
Report by the British Geological Survey (BGS) on the the Carboniferous and Devonian petroleum systems of the Central North Sea and Mid North Sea High area. Published on the Natural Environmental Research Council (NERC) Open Research Archive.
The Geological Society of London Memoir Number 20 written by geologists and and geophysicists, from both industry and academia, who work on hydrocarbon fields and discoveries across the UK Continental Shelf. These including the North Sea and contain information on the tectonic and stratigraphic framework, as well as stories behind the major plays and discoveries.
Geological and petroleum atlas written by a collaboration of the Geological Society of London (GSL), the Geological Survey of Denmark and Greenland (GEUS) and the Norwegian Petroleum Society (NPS). Funded by and based on data supplied by the oil and gas industry.
The Geological Society of London Special Publication Number 403 that focuses on the regional depositional setting of these deep-marine systems, stratigraphic and palaeogeographical context for exploration, and their development case histories.
Selection of links to educational websites, geoscience activities and organisations to help provide activities and ideas. You can also visit the Geoscience Online Teaching Resources google spreadsheet created by Prof. Julia Libarkin, with many, many contributions from across the world of Geoscience.
A Lego based activity created by Dr. Stephanie Zihms to communicated how geologists use outcrop and core data to understand the subsurface, including synclines, anticlines and a simple oil reservoir.
Collection of online Earth system science activities with scientific data sets and analysis tools that enables users to explore some aspect of the Earth system.
Extensive catalogue of activities across the earth sciences to support teachers in all levels of education and in number of languages. Additional educational resource links, blog, research and publications.
A national provider of CPD in Earth science to UK teachers with an extensive list of teaching resources.
Compilation of geoscience education resources from a variety of organizations, providing free Earth and Space Science resources, from lessons to outreach and teacher professional development. Supported by the American Geosciences Institute (AGI).
A free educational outreach project for schools developed and run by the School of Earth & Environmental Sciences at the University of St Andrews, with a sister project at University College London.
Collection of free educational resources from the Geological Society of London, includes rocks and fossils, geological hazards and geological time.
An organisation dedicated to creating access and inclusion for persons with disabilities in the Geosciences through a range of resources and research available to academia, industry and the public. Includes the Diversity in Geoscience UK (DiG-UK), a newly-established UK chapter of IAGD as the Geological Society of London.
Aims to promote geoscience education internationally at all levels while enhancing the quality of geoscience education and raising public awareness of geoscience. Includes teaching activities (links to Earth Learning Idea), learning best practice and free geoscience textbook resources.
An extensive website that catalogues a wide range of geological resources, including textbooks, software, webinars, virtual thin sections and field trips and much more. It is also possible to share your own educational material with the website.
A range of earth science activities and resources for 8-14 year old school students. Created by the Science Partnership which is a collaborative project lead by the California State University, East Bay and the Alameda County Office of Education.
Online resources for educators in the geosciences and related fields, include classroom activities, course descriptions, information about pedagogical strategies, topical collections, and more. Managed by the National Association of Geoscience Teachers with support from the Science Education Resource Center (SERC).
An interactive Geology Toolkit by the Open University, featuring a geology timeline, rock analyser, rock cycle, landscape features and safety tips.
Selection of links to virtual geological resources that could be used with your core material, including to help understand what geology may look like in outcrop or under the microscope, to how we understand the subsurface. We also recommend you visit the Geoscience Online Teaching Resources google spreadsheet created by Prof. Julia Libarkin, with many, many contributions from across the world of Geoscience.
Aims to provide open-source geological data and visualisations for researchers, educational institutions and the general public. Led by Adam Cawood and Clare Bond at the University of Aberdeen.
A database of images of a range of sedimentary, igneous and metamorphic rocks created by the Earth Science Education Unit (ESEU). Includes exposure, hand specimen, close-up, thin section, x-polars and rock in use.
Contains a collection of structural geology images and is sponsored by Tectask, the Commission on Tectonics and Structural Geology of the IUG.
A a repository and viewer for >100 high quality virtual 3D geological outcrops and geoscience models from across the world which are free to use for non-commercial teaching and research purposes. It is a part of a long-term initiative from the Virtual Outcrop Geology (VOG) Group which is based in the NORCE Norwegian Research Centre in Bergen, Norway, and the University of Aberdeen, Scotland.
Interactive field trips across the globe on a range of topics run by Arizona State University.
Contains a number of the field trips run by Ocean and Earth Science at the University of Southampton, with each module including a series of panoramic and 3D images on a range of scales.
A range of mapping exercises, 3D geological and topographic maps provide by the University of Leeds.
Has a vast selection of virtual thin sections and accompanying hand samples from collections that are currently held in museums, universities and other institutions around the world.
The following links are for data repositories, often Geological Surveys and Government Organisations. They include a range of subsurface data including core, well and seismic data. Please note that not all of these repositories are based in the UK and or include North Sea related geology.
Wide range of datasets available through the OpenGeoscience portal, including Maps and Sections, Thin Sections and Research Reports. Onshore and Offshore data can be searched through the Map Viewers which contains geological maps, geochemical, geophysical and engineering information, subsurface data, GIS layers and more. The Offshore Hydrocarbon Wells Database which contains >300 km of scanned core material and >4.5 million cutting samples from ~8000 wells.
Surface data from petroleum exploration on and around the Faroe Islands, includes a range of core, well and seismic data, in addition to GIS shapefiles. Please be aware not all data is available online or for free.
Database of largely onshore boreholes across Sweden, with additional GIS layers of information. The Geological Survey of Sweden has on ongoing project at the national drill core archive in Malå to scan ~200, 000 meters of drill core from Norrbotten and Västerbotten. Details on the
project, the core scans available and how to use the map view are available here.
This is an international marine research collaboration which has conducted a vast number of research expeditions throughout the world's oceans. Access to a wide range of databases is available through a number of portals, with data geochemical, petrophysical and core data. Additional legacy DSDP/ODP/IODP core data can be searched via this map.
An impressive database of over 1,000,000 metres of core from over 3,000 boreholes across the seven Australian Geological Survey jurisdictions. The data is available through the AuScope project, with more information here.
Access to a wide range of material, including relinquishment reports, information on hydrocarbon fields and discoveries, licence rounds and awards, offshore boundary polygons and subsurface data, such as seismic volumes and wireline logs. Available through the Open Data portal and the National Data Repository (NDR), most data is available from free or to be viewed online. Please note, industry, academia and the wider public may have to register to search, view and download disclosed information.
An intermediate data release pack from BGS of borehole GGC01, site 10 of the UK Geoenergy Observatories Glasgow facility. Contains sedimentology, discontinuity and engineering logs, composite and digital wireline logs, sample recovery information and a range of core logs and images.
Access to a wide range of products and data, including geological maps, publications, images and GIS data. Well data can be searched via the USGS Well Catalog or by looking through the available Links to State Well Data. Core data can be found through the Links to Core Repositories. The curation and availability of data varies across the individual geological surveys.