Energy, Resources and the Environment

The mysterious subsidence of the seafloor due to oil production – How to predict it with a simple model?

ERE Matters June 22, 2015

by Daniel Keszthelyi

Physics of Geological Processes group at the Department of Physics, University of Oslo

Over 40 years of oil production from the Ekofisk field caused the overlying seafloor to sink over 9 meters during the years and while there have been numerous researches on the topic; the clear understanding of what happens with the reservoir rocks during production is still missing. We created a simple model of compaction with physics-based assumption to estimate the magnitude of the subsidence.

The Ekofisk field situated some 320 km off the coast of Norway is one of the largest petroleum fields of the country. Oil is produced from carbonate rocks (chalk) lying almost 3 km below the seabed to an oil platform standing in 76 meter deep water. The depletion of oil from the carbonate rocks caused a dramatic decrease in the pore pressure in these rocks and in turn a large increase in the effective stress acting to them. This increased effective stress then led to their compaction which was much more significant than expected by previous models.

The compaction has positive effect on the oil production as it pushes out oil from the rock; however it also puts at risk the surface facilities (oil platform and pipelines) and decreases the permeability of the rock making the flow of fluids inside them more difficult.

Our new model of compaction is based on very simple assumptions and describes rock as a collection of pores where these pores are material weaknesses. Imagine a sheet of paper with a small cut made in the middle of the paper. Then if you try to tear or shear the paper slowly you can see that this cut starts to grow until the sheet of paper is torn into two pieces. Similar things happen to the rock if effective stresses are increased: the pores – like the small cut in the middle of the sheet of paper – will become nuclei of new fractures and eventually a fracture network will be created. According to linear elastic fracture mechanics the larger the pore the less stress is needed to involve it into fracturing and vice-versa the larger the stress the smaller pores can be fractured: so with increasing effective stresses considerably more fractures can be created.

The Ekofisk field: location, mechanism of compaction and predicted subsidence (by Daniel Keszthelyi)

Fluids originally inside the pores can flow into these new fractures and if the fluid is water or partly water it can dissolve the material of rock: in carbonates calcite. The exact mechanism is called pressure solution which vaguely speaking means that the solubility of calcite depends on the pressure and therefore it can dissolve at grain-grain contacts along the new fracture and precipitate anywhere else. According to our model this dissolution will lead to the compaction of the carbonate rock.

The speed of dissolution can be calculated from pressure solution theories and the number of fractures can be calculated by statistical means and therefore the speed of deformation (the strain rate) can be predicted knowing some parameters without using any fitting parameters. All we have to know is porosity, pore size distribution, effective stress, water saturation, temperature and how the solubility of calcite depends on pressure and temperature: all of these can be measured independently in laboratory experiments.

If we apply this model to the Ekofisk field using an estimated pressure history of the reservoir we get quite good agreement with the measured subsidence values. This means that with a very simple model with no fitting we are able to predict the subsidence of the Ekofisk field. Furthermore, with simple modifications might also help to better understand other subsidence cases related to oil, gas or water production.

This post is based on the EGU talk Compaction creep by pore failure and pressure solution applied to a carbonate reservoir by Daniel Keszthelyi, Bjørn Jamtveit and Dag Kristian Dysthe. Daniel Keszthelyi is a PhD candidate at the Physics of Geological Processes group at the Department of Physics, University of Oslo. For further information please contact daniel.keszthelyi @fys.uio.no.

The Pore Space Scramble

ERE Matters June 15, 2015

by Alexandra Gormally^a and Michelle Bentham^b
^aLancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK; ^bBritish Geological Survey, Keyworth, Nottingham, NG12 5GG, UK

The underground is being used more to help us meet some of the challenges facing humans from tackling climate change, waste disposal to ensuring energy security. The notion of ‘pore space’ and its commodification, has gained much momentum over the last few years in policy circles, industry and the natural (geo)sciences alike. However ‘pore space’ and the use of the underground is now also being discussed within the social sciences too. Human geographers in particular are starting to critically discuss some of the ways in which society uses, perceives and interacts with the subsurface in the past, present, and different ways of how this might happen in the future. Through this examination, social scientists are beginning to interact with geoscientists. This has led to a collaborative engagement between human geographers in the Lancaster Environment Centre, with geoscientists at the British Geological Survey. Through this collaboration the notion of The Pore Space Scramble was born.

