Julian Mecklenburgh

Synchrotron tomographic quantification of strain and fracture during simulated thermal maturation of an organic‐rich shale, UK Kimmeridge Clay

Analyzing the development of fracture networks in shale is important to understand both hydrocarbon migration pathways within and from source rocks and the effectiveness of hydraulic stimulation upon shale reservoirs. Here we use time-resolved synchrotron X-ray tomography to quantify in four dimensions (3-D plus time) the development of fractures during the accelerated maturation of an organic-rich mudstone (the UK Kimmeridge Clay), with the aim of determining the nature and timing of crack initiation. Electron microscopy (EM, both scanning backscattered and energy dispersive) was used to correlatively characterize the microstructure of the sample preheating and postheating. The tomographic data were analyzed by using digital volume correlation (DVC) to measure the three-dimensional displacements between subsequent time/heating steps allowing the strain fields surrounding each crack to be calculated, enabling crack opening modes to be determined. Quantification of the strain eigenvectors just before crack propagation suggests that the main mode driving crack initiation is the opening displacement perpendicular to the bedding, mode I. Further, detailed investigation of the DVC measured strain evolution revealed the complex interaction of the laminar clay matrix and the maximum principal strain on incipient crack nucleation. Full field DVC also allowed accurate calculation of the coefficients of thermal expansion (8 × 10−5/°C perpendicular and 6.2 × 10−5/°C parallel to the bedding plane). These results demonstrate how correlative imaging (using synchrotron tomography, DVC, and EM) can be used to elucidate the influence of shale microstructure on its anisotropic mechanical behavior.

A new belt-type apparatus for neutron-based rheological measurements at gigapascal pressures

We have developed a new solid-media apparatus for performing rheological investigations at multi-gigapascal pressures. The pressure cell consists of a simple belt design and fits in a modified 250 tonne Paris–Edinburgh press. Elastic strains are measured by neutron diffraction, on the ENGIN-X experimental station at ISIS. Stresses are estimated from the measured strains in combination with published values of the elastic moduli. As an exemplair of the method, we present data from initial deformation experiments on pyrope garnet at 1.5 GPa and 873 K. Data collection times are as short as 60 min and the elastic strain resolution is better than 10−4. We anticipate, however, that by interrupted testing, strain rates as low as 10−9/s, or lower, will be measurable.

Rock mechanics constraints on mid-crustal low-viscosity flow beneath Tibet

Abstract It has been inferred from various types of geophysical data that the Tibetan middle and upper crust is detached from the underlying lower crust and mantle by a weak, mid-crustal zone involving partial melting at about 30–35 km depth. Previous modelling of the flow has used an arbitrary mid-crustal rheology to match the constraints imposed by the overall flow regime. Here we show that extrapolation of experimental rock mechanics data for solid-state flow of a quartz-dominated Tibetan middle and upper crust, plus flow of partially molten synthetic ‘granitoid’, are consistent with the geophysical constraints and provide an experimentally constrained basis for the modelling of crustal rheology involving partially molten rocks.

Hydraulic conductivity of bedding-parallel cracks in shale as a function of shear and normal stress

Abstract Conductivity of fluids along fractures in all rocks is reduced by increasing normal stress. For sandstones and other hard rocks the onset of shear failure along planar cracks is thought to enhance fluid flow owing to a small amount of dilatancy, yet such effects are poorly quantified. Here we determine experimentally how independently increasing normal and shear stress affects fluid flow along fractures in shale. Gas flow along bedding-parallel planar interfaces was measured for flow parallel and normal to the shear direction. Increasing shear stress causes accelerating reduction of conductivity, even before the onset of macroscopic slip. Such reduction in fluid flow rate is non-recoverable, and the combined effects of normal and shear stress can reduce fracture conductivity by more than 3 orders of magnitude over the range of shale reservoir conditions. Bedding plane-parallel slip is common in shales; it can result in a large enhancement of permeability anisotropy, because flow across bedding planes becomes inhibited. This can impact upon the geometry of developing hydraulic fractures, encouraging complexity and favouring lateral relative to vertical growth. The results will facilitate modelling of fluid flow through fracture networks. Supplementary material: A CSV file containing all experimental conditions and tabulations of results is available at https://doi.org/10.6084/m9.figshare.c.3721831.

