Impact of Reservoir Heterogeneity on CO2-Plume Geothermal Performance in the Upper Jurassic Malm Carbonate Aquifer: A Stochastic Numerical Assessment

CO2 Plume Geothermal (CPG) offers a dual-benefit strategy combining geothermal heat extraction with geological CO2 storage in deep saline aquifers. However, reservoir heterogeneity impacts remain poorly quantified in carbonate reservoirs where facies architecture and diagenetic modification control permeability. This study quantifies the influence of reservoir heterogeneity on CPG performance in the Upper Jurassic Malm carbonate aquifer of the South German Molasse Basin over a 30-year operational period. Two-dimensional thermo-hydraulic simulations compare a homogeneous reference model with stochastic heterogeneous ensembles across four well spacings and a range of geostatistical parameters. Well spacing emerges as the dominant control on cumulative thermal recovery, with 400 m providing the highest median and worst-case recovery under heterogeneous conditions. Relative to the homogeneous baseline, heterogeneity reduces mean cumulative thermal recovery by 13.7–21.9% through preferential CO2 flow along high-permeability grainstone pathways and reduced effective reservoir sweep. Despite this penalty, heterogeneity delays thermal breakthrough at close spacing by up to 33.1%, whereas at wider spacing preferential inter-well pathways can accelerate breakthrough. Cumulative recovery exhibits substantial stochastic variability, with coefficients of variation ranging from 43.7–58.7%. Heterogeneity also reduces net CO2 storage by 9.6–14.8%, while storage increases with well spacing, demonstrating that thermal recovery and storage are not co-optimized at a single layout. The results provide probabilistic performance bounds for pre-feasibility assessment of CPG in matrix-dominated heterogeneous carbonate reservoirs.