Stephan Matthai is a geoscientist and reservoir engineer renowned for his computer-simulation research on subsurface fluid flow and deformation in heterogeneous porous media, with a focus on coupled multiphase flow, geomechanics, and reactive transport in fractured and faulted rocks for geo-energy applications. His team has contributed to projects spanning hydrocarbon extraction, gas storage and geological CO2 sequestration, enhanced geothermal systems, nuclear waste repository safety, and hydrothermal ore deposits. His career has been marked by major industrial collaborations, including leading the development of the Australian Carbon Geo-Sequestration Simulator, contributing to the IEA working group on EOR, directing the Reservoir Engineering Institute at Montanuniversität Leoben, and co-leading a long-running industry consortium on naturally fractured reservoirs at Imperial College London. In 1994, he invented the “Complex Systems Modelling Platform” THMC simulation framework, which evolved into Open-CSMP++, now supporting an international research network and advanced modelling applications in multiphase fracture flow, geothermal systems, and carbon geosequestration.