Microstructural controls on the pressure-dependent permeability of Whitby Mudstone

Abstract A combination of permeability and ultrasonic velocity measurements allied with image analysis is used to distinguish the primary microstructural controls on effective-pressure-dependent permeability. Permeabilities of cylindrical samples of Whitby mudstone were measured using the oscillating pore-pressure method at confining pressures ranging between 30 and 95 MPa, and at pore pressures ranging between 1 and 80 MPa. The permeability–effective pressure relationship is empirically described using a modified effective pressure law in terms of confining pressure, pore pressure and a Klinkenberg effect. Measured permeability ranges between 3×10−21 and 2×10−19 m2 (3 and 200 nd), and decreases by approximately one order of magnitude across the applied effective pressure range. Permeability is shown to be less sensitive to changes in pore pressure than changes in confining pressure, yielding permeability effective pressure coefficients (χ) between 0.42 and 0.97. Based on a pore-conductivity model, which considers the measured changes in acoustic wave velocity and pore volume with pressure, the observed loss of permeability with increasing effective pressure is attributed dominantly to the progressive closure of bedding-parallel, crack-like pores associated with grain boundaries. Despite only constituting a fraction of the total porosity, these pores form an interconnected network that significantly enhances permeability at low effective pressures. Supplementary material: A CSV file containing all experimental conditions and a tabulation of results is available at https://doi.org/10.6084/m9.figshare.c.3785741

Geomechanical and petrophysical properties of mudrocks: introduction

Abstract Mudstones (shales) are of particular importance as the source rocks for oil and gas, and increasingly so as the reservoirs for unconventional hydrocarbons. They are also the most common sedimentary rocks on Earth, and, hence, are frequently encountered in excavations and foundations for buildings. These factors point to a pressing need to develop an increased fundamental understanding of their geomechanical and petrophysical properties. The mineral content of mudstones has a dominant effect on their mechanical properties. Presence of clay minerals within them results in plasticity and ductility that can pose particular engineering challenges, but swelling clays in particular can lead to serious problems of mechanical stability of boreholes and in construction. Good hydraulic fracture performance is linked to brittleness and high elastic moduli. This in turn is favoured by high silica or carbonate content and diagenetic cementation. Permeability to fluids depends on the interconnectivity of storage pores through orientated crack networks. New advances in imaging technologies are permitting very-high-resolution three-dimensional imaging down to the nanometre scale. Such studies will eventually lead to technological advances that exploit more effectively these enigmatic rocks.

Rock deformation from field, experiments and theory : a volume in honour of Ernie Rutter

Ernie Rutter has made, and continues to make, a significant impact in the field of rock deformation. He has studied brittle and plastic deformation processes that occur within both the oceanic and continental crust, as well as other key properties such as the permeability and seismic velocities of these rocks. His approach has been one that integrates field observations, laboratory experiments and theoretical analyses. This volume celebrates Ernie’s key contribution to rock deformation and structural geology by bringing together a collection of papers that represent this broad approach. The papers within the volume address key issues that remain within these fields. These range from fundamental studies of brittle and plastic behaviour along with the resultant structures and microstructures from both the field and laboratory, to applied problems where a better understanding of the deformation and properties of the crust is still needed.

Towards an improved understanding of the mechanical properties and rheology of the lithosphere: an introductory article to ‘Rock Deformation from Field, Experiments and Theory: A Volume in Honour of Ernie Rutter’

Ernie Rutter’s influential contribution to our understanding of rock deformation spans the whole continental crust, from deformation ‘just under the grass’ (e.g. Rutter & Green 2011) to melt migration in the lower crust (e.g. Rutter & Neumann 1995). Ernie has also worked on many aspects of the deformation of oceanic crust (Rutter & Brodie 1988); however, here we reflect on the wide range of deformation conditions Ernie has studied by presenting key aspects of the strength and rheology of the continental lithosphere as we understand it today, with the aim of providing the context for the contributions included within this volume. The continental crust forms just over a third of the Earth’s surface and it is different from the crust of any other planet in our solar system. It is rich in those elements that partition in a silicate melt and its formation and evolution determined the composition of the mantle and that of the atmosphere (Hawkesworth 2006). Thanks to its physical and chemical fingerprint, the Earth’s crust has sustained life uninterrupted for 3.8 Ga and it is the source of major natural resources such as hydrocarbons and mineral deposits and others such as geothermal energy. The continental crust is less dense than its oceanic counterpart, and is therefore more buoyant. It forms a substantial part of the cold thermal layer of the Earth and its rheology (from Greek r1v rheō, ‘flow’ and -logia, -logia, ‘study’; ‘the study of the flow of matter’) is a key factor controlling plate tectonic processes at the Earth’s surface and in its interior (Davies 2011). Beneath the continental crust lies the lithospheric mantle and together they form the continental lithosphere. Considerable variations in structure, thickness and composition of the continental lithosphere mean that, despite its importance, we currently have limited understanding of the rheology of this complex system (Thatcher & Pollitz 2008). Significant advances in our knowledge of continental rheology have been possible through:

