Rick Allis

Techniques, analysis, and noise in a Salt Lake Valley 4D gravity experiment

Repeated high-precision gravity measurements using an automated gravimeter and analysis of time series of 1-Hz samples allowed gravity measurements to be made with an accuracy of 5 Gal or better. Nonlinear instrument drift was removed using a new empirical staircase function built from multiplestationloops.Thenewtechniquewasdevelopedbetween March 1999 and September 2000 in a pilot study conducted in the southern Salt Lake Valley along an east-west profile of eight stations from the Wasatch Mountains to the Jordan River. Gravity changes at eight profile stations were referenced to a set of five stations in the northern Salt Lake Valley, which showed residual signals of 10 Gal in amplitude,assumingareferencestationneartheGreatSaltLake to be stable. Referenced changes showed maximum amplitudes of 40 through 40 Gal at profile stations, with minima in summer 1999, maxima in winter 1999‐2000, and some decrease through summer 2000. Gravity signals were likelyacompositeofproduction-inducedchangesmonitored by well-water levels, elevation changes, precipitation-induced vadose-zone changes, and local irrigation effects for whichmagnitudeswereestimatedquantitatively.

Fracture development within a stratovolcano: The Karaha-Telaga Bodas geothermal field, Java volcanic arc

Abstract Karaha-Telaga Bodas, a vapour-dominated geothermal system located in an active volcano in western Java, is penetrated by more than two dozen deep geothermal wells reaching depths of 3 km. Detailed paragenetic and fluid-inclusion studies from over 1000 natural fractures define the liquid-dominated, transitional and vapour-dominated stages in the evolution of this system. The liquid-dominated stage was initiated by a shallow magma intrusion into the base of the volcanic cone. Lava and pyroclastic flows capped a geothermal system. The uppermost andesite flows were only weakly fractured due to the insulating effect of the intervening altered pyroclastics, which absorbed the deformation. Shear and tensile fractures that developed were filled with carbonates at shallow depths, and by quartz, epidote and actinolite at depths and temperatures over 1 km and 300°C. The system underwent numerous cycles of overpressuring, documented by subhorizontal tensile fractures, anastomosing tensile fracture patterns and implosion breccias. The development of the liquid system was interrupted by a catastrophic drop in fluid pressures. As the fluids boiled in response to this pressure drop, chalcedony and quartz were selectively deposited in fractures that had the largest apertures and steep dips. The orientations of these fractures indicate that the escaping overpressured fluids used the shortest possible paths to the surface. Vapour-dominated conditions were initiated at this time within a vertical chimney overlying the still hot intrusion. As pressures declined, these conditions spread outward to form the marginal vapour-dominated region encountered in the drill holes. Downward migration of the chimney, accompanied by growth of the marginal vapour-dominated regime, occurred as the intrusion cooled and the brittle-ductile transition migrated to greater depths. As the liquids boiled off, condensate that formed at the top of the vapour-dominated zone percolated downward and low-salinity meteoric water entered the marginal parts of the system. Calcite, anhydrite and fluorite precipitated in fractures on heating. Progressive sealing of the fractures resulted in the downward migration of the cap rock. In response to decreased pore pressure in the expanding vapour zone, walls of the fracture system within the vapour-dominated reservoir progressively collapsed. It left only residual permeability in the remaining fracture volume, with apertures supported only by asperities or propping breccia. In places where normal stresses acting on the fracture walls exceeded the compressive strength of the wall rock, the fractures have completely collapsed. Fractures within the present-day cap rock include strike- and oblique-slip faults, normal faults and tensile fractures, all controlled by a strike-slip stress regime. The reservoir is characterized by normal faults and tensile fractures controlled by a normal-fault stress regime. The fractures show no evidence that the orientation of the stress field has changed since fracture propagation began. Fluid migration in the lava and pyroclastic flows is controlled by fractures. Matrix permeability controls fluid flow in the sedimentary sections of the reservoir. Productive fractures are typically roughly perpendicular to the minimum compressive stress, σ3, and are prone to slip and dilation within the modern stress regime.

Natural CO2 Reservoirs on the Colorado Plateau and Southern Rocky Mountains, USA: A Numerical Model

Publisher Summary Gas reservoirs within the Colorado Plateau and Southern Rocky Mountains region are natural laboratories for studying the factors that promote long-term storage of CO2. They also provide sites for storing additional CO2 if it can be separated from the flue gases of coal-fired power plants in this part of the United States. These natural reservoirs are developed primarily in sandstones and dolomites. In many fields, stacked reservoirs are present, indicating that the gas has migrated up through the section. There is also evidence of geologically young travertine deposits at the surface and CO2-charged groundwater and springs in the vicinity of known CO2 occurrences. These near-surface geological and hydrological features also provide examples of the environmental effects of leakage of CO2 from reservoirs. The confidence in the predictions of numerical simulations by modeling a natural system of CO2 reservoirs and comparing the modeling results with a number of observations has been discussed in the chapter. A numerical model based on the Farnham Dome CO2 reservoir structure located in east-central Utah has been developed. This reservoir is typical of those found on the Colorado Plateau with stacked CO2 reservoirs contained within a Laramide, dome-like structure. Subsequent modeling of the chemical interactions between reservoir brine, CO2 gas and a simplified mineral assemblage using the simulator CHEMTOUGH2 produces results that are generally consistent with the water chemistry observed in basins of Eastern Utah.

An opportunity of geothermal proportions in sedimentary basins

John Holbrook, Joseph N. Moore, Derek Elsworth, Karin A. Block, Rick Allis, Herbert Einstein 1 School of Geology, Energy, and the Environment, Texas Christian University, Fort Worth, TX 76129 John.Holbrook@tcu.edu 2 Energy and Geoscience Institute, University of Utah, Salt Lake City, Utah, 84108 jmoore@egi.utah.edu 3 Energy and Mineral Engineering, G3 Center and EMS Energy Institute, Pennsylvania State University, University Park, PA 16802 elsworth@psu.edu 4 Department of Earth and Atmospheric Sciences, City College of New York. New York, NY 10031 kblock@ccny.cuny.edu 5 Utah Geological Survey, Salt Lake City, Utah, 84105 rickallis@utah.gov 6 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 einstein@mit.edu