| Title | The Geothermal Stimulator: a High-Temperature, High-Pressure Device for Inducing Thermal Fracture in Rocks |
|---|---|
| Authors | Paul A. Siratovich, Marlene Villeneuve, Ben Kennedy, Darren Gravley, Jim Cole and Jonathan Davidson |
| Year | 2013 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Thermal Cracking, Acoustic Velocities, Thermal Stimulation, Porosity, Density, Mechanical Properties, Elastic Properties, Stimulation Technology, Laboratory Testing, Thermal Stress |
| Abstract | Thermal cycling of rock by heating and rapid quenching in water significantly affects its physical, mechanical and elastic properties. Understanding the processes that lead to these changes is relevant to nuclear waste repositories, tunnel excavations and geothermal energy production. In this study we present a novel technique where specially designed equipment simulates the wet, cyclic thermal stimulation processes employed by the conventional geothermal industry. To enhance productivity and injectivity of geothermal wells, operators commonly inject fluids cooler than the reservoir into wells at pressures less than the natural fracture gradient. By thermally stimulating core samples in a pressure vessel capable of attaining 350°C and 24 MPa, we attempt to replicate conditions encountered at depth in geothermal reservoirs during stimulation procedures. We establish baseline physical and acoustic properties, (compressional (Vp) and shear (Vs) acoustic wave velocities), porosity and density, and dynamic elastic modulii. We compare these baseline properties to those of specimens subjected to four different heating and cooling cycles: (1) heated to 300°C and slowly cooled without quenching, (2) heated to 300°C and quenched, (3) heated to 300°C, quenched and repeated, and (4) heated to 300°C quenched and repeated for three cycles. We observed a decrease in acoustic wave velocities and elastic modulii in the thermally treated samples when compared to the baseline samples. The study has shown that microscopic fractures form as rocks are heated and rapidly quenched by water under simulated geothermal reservoir conditions, and those fractures attenuate acoustic velocities and significantly affect the dynamic and static stiffness modulii of the samples. |