Experimental and Numerical Modelling of Thermal Stimulation in Geothermal Core Studies

Villeneuve, M. C. (University of Canterbury) | Siratovich, P. A. (University of Canterbury)

OnePetro 

Abstract

We present an integrated analysis of laboratory-based thermally induced fracturing and results of numerical models to give insight to the role of thermal shock fracture on geothermal reservoir rocks. Thermal stimulation is a reservoir permeability enhancement technique applied to commercial geothermal reservoir rocks to enhance fluid injection capabilities for spent power plant working fluids. The process is well known to enhance permeability but the thermodynamic and physical constraints of the process are less certain. In an attempt to constrain the interaction and the ideal conditions that lead to permeability enhancement, experimental procedures were carried out to mimic the conditions that a reservoir rock would experience during a thermal stimulation using temperature differentials ranging from 50-300°C. Samples underwent such thermal gradients under controlled laboratory conditions and were characterized for the changes to permeability, porosity, ultrasonic velocities, dynamic elastic moduli and petrological changes. The thermal stimulation was simulated in a FLAC2D thermal numerical model to investigate the nature of the thermally induced changes in the sample. The development of this model also allows us to investigate the relationship between geological characteristics and the ability to thermally stimulate any type of rock. The results indicate that numerical thermal shocking experiments are corroborated by laboratory-based results. The implication of this study is that the refined numerical models present an insight to the conditions and constraints under which thermal stimulation can prove to enhance permeability that could not be gained through purely laboratory-based studies.

Introduction

Thermal cracking of rocks is a process that can enhance permeability, reduce strength and have a significant effect on the competency of rocks in engineering applications. Thermal cracking as a result of induced temperature gradients in rock needs to be a consideration when rocks have undergone thermal stress in environments such as nuclear waste repositories, stone structures subjected to fires and geothermal reservoir rocks. Unlike the latter examples where thermal gradients can compromise structural integrity, geothermal reservoirs can benefit from thermal cracking as a permeability enhancement technique. The application of thermal stimulation to geothermal reservoirs has been shown to help improve the output and injection capacity of many geothermal wells worldwide (e.g. Axelsson and Thórhallsson, 2009; Grant et al., 2013) The process generally involves injection of water into wellbores that is cooler than the geothermal reservoir and through a combination of contraction and thermal cracking, permeability is enhanced. However, the processes that constrain successful stimulation are less well constrained and require both laboratory and numerical simulation to attempt to understand the underlying mechanisms.