So, what is pore space and why might there be such a scramble for its use? Pore spaces are voids between rock grains that contain liquid or gas. The connection of pores then form pathways in which water, oil or gas can move. The interest in pore space therefore, is of interest due to its potential to store materials such as heat, gas or water. Uses of the pore space form a complex system so to simplify this we frame our discussion around the use of pore space for the long-term storage of CO₂ as a result of Carbon Capture and Storage (CCS). CCS is a suggested route to decarbonising the power and industrial sectors.

Going down in scale: from the outcrop to the pore space (by the British Geological Survey)

There is strong political will behind CCS both at the European level and in the UK itself [1], the UK setting out a 3 phase road map to commercialise CCS going forward. Given this political drive, it is not only necessary to understand the technical capabilities and practical ability to take this technology forward it also raises many questions around society and governance of such a system into the future. For example, who has precedence over this space and how does it compete with other energy infrastructure on both the surface and subsurface (eg. oil & gas industry, windfarms)? What are the legal and regulatory standpoints of this space i.e. who owns the pore space and therefore legally be able to utilise its use? What are the long-term stewardship plans of this space (eg. 10, 100, 1000 years), how does this effect liability and how might the precedence of such industries change over time? Ultimately, how do we ethically and justly govern such a space both presently and when projected into the future?

Demands on the (sub)surface (by the British Geological Survey)

We do not currently have answers for these questions but are initiating and undertaking research in this area as well as highlighting the need for policymakers, scientists, academic and publics to negotiate the challenges the subsurface will face going forward.

Reference:

DECC (2012). CCS Roadmap: Supporting deployment of Carbon Capture and Storage in the UK < www.gov.uk/government/uploads/system/uploads/attachment_data/file/48317/4899-the-ccs-roadmap.pdf > Accessed: 10/06/2015.

Funding opportunity for Early Career Researchers to attend GSA Baltimore

ERE Matters June 8, 2015

The Heritage Stone Task Group in southern Europe is a Task Group within the IUGS. In March, HSTG had a proposal accepted as Project 637 of the International Geoscience Programme (IGCP 637). With this acceptance, IGCP 637 offered $US6,000 in 2015 to support conference participation.

HSTG has decided that this funding should be used in 2015 to support attendance to our session in the GSA Baltimore conference. Amounts not exceeding $US2000 will likely be available. We have been asked by the IGCP Secretariat to give preference to supporting scientists from developing countries or who are young or women scientists. Recipients will also be expected to make a conference presentation in our session, related to natural stones, architectonic heritage and related issues.

Early Career researchers who are interested should send a message showing interest and a short cv, with a potential title for the contribution in the HSTG session, to the HSTG secretary general Barry Cooper: Barry.Cooper@unisa.edu.au

Applicatons will be received up to 30 June 2015.

Please contact Dr Lola Pereira for further information (mdp@usal.es)

Down by the River: Environmental Impact of Energy Generation Along the Colorado River

ERE Matters June 5, 2015

In our hunt for energy, we turn in many directions, especially those that will affect the environment to a lesser extent than the conventional fossil fuels. Though renewable energy is a sustainable form of energy production – it is after all infinite – it does not always mean that this form of energy production is without impact.

In 1963 the Glen Canyon Dam was built across the Colorado River, running through the Grand Canyon. Doing so created Lake Powell and helped in the generation of hydroelectric power. By 1974, researchers discovered the impact the dam had further downstream along the Colorado River, with shrinking sandbars as they no longer were replenished by sediment trapped in Lake Powell, behind the dam. Since then, scientists have been trying to get insight into the possibility of controlled flooding of the river to maintaining, or growing, the number of sandbars in the Colorado River. A new High Flow Experimental Release (HFE) Protocol could be the solution. However, care needs to be taken to protect both the downstream eco-system, as well as ensuring sufficient power generation by the dam.

Read more in this week’s EOS article 🙂