uAbstract

uFigures

uIntroduction

uConventional Coring

uPressure Coring

uThe HYACE/HYACINTH Pressure Coring System

uMeasurements and Experiments on Pressure Cores

uPotential Future Developments and Applications of Pressure Coring Systems

uReferences

uAbstract

uFigures

uIntroduction

uConventional Coring

uPressure Coring

uThe HYACE/HYACINTH Pressure Coring System

uMeasurements and Experiments on Pressure Cores

uPotential Future Developments and Applications of Pressure Coring Systems

uReferences

In the following we present a brief outline of the history of pressure coring and an outlook on future developments and applications. Starting with the shortcomings of conventional coring, we give an overview of a set of tools developed to acquire pressurized cores from the seabed. The successful deployments on several expeditions encouraged us to start a new project with the aim to develop more sophisticated pressure coring tools which can be used not only for gas hydrate research and the investigation of the deep marine biosphere for which they have originally been invented. Potential future applications will be in the fields of hydrocarbon exploration, earth and environmental science of the deep sub-seafloor, and the study of mechanisms of biogeochemical cycles.

Conventional Coring

The recovery of cores from boreholes nowadays is a standard operation in oil & gas exploration, geotechnical survey, and scientific drilling both offshore and on land. The cores provide valuable data for engineers and scientists; however, the main disadvantage of this type of coring is that the cores are no longer in the same condition as deep in the sediment.

A core recovered from the depth is disturbed mechanically as a result of the coring process.  The temperature has changed and most important the lithostatic and hydrostatic pressures are released during the retrieval of the core.

As a result mechanical, physical, and chemical properties as well as living conditions for microorganisms of the deep biosphere are significantly altered. The changes include the equilibrium of gases, fluids and solids, phase boundaries, wettability, solubility of gases and solids, and the integrity of the mechanical structure. Scientists try to take this into account when they analyse the cores in their laboratories; however, it is not always possible to deduce the original conditions from the available data.

Pressure Coring

The shortcomings of the usual coring technologies especially apply to two fields of research which have come into the focus of the scientific community worldwide: gas hydrates and the deep biosphere.

Gas hydrates (mainly methane hydrates) have been found in the seabed of the continental margins all around the world. They contain vast amounts of hydrocarbons which make them a possible energy resource. They are stable only under relatively high pressure and low temperature and will decompose rapidly if the in situ pressure is released and/or the temperature is increased. Dissociation of gas hydrates in areas of conventional oil and gas production gives cause to geohazard assessments. Release of large quantities of methane due to global warming may have an effect on the climate and lead to subsea landslides.

The deep biosphere accounts for perhaps 10% of the total global biomass and in layers of several hundred meters of sediment may contain more than 50% of the global bacterial biomass. Thus they may play an important role in the global cycling of elements. Obligate barophilic bacteria need high pressure to survive and can't be studied under atmospheric conditions.

For this reason pressure coring systems have been developed in order to maintain the sediment structure, the gas hydrate stability, and the biochemical conditions. Primarily temperature and pressure have to be conserved during the sequences of sampling, retrieval, transfer, storage and downstream analysis.  Thus, pressure coring and sampling became an indispensable part of offshore gas hydrate expeditions during the last few years.

The HYACE/HYACINTH Pressure Coring System [1]

The most successful tools today are the suite of tools developed within the EU funded projects HYACE (Hydrate Autoclave Coring Equipment) [2] and HYACINTH (Deployment of HYACE tools In New Tests on Hydrates) between 1997 and 2005. These projects were mainly a collaboration between the Technical Universities of Berlin and Clausthal, the Cardiff University, and the industrial partners Fugro and Geotek Ltd.

The suite of research technologies developed by Technical University Berlin (TUB) and European partners in the EU Projects, as well as continuous improvements of these prototypes were considerably successful and made the tools more and more reliable.

Other down-hole autoclave coring tools for deep drilling worth mentioning are ODP’s Pressure Core Sampler (PCS) from the U.S., and the Japanese Pressure and Temperature Maintaining Coring System (PTCS).  In contrast to the more flexible and economic HYACINTH wire line tools, such as the Hyace Rotary Corer (HRC), or the Fugro Pressure Corer (FPC) the PTCS and PCS cannot be interfaced with core transfer or subsequent processing tools.  Consequently their applications and advantages are limited to pressurized core retrieval only.

The HYACINTH system is the only system with the ability to transfer a core out of the coring tools autoclave into other pressure chambers for measurement, experiments, or storage.

After the end of the EU project in 2005 the rotary corer and transfer system was handed over to Fugro BV and Geotek Ltd. Several improvements have been made as a result of the lessons learned on various projects. The present set of tools are the coring tools FPC, an improved FRPC (Fugro Rotary Pressure Corer - the former HRC), and the transfer system PCATS (Pressure Core Analysis and Transfer System) with its analytical portion, the Geotek MSCL-P.

