Hydraulic Fractures Are Pressure Relief Valves

Abstract

Sometimes, it is helpful to question and revisit our conceptual understanding of how processes work rather than rely on complex coupled simulations that incorporate ever-more physics and complexity. This is especially true for hydraulic fracturing because it involves a system with exceptionally high uncertainty which then limits the utility of having precise physics calculations. It is also true when the application scope changes for the technology, such as transitioning from oil and gas (O&G) to enhanced geothermal systems (EGS). Conceptually, hydraulic fracture initiation and extension requires injecting fluid into a well at a sufficiently high rate that the pressure reaches the limit required for tensile opening. If the injection rate increases beyond this limit, the typical assumption will be that pressure will continue to rise. However, the actual behavior is more interesting and potentially quite useful for geothermal applications. In this study, we elucidate how injection pressures behave as flow rate is increased from zero to ludicrous speed to show how hydraulic fractures can be thought of as functionally akin to pressure relief valves. We then discuss how this concept can help to understand fracture behavior in EGS and what this could mean for its technological viability.