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November 02, 2006

Monitoring the Physical and Chemical Conditions Affecting
the Hydrocarbon System Within the Hydrate Stability Zone
of the Northern Gulf of Mexico

presented at the
Minerals Management Service
Information Transfer Meeting
December 5-7, 2000
Kenner, Louisiana

by

J. Robert Woolsey, Director
The Center for Marine Resources and Environmental Technology
The University of Mississippi

INTRODUCTION

A definite need exists to address questions and problems which relate to the shallow hydrocarbon system of the Continental Slope, northern Gulf of Mexico, within the hydrate stability zone. The possibility of designing and developing a remote, multi-sensor, monitoring station for long term investigation and research of this problematic near-sea floor hydrocarbon system has been discussed for some years. During March 22-26, 1999, a workshop entitled "New Concepts in Ocean, Atmosphere and Sea Floor Sensor Technologies for Gas Hydrate Investigations and Research" was held in Biloxi, Mississippi. It was sponsored primarily by the Center for Marine Resources and Environmental Technology (CMRET), of the University of Mississippi, Oxford. It was attended by an international group of participants which included a delegation from the Russian Academy of Sciences. One day of the workshop was devoted to intensive discussion sessions that addressed practical aspects of assembling and deploying such a station. By June, 1999, those and subsequent discussions led to a written document that described a project that would produce a station primarily for studying sea floor stability. That document was well received by government, industry, and academia. The interest it generated had led to further discussions and meetings and the project concept has been revised incrementally to include a widening spectrum of interests.

As of December, 2000, the intention is to assemble a station which will monitor physical and chemical parameters of the sea water and sea floor sediments on a more-or-less continuous basis over an extended period of time. The project will initiate collection of a data base for assessing and modeling the stability of the sea floor and factors associated with the formation and dissociation of gas hydrates. This data base will also be available for planning hydrate area exploration/delineation and, eventually, for analyzing the impact of producing hydrates from the ocean floor. It is also possible to expand the capabilities of the station to include biological monitoring. This would allow the study of chemosynthetic communities and their potential as a source for natural products/drug discoveries as well as an assessment of their environmental health and interactions with geologic processes.

The near-sea floor hydrocarbon system of the northern Gulf of Mexico is manifested by a variety of unusual features and phenomena. One of the more interesting and perplexing is the occurrence of gas hydrates, an ice-like mineral composed of natural gases and water. Gas hydrates are stable within a hydrate stability zone (HSZ) defined by pressure, temperature and gas composition. If the gas is almost pure methane, the proper conditions typically are found at ocean depths greater than 400m. The vertical extent beneath the sea floor is largely controlled by the geothermal gradient. Gas hydrates commonly form along faults and intersecting fractures near the sea floor where the hydrocarbon gases that migrate up from deep reservoirs interact with sea water. Mounds form on the sea floor that contain gas hydrates and various minerals deposited by bacteria feeding on the hydrocarbons. Variations in pore pressure, temperature distribution, chemical composition of the gas, and gas flow rate combine to determine whether hydrates within the mounds accumulate or dissociate. Many geoscientists familiar with Recent geologic processes in the Gulf of Mexico think that events which produce changes to the mounds also trigger episodes of sea floor instability.

Hydrates in direct contact with a large volume of sea water are stable only marginally. This causes parts of the mounds where hydrates outcrop to be more-or-less ephemeral, capable of changing greatly within a matter of days. The eddies of warm water that periodically separate from the Loop Current, a major current that runs northward through the Yucatan Straight and eastward through the Straits of Florida, are potentially prominent influences on outcropping hydrates because they can raise bottom temperatures by as much as 2-3 degrees C and thereby impose a quasi-cyclicity of sea floor hydrate formation and dissociation. Changes in pore pressure, gas composition, and flow rate may also be major influences but their causes are not well understood. They may relate to tectonic activity associated with salt movement.

Hydrates contained in sediments are stable when the host sediment is within the HSZ. If hydrocarbon gases migrating up faults encounter sediments of sufficient permeability within the HSZ, hydrates can form within pore spaces, along bedding planes, and within fractures. When hydrates act to cement the sediment grains, the shear modulus, and thereby the bearing capacity, of the sediment increases. When outside influences, such as thermal distortions due to salt movement, fluid migration, or drilling activity, act to dissociate the hydrates; the bearing capacity decreases and the potential for sea floor instability is created.

Common indicators of bearing capacity are the speeds at which compressional (P) and shear (S) waves propagate below the sea floor and the efficiency of P-to-S conversion (PS) at reflecting horizons. A comprehensive monitoring station would be capable of measuring these and many other parameters that are relevant to the formation and dissociation of gas hydrates.

