On March 3, 2000, 45 representatives of the petroleum, geotechnical, and scientific drilling communities met in Houston for the "GeoFluids of Overpressured Strata in the Gulf of Mexico Workshop". Participants set an ambitious goal for the application of scientific drilling on the continental slope: the development of a "grand unified theory" that will describe the hydrodynamics of continental slopes. To achieve this synthesis, it was proposed that generations of observational and theoretical work within the geological, hydrogeological, and the geotechnical communities be coupled with new observations from the deepwater continental slope to clarify the state and interactions of pressure, stress, sedimentation, and deformation in the first 1000 meters below the seafloor. The resulting model will allow prediction of both hydrodynamic behavior and in situ conditions in continental slope sediments using information gleaned from remotely sensed data. These advances will increase the understanding of slope stability, fluid-flow in faults, margin-ocean fluxes, seep communities, and basinal fluid flow. Measurements made will allow the petroleum industry to optimize deepwater exploration and production. Finally, the proposed experiments will position ocean drilling for a new generation of both riser and riserless drilling science within the International Ocean Drilling Program (IODP).
The 1-day workshop was funded by Conoco and the Joint Oceanographic Institute/United States Scientific Support Program. It brought together 32 representatives of the petroleum industry, 3 geotechnical specialists, 8 academic scientists affiliated with ODP, and 2 ODP-TAMU (Texas A&M University) representatives. The morning was spent defining common technical problems and bridging disciplinary language divides. Industry representatives described their experiences drilling overpressures in the shallow subsurface. Geotechnical specialists described the array of geotechnical measurements that are possible. ODP veterans described the capability of the drillship JOIDES-Resolution, and the successes and failures of previous hydrogeological experiments. The real work began in the afternoon. First, technical questions of greatest common interest were ranked as a group. Second, three subgroups were formed to address: a) what do we want to measure? b) where do we want to measure it? and c) how will industry and ODP collaborate to achieve these measurements?
In certain regions of the ocean floor, the uppermost 1000 meters beneath the sea floor of the continental slope is a low effective stress zone (a zone where the difference between pore pressure and confining stresses are small) (Fig. 1). In this regime, overpressure, slope failure, fluid migration, and biological vent communities coexist and their interactions can be examined. We would like to know where sediments are overpressured, what their rheology is, what their permeability is, how permeability couples with stress, and how overpressure and seeps are coupled. An understanding of how overpressure controls fluid expulsion and debris flows may allow scientists to risk the probability of occurrence of these geohazards. The proposed analyses will yield insight into other low effective stress zones such as accretionary prisms (fluid flow in faults) and deep overpressured basins (basinal fluid flow).
A better understanding of this hydrodynamic system will benefit industry in several ways: 1) it will assist our understanding of hydrocarbon migration, hydrocarbon trapping, and rock rheology in deeper low effective stress zones; 2) it will strengthen our ability to design wells so we can safely and economically reach hydrocarbon targets; and 3) it may lower the tremendous costs of developing our deepwater deposits where single structures can cost billions of dollars.
"Shallow-Water Flow" is a particularly vexing drilling problem that occurs when overpressured and unconsolidated sands are encountered in the first 1000 meters below the sea floor (Ostermeier et al., 2000). In this environment, it is difficult to control the fluid pressure in the well bore so that it lies between the formation’s pore pressure and the fluid pressure that fractures the rock. Drilling these overpressured sands without proper pressure balance can result in uncontrolled flows, formation compaction, and structural well damage. The "Shallow-Water Flow" problem has cost the petroleum industry hundreds of millions of dollars. Although less well known, the ODP has experienced poor core recovery, low penetration, and drilling problems that may result from shallow-water flow at ODP Sites 1071 and 1072 and 948.
What Processes, Where to Measure Them, and What to Measure
Workshop participants selected five criteria and/or phenomena to examine with scientific drilling: 1) fluid expulsion features; 2) slump features and slope failures; 3) known zones of shallow water flow; 4) locations with different deposition rates; and 5) sites that are world-wide analogs. Five Gulf of Mexico locations were selected that met some or all of these criteria (Fig. 2).
To understand the hydrodynamic behavior of these systems, a range of traditional (e.g. whole core, wireline logs, logging while drilling) and less traditional (e.g. pore pressure, stress, permeability) scientific drilling measurements must be made. Industry and academic presentations described application of the piezocone, the piezoprobe, the cone-pressuremeter, and the wireline packer. Together, these tools yield a range of geotechnical measurements including state of stress, strength, stiffness, and permeability. Workshop participants concluded that although the JOIDES-Resolution had limited geotechnical measurement capability, there was extraordinary opportunity to use existing technology to make geotechnical measurements with this drillship.
Workshop attendees concluded that a joint industry-ODP leg in hydrodynamics of the continental slope should be aggressively pursued because it would impact an important fundamental and applied science problem. The following list describes some of the areas of collaboration that were identified at the workshop.
1) Pursue joint ODP-industry discussions to explore development and application of geotechnical tools and PWD (pressure while drilling) on the JOIDES-Resolution. Where contractor tools exist, pursue lease arrangements.
2) Work with industry to release pre-drill site data (e.g. seismic, drilling, previous geotechnical measurements). Bring to bear industry drilling experience in deepwater.
3) Integrate research efforts of industry and academia. Pursue possibility of limited shipboard stays for industry scientists.
Three major results from this workshop are completed or in progress. First, an ocean drilling proposal was submitted http://www.joides.geomar.de/cgi-bin/abstracts.cgi? number=589-Full).Second, workshop participants are preparing a detailed workshop report that outlines the scientific vision, the current capabilities, and the desired capabilities for the scientific drilling in the hydrodynamics of the continental slope. Third, workshop participants are working with ODP engineers and scientists to evaluate the possibility of using industry geotechnical tools on the J.R. Workshop results can be viewed at (http://hydro.geosc.psu.edu/Odp/odp.html).
Ostermeier, R.M., Pelletier, J.H., Winker, C.D., Nicholson, J.W., Rambow, F.H., and Cowan, K.M., 2000, Dealing with Shallow-Water Flow in the Deepwater Gulf of Mexico. Offshore Technology Conference Paper #11972. 2000 Offshore Technology Conference, May 1-4.
Peter Flemings is an Associate Professor in the Department of Geosciences at The Pennsylvania State University. Alan Huffman is the Manager of the Seismic Imaging Technology Center of Conoco. Bob Bruce is a Geophysical Advisor for BHP Petroleum. Jean Benoit is a Professor in the Department of Civil Engineering at the University of New Hampshire. Paul Mayne is a Professor in the School of Civil & Environmental Engineering at the Georgia Institute of Technology.
Figure 1: The first 1000 meters below the sea floor on the continental slope of the Gulf of Mexico is a zone of fluid expulsion, biological communities, faulting, and slumping. Scientific drilling within the shallow low effective stress zone will sample rocks that are in a stress state similar to deeper overpressured systems.
Figure 2: The workshop identified five sites of high priority for scientific drilling. 1) The Mississippi Fan has high and recent sedimentation. 2) The Mississippi Canyon slope region is a zone of known shallow water flow. 3) The Sigsbee Escarpment records the interaction of salt deformation, slumping, turbidite deposition and fluid flow. 4) The Mexican Ridge Folds have low and variable sedimentation rates. 5) In the Green Canyon Region moderate sedimentation rates and higher near-sea floor structural relief are present.