Immense potential for providing clean, sustainable energy through geothermal lies in producing heat from enhanced geothermal systems (EGS) reservoirs that exist several kilometers deep in the subsurface. However, in order to develop these reservoirs, we must overcome two challenges. The first challenge is that EGS reservoirs are often dominated by tight (low porosity, Φ, and low permeability, k) crystalline rocks which require stimulation to increase k. While hydraulic fracturing is currently the primary stimulation method, there is great interest in developing complementary stimulation practices that can facilitate stimulation or can enhance the safety of stimulation. ‘Thermal shocking’ is one innovative stimulation method which involves cyclically injecting surface water at pressures lower than those used in hydraulic fracturing. The surface water is relatively cooler than hot reservoir rocks, and a thermal gradient is created as the water reaches the hot rocks’ mineral crystals. The crystals undergo a sudden thermal change, contract or expand in response to induced thermal stresses, and generate thermal cracks that serve as fluid flow channels. Injection is often paused allowing the reservoir to reheat, and additional thermal cracking is induced with more injections or ‘cycles’. Because this is a relatively new technique, little is known of how to optimize the effectiveness of thermal shock stimulation. Here, we present time-lapse k measurements taken under reservoir conditions to investigate whether the number of thermal shock cycles can be optimized for increasing flow in three unique, tight lithologies relevant to EGS: granodiorite, basalt, and carbonate. The second challenge in developing EGS is that reservoir operators must remotely monitor subsurface processes, often using geophysical methods like mapping changes in acoustic (seismic) P- and S-wave velocities (Vp and Vs respectively). Although permeability enhancing microcracks create more productive reservoirs, they are very difficult to map with techniques like seismic waves. This is because we are far from understanding mechanisms behind velocity changes in tight rocks containing pore spaces that exist as subtle, thin, microcracks. Furthermore, little is known about how to optimize repeated thermal shocking using seismic data interpretations. In this work, we use time-lapse Vp and Vs measurements, taken under reservoir conditions, to assess the impact of multiple cycles on the three tight lithologies of interest. Our measurements will inform under which reservoir conditions seismic is useful for detecting the influence of multiple cycles. In previous studies, our time-lapse k, Vp and Vs measurements have been conducted on all three lithologies given a single stimulation cycle. We found that for a single cycle, lithology greatly dictated the extent of k increase, and thermal cracking could be detected through a reduction in Vp and Vs at low effective pressure (Peff). We also conducted preliminary time-lapse k measurements of a carbonate after several thermal shock cycles which revealed no more than two cycles are needed to reach a maximum k value. For this workshop, we will extend our repeated thermal shock cycle study to include the basalt and granodiorite lithologies, and also integrate time-lapse Vp and Vs measurements under reservoir conditions. Ultimately, our experiments will contribute to a dataset which will help guide best practices for optimizing safe, sustainable EGS reservoir stimulation with thermal shocking.

Press the Back button in your browser, or search again.

Copyright 2024, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.