Effect of water on brittle creep of Indiana limestone

Abstract

<p>The effect of water on time-dependent deformation (creep) behaviour in intact limestone is not yet fully understood. Using a triaxial deformation apparatus, we investigated creep of an allochemical limestone (Indiana) using three different pore fluid chemistries: standard demineralized water, a saturated CaCO<sub>3</sub> solution and Argon gas (i.e. in the absence of water). Room temperature experiments are performed at 10 MPa effective confining pressure and a pore fluid pressure of 20 MPa. For each pore fluid type, a reference triaxial test was performed at 10 MPa effective confining pressure, using a constant strain rate of 10<sup>-5</sup> s<sup>-1</sup>,<sub></sub>to ascertain the failure stress for samples with this porosity under these conditions. To investigate the stress-dependence of creep, samples of a narrow porosity band are loaded to 75, 80 or 90% of the failure stress, after which the stress is held constant and we monitor the subsequent time-dependent deformation. We use a combination of axial and volumetric strain measurements and acoustic monitoring. The porosities of our sample sets were chosen to enable comparison between the strain rates obtained of samples loaded to different stresses for each pore fluid type, where the porosities were 16.25 +/- 0.15 %, 17.25 +/- 0.05%, and 16.6 +/- 0.15 % for demineralized water, CaCO<sub>3</sub>-saturated solution and Argon respectively.</p><p> </p><p>Our initial results indicate that creep rates of samples saturated with demineralized water and saturated CaCO<sub>3</sub> solution are 10<sup>-7</sup> – 10<sup>-9</sup> s<sup>-1</sup>. Creep rate depends exponentially on stress, with an exponent of ~8.2 for demineralized water and ~9.6 for CaCO<sub>3</sub> saturated solution. Such high exponents are generally associated with brittle creep processes, though using a microstructural model for pressure solution indicates that solution transfer processes will lead to creep rates of 10<sup>-3</sup>-10<sup>-11</sup> s<sup>-1</sup> under these conditions for grain sizes between 0.1 and 100µm, which is roughly the range of grain sizes present in Indiana limestone. An initial experiment with Argon shows a decrease of ~ 1 order magnitude in creep rate compared to an experiment with water at a similar stress. Our preliminary results imply that Indiana limestone deforms via a combination of fluid-assisted solution-transfer and brittle creep processes, however more Argon experiments and microstructural analyses are required to confirm the rate-controlling deformation mechanisms.</p>