Self‐Organizing Fluid Convection Patterns in an en Echelon Fault Array

• Published in: Geophysical research letters 2018-05, Vol.45 (10), p.4799-4808
• Main Authors: Patterson, James W., Driesner, Thomas, Matthai, Stephan K.
• Format: Article
• Language: English
• Subjects: Cold water; Computational fluid dynamics; Computer simulation; Convection; Convection cells; Convection cooling; Convection patterns; Cooling; Crack initiation; en echelon; Enthalpy; faults; Flow rates; Fluid flow; Fracture mechanics; Fracture permeability; Fractures; Free convection; geothermal; Geothermal energy; Heat; Heat flow; Heat transfer; Heat transmission; Heating; Heating and cooling; Heterogeneity; Numerical simulations; Organizations; Permeability; Rocks; Site selection; Synchronism; Synchronization; Temperature changes; Temperature distribution; Temperature fields; Transmissivity; Water circulation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL078271

We present three‐dimensional numerical simulations of natural convection in buried, vertical en echelon faults in impermeable host rock. Despite the fractures being hydraulically disconnected, convection within each fracture alters the temperature field in the surrounding host rock, altering convection in neighboring fractures. This leads to self‐organization of coherent patterns of upward/downward flow and heating/cooling of the host rock spanning the entire fault array. This “synchronization” effect occurs when fracture spacing is less than the width of convection cells within the fractures, which is controlled by fracture transmissivity (permeability times thickness) and heterogeneity. Narrow fracture spacing and synchronization enhance convective fluid flow within fractures and cause convection to initiate earlier, even lowering the critical transmissivity necessary for convection initiation. Heat flow through the en echelon region, however, is enhanced only in low‐transmissivity fractures, while heat flow in high‐permeability fractures is reduced due to thermal interference between fractures. Plain Language Summary Fractures and faults in the subsurface may, under the right conditions, allow water to naturally circulate, with hot water rising and cold water sinking. This water circulation heats or cools the host rock within some distance from the fracture. We show that the resulting temperature change affects water circulation in nearby fractures and may form large‐scale patterns of heating and cooling, even when the fractures are disconnected. The primary factors controlling this interaction are the fractures' ability to facilitate fluid flow (permeability) and the spacing between them. Fluid flow rates increase when fractures are closely spaced, but total heat flow through the fractures may increase or decrease, depending on fracture permeability. Understanding this behavior helps us to better understand how fluid moves underground and may aid site selection for future geothermal energy projects. Key Points Convection self‐organizes into large‐scale patterns across closely spaced fractures despite lack of hydraulic connectivity Narrower fault spacing enhances convection but reduces total heat flow through high‐transmissivity fractures Critical fault permeability required for convection initiation is reduced as fracture spacing narrows