The degree to which biological factors come into play is the subject of increasing amounts of research. For example, over time, bacteria consume the hydrocarbon molecules within the hydrate crystals and precipitate minerals, such as calcium carbonate, which cement sediment particles and increase bearing capacity.

Sea Floor Positioning System: The first system to be installed at the monitoring station site will be a sea floor navigation system by which the locations of other systems are referenced. It will be a long baseline system that encompasses an area of about 10 square miles (25 km²), somewhat larger than that required to deploy all other systems. It will remain in place throughout the lifetime of the station.

Vertical Line Arrays/Sea Floor Probes: The heart of the station will be a net of vertical arrays (VLA's) and sea floor probes (SFP's). The VLA's will occupy the lower portion of the water column while the SFP's will be placed in shallow bore holes (10m) in the sea floor in close proximity to the anchor points of the VLA's. VLA and SFP sensors will be selected to monitor dynamic oceanographic and geologic processes acting on the lower water column and the near-sea floor sedimentary section.

An issue to be addressed in consideration of appropriate systems for S-wave measurement is whether the 3-component instruments should be seismometers or accelerometers. Seismometers are more traditional but accelerometers can have broader bandwidths and are more shock resistant. When ocean bottom seismometers (OBS's) are deployed on the surface of sea floor sediment, their data can be degraded if the coupling between the instruments and the sediment is poor. That is probable in the Gulf of Mexico where sediments are often very soft. The sea floor earthquake measurement system (SEMS IV) of the Minerals Management Service reports great success using OBA's pushed into soft sediments to monitor earthquakes offshore California. An experiment will be conducted during the site selection phase in which OBS's and OBA's will be pushed into the bottom by a remotely operated vehicle (ROV). Recordings from each will then be compared to determine if OBA's rather than OBS's should be used at surface locations. Regardless of the outcome, the OBA's will be the only practical sensors for the SFP's.

Thermistors: The thermistors mounted on the sensor string of the SFP will be designed to monitor heat flow density from which heat flow gradient estimates may be determined. These data will in turn provide estimates for the depth dimensions of the HSZ as well as provide a base for mapping heat flow anomalies relating to localized hydrocarbon fluid flow (primarily gas).

The subbottom thermal measurements will be accompanied by pore-fluid pressure measurements to provide information pertaining to hydrate dissociation and its effects on the physical/mechanical properties of the sediments. The results will be described in terms of both absolute pressure (and how it changes due to a small amount of gas release), and load partitioning between sediment matrix and pore fluid during tidal cycles and meteorological events.

Thermistors mounted on the VLA will add to the thermal study by enabling the monitoring and input of bottom-water temperature variations.

Optical Spectroscopy: Using a technique similar to that employed by the Mars Rover, an optical spectrometer will be used to identify and quantify hydrocarbon gases present in the sea water. Samples will be illuminated by laser light shining through an optic fiber and the back scattered light collected by other optic fibers. Spectral analysis of the back scattered light will provide information concerning the chemical composition of the gas in each sample. Of particular interest is a recent advancement in mid infrared (MIR) spectroscopy, under development, which utilizes a miniature cryogenic cooler at the sensor which has been demonstrated in the laboratory to produce very significant analytical results with mixed hydrocarbon gases. An obvious and very useful application of this technology would be its incorporation in the sea floor mounted tip bucket, a free gas volume measurement device presently in use on small gas vent sites. For instance, a sample might be analyzed after a programmed number of tips of the bucket. A less conventional but equally valuable application would be the installation of optical sensors on the SFP string for measurement of pore fluid chemistry insitu.

Current Measurements: An acoustic doppler current profiler (ADCP) will be installed on the sea floor to monitor current flow at several levels within the lower approximate 200m of the water column. These data will also be processed for backscatter intensity to yield an estimate of suspended particulate mass, a parameter affected by both sediment resuspension due to water currents and by sediment mass movements. In addition, a number of three-axis acoustic current meters will be installed near the VLA to assist in the determination of VLA receiver geometry. Each of these meters will record temperature, pressure, and optical backscatter as well as current flow.

Underwater Vehicles: A variety of underwater vehicles will be used at various stages of the project: deep-tow and bottom-tow devices, tethered ROV's, manned and eventually autonomous underwater vehicles (AUV's). The latter will carry sensors that are more effective when moved about, thus improving spatial resolution while the former group will be vital to reconnaissance and station assembly. The locations of underwater vehicle activities will be determined in relation to the long baseline navigation system.

Towed vehicles and ROV's will be deployed from surface ships during site selection and calibration. Towed vehicles will reconnoiter areas with survey tools such as side scan sonar, seismic/acoustic subbottom profilers, and the electromagnetic profiler. ROV's and manned vehicles will be used to survey smaller areas with greater precision and to deploy and service instruments such as OBS's and OBA's, cameras, conductivity-temperature-density (CTD) probes, pore-water samplers, pH meters, dissolved oxygen (O₂) probes, and the optical spectrometer. Together, they will provide close-up images of the sea floor and measurements to identify hydrate zones, hydrocarbon seeps and chemosynthetic communities.

