Previous Talks

Wednesday, October 10th, 2007

Laboratory study of hydrothermal deformation in unconsolidated St Peter quartz sand

Stephen L. Karner - ExxonMobil Upstream Research Company, Houston, TX

Abstract

The temperature sensitivities of dilatant fracture and frictional strength of sheared surfaces are known to be subtle. However, few studies have explored the temperature dependence for granular compaction when reactive fluids are present. I report on an investigation of hydrothermal deformation in granular quartz as observed from laboratory experiments on St Peter sand (grainsize fraction 250-350 μm). The experiments were conducted in triaxial testing apparatus housed at the John Handin Rock Deformation Laboratory, Texas A&M University. The tests were performed at temperatures between 25-200 oC. Throughout each test we monitored pressures (constant pore fluid pressure of 12.5 MPa, confining pressures between 15-300 MPa), pore fluid volume, temperature, sample strain, axial stress, and acoustic emissions. Two sequential modes of stressing were imposed that simulate stress levels comparable to those of burial within sedimentary basins: 1. hydrostatic loading achieved by increasing confining pressure at a constant rate; and 2. triaxial deformation for which samples were axially shortened after a target hydrostatic pressure had been reached.

At all temperatures, quartz sand compacts with increasing hydrostatic load by a combination of elastic and inelastic mechanisms. Accelerated acoustic emission and volumetric strain rates occur above a critical pressure (P*), corresponding to macroscopic yield. The results show a reduction of P* as temperature increases (P*~105 MPa at 25 oC, ~95 MPa at 150 oC). During triaxial loading, all samples initially show quasi-elastic deformation followed by a substantial change of volumetric strain rate at yield. Yet, the style of post-yield deformation varies with initial hydrostatic pressure. Samples dilate with low AE rates and minimal grain crushing when triaxially deformed at low effective pressures; and samples compact with high AE rates and considerable grain cracking/crushing at greater pressures. When considered in terms of distortional and mean stresses, the yield strength data for a given temperature define an elliptical envelope consistent with critical state and CAP models from soil mechanics. For the conditions tested, triaxial yield data at low effective pressure are essentially temperature-insensitive whereas yield levels at high effective pressure are lowered as a function of elevated temperature.

The yield data are used to estimate activation energies that can be compared to models in which either pressure solution or subcritical (stress corrosion) cracking is considered to play a role. For the experiment conditions tested, the observations of grain crushing, AE rates, the temperature dependence of cataclastic compaction, and the estimates of activation energy indicate that deformation occurs in a manner consistent with Arrhenius behavior expected for thermally-assisted subcritical crack growth. Taken together, the results indicate that increased stresses and temperatures associated with subsurface burial will significantly alter the yield strength of deforming granular media in systematic and predictable ways. The results have implications for the strength of sediments in a variety of tectonic regimes, as well as for strength and stability of frictional fault zones (assuming that triaxial deformation can be related to shear deformation by accounting for differences in stress states and geometry).

Speaker Biography

Steve Karner is a Research Specialist in the Structure, Petrophysics & Geomechanics team at ExxonMobil's Upstream Research Company. His current technical focus relates chiefly to results derived from laboratory-based geomechanics testing of geologic materials with application to reservoir properties and wellbore analysis.

Steve obtained BSc and BSc(Honours) degrees in geology & geophysics from the Flinders University of South Australia in 1986 and 1987, respectively; a MA degree in geology from Queens College C.U.N.Y. in 1993; and PhD in geophysics from Massachusetts Institute of Technology in 1999. He spent 2 years working in petroleum-related companies in Australia before starting his graduate studies. Prior to his 2006 arrival at ExxonMobil-URC, Steve spent 4 years at Texas A&M University as a post-doc and research scientist followed by 2 years in the geothermal energy program at the Idaho National Laboratory.