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Understanding the Micro-to Macro Behaviour of Rock-Fluid Systems

A major programme of work being funded by NERC in the area of geoscience relates to understanding rock-fluid systems across the range of scales from the micro to the macro. Here Richard Shaw, from the British Geological Survey, who is the Scientific Co-ordinator of the programme outlines the programme's objectives and themes.

Objectives
The programme is focused on developing our understanding of the relationships between measured and modelled sub-surface fluid flows spanning the range of spatial and temporal scales relevant to fluid resource management. It is motivated by the growing recognition that assumptions of uniformity at certain scales are inadequate for extrapolating fluid behaviour both in time and space. A clear example is the power law behaviour which characterises many aspects of observed rock properties seemingly independently of rock type. Developments in the areas of geological and geo-fluid observations and modelling that focus on scaling relationships in generic rock are needed to provide a clearer physical understanding on which to base more effective geofluid management.

To make progress towards this objective, research spanning a wide spectrum of observation and simulation scales can be divided into four themes:

• Understanding the natural processes which lead to scaling relationships between size and magnitude of rock and flow heterogeneity;
• Quantification of essential fluid flow properties and their spatial pattern from measurements;
• Identification of appropriate statistical models and scaling laws describing rock property heterogeneity and fluid-rock interactions in geological media;
• Understanding the relationships between rock property distributions and flow model parameter distributions.

In addressing fluid-rock interactions, the µ2M programme encourages combinations of academic and industry experience, techniques and data. Under the current five year programme, it is expected that research groups will address specific issues encompassing one or perhaps two of these areas. In the longer term, as this initiative matures, then the four areas should coalesce to create a coherent single research field. The ultimate aim is to develop reliable methods of predicting fluid flow in natural, heterogeneous rocks and to provide quantification of the sensitivities of such predictions to both parameter and model uncertainties.

An aim of µ2M is to encourage cross fertilisation of ideas between academia and industry and between different disciplines, as well as encouraging research aimed at breaking new ground.

Programme Themes

Theme 1: Understanding the processes leading to rock property/fluid flow scaling.
Depositional, climatic, tectonic, structural and geochemical processes are all involved in the development of the pathways for fluid flow within a geological medium. Areas of interest include among others: the evolution of fracture and porous media permeability distribution from micro- and macro-scale studies of rock heterogeneity; geochemical sealing and re-sealing of pores under changing stress fields and chemical environments; chemical kinetics of rock dissolution in actively flowing systems; the coupling between fluids and stress in actively deforming rock; mineral and chemical signatures as indicators of permeability/porosity development; patterns of self-organisation and associated power-law behaviour resulting from coupled non-linear geological processes.

The development of conceptual or generic models of the evolution of scaling rock properties which control flow will provide valuable insights into heterogeneity and structure distributions that may not otherwise be attainable.

keywords: genesis modelling, coupled geological processes, self-organisation, microscale heterogeneity

Theme 2: Quantification of essential fluid flow properties and their spatial variation.
The integrated interpretation of the full range of observations, including geophysical data, to extract information on rock geometry and property distributions that control fluid flow needs further development. Geophysical inversion currently provides valuable constraints on rock geometries and may provide viable information on the geomechanical and hydraulic behaviour of a rock mass. Further research may enable these techniques to be refined and validated.

Increasing data resolution and bandwidth of non-invasive measurements together with new insights from rock evolution modelling (Theme 1) open new possibilities for defining both geometric and hydraulic data patterns in rocks at all depths. Combining measurement methods with insights into the natural processes governing property distributions may ultimately produce efficient and accurate data inversion methods for permeability and multi-scale flow predictions. In addition to geophysical data, geochemical data sets offer important indicators of fluid migration patterns. New insights into the reliability of these data obtained across a range of space scales are needed to improve the use of this data resource.

Note that development of new measurement tools lies outside the scope of this project. The output of this theme feeds directly into Theme 3.

keywords: geophysical measurement inversion, geochemistry, multiscale heterogeneities

Theme 3: Statistical models and scaling laws for rock heterogeneity and fluid-rock interactions
Geostatistical models encompassing fractal, multi-Gaussian, Boolean, Markov as well as non-parametric, non-Gaussian approaches have all been adapted for the statistical characterisation of the parameter distributions of fractured and porous media at a range of spatial scales.

For robust application of these methods, only low order statistical moments have been extracted from the available observation data. While the expectation values derived for intermediate points between observation values have been found to be well conditioned, the textural patterns in real geological media controlling fluid flow have not been well reproduced. Biases in the fluid flow realisations generated using low order statistical models of rock properties are recognised but the magnitude of prediction errors are not satisfactorily characterised. New research is needed to understand more completely the application of such models to fluid flow patterns in geological media.

Research is also needed to integrate new measurement methods with new statistical models to reduce the bias in the output of such models. Errors in prediction are as much related to uncertainties in the conceptual models describing the behaviour of the rock mass as they are to uncertainties in the parameterisation of these models. There is therefore a need to improve the conceptual models as well as the parameterisation and the addressing of parameter uncertainty.

