| Abstract |
The Finnish energy company St1 Deep Heat Ltd is drilling a 6 km deep EGS well doublet 15km west of central Helsinki to pilot test delivering 80oC water to the city of Espoo district heating plant. As of 2018, a 6.1 km deep injection well, OTN-III, has been stimulated for a period of 50 days with injection of 18000 cubic meters of water in five stages over a 1km length of open hole at 5-6km depth. The OTN-III open hole stimulation section was enveloped by stimulation-induced microseismicity that varied episodically in space and time along the well. Moderate clusters appeared above and along the open hole, with a dominant cluster expanding below the open hole. As the fluid injection sequence stepped in time through the five open-hole injection intervals, fluid injected from each the five stages reached all of the microseismicity cluster volumes. The