Thermal Stimulation of the Rotokawa Andesite: A Laboratory Approach
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Thermal stimulation of geothermal wells is a production enhancement technique that is an attractive option to operators of geothermal fields as a way to enhance and revitalize well performance capabilities through injection of cold water into the geothermal reservoir. This thesis presents a review of thermal stimulation procedures that have been carried out at various geothermal fields worldwide, and then sets out to demonstrate through laboratory experiments the effects of thermal stimulation on typical reservoir rocks.
Thermal damage to crustal rocks is important in many fields of practical engineering applications. Thermal fractures have been discussed in many studies, however their formation under fully water saturated conditions as a result of rapid quenching is not fully understood. In this study, a new methodology is designed to replicate thermal stimulation in such an environment, using an apparatus that allows rocks to be heated to 350°C at up to 22 MPa confining pressure and rapidly quenched with cold water to ambient temperature while maintaining system pressure. The results indicate that through thermal cycling in the apparatus, porosity was increased, density decreased, acoustic velocities attenuated and mechanical properties significantly altered. Maximum damage occurred during the first thermal cycle, a product of the thermo-mechanical Kaiser effect such that rocks should not experience additional damage unless a previous maximum stress is surpassed.
The thesis details a comprehensive evaluation of the Rotokawa Andesite sourced from the Rotokawa Geothermal field located in the Taupo Volcanic Zone, New Zealand. The importance of microstructural fabrics on the physical properties of this reservoir lithology is demonstrated. The mineralogical and petrological fabrics of the rocks are coupled with detailed studies of the microstructural fracture networks, including measurements of porosity, density and permeability. Acoustic wave velocities and dynamic elastic moduli were determined. Uniaxial compressive strength testing coupled with acoustic emission have helped to determine the behavior of the rock under deformation and provided data to characterize the static elastic moduli of the rocks. These data are then utilized to build empirical, micromechanical and geometric relationships.
To better constrain important engineering concerns such as wellbore stability, reservoir forecasting and stimulation procedures, thermal property measurements were carried out on samples recovered from the Rotokawa Andesite. In particular, measurements of linear thermal expansion, thermogravimetric analysis, and differential scanning calorimetry were measured utilizing varied experimental heating rates of 2, 5 and 20 K/min. The property analyses were carried out to determine if heating rates influenced the measurement of thermal properties, specifically thermal expansion coefficients and strain rate in the samples. Results indicate that thermal expansion is not heating rate dependent within the range investigated though the strain rate is significantly dependent on heating rate, with higher strain rates observed in conjunction with higher heating rates. By using a one dimensional stress model, a failure criterion can be established for the Rotokawa Andesite when subject to thermal stressing. The importance of this study is to further understand the critical heating and cooling rates at which thermal stress causes cracking within the Rotokawa reservoir. This can enhance permeability but can also affect wellbore stability, so constraining these conditions can be beneficial to resource utilization.
To test effects of thermal stimulation in the laboratory, Rotokawa Andesite core was heated to 325ºC at pressure of 20 MPa and quenched rapidly to 20ºC while maintaining a pressure of 20 MPa. Permeability increased by an order of magnitude over original pre-treatment values. Ultrasonic velocities also reflected a significant change after stimulation testing. Scanning electron microscopy showed significant microstructural change to samples and supplemented physical property investigations. The results imply that thermal stimulation can be successfully repeated in the laboratory and is coupled with both thermal and chemical components. The results of these investigations are of profound importance for effective utilization and maintenance of the Rotokawa Geothermal field and the results also have implications for geothermal fields worldwide.