Recent developments in self-similar systems in which power law scaling is found across a wide spectrum of the spatial scales from the microscopic to the macroscopic also demand greater attention. Such laws may allow better inference of behaviour at large or small scales to be made from data obtained using current laboratory and field measurement methods. Research on the mathematics of heterogeneous ``noisy" (eg 1/f noise) systems previously developed in unrelated disciplines may provide new knowledge if applied to geological media.

keywords: geostatistics, scaling laws, 1/f noise, uncertainty

Theme 4: Relationship between rock property and flow parameter distribution.
Simulators, whether used for data inversion from field experiments or for analysis of field scale flow and transport patterns, require consistent unbiased input data sets derived from the available field data. Simulators needed for flow modelling at one scale demand up-scaled property distributions derived from models of finer scale variations in the geological media. Further research is needed to connect up-scaling approaches to the geological modelling in Themes 1 and 3 and to derive formal scientific statements of the relationships between model properties, their domain of application and the range of geological media to which they are potentially applicable.

Effort is also justified in analysing the level of detail to which modelling of geology should be taken prior to the upscaling procedure, which in some sense averages over a lot of that detail. Useful analogues are likely to be found in the fields of statistical mechanics and (non-equilibrium) thermodynamics. These may suggest how the statistical parameters of the underlying geology and, equally important in a commercial development, their uncertainties may be incorporated directly in fluid flow simulation.

Mechanical, thermal and chemical processes can significantly influence rock transport properties even during exploitation lifetimes. Hydraulic modelling may not be valid unless these additional effects are incorporated, particularly in fractured rock. Understanding the characteristics of the coupled non-linear dynamic system, particularly spatial patterns and scaling relationships, should point the way towards means of efficient modelling of all of the pertinent physics, compatible with the small amount of information usually available in commercial resource development.

keywords: upscaling, coupled THM modelling, non-linear dynamic processes, uncertainty

Programme Awards
The two rounds of calls for proposals have been completed and fifteen awards have been made. A list of these projects is provided below. There will not be further calls under the Programme.

A web site for the Micro-to-Macro Programme is maintained by the data managers, the British Geological Survey at http://www.bgs.ac.uk/micromacro/about.html where project updates on most individual projects and links to some of the research departments can be found.

Contact
For further information on the programme please contact Dr Richard Shaw, Scientific Co-ordinator, Micro-to-Macro, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK Tel: +44 (0) 115 9363545 Fax: +44 (0) 1159363150, rps@bgs.ac.uk

Projects (it is hoped to include summary articles describing results arising from these projects in future editions of the Newsletter).

Prof H Huppert (University of Cambridge) Analysis of reaction and flow in stochastically heterogeneous porous media
Prof CM Graham & Dr RS Haszeldine (University of Edinburgh) New micro-geochemical traces of fluid and solute transport in sedimentary basins
Dr E Liu (British Geological Survey, Edinburgh) & Dr J Hudson (University of Cambridge) Determination of hydraulic properties of distributed fractures using seismic techniques
Prof PG Meredith (University College, London) & Dr IG Main (University of Edinburgh) Scaling properties of fluid flow in fractured rocks
Prof ML Coleman (University of Reading) Controls on matrix and fracture flow from geochemical analysis of produced oil
Dr AJ Barker (University of Southampton) & Prof DJ Sanderson (Imperial College) Localised flow in fractured rock masses: Mechanisms, modelling and characterisation
Dr PJ Bloomfield (British Geological Survey, Wallingford) & Prof J Barker (University College, London) Modelling porosity development and flow in heterogeneous media
Prof RJ Knipe & Prof DB Ingham (University of Leeds) Scaling of fluid behaviour associated with flow through complex geological structures
Prof DJ Vaughan, Dr RA Wogelius, Dr S Boult, Dr C Merrifield (University of Manchester) & Dr DJ Large (University of Nottingham) Quantifying the effects of biofilm growth on hydraulic properties and on sorption equilibria: µ2M measurements
Dr AC Barnicoat, Prof BW Yardley (University of Leeds), Dr JJ Wilkinson (Imperial College), Prof CM Graham (University of Edinburgh) & Dr AJ Boyce (Scottish Universities Research Reactor Centre) Multi-scale fluid-flow path analysis: Calibration and modelling using mineralisation systems
Dr J McCloskey & Dr P Morrow (University of Ulster, Coleraine) Fully determined fluid velocity fields for complex 2D media with multi-scaled heterogeneity
Dr RS Haszeldine, Prof C Graham, Dr C Macaulay (University of Edinburgh), Dr PW Corbett (Heriot Watt University) & Prof E Fallick (Scottish Universities Research Reactor Centre) Cementation of oilfield sandstones: Micro-geochemical tracers, reveal macro-fluid hydrogeology
Prof A Hurst (University of Aberdeen) The scaling behaviour of fluid flow in rock fractures
Prof R Mackay (University of Birmingham) Quantifying the scaling of physical transport in structures heterogeneous porous media
Prof JA Barker & Prof D Preiss (University College, London) Novel flow and transport models for systems exhibiting non-integer flow dimensions
Dr R Worden (University of Liverpool) & Prof. A Aplin (University of Newcastle) Mudstone Microstructure Evolution During Burial and Diagenesis and Effect on Caprock Sealing Capacity (Small Award)
Dr T Atkinson (supervisor) (University College, London) Tracer Breakthrough, Hydraulic Properties and Fracture Networks at Sub-Continuum Scales in Aquifers - (Ph.D. studentship)

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