# Well-Doublets: A First-Order Assessment of Geothermal SedHeat Systems
## 4 Heat-and Flow-Transport Challenges
### Reservoir Geological Aspects  
A first-order factor in the productivity of a geothermal SedHeat well array is the stratigraphy and flow properties of the reservoir. Figure 8 is a representation of a doublet in a typical SedHeat situation. The target stratum is bounded by impermeable strata: U shale is the upper advective flow boundary, L shale is the lower advective flow boundary. Given typical permeability contrasts in geological media, it is usually assumed that the SedHeat fluid reservoir is bounded by impermeable strata, such that advective flow is confined to the more permeable intervals. In litharenite, SedHeat reservoirs (sand-silt-shale), the productive interval contains granular beds (sandstones) of differing permeability; the most common distribution is to find higher values in the lower beds, grading upward to lower permeability in the uppermost beds (also characteristic of transgressive sedimentary regimes). Commonly, there are intraformational low-permeability strata, deposited under more quiescent flow conditions, called silts or shales; they are of much lower permeability than the low k sandstone, usually by more than two orders of magnitude. Furthermore, because of processes in the original depositional environment, these low-permeability strata may be continuous at the doublet spacing scale (shale C), or they may be laterally discontinuous (shale A and shale B). The complete disposition of the various strata between the doublet pair is difficult to specify, even with excellent seismic investigation; usually, stratigraphic details are known only from the geophysical logs available from the production and injection wells. \begin{table}
\begin{tabular}{c c} \hline \hline
**Human Controlled Parameters** & **Natural Subsurface Physical Parameters** \\ \hline Double well spacing & Porosity of aquifer \\ Double distance & Permeability of aquifer \\ Flow rate & Conductivity of aquifer \\ Production recovery cycle & Specific heat capacity \\ Well types (horizontal/vertical) & Specific heat capacity of the brine \\ Re-injection water temperature & Density of brine \\  & Initial aquifer temperature \\  & Aquifier thickness \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Effective parameters for heat production from a doublet geothermal system. One may assume that in the stratigraphic disposition shown, the permeability of the lowermost sandstone is several times that of the central sandstone, which in turn is several times that of the upper sandstone (a permeability range for the sandstones might be from 1 D to 0.1 D). The silty shale interbeds (labeled \"shale\") have permeability values less than 0.001 D, so they may be considered functionally impermeable in the context of advective fluid flux. Thus, heat in these strata (water and mineral phases) can migrate to the sandstone only by conductive heat transport, and is thence taken to the production well advectively (blue arrows). Similarly, the heat in the bounding strata (U-, L-shale) can only flow conductively and be exploited when the advectively flowing fluid in the sandstone drops below the original temperature. Another issue in a natural, unbounded reservoir is that flow is not constrained by lateral no-flow boundaries. Assuming an original pressure P\({}_{0}\) and similar pressure build-up and draw-down cones around each well (blue line), part of the injected fluid will dissipate into the farfield to the right, driven by the positive \(\Delta\)P, and at the production well, part of the warm fluids will come from the farfield to the left, driven by the pressure sink -\(\Delta\)P. The heterogeneity that is characteristic of all sedimentary rock geothermal reservoirs invariably decreases the energy recovery and the energy recovery rate. The most optimistic recovery scenario is the isotropic, constant permeability case, with the further assumption that the local permeability is not impaired by geochemical processes or fine-grained solids migration into the reservoir.