Mike Lovell

Downhole images: Electrical scanning reveals the nature of subsurface oceanic crust

High-resolution electrical images of oceanic sediments exposed by drilling are permitting scientists to make detailed evaluation of the record preserved in the rocks, particularly in intervals where little core was recovered. The images are generated from measurements taken with a slimhole Formation Micro-Scanner (FMS), developed by Schlumberger specifically for the Ocean Drilling Program (ODP). The new measurement technique was used in May 1989 on ODP Leg 126 in two holes drilled in the Izu-Bonin intra-oceanic volcanic arc (Figure 1), which extends south of Honshu to Iwo Jima, Japan.

Sediment-hosted gas hydrates : new insights on natural and synthetic systems

Abstract In the publics imagination, hydrates are seen as either a potential new source of energy to be exploited as the world uses up its reserves of oil and gas or as a major environmental hazard. Scientists, however, have expressed great uncertainty as to the global volume of hydrates and have reached little agreement on how they might be exploited. Both of these uncertainties can be reduced by a better understanding of how hydrates are held within sediments. There are conflicting ideas as to whether hydrates are disseminated within selected lithologies or trapped within fractures comparable to mineral lodes. To resolve this, hydrates have to be examined at all scales ranging from using seismics to microscopic studies. Their position within sediments also influences the stability of methane hydrate in responding to pressure and temperature and how the released gas might transfer to the ocean, atmosphere, or to a transport mechanism for recovery. These results also run parallel with the studies of carbon dioxide hydrate, which is being considered as a potential sequestion medium.

Short pulse multi-frequency phase-based time delay estimation

An approach for time delay estimation, based on phase difference detection, is presented. A multiple-frequency short continuous wave pulse is used to solve the well-known phase ambiguity problem when the maximum distance exceeds a full wavelength. Within an unambiguous range defined with the lowest frequency difference between components, the corresponding phase difference is unique and any distance within this range can be determined. Phase differences between higher frequency components are used to achieve a finer resolution. The concept will be presented and the effectiveness of the approach will be investigated through theoretical and practical examples. The method will be validated using underwater acoustic measurements, simulating noisy environments, demonstrating resolutions better than a 50th of a wavelength, even in the presence of high levels (-5 dB) of additive Gaussian noise. Furthermore, the algorithm is simple to use and can be easily implemented, being based on phase detection using the discrete Fourier transform.

Non-Fickian diffusion and the accumulation of methane bubbles in deep-water sediments

In the absence of fractures, methane bubbles in deep-water sediments can be immovably trapped within a porous matrix by surface tension. The dominant mechanism of transfer of gas mass therefore becomes the diffusion of gas molecules through porewater. The accurate description of this process requires non-Fickian diffusion to be accounted for, including both thermal diffusion and gravitational action. We evaluate the diffusive flux of aqueous methane considering non-Fickian diffusion and predict the existence of extensive bubble mass accumulation zones within deep-water sediments. The limitation on the hydrate deposit capacity is revealed; too weak deposits cannot reach the base of the hydrate stability zone and form any bubbly horizon.Graphical abstract

Petrophysically determined lithofacies at the Nankai Trough Accretionary Prism: NanTroSEIZE, IODP Expedition 314

Abstract: Characterizing the physical properties and identifying boundaries within active accretionary prisms is necessary in understanding their behaviour and recent movement. In such unstable conditions core recovery is not always reliable, especially around fault zones. Integrated Ocean Drilling Program (IODP) Expedition 314 was the first stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), and used logging-while-drilling (LWD) technology to record continuous physical property data. We use iterative non-hierarchical cluster analysis (INCA) to quantitatively define the characteristics of the slope sediments and sediments within the accretionary prism at Sites C0001 and C0004. A new and detailed log-based lithostratigraphy is developed, and positions of major boundaries, defined by 3D seismic profiles and initial interpretation of log responses, are refined. The results produce clusters that clearly distinguish the slope sediments and characterize formations within the accretionary prism. Boundaries that correlate to the seismic-defined unconformity between the slope sediments and the accretionary prism, and a boundary within the accretionary prism that corresponds to a megasplay fault previously unresolved by log analysis and borehole images, are identified. Our study demonstrates that INCA analysis of LWD data can accurately define boundaries and characterize sediments in environments where core recovery may be incomplete.

