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An innovative program called 30StresS© has been developed
at SwRI to analyze the tendencies of faults and fractures to
slip or dilate. Here, 30Stress highlights in purple the region
of maximum dilation tendency along the flank of a salt dome.
Sand and shale layers surrounding salt domes such as those
in the Gulf of Mexico region often contain trapped oil and
gas. Information regarding dilation tendencies on salt dome
flanks can be used to assess the risk of trap leakage.
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S tress is always just beneath the surface - the surface of the
Earth, that is. And where faults and fractures exist in the brittle
rock layers of the Earth's crust, stress can have quite an impact,
from triggering earthquakes to redirecting natural resources.
Using 3DStress©, a unique computer program developed at
Southwest Research Institute (SwRI), geologists and geophysicists can now quickly and accurately determine the tendencies
of faults and fractures to slip or dilate.
Slip can result in an earthquake, whereas dilation widens
existing fractures, creating channels through which water,
hydrocarbons, or magma can travel. 3D Stress is thus a valuable
tool for oil and gas exploration, earthquake and volcanic hazard
analyses, and groundwater flow and transport analyses.
The program applies computed stresses in three dimensions to fault traces on maps or three-dimensional surfaces of
known or suspected faults. Fault data exported from 3DMove©,
developed by Midland Valley Exploration in Glasgow, United
Kingdom, and ARC-INFO©, d eveloped by the Environmental
Systems Research Institute in Redlands, California, can be
loaded into 3DStress, as can user-created files.
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The reddish-orange areas in these
3DStress slip tendency analysis
plots of faults and bedding horizons indicate regions of predicted
maximum slip tendency for a set of
given stress conditions. Threedimensional stress states are
entered into 3DStress through a
straightforward graphical user
interface, and results can be displayed in two or three dimensions.
Three-dimensional stress states are entered through a graphical user interface. Results, which can be displayed in two or three
dimensions, can be interactively adjusted by modifying stresses to
investigate slip tendency and direction on each individual surface.
The ratio of shear stress (parallel to a fault's surface) to normal stress (perpendicular to the surface) d etermines slip tendency. Fault dilation tendency is computed from the threedimensional stress state and the stress normal to a fault or extension fracture surface. Previously, these complex calculations
were done m anually for one fault orientation at a time. With
3DStress, calculations are m ade for all fault and fracture orientations and simultaneously displayed using stereo graphic projections, fault maps, and three-dimensional models of faults.
"Oil and gas companies can use 3DStress before drilling to
determine the risk of leakage from a hydrocarbon trap," says
Dr. David A. Ferrill, structural geologist at SwRI. Traps formed
by faults, folds, and stratigraphic variations prevent the upward
and lateral migration of naturally buoyant gas or oil. Dilatant
faults can channel hydrocarbons away from a trap, resulting in
little or no hydrocarbon accumulation.
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Volcanic hazard analyses rely to a great extent on probabilistic techniques. Ferrill explains that 3DStress provides a
deterministic complement to these techniques by revealing the
dilation tendencies of faults that could, under the right conditions, act as conduits for ascending m agma. The program can
also be used to predict the effects of changing stress fields on
existing faults in earthquake-prone areas.
"Groundwater flow in fractured aquifers is strongly controlled by fracture orientation and aperture, the product of dilation," Ferrill adds. "3DStress provides a rapid m eans for
evaluating dilation tendency in naturally fractured aquifers and
can thus be used to predict groundwater flow direction and
contaminant migration."
3DStress was developed under sponsorship of the U.S.
Nuclear Regulatory Commission. Codevelopers are Ferrill and
D . Brent H enderson of the Center for Nuclear Waste Regulatory Analyses, SwRI, and Dr. Alan Morris, associate professor at the University of Texas at San Antonio. Commercial
licensing is being handled by Midland Valley Services in
Boerne, Texas . •:.
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