Integration of heat-energy recovery and carbon sequestration

Abstract

Cold mixed CO2–water injection into geothermal reservoirs can be used to integrate geothermal-energy production and subsurface CO2 storage. This article studies this process in a 2D geothermal reservoir derived from the Delft Sandstone Member below the city Delft (The Netherlands). In this process, often regions of two-phase flow are connected to regions of single-phase flow. Different systems of equations apply for single-phase and for two-phase regions. For this purpose, we have applied a new and effective solution approach, the so-called “negative saturation” (NegSat) solution approach. The results show that permeability and porosity heterogeneities in a geothermal aquifer significantly influence both heat extraction and CO2 storage. Hence, reservoir characterization plays an important role in assessing the benefit of CO2 storage and energy extraction. The CO2-trapping mechanism is more efficient in the heterogeneous-permeability field because of the larger permeability heterogeneity contrasts and consequently larger capillary effects, compared to the homogeneous-permeability field. In particular, CO2 banks are mainly formed in the highly permeable zones that are surrounded by less permeable zones. However, the existence of non-isolated highly permeable zones for some injected CO2 concentrations leads to earlier breakthrough. Moreover, heterogeneity considerably weakens gravity effects. The frequent occurrence of evaporation and condensation, which is particularly effective close to the bubble point, substantially delays CO2 breakthrough and leads to a larger amount of heat-energy production and CO2 storage. Based on the simulations, it is possible to construct a plot of the recuperated heat energy versus the maximally stored CO2 for a variety of conditions.