Stress-Sensitive Hollow Proppants for Enhanced Heat Transfer in Closed-Loop Geothermal Systems

Closed-loop geothermal systems (CLGS) face a persistent bottleneck in heat extraction due to the absence of forced convection. Fractured-CLGS was proposed as solution. This study introduces hollow metallic proppants as a new class of heat-transfer media and evaluates their stress-dependent performance using a decoupled discrete element, finite element, and finite volume modeling approach. Results show that copper, aluminum, brass, and bronze consistently outperform sand, with thermal conductivity two to ten times higher across closure stresses up to 144 MPa. Unlike conventional sand or ceramic proppants that maintain static thermal conductivity, metallic proppants exhibit stress-sensitive conductivity. As fractures close, particle rearrangement and contact growth establish thermally conductive bridges that enhance lateral heat transfer. Comparative analysis further reveals trade-offs among thermal conductivity, density, cost, corrosion resistance, and mechanical strength. Copper achieves the highest conductivity but at high density and cost. Aluminum offers a balanced performance with efficient placement. Brass and bronze provide durability in chemically aggressive environments. Stress-sensitive hollow metallic proppants therefore offer a practical pathway to extend the efficiency and longevity of CLGS.