ABSTRACT:

We report on hydrostatic and triaxial hydrothermal deformation experiments of water-saturated St Peter quartz sand at temperatures between 24 and 225 °C. At all conditions, quartz sand deforms by a combination of elastic and inelastic mechanisms. During triaxial loading, all samples initially show quasi-elastic deformation followed by a substantial change of volumetric strain rate at yield. Samples dilate with low acoustic emission (AE) rates and minimal grain crushing when deformed at low effective pressures, and compact with high AE rates and substantial grain cracking/crushing at greater pressures. Yield strength data for a given temperature define an elliptical envelope consistent with critical state and CAP models from soil mechanics. Yield strength at low effective pressure is temperature-insensitive whereas yield strength at high effective pressure is lowered at elevated temperature. Deformation is consistent with the Arrhenius behavior expected for thermally-activated subcritical crack growth, as are activation energies estimated using the high pressure, temperature-sensitive yield data. The results indicate that increased stresses and temperatures associated with subsurface burial to the depths associated with currently productive hydrocarbon reservoirs are sufficient to significantly alter the yield strength of deforming granular media in systematic and predictable ways. Hence, models for reservoir deformation should take thermally-activated damage mechanisms into account.

1. INTRODUCTION

Deformation in granular media has received significant attention from researchers in a multitude of disciplines. To date, much of the hydrocarbon industry interest in granular mechanics has related to deformation impacting site evaluation, wellbore stability and performance of clastic reservoirs related to fluid extraction and injection (see [1] for a comprehensive description). Typically, these applications use strength and elastic properties gathered from room-temperature experiments designed to investigate the stress- and rate-dependence of elastic properties and yield behavior of clastic deposits in sedimentary basins [e.g. 2-5]. Limiting laboratory studies to room-temperature conditions is often regarded as being sufficient for industry purposes due to the fact that present-day temperatures in a majority of siliciclastic hydrocarbon reservoirs are moderately low (<125 °C). At these conditions, the reaction kinetics of fluid-rock systems may be sufficiently slow to have little impact on deformation behavior when considered in terms of the lifetime of a field (~ 5-30 years, see [6]). Recently, however, the hydrocarbon industry has faced significant challenges related to reservoir stresses and temperature brought about by the need to explore for hydrocarbons at deeper levels in sedimentary basins, or in areas with higher than average geothermal gradients (see Figure 1). While a few laboratory studies address the role of waterrock reactions on strength and flow properties of granular rocks [e.g. 7-13], the relative paucity of existing literature detailing hydrothermal deformation in porous media leaves many questions unresolved. To study the impact of elevated temperature on the yield behavior of quartzose reservoirs, we performed a suite of triaxial and hydrostatic experiments on St Peter quartz sand. The testing program was explicitly designed to explore the thermal sensitivity of yield behavior associated with critical state and CAP plasticity models historically generated by soil mechanics work [e.g. 14].

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