For the time being, the HYACINTH operation sequence requires the following tools and procedures to bring the in situ deep seabed conditions to the controlled situation of the lab where measurements and experiments can be carried out:

HYACE pressure coring tools

Two types of wireline pressure coring tools were developed in the EC-funded HYACE/HYACINTH programs: A percussion corer and a rotary corer, which were designed to cut and recover core in a range of lithologies where gas hydrate bearing formations might exist. On completion of coring, the drill string is lifted to extract the core barrel from the sediment.  Once the core barrel is free from the sediment the wireline pulls the core barrel liner containing the core into the autoclave. A specially designed flapper valve is used to seal the bottom end of the autoclave after the core has been retrieved.

Fugro Pressure Corer (FPC): The HYACE percussion corer was developed by Fugro Engineers BV and is known as the Fugro Pressure Corer (FPC). The FPC uses a water hammer, driven by the circulating fluid pumped down the drill pipe, to drive the core barrel into the sediment up to one meter ahead of the drill bit. It is suitable for use in soft to medium hard sediments.

HYACE Rotary Corer (HRC) [2]: The HYACE rotary corer was developed by the Technical University Berlin and the Technical University Clausthal. The HRC uses an inverse Moineau motor driven by the circulating fluid pumped down the drill pipe to rotate the cutting shoe. Although originally designed for hard lithified material, the tool can now be used with a new designed helical bit (the "Viking") in soft and sticky material as well.

HYACINTH core transfer

In order to remove the core from the pressure corer autoclave, the autoclave is connected to the shear transfer chamber (STC) with quick-clamps and then pressure-balanced with the autoclave before opening the ball valves. The top half of the pressure core, containing the piston and other components, are captured by a catcher on the end of the manipulator, and the full core is withdrawn from the autoclave into the shear transfer chamber. When the ball valves are closed, the autoclave can be removed from the system.

The STC, now containing the core at full in situ pressure, is attached to the Geotek MSCL-P (Multisensor Core Logger-Pressure), pressures are balanced, and ball valves are opened. The core can now be pushed and pulled through the sensors using the manipulator under computer control.  Once the analyses are completed, cores can be depressurized to collect and evaluate the gas content.

PRESS (Pressurized Core Sub-Sampling and Extrusion System) [3, 4, 5]

PRESS, the sub-sampling system developed by TU Berlin, is able to cut contamination controlled well defined pressurized subsections of adjustable length from drilled HYACINTH pressure-cores.  These whole round subsections can either be transferred under pressure into transportation chambers (PARR vessel) for shipment to other laboratories or are input feeds for an axial extrusion of the very central core plug into the pressurized cutter and diverter unit (DeepIso Bug, Univ. Cardiff) for subsequent incubation, cultivation, isolation, characterization and further microbiological research to examine high-pressure adapted bacteria or remote biogeochemical processes at highest pressures under well-defined research conditions of the laboratory: sterile, anaerobic and without depressurisation.

Storage Chambers

The HYACINTH storage chamber is a cylindrical pressure vessel made of stainless steel or high-strength aluminum alloy and a ball valve at one end. They can be attached to the transfer system by means of specially designed quick clamps. They can be used for intermediate storage on the drillship or for long term storage and transfer to shore-based institutions for further investigations.

Measurements and Experiments on Pressure Cores

All HYACINTH pressure cores have been recovered in order to investigate gas hydrates. The use of the transfer system has allowed pressure cores to be investigated with the structure and morphology of the gas hydrate undisturbed. Gas hydrates can take on a large variety of morphologies and structures: big chunks and nodules, thin layers, complex vein structures and even microscopic structures of a pore-filling matrix. All these often-complicated structures can be investigated on pressure cores only. [6, 7, 8]

An impressive example of the distribution of gas hydrate in a pressure core is given in figures 1 and 2. Figure 1 [6] shows an X-ray image taken with the Geotek MSCL-P from a core collected on Expedition 1 of the Indian National Gas Hydrate Program (NGHP1).

Some X-ray computed tomographic (CT) scans have been made of selected cores which have been brought ashore by means of the HYACINTH storage chambers. The cores were analyzed using equipment in hospitals.  An example of cross-sections of X-ray CT data is given in figure 2 [6].

Examples of the gamma density and P-wave velocity profiles of pressure cores as well as X-ray images taken on a rotated core taken from different angles can be found in Schultheiss (2008) [7].

