Multi-fluid geothermal energy production and storage in stratigraphic reservoirs

Stratigraphic reservoirs (in sedimentary formations) are an under-explored play concept because they are typically associated with a conductive thermal regime, requiring greater depths to reach economic temperatures than hydrothermal upflows. On the other hand, stratigraphic reservoirs offer the advantages of higher permeability (and transmissivity), extending over much larger areas (> 100 km²) than typical upflows (< 3 km ²), and have lower, predictable drilling risk. These make an attractive target for geothermal development, but several challenges need to be addressed. The primary challenge is to maximize heat extraction, while minimizing drilling and extraction costs. To increase extraction efficiency, we propose injecting supplemental fluids (CO₂ and/or N₂) to augment reservoir pressure, thereby enhancing fluid production rates. Because N₂ can be readily separated from air, pressure augmentation can occur during periods of low grid power demand, which will reduce costs and enable energy storage. A well pattern consisting of a minimum of four concentric rings of horizontal producers and injectors is proposed to conserve pressure from injection operations, minimize loss of supplemental fluids, generate large artesian flow rates that take advantage of the large productivity of horizontal wells, and segregate the supplemental fluid and brine production zones. We present simulations of this approach for an idealized reservoir model, consisting of a relatively permeable sedimentary formation, vertically confined by two impermeable seal units. More realistic (heterogeneous) geologic settings and wellbore flow effects will be considered in future studies to more rigorously evaluate the potential economic advantages of this approach.