Abstract

Many processes in subsurface applications result in heating or cooling operation. Processes such as steam flooding, thermally induced fracturing, perforation, deep disposal of nuclear waste, and extraction of energy from geothermal reservoirs can impose a thermal field in geological reservoirs. That is, heat flux can induce stress in the rock and can have a pronounced effect on the physical and mechanical properties of rock bodies. Many researchers are exploring this method for fracturing applications. Either applying a thermal shock due to cooling the formation or imposing an instantaneous heating using plasma jet technology. In this study, data were collected from the literature for physical and mechanical properties of sandstones that were subjected to different modes of thermal treatment. The objective is to investigate the impact of different thermal fields on sandstone properties. The main properties that were considered under this study were permeability, Young's Modulus, and unconfined compressive strength. The data were normalized to obtain fair comparison and to understand the general trend at which those properties change as a function of temperature. Overall, the data shows that the permeability of sandstone increases when a cooling mode of treatment is imposed. Furthermore, sandstones subjected to thermal shock with Nitrogen shows a higher rate of increase in permeability. On the other hand, heating decreases the permeability of sandstone. Cooling reduces the unconfined compressive strength and Young's modulus, while heating resulted in inconclusive results.

Introduction

Applying a sudden thermal gradient can induce thermal stress in reservoir rock and open cracks in the formation. Rock as other material will experience a volumetric strain as a result of heating/cooling, as shown in Fig.1. Several studies were performed to evaluate the effect of thermal treatment in rock properties. An essential step before applying thermally induced fracturing or any subsurface processes that involve thermal alteration is to study the effect of heat on basic rock mechanical properties. Usually, the temperature effect is ignored in the analysis of hydraulic fracturing, but if the induced thermal gradient is large, then this effect has to be considered with respect to the mechanical and physical properties.

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