The use of reactive-transport models for simulating geochemical processes in large-scale thermal subsurface systems is important in the formulation of strategies for geologic carbon sequestration, hydrogen storage, and various others engineering applications. Key geochemical processes include mineral precipitation and dissolution, redox and biogeochemical reactions, and dissolution. When CO2 is injected into the subsurface, it disturbs existing geochemical equilibrium, promoting conditions that induce mineral dissolution, salt precipitation near the wellbore due to water vaporization, and possible dolomitization. Under certain circumstances, these geochemical effects are significant and their accurate representation becomes essential. Reactive-transport models incorporate equations for fluid flow, mass transport, and geochemical reactions to predict the spatial and temporal evolution of mineral saturation states, mineral precipitation/dissolution rates, and other parameters. In the case of thermal CO2 injection, these models must account for the effect of temperature on mineral solubility and precipitation kinetics. For instance, at high temperatures, calcite may precipitate from the brine due to the increase in carbonate-ion activity caused by CO2 dissolution. Conversely, at lower temperatures, silica may precipitate due to the decrease in solubility caused by CO2-induced acidification. We present a reactive-transport model using a thermal compositional simulator and a geochemistry package - Reaktoro. We present a novel framework that allows for a flexible coupling between flow and geochemistry using a reduced conservative species description of the thermodynamic state. The geochemistry module resolves complex geochemical reactions between the mineral assemblage and the species defined in the aqueous phase. Through a carefully crafted transformation matrix, the fully speciated geochemical state is transformed into the reduced variable set used by the flow simulator. This information is used to update the source terms into the conservation equations, thereby computing the pore volume and permeability changes. Due to the sequential nature of this coupling, tailored solution methods can be applied to each system such as sub-timestepping for resolving reaction kinetics and advanced preconditioners and linear solvers for the flow system. We present simulation results on 2D and 3D heterogenous subsurface models that show mineral precipitation and dissolution phenomena with focus on CO2 injection and storage. These include both equilibrium as well as kinetically, Palandri-Kharaka model, driven reactions. We also present results on reactions such as salting out of NaCl around the wellbore, dolomitization and mineralization due to CO2 injection. To show the flexibility of the proposed framework, results are presented on modeling peridotite (mostly olivine and pyroxene) reactions with dissolved CO2.