Thermal effect on the physical properties of carbonate rocks

[1] Detailed experimental studies of the development of permeability of crustal rock during deformation are essential in helping to understand fault mechanics and constrain larger-scale models that predict bulk fluid flow within the crust. Permeability is particularly enhanced in the damage zone of faults, where microfracture damage accumulates under stress less than that required for macroscopic failure. Experiments performed in the prefailure region can provide data directly applicable to these zones of microfracture damage surrounding faults. The strength, permeability, and pore fluid volume evolution of initially intact crystalline rocks (Cerro Cristales granodiorite and Westerly granite) under increasing differential load leading to macroscopic failure has been determined at water pore pressures of 50 MPa and varying effective pressures from 10 to 50 MPa. Permeability is seen to increase by up to, and over, 2 orders of magnitude prior to macroscopic failure, with the greatest increase seen at lowest effective pressures. Postfailure permeability is shown to be over 3 orders of magnitude higher than initial intact permeabilities and approaches the lower limit of predicted in situ bulk crustal permeabilities. Increasing amplitude cyclic loading tests show permeability-stress hysteresis, with high permeabilities maintained as differential stress is reduced and the greatest permeability increases are seen between 90 and 99% of the failure stress. Prefailure permeabilities are nearly 7 to 9 orders of magnitude lower than that predicted by some high-pressure diffusive models suggesting that if these models are correct, microfracture matrix flow cannot dominate, and that bulk fluid flow must be dominated by larger-scale structures such as macrofractures. We present a model, based on our data, in which the permeability of a highly stressed fault tip process zone in low-permeability crystalline rocks increases by more than 2 orders of magnitude. Stress reduction related to the onward migration of the fault tip close damage zone cracks, while some permeability is maintained due to hysteresis from permanent microfracture damage.

/pdf/experimental-measurements-of-permeability-evolution-during-4y7a8inhnr.pdf

Experimental measurements of permeability evolution during triaxial compression of initially intact crystalline rocks and implications for fluid flow in fault zones

The reliable assessment of volcanic unrest must rest on an understanding of the rocks that form the edifice. It is their microstructure that dictates their physical properties and mechanical behavior and thus the response of the edifice to stress perturbations during unrest. We evaluate the interplay between microstructure and rock properties for a suite of edifice-forming rocks from Volcan de Colima (Mexico). Microstructural analyses expose (1) a pervasive, isotropic microcrack network, (2) a high, subspherical vesicle density, and (3) a wide vesicle size distribution. This complex microstructure severely impacts their physical and mechanical properties. In detail, porosities are high and range from 8 to 29%. As a consequence, elastic wave velocities, Youngs moduli, and uniaxial compressive strengths are low, and permeabilities are high. All of the rock properties demonstrate a wide range. For example, strength decreases by a factor of 8 and permeability increases by 4 orders of magnitude over the porosity range. Below a porosity of 11–14%, the permeability-porosity trend follows a power law with a much higher exponent. Microstructurally, this represents a critical vesicle content that efficiently connects the microcrack population and permits a much more direct path through the sample, rather than restricting flow to long and tortuous microcracks. Values of tortuosity inferred from the Kozeny-Carman permeability model support this hypothesis. However, we find that the complex microstructure precludes a complete description of their mechanical behavior through micromechanical modeling. We urge that the findings of this study be considered in volcanic hazard assessments at andesitic stratovolcanoes.

Microstructural controls on the physical and mechanical properties of edifice‐forming andesites at Volcán de Colima, Mexico

Quantification of microcrack characteristics and implications for stiffness and strength of granite

High temperature may lead to the development of new microcracks or growth of pre-existing microcracks within granite, varying its physical and mechanical properties. Experiments were conducted to study the evolution of the physical and mechanical properties of granite specimens from room temperature to 800 °C. The specimens were heated in heating furnace and uniaxial compression tests were done using MTS servo-controlled testing machine. The results indicate five phases in the variation of physical and mechanical properties with temperature: from room temperature to 100, 100–300, 300–400, 400–600, and 600–800 °C. The first phase corresponds to the vaporization-escaping interval of adhered water, bound water, and structural water. Larger changes of physical and mechanical parameters in the temperature range of 300–600 °C, mostly 400–600 °C, are probably caused by the transition from the brittle state to plasticity (or ductility) of granite, and 400 °C may be a critical threshold of its thermal damage. These results confirm the important link among physical and mechanical properties in response to thermal treatment.

Thermal damage pattern and thresholds of granite

Thermal cracking experiments have been conducted to investigate the role of anisotropic thermal expansion of the constituent minerals of a basalt, a microgabbro and a microgranite in changing the physical properties of the rocks. Samples of an Icelandic basalt, a microgabbro from Sweden, and a fine-grained microgranite from Ailsa Craig, Scotland, have been selected for these experiments. Samples were heated to a range of peak temperatures up to 800°C at ambient pressure. Thermal cracking was monitored through acoustic emission and elastic wave velocity measurements. Fluid permeability of the basalt was measured under hydrostatic conditions at elevated confining pressure using the steady state flow technique.

Acoustic emission and fluid permeability measurements on thermally cracked rocks

Hydrostatic and triaxial compression experiments have been conducted to investigate the evolution of fluid permeability and fluid storage capacity in Tennessee sandstone, a low-porosity 'tight' reservoir sandstone. A newly-developed single-ended transient pulse permeability measurement technique has been used for this study. Under hydrostatic stress, both permeability and specific storage are demonstrated to be dependent upon effective confining pressure. The evolution of axial permeability as a function of increasing compressive deviatoric axial stress has been investigated in triaxial compression experiments. All samples were deformed within the brittle faulting regime. Both permeability and specific storage decrease during compaction, and subsequently increase when axially-aligned dilatant microcracking becomes dominant. Brittle failure is sometimes accompanied by a transient increase in permeability by as much as 1.5 orders of magnitude. Following brittle failure the permeability is controlled by the properties of the fault rock. Permeability decreases as stable sliding occurs on the fault up to large axial strains, such that the fault may act as an impermeable seal to migrating fluids.

G.M.J. Keaney

Papers

Acoustic emission and fluid permeability measurements on thermally cracked rocks

Laboratory Study of Permeability Evolution in a Tight' Sandstone under Non-Hydrostatic Stress Conditions