E. H. Rutter: a biography

Ernest Henry Rutter was born in January 1946 in Sunderland. His father, George, was a sergeant in the army and then worked for the Post Office and his mother, Irene, worked in retail. From an early age, Ernie showed aptitude for all things scientific and especially practical science. At 11 he was selected for a technical high school where his interest in science and electronics was encouraged. Indeed, his chemistry experiments led to an explosion in the garden shed and partial defoliation of his neighbour’s garden. During his studies for university entrance he was enthused by electronics and constructed his own television from constituent parts. With this knowledge, he worked in his spare time as a television repair man in Sunderland. During his teenage years he also developed a love of classical music, regularly attending concerts at Newcastle City Hall. He gained entrance to Imperial College, London to study Geology in 1964. He was interested in all things geological and in fieldwork in particular. He retained his enjoyment of practical science, building his own cathodoluminescence microscope in the undergraduate petrology laboratory. He conducted his undergraduate thesis work on the basic igneous rocks of Løkken in Norway, having persuaded a mining company that their exploration would be aided by knowledge of the structure of the area. The results from his undergraduate thesis work on massive sulphide deposits were published in 1967 and 1969. In his final year, he was particularly intrigued by the fluid mechanics of how ammonites propel themselves through water, and considered the possibility of a palaeontological career, but chose to undertake a PhD in the field of Structural Geology and Rock Mechanics instead. Palaeontology’s loss was Structural Geology’s gain. Ernie graduated from Imperial College in 1967 with a First Class Honours degree. At that time, Structural Geology at Imperial College was a subject in the ascendancy. At this time he was very influenced by a publication by Professor Derek Flinn (of the University of Liverpool), who introduced concepts of deformation mechanisms based on material science literature to geologists (Flinn, 1965 in The Controls of Metamorphism Volume). He was greatly influenced and encouraged at Imperial College by Professors Neville Price and John Ramsay. They recognized the potential for Structural Geology of quantifying the mechanics of rocks under pressure and temperature which, at that time, was largely an area of research dominated by universities in the USA. When Neville Price gained Natural Environment Research Council funding to establish a Rock Deformation Laboratory at Imperial College, Ernie was appointed as a graduate student to lead this endeavour. Ernie’s hands-on, practical approach and knowledge of electronics, in addition to his geological skills, made him an ideal candidate, although it is hard to imagine a graduate student being placed in such a position today! The designs and drawing for the first deformation apparatus used in the Rock Deformation Laboratory were provided by Professor Hugh Heard from the Lawrence–Berkeley Laboratory in California. At this time, Ernie established his long-running working relationship with Mr Robert Holloway, who was appointed as the mechanical technician for the new laboratory. Together they constructed six Heard-type apparatuses and Ernie ran his first experiments on the deformation of carbonate rocks. Despite the enormous task of initiating a laboratory from scratch, Ernie coped admirably and was appointed to the academic staff in 1969, a year before the end of his PhD. Ernie had to juggle preparing lectures and teaching with writing up his thesis at the same time. Around this time the first publications from the laboratory appeared. What followed was an extraordinary contribution to the field of Rock Deformation spanning almost every imaginable aspect from high-temperature plastic deformation to brittle rock mechanics. Ernie was also a key factor in an immensely successful taught Master’s programme in Structural

Recrystallization Microstructures during Creep of MgO Single Crystals

In order to gain a better understanding of the mechanisms that control recrystallization and nucleation processes three MgO single crystals (ps1, ps2 and ps3) were deformed in axial compression parallel to 〈100〉 in a 1 atmosphere creep rig at a temperature of 1400°C, stress of 32 to 35 MPa and up to 31% strain. Quasi steady-state strain rates of 10 s were measured. All specimens deformed heterogeneously. The microstructures were investigated using automated electron backscatter diffraction mapping (EBSD). In all samples orientation maps of sections cut parallel to the loading direction, show continuous misorientation across the crystals from a reference orientation. The maximum deviation angle measured is 36°. While in ps2 (17% strain) very little substructure can be observed, sample ps1 (24% strain) and ps3 (31% strain) are characterised by a network of low angle boundaries (1° to 5°) and incipient sub-grain boundaries of 5° to 10°. These can be interpreted to have formed by sub-grain rotation recrystallization assisted by climb of dislocations. Continuous small circle dispersion of the poles to {100} and {110} with [001] as the rotation axis, suggests {110}〈110〉 as the most likely active slip systems, while slip on {100}〈110〉 may be triggered by heterogeneous deformation.