Developments of acid fracturing technology for enhanced geothermal systems: A review

In Enhanced Geothermal Systems (EGS), heat extraction efficiency can be improved through various reservoir stimulation techniques. Acid fracturing is widely used in the petroleum industry. It combines the advantages of other commonly used stimulation methods to stimulate reservoirs and reduce the risk of induced seismicity effectively. This technique is promising in geothermal reservoirs, especially in carbonate formations. A thorough understanding of the stimulation mechanisms and practical experiences with acid fracturing in EGS will facilitate the application of this technology. This paper initially provides an overview of stimulation strategies in EGS, followed by an introduction to the fundamental principles of acid fracturing. It examines the stimulation mechanisms through three lenses: hydraulic, chemical reaction, and thermal effects. The paper compares acid fracturing with other established stimulation techniques, analyzing the distinct characteristics and applicability of each approach, and summarizing the benefits. Subsequently, it reviews acid fracturing processes applied in geothermal projects or those with potential for future use. A review of typical EGS acid fracturing projects globally is presented, highlighting the specific processes and stimulation outcomes of these projects. Finally, future research directions are presented. The review shows that the stimulation mechanism still needs to be further clarified; numerical simulation studies need to be expanded from conventional to deep and three-dimensional acid fracturing, and mature simulation tools need to be developed; high-performance acid systems applicable to a wider range of lithologies need to be developed; and the role of carbon dioxide in acid fracturing continues to be explored.