This article highlights some of the research thrusts at The University of Texas at Austin’s Jackson School of Geosciences.

Tight Gas Sandstones

All tight sandstones are not alike, notes Shirley Dutton senior research scientist at the Jackson School's Bureau of Economic Geology. "Some areas produce better than others (sweet spots), and it's still poorly understood what controls the production," said Dutton. One factor is how the sand was originally deposited. Depositional history determines the regional distribution, geometry, and texture of the reservoir sandstones. However, Dutton noted, production characteristics of tight gas sandstones are in large part controlled by the diagenesis, or chemical and physical changes, that the sediment has undergone after deposition. Extensive cementation is commonly the reason for low permeability. Finally, natural fractures can be locally abundant in tight gas sandstones and may play a key role in production. Open natural fractures provide pathways for fluid movement and affect the way that hydraulic fractures grow.

Dutton and her colleagues are using seismic data and rock samples to study deeply buried sandstones (>15,000 ft) below the shallow water of the Texas Gulf Coast. High temperatures and pressures make drilling 20,000 or 30,000 feet deep an especially risky proposition, making it all the more important to devise methods to gauge the economic viability of resource plays at these depths. "For example, we are investigating whether there is a reservoir basement, so we could say, 'If you get beyond a certain temperature or depth, an economic rate of production becomes unlikely,'" said Dutton.

As producers in the Barnett Shale have learned, fractures are essential for gas production. Shale's susceptibility to artificial fractures varies greatly and geoscientists are currently trying to find out what makes some shales break easier than others. Variations in the mineral content of a rock and the particle types it contains all contribute to its mechanical strength, notes Steve Ruppel, Senior Research Scientist at the Bureau. Shale encompasses a really broad range of mineralogic and sedimentary properties. He and his team are studying how their composition varies across different areas, such as the Fort Worth Basin and the Permian Basin.

There are environmental considerations with shale gas development, water being a primary one. Not only are producers

competing for surface water resources required for the drilling process but water contaminated with hazardous chemicals and elements generated in the production process needs to be disposed of. Beyond water, there are the surface impacts of drilling and production. Unconventional resources require closely-spaced wells covering large areas—often multiple counties in a given play.

Determining the amount and distribution of methane hydrates near the ocean floor is currently the primary research focus in the field, according to Bob Hardage, Senior Research Scientist at the Bureau. Hardage and his team are working with multi-component seismic data to create detailed profiles of near-seafloor strata beneath the Green Canyon area in the Gulf of Mexico where heavier thermogenic gases (ethane and propane) fill some of the hydrate cages and cause the hydrate to be stable over a wider range of pressure and temperature than is pure-methane hydrate. By sending sound waves through the ground and measuring how fast they travel, the team can gain clues about the composition of the sediments, such as potential hydrates content. Rather than the commonly used compression waves, Hardage and his team use shear waves, which provide better resolution. Based on their data, Hardage and his team have developed rock physics models that relate seismic wave velocities to different percentages of hydrate concentrations in various mixtures of sand, clay, and brine.

Tom Shipley and Nathan Bangs, senior researchers at the Jackson School's Institute for Geophysics, also focus efforts on gas hydrates. They were among the first to clarify the properties of the geophysical diagnostic of the presence of hydrates, the Bottom Simulating Reflector, or BSR. Their work helped show that the BSR was related to a  phase change between methane in the hydrate form and the free gas form.