Mapping hydrate stability zones offshore Scotland

Abstract One practical method to reduce environmentally damaging greenhouse gas emissions is through the geological storage of carbon dioxide. Deep, warm storage of carbon dioxide is currently taking place at Sleipner, North Sea and Weyburn, Canada. It is, however, also possible to store carbon dioxide as a liquid and hydrate in cool, sub-seabed sediments. Offshore north and west of Scotland seafloor pressures and temperatures are suitable for hydrate formation. In addition to the possibility of natural methane hydrate being present in this region, conditions may also be favourable for carbon dioxide storage as a liquid and hydrate. A computer program has been developed to calculate the depth to the base of the carbon dioxide and methane hydrate stability zones in two offshore regions: the Faeroe–Shetland Channel and the northern Rockall Trough. Results predict that methane hydrate remains stable to a maximum depth of 650 m below the seabed in the Faeroe–Shetland Channel, and 600 m below the seabed in the northern Rockall Trough; the carbon dioxide hydrate stability zone extends below the seabed to a depth of 345 and 280 m, respectively. No physical evidence for the existence of natural hydrate in these regions has been confirmed. Suitable conditions for carbon dioxide storage as a liquid and hydrate exist, and should this storage method be developed further, a more refined program and greater offshore investigations to improve data sets would be necessary to scope the full potential.

Bat-inspired distance measurement using phase information

This paper shows the use of phase measurement to estimate the distance to a target. Inspiration for this work comes from the observation that bats have been shown to have exceptional resolution with regard to target detection when searching during flight. Au and Simmons [“Echolocation in dolphins and bats,” Phys. Today 45(7), 40–45 (2007)] concluded bats with a center frequency of about 80 kHz (i.e., 4‐mm wavelength), and 40‐kHz bandwidth can have a resolution of distance in air approaching 20 μm. For this frequency, we see that the resolution achieved by the bat is about 200 times better than λ/2 (i.e., 2 mm at this frequency), which is usually used as a guide for resolution for analog systems. Moreover, Au and Simmons show, using time‐frequency analysis, that there are essentially two frequencies present at any particular time within a single bat pulse. Considering this use of two frequencies we may infer a distance. A new bat‐inspired algorithm is presented. This is based on phase measurement and, when...

Methane hydrates: problems in unlocking their potential

Methane hydrates have been recovered or postulated for virtually all continental margins around the world and a few areas onshore. Volumes of about 2 × 10 14 m 3 have been estimated for this potential resource. However, only a few sites have been suggested offshore northwest Europe, despite extensive hydrocarbon exploration and academic studies of the margin. Reasons for this anomaly are unclear. To aid the search a new hydrate stability zone map for the UK is presented. As well as identifying a resource, hydrate studies are also important in assessing geohazards to deep-water exploration and development. Stability, processes and distribution information contribute to the wider climate change debate as methane hydrates are estimated to hold a significant part of the global organic carbon budget. To quantify reserve potential and to identify suitable methods of methane extraction, a full understanding of how hydrates are held within sediments is required. Although modelling (physical and theoretical) can contribute to an understanding, it is important to evaluate in situ conditions to ‘ground truth’ acoustic data and imagery. How hydrate is held and its control of dynamic geotechnical behaviour within the sedimentary system is still very poorly understood. Parameters such as pore size, fluid saturations, sediment mineralogy and cementation will affect hydrate morphology, distribution, behaviour (during dissociation) and potential recovery from porous media. Assessing physical parameters and processes under in situ conditions provides the next step along the route to exploiting methane hydrates as a resource. The requirement to recover samples under in situ pressures and temperature conditions provides a significant technological challenge that has been attempted over the last few years with some success. Currently, the European HYACINTH project is developing systems to recover, analyse and manipulate hydrate-bearing sediments under in situ pressures and temperatures. On Leg 204 of ODP this equipment was used for the first time to recover hydrate cores at in situ pressure, transfer them without loss of pressure into laboratory chambers and to log them geophysically. As the database of in situ properties grows, integrated laboratory studies of synthetic sediment-hosted hydrates can be developed to provide important benchmarking, which is crucial for the study of rare natural core samples.

A novel integrated approach to estimating hydrocarbon saturation in the presence of pore-lining chlorites

Pore-lining chlorites are often associated with a low-resistivity contrast between corresponding reservoir units, making the identification and quantitation of hydrocarbon-bearing intervals difficult. In many low-resistivity situations, the traditional approach of using Archie’s equation to determine saturation from electrical resistivity fails, and modified Archie equations derived specifically for ‘shaly sands’ have been developed. In chlorite-bearing intervals, however, the effect of chlorite can be such that both Archie and the so-called shaly-sand models are inappropriate. Under these circumstances, calculating saturation from electrical resistivity can be circumvented by detailed analysis of the sedimentology and petrophysics, enabling the construction of a saturation height model based on core data. In this novel study we integrate a detailed core-based sedimentological facies scheme with wireline log data and petrophysical core data to demonstrate a clear link between chlorite occurrence, petrophysical characteristics and saturation height. Through this innovative approach, saturation is estimated without recourse to resistivity logs and improves hydrocarbon saturation estimates in chlorite-bearing reservoirs.

Bioinspired Low-Frequency Material Characterisation

New-coded signals, transmitted by high-sensitivity broadband transducers in the 40–200 kHz range, allow subwavelength material discrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra enable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as well as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction errors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only been matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low frequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly from past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for application in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing and evaluation, geophysics, and medical physics.