A future consideration, not presently factored, will be the use of AUV's. It is anticipated that the AUV will carry imaging instruments and measuring probes but will be used primarily during periods when the station is not attended by surface ships. It will be guided by the long baseline navigation system to carry imaging systems along specific transects or to take measurements at specific locations. It will be guided by genetic algorithms and other software to search designated sectors for new targets. Given appropriate development, it will operate from a docking facility near the monitoring station where data can be downloaded, instructions received, and batteries recharged. The dock will be connected by optic fiber to a site, probably an oil platform, where the images and spectral data can be transmitted ashore and instructions received. Electric power for recharging batteries will also be obtained from that site.

A suite of experiments will be carried out in the spring of 2001 that, summed together, will approximate the operation of a monitoring station. The experimental site is expected to be in Mississippi Canyon Block 798 where verbal permission, subject to formal request, has been received from the lease holder. The topography of the bottom is relatively gentle there and the presence of hydrate mounds has been confirmed. Additional investigation was done using OBS deployments and high resolution seismic profiling during a June, 1998, cruise funded by CMRET, the U.S.Geological Survey, and the U.S.Department of Energy. The existing data are sufficient to provide a reasonably detailed understanding of the configuration of the subbottom there.

Systems used in the Block 798 will include an autonomous VLA, several OBS's and OBA's, and several resistivity and heat flow conventional gravity probes. The instruments will be deployed for a period of 7-10 days. High resolution seismic profiles will be recorded using surface-deployed seismic sources of various energy levels. Signals from these sources also will be recorded by the VLA, OBS's, and OBA's. In addition, the sounds of ships will be recorded to test the usefulness of sources of opportunity; probes will be deployed to obtain resistivity and heat flow data; and gas samples will be collected by an ROV.

For a period of some months commencing in 2002, Conoco has offered to provide facilities in the Green Canyon area (Marquette and Joliet fields) for the purpose of performing experiments related to the monitoring station. The offer includes limited access to platforms, electrical power, and satellite communications. It is expected that one of the first experiments to be performed using these facilities will be a test of the feasibility of measuring fluid flow rates (from sea floor vents) using the sound of bubbles, and possibly the deployment of a video monitoring system.

If the monitoring station has access to sufficient electrical power, several video systems with pan and tilt capability will be installed for scientific, educational, and public outreach purposes. Not only will the live images be useful to scientists for visual evaluation of sea floor conditions as they change in response to either experimental or natural stimuli, but they will be made available to K-12 educators for use in classrooms as well. Students at any facility with access to the Internet will be able, on a predetermined schedule, to pan, tilt, and zoom the cameras, engage different lights, and compare the images with other data and with images from other, similar systems.

A short list of potential monitoring station sites will be identified on the basis of their geologic framework and the applicability of available and appropriate technologies tested during the initial experiments. The final choice will be the responsibility of the project's board of scientific supervisors.

The process of selecting a site for the monitoring station will commence with evaluation of candidate sites for which pertinent information already exists. Many sites have been studied over a period of years, especially by the Coastal Studies Institute of Louisiana State University and the Geochemical and Environmental Research Group of Texas A&M University. These will have priority in the selection process. Appropriate physical conditions form only part of the criteria for a suitable site. Much of the sea floor in the Gulf of Mexico is under lease to private parties and sea floor instruments cannot be installed in leased areas without permission of the lease holder(s). Another criterion for site selection is reasonable proximity to a telemetering site and an electrical power source.

The most efficient method for reconnoitering potential sites is reflection seismic profiling. A substantial body of exploration seismic data exists and some of it is available for review. The resolution of exploration data is generally low, however, and higher resolution data will be required prior to final site selection. The higher resolution data will make it possible to resolve layers a fraction of a meter thick near the sea floor while providing somewhat lower resolution up to a kilometer below the sea floor.

Seismic profiling can provide information concerning reflection coefficients and, to some extent, speeds of seismic propagation but other types of geophysical data will be required before a final site choice can be made. Optical, side scan and swath-bathymetry images as well as shear-wave, geoelectric and heat flow measurements will be collected at sites on the short list. Core samples will also be taken as necessary.

A period of two years will be required to construct, integrate station systems, and deploy the VLA's/SFP's, the end of the second year marking the beginning of operations of the initial station on the sea floor. Most probably a third year will be required to complete the station as presently planned. A few systems require at least some development before they can be constructed. These include certain of the SFP sensors and the optical spectrometer.

As noted previously, the VLA's and SFP's will represent key components of the monitoring station. Considering the architecture and economics involved, the VLA's are retrievable systems while the SFP=s, by design and location, must be considered more-or-less permanent in their placement and expendable.

