GRANTEE: RENSSELAER POLYTECHNIC INSTITUTE
Department of Earth and Environmental Sciences
Troy, New York 12180-3590
GRANT: FGO2-95ER144532
TITLE: Transport Phenomena in Fluid-Bearing Rocks
PERSON IN CHARGE: E.B. Watson (518-276 8838; Fax 518-276 8627; E-mail watsoe@rpi.edu)
Objectives: The objective of this project is to shed light on chemical transport in the Earth through: 1) development and implementation of a technique for measuring mineral solubilities and diffusivities of dissolved mineral components in aqueous fluids at P-T conditions of the Earth's crust and upper mantle; and 2) characterization of the grain-scale permeability of fluid-bearing rocks under conditions of chemical and mechanical equilibrium.
Project Description: Deep in the Earth, fluid-assisted geochemical transport is controlled mainly by: 1) the solubilities of rock components in the fluid of interest; 2) the diffusion characteristics of the dissolved solutes; and 3) the permeability of the rock to fluid flow. (Under circumstances where Pfluid Ptotal and an open fracture system is not sustainable, the permeability of interest is that dictated by the equilibrium microstructure of the rock.) All three of these properties are poorly constrained: Solubility data are scarce at pressures in excess of 1 GPa, and information concerning solute diffusion and rock permeability is virtually nonexistent. The Rensselaer project involves the development and implementation of techniques to characterize these key properties.
For the solubility and diffusion measurements, the principal methodology is experimentation at high pressures (0.5-3.0 GPa) and temperatures (500°-900°C) in a conventional solid-media, piston-cylinder apparatus, using noble-metal capsules (or cells) developed for the purpose. In the permeability study, the piston-cylinder apparatus is used to fabricate metal-jacketed "rock" samples exhibiting near-equilibrium microstructure in the presence of aqueous fluid. After recovery from the piston-cylinder apparatus, the samples are mounted for permeability characterization at room conditions using conventional gas-flow techniques.
Following completion (in FY96) of a study of aqueous SiO2 diffusion (now in press, Contributions to Mineralogy and Petrology), FY97 was dedicated to permeability measurements. For fluid-bearing rocks exhibiting equilibrium microstructure, the permeability (k) is a function of grain size (d), porosity (f) and the extent of pore connectivity (which is determined to some extent by the dihedral angle, q). Two synthetic rock types quartzite and marble were chosen for initial study because of their ease of fabrication and their differing q values. A series of porous, H2O-bearing marbles (q ~ 68°) was prepared at 800°C and 1.0 GPa, in which H2O abundance (hence porosity) ranged from ~0.8 to 19 volume per cent. Similarly, quartzites were fabricated at 850°C and 1.4 GPa with varying amounts (~0.3 - 17 volume %) of both pure H2O (q ~ 44°) and brine (q ~ 38°). Due to differences in initial grain size and synthesis duration, the average grain size of the synthetic rocks was variable, ranging between ~50 and 100 mm. In all cases, however, the grain size distributions resembled those expected of texturally-equilibrated samples.
When normalized to a single average grain size (assuming a d2 dependence of k), the measured permeabilities of the synthetic marbles and quartzites depend systematically upon f, as shown on a logk vs. f plot (Figure 1). For samples with q<60° (i.e, quartzites containing pure H2O or brine), k is insensitive to q. For the marbles (q>60°) the logk vs. f curve is slightly below those of the quartzites, but in this case f®zero for porosities below about 1 volume %. Figure 1 reveals that measured permeabilities resemble those predicted from idealized models, the Blake-Kozeny-Carman relation providing the best representation of the data.
Figure 1. Permeability (k) vs. porosity (f) for (a) quartz + water (filled diamonds) and quartz + brine (open diamonds), and (b) calcite + water (arrow shows porosity of sample of zero permeability). All data and curves normalized to grain diameter of 1 mm (see text). Curves show k-f relations proposed by others: VBW from von Bargen & Waff (1986) for q = 50°; C50 and C70 for q = 50° and 70°, respectively, of M. Cheadle (as cited in McKenzie, 1989 and pers. comm. 1997); MS from measurements of permeability on sintered glass spheres by Maaløe and Scheie (1982); BKZ shows the Blake-Kozeny-Carman relation as discussed in Dullien (1992), modified with constant K set at 500. Inset in (a) shows details at low f where k-f relation breaks down, presumably due to closure of some channels, which increases the "effective" grain size.