Exploration and Production Technologies

Acoustical Imaging and Mechanical Properties of Soft Rock and Marine Sediments

DE-FC26-01BC15302

Program
This project was selected in response to DOE's Oil Exploration and Production solicitation DE-PS26-01NT41048 (focus area: Critical Upstream Advanced Diagnostics and Imaging Technology). The goal of the solicitation is to continue critical upstream cross-cutting, interdisciplinary research for the development of advanced and innovative technologies for imaging and quantifying reservoir rock and fluid properties for improved oil recovery.

Project Goal 
The focus of this project was to provide research to allow an expansion of time-lapse seismic technologies to image damage occurring in oil and gas reservoirs. The study attempted to provide an answer to two questions: 1) can 3-D seismic imaging be used to define and outline regions of rock that are 
susceptible to drilling damage and production damage prior to drilling a well; and 2) to track, after production has started (via time lapse 4-D seismic imaging) the development of any rock damage such as compaction and subsidence.

Performer
University of Oklahoma
Norman, OK

Project Results
The project determined that field seismic imaging of rocks can be utilized to highlight regions of rocks that have the potential for generating damage during drilling and for long-term tracking of mechanical damage during the extraction of petroleum reserves.

The project generated a basic database on acoustic (seismic) properties of weak rocks and soft sediments when subjected to high stresses. Perhaps the most important advance in the project was a demonstration that rock damage can be time-lapse imaged as it develops within a highly stressed region of rock. This laboratory experiment suggests that damage within a petroleum reservoir could be imaged as well by repeating 3-D seismic surveys over the life of the reservoir (i.e., 4-D imaging). 

Benefits
The easy to drill and produce reservoirs already have been depleted during past oil and gas production operations. This leaves much of the existing reserves in more-expensive, problematic reservoirs. Many of these reservoirs are in soft rock or unconsolidated sands that are mechanically unstable and generate costly drilling and production problems such as casing failures, reservoir compaction, and sand production. Many of these problems can be mitigated if the operators know beforehand that a problem may occur or is beginning to develop within an existing field. If successfully applied, this seismic technology could save billions of dollars by alerting operators to potential or actually occurring rock damage. It may make many problematic (e.g, unpredictable and unstable) reservoirs more economic to drill and produce.

Background
Damage to weakly cemented, unconsolidated sands or soft rocks (like chalk) during the production and drilling of reservoirs is a costly problem for the oil and gas industry. For example, the unexpected compaction (and its resultant damage) in the Ekofisk chalk resulted in over $1 billion in remedial work being applied to production facilities overlying the reservoir. Reservoir compaction can also result in casing failures, loss of reservoir permeability, and damage to surface production facilities. Another example of the problem generated by drilling unconsolidated sands is the problem of shallow water flows. Such flows occur at shallow depths below the seafloor (less than 2,000 feet) but in deep water (2,000 - 4,000 feet). These flows occur when the unconsolidated sands suddenly flow up the annulus of the borehole and flow onto the seabed. The flow can cause washouts and loss of the supporting surface casing. For example, the damage to the Ursa development project on Mississippi Canyon Block 810 resulted in the loss of $150 million for the partners in the project.

Project Summary
The research during this project concentrated on developing a correlation between rock deformation mechanisms and their acoustic velocity signatures. Among the milestones:

  • The experiments on unconsolidated sands indicated that shear wave velocities sharply decrease and Vp/Vs ratios markedly increase during liquefaction of sand at high pressure in undrained triaxial experiments and during plasticity of sand in drained triaxial experiments.
  • Experiments were conducted to determine the acoustic emission signature of deforming high- porosity rocks (in comparison to their low-porosity, high-strength counterparts). The results were highly variable-typically sands and sandstones generate much acoustic emission during damage and high-porosity, soft chalks did not.
  • The project demonstrated that the elastic and poroelastic moduli are strongly affected by applied stress. In fact, the rock can develop a significant stress-induced anisotropy-a factor extremely important to geophysicists processing seismic data.
  • Research showed that damaged regions of rock could be seismic-tomographically imaged as the damage is occurring. The development of a growing, propagating shear fracture and its associated damage halo was successfully imaged during a triaxial compression experiment.

Current Status
The project has led to the development of a spin-off company called Rock Dynamics (located in Norman, OK). The company was set up to directly facilitate technology transfer of the concepts developed in this DOE project to the oil and gas industry. The company has been involved in 1) transferring the new 
laboratory technologies developed in this project to several oil and gas company R&D facilities, 2) refining the technology in laboratory acoustic tomographic imaging systems for eventual marketing to geotechnical testing laboratories and the oil and gas R&D facilities, 3) continuing research into acoustic 
and seismic imaging of rock damage, and 4) presentation of scientific results to various R&D laboratories. Research also continues in the PoroMechanics Institute at the University of Oklahoma in the Rock Mechanics Consortium of Oil and Gas Companies.

Publications 
Scott, T.E., and Abousleiman, Y., Acoustical Imaging and Mechanical Properties of Soft Rock and Marine Sediments, DOE Final Technical Report #15302, DOE Award Number: DE-FC26-01BC15302, 2004.

Scott, T.E., and Abousleiman, Y., A determination of the stress-induced dynamic moduli of a porous medium subjected to various deformational pathways., Proceedings from the Second Biot Conference on Poromechanics, pp. 795-799, 2002

Scott, T.E., and Abousleiman, Y., An experimental method for measuring anisotropic poroelastic Biot's effective stress parameters from acoustic wave propagation, Proceedings from the Second Biot Conference on Poromechanics, pp. 801-806, 2002

Scott, T.E. and Abousleiman, Y., Ultrasonic imaging of a shear failure during triaxial testing, Proceedings from the North American Symposium on Rock Mechanics (Alaska Rocks), 2005.

Scott, T.E. and Abousleiman, Y., Acoustic measurements of the anisotropy of dynamic elastic and poromechanics moduli under different stress paths, to be published in the September 2005 issue of the Journal of Engineering Mechanics, 2005.

Project Start: January 2, 2002
Project End: December 30, 2003

Anticipated DOE Contribution: $289,887
Performer Contribution: $78,218 (21% of total)

Contact Information
NETL - Virginia Weyland (virginia.weyland@netl.doe.gov or 918-699-2041)
U. of Oklahoma - Thurman Scott (gene@rmg.ou.edu or 405-325-2900)

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