Double-direct shear configuration consisting of three granite blocks. A normal load is applied perpendicular to the sample via the horizontal ram and the center block is sheared at constant displacement rate via the vertical ram. Piezoceramic sensors are placed inside steel loading blocks and positioned adjacent to the fault zone.
Image: Chas Bolton
A team of Penn State researchers will soon have a better understanding of the deformation properties and poromechanical behavior of rock samples containing anhydrite, thanks to a $450,000 Chevron grant.
Geologic layers and rock formations in the Earth’s crust have large fractures called faults, which contain interconnected channels. Water flows through and along these channels. However, anhydrite, a mineral created by the evaporation of seawater, and other similar minerals, can become smeared along the shear zones of the fault and clog fluid flow, acting as a pressure seal. Such fault seals can also hinder the extraction of natural resources such as petroleum.
Despite the importance of extracting natural resources in these locations, there is still a poor understanding of the faults’ frictional properties and permeability.
“Faults tend to be barriers to flow,” said Chris Marone, professor of geosciences at Penn State and affiliate at the EMS Energy Institute, who is the principal investigator on the project. “The idea for this project is to look carefully at how that fault seal process works. This topic is of great importance for earthquake prediction and for the energy industry, who would like to image fault zone properties using seismic waves.”
Marone’s investigation will explore fault zone structure and compare layered faults with mixtures of the same bulk composition using synthetic samples and reservoir rocks obtained in the field and from the . To do this, they will use a new testing device that allows for the application of true triaxial stresses, corresponding to the vertical stress and both horizontal stresses for a rock sample at depth, with fluid flow and acoustic measurements during deformation. Experiments will include measurements of formation permeability, elastic wave properties, deformation data and frictional strength for a range of conditions related to fault seal and seal breach.
“We’ll deform the samples slowly, little by little,” Marone said. “Then we will watch how permeability evolves and measure the evolution of elastic properties using acoustic waves travelling through the sample.”
The team hopes the work will lead to models that can better predict the evolution of permeability and seal in areas of intermixed evaporite- dolomite-clay, locations where anhydrite is often found. They also hope the work will improve their understanding of petroleum seal and seal development in marine-sediment basins.
“The project has, at the outset, some fairly simple goals that will push the envelope of what we know about fault zone permeability and shear strength,” Marone said. “Ultimately, we hope to improve our understanding about the way elastic properties and permeability co-evolve with deformation.”
This four-year project, titled “Seal and poromechanical properties of anhydrite-carbonate and anhydrite-shale mixtures” began in September.
Double-direct shear configuration placed inside biaxial loading frame. An acrylic block is placed in series with the vertical ram to modify the system stiffness and to help produce a range of slip behaviors. A direct current displacement transformer (DCDT) is placed on the center block to measure fault displacement. Piezoceramic sensors are placed inside steel loading blocks and positioned adjacent to the fault zone.
IMAGE: Chas Bolton