Deployment: Following assembly and testing of the SFP, it will be deployed using a modified version of existing CMRET remote sea floor drilling technology. The present version is designed to remotely emplace and extract casing of up to 11.4cm (4.5 in.) O.D. by 12m (40 ft.) length for the purpose of taking cores and implanting sensor probes. In the case of the latter, the probe, consisting of a string of various sensor elements, which may or may not be incased in a gel-filled plastic tube, is emplaced in the delivery casing. To meet the requirements of the proposed project, a fishtail drill point of greater diameter than the selected casing will be loosely inserted at the lower end of the casing and provided with a slot to permit torquing of the casing during the emplanting procedure. The sensor string or tube is attached to the fishtail drill point such that on extraction of the casing, the sensor string, remains in the hole, anchored to the bottom by the detached drill point.

With this program in mind the CMRET remote sea floor drill (RSD) design modification plans have been completed and await implementation, which will permit system operation to 1000m. Basically, the modifications involve a battery powered, d.c. electric-hydraulic system supplying an existing bottom mounted semi-rotary (reciprocal) casing drive mechanism fitted with hydraulic cylinders for pull-down and pull-up. The existing system is operated via a pressure vessel-encased computer system controlled by a computer on deck. Additional modifications will include the installation of a locator pinger and a video camera and light to assist in site location. Once modified, the RSD may be deployed from offshore vessels of opportunity, suspended from a single coaxial communication/suspension cable.

Operation: Initial operation of the VLA's/SFP's will be in an autonomous mode, utilizing battery power (pressure compensated) and data recording. While recording of thermal, pore pressure, and chemical data can be recorded for extended periods, the high volume typical in acoustic data collection will require down loading at three to five day intervals. Since the acoustic source for both P and S wave studies will be deployed from an appropriate surface vessel, servicing batteries and downloading the recorders can be accomplished by means of ROV's. It is, however, the long term intent to utilize power and communication uplink where available at the sea floor. The station will produce many channels of data on a more-or less continuous basis. The total data volume will be large and recovery is not a trivial problem. Present plans are to digitize each channel on site and transmit the digital signals via optic-fiber cable to a structure, such as an oil platform, where they can be telemetered to an onshore processing facility.

Aspects of installing a station to monitor an area of the sea floor near hydrate mounds on the continental slope of the northern Gulf of Mexico have been discussed in depth by some of the world=s foremost experts in appropriate fields. There is unanimous agreement that not only is the concept feasible but that most of the necessary components exist and already have been used in deep ocean applications. A few components still require some development, though initial experiments and site selection will commence in the spring of 2001.

The monitoring station will commence initial operation by the end of 2003 and will begin collection on a more-or-less continuous basis in 2004. Physical and chemical information concerning sea floor stability and the accretion/dissociation of gas hydrates will be the initial primary focus. If that information reveals factors as expected, which elicit responses from chemosynthetic communities residing nearby, the stations=s capabilities could be expanded to include biologic monitoring. This should provide an excellent means to explore the interactions between life forms and physical/chemical stimuli as well as how biologic agents produce or modify geologic materials and processes.

Transfer of technology and science to industry and government agencies will be regarded as a primary responsibility. Also, an effort will be made to make activities and results of the monitoring station available for use in classrooms. The access will be in near-to-real time and, as much as possible, interactive.

A Cooperative Effort with Support/Participation From:

DEPARTMENT OF COMMERCE
National Oceanographic and Atmospheric Administration
National Undersea Research Program

DEPARTMENT OF DEFENSE
Naval Research Laboratory
Naval Oceanographic Office

DEPARTMENT OF ENERGY
Federal Energy Technology Center

DEPARTMENT OF INTERIOR
Minerals Management Service
U.S. Geological Survey

Geological Survey of Canada
Conoco
Marathon
Specialty Devices, Inc.
Florida State University
Louisiana State University
Mississippi State University
Texas A&M University
University of Georgia
University of Southern Mississippi
University of Texas, Austin
University of Victoria, British Columbia
Woods Hole Oceanographic Institution

Facilitated by:
The Center for Marine Resources and Environmental Technology
The University of Mississippi

List of Acronyms

ADCP acoustic doppler current profiler
AUV autonomous underwater vehicles
CMRET The Center for Marine Resources and Environmental Technology
CTD conductivity-temperature-density
HSZ hydrate stability zone
MFP matched field processing
MIR mid infrared
OBA ocean bottom accelerometers
OBS ocean bottom seismometers
P compressional waves
PS compressional to shear conversion
ROV remotely operated vehicle
RSD remote sea floor drill
S shear waves
SEMS IV sea floor earthquake measurement system
SFP sea floor probes
VLA vertical line arrays