The DeepIsoBUG developed at the Cardiff University uses the sub-samples from the PRESS system as sterile, anoxic, high pressure input feed for the cutter and diverter unit. The samples, lured in liquid medium, are then transferred into a number of high pressure vessels. These can be incubated under a range of different conditions thereby enriching a range of different high-pressure adapted bacteria or studying biogeochemical processes. Finally, pure cultures can be obtained from positive enrichments within a high-pressure isolation chamber for further study and characterization. [4, 5]

A device capable of making physical measurements on a natural core held under in situ pressure was designed, built and used by scientists from Georgia Institute of Technology. The instrumented Pressure Testing Chamber (IPTC) was used to measure successfully P-wave and S-wave velocities, electrical conductivity, and strength of cores. The difference of the results compared to measurements in conventional cores confirms the importance of measurements on pressure cores. [9]

Potential Future Developments and Applications of Pressure Coring Systems

The entire system will consist of a rotary pressure coring tool, which is deployed on a wire through the main drill string, a transfer system that allows for the retrieval of the core out of the corers autoclave section without loss of pressure and also executes sub-sampling like cutting and transfer of smaller core samples into specially designed investigation or storage chambers for long term storage of the pressurized cores.

The new features of the tools to be developed include:

Higher pressure rating:

·         There is a growing demand for pressure coring in deeper water and deeper into the seabed.

·         Not only in gas hydrate research but also in other fields, such as paleoceanography and microbiology scientists aim at depths deeper than 2500 m below sea level resulting in operating pressures beyond 250 bars.

·         Moreover, a pressure rating of 600 bar was asked for by some scientists, which is still in the range of the drillship JOIDES Resolution.

·         However, autoclaves suitable to resist pressures like this require new design principles in terms of geometry and material.

Core Length:

Versatility:

Instrumentation - Measurement – Monitoring:

Easier handling, faster redressing after deployment, increased safety, reliability:

Contamination:

The proposed construction concept aims for a routine industrial pressure coring system that is cost-effective and interfaces easily with dedicated upstream tools for very different and specific scientific tasks.  The recent successful beginning of in situ technology in scientific drilling will result in broad applications and further development during the forthcoming years.

Future applications include, but are not limited to: Hydrocarbon exploration in shales and other tight formations, conventional oil & gas exploration, coalbed methane, CO₂-sequestration, mechanisms of biogeochemical cycles, earth and environmental science of the deep sub-seafloor, and other fields of research where sediments and rocks are investigated that have pressure-sensitive characteristics.

This article is a "call for collaboration" to all those currently working in the fields of research mentioned above or other fields where it might be useful to investigate cores under in situ pressure!

References

[1] Schultheiss, P.J., T.J.G. Francis, M. M. Holland, J.A. Roberts, H. Amann, Thjunjoto, R.J. Parkes, D. Martin, M. Rothfuss, F. Thyunder and P.D. Jackson, 2006,  Pressure coring, logging and sub-sampling with the HYACINTH system, in  Rothwell, R.G. (ed.) New Techniques in Sediment Core Analysis: Geological Society London, Special Publications 267, p. 151-163.

[2] Rothfuss,  M., et al., 2003, Gewinnung von Bohrkernen aus marinen Gashydraten unter in situ Bedingungen mit dem HYACE Rotary Corer (Retrieval of cores from marine gas hydrates under in situ conditions with the HYACE Rotary Corer), in Proceedings of the DGMK Spring Conference, Celle, Germany,  p. 565-576.

[3] Müller, W.H., E. Anders, and H. Amann, 2006, In-situ Sampling, Transfer and Investigation Methods in Scientific Drilling: Technology Progress Report from IODP Leg 311, Cascadia and Beyond; Poster presentation, IODP-ICDP Kolloquium, Greifswald, March 27-29, 2006; Abstract, p. 94-95.

[4] Parkes, R.J., D. Martin, W.H. Müller, E. Anders, H. Amann, X. Wang, and K. Dotchev, 2006, Sub-Sampling and Microbiological Experiments on High-Pressure Cores without Depressurisation; Abstract of poster presentation, EuroForum, Cardiff, May 8-9, 2006.

[5] Parkes, R.J., H. Amann, M. Holland, D. Martin, P.J. Schultheiss, E. Anders, X. Wang and K. Dotchev, 2008, Technology for High-Pressure Sampling and Analysis of Deep Sea Sediments, Associated Gas Hydrates and Deep Biosphere Processes, in AAPG Special Volume on Gas Hydrates, Ed. Tim Collett, In Press.

[6] Holland, M.E., P.J. Schultheiss, J.A. Roberts, and M. Druce, 2008, Observed Gas Hydrate Morphologies in Marine Sediments: in Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.

[7] Schultheiss, P.J., M.E. Holland, and G.D. Humphrey, 2008, Borehole Pressure Coring and Laboratory Pressure Core Analysis for Gas Hydrate Investigations: OTC 19601, Offshore Technology Conference, Houston, Texas, May 2008.

[8] Schultheiss, P.J., M.E. Holland, J.A. Roberts, M. Druce, and G.D. Humphrey, 2008, Pressure Core Analysis: The Keystone of a Gas Hydrate Investigation: in Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.

[9] Yun, T., G. Narsilio, J. Lee, J. Santamarina, and C. Ruppel, 2005, Physical properties of a Keathley Canyon pressure core maintained at in situ pressure and measured in a new instrumented pressure testing chamber, American Geophysical Union Fall Meeting, San Francisco, CA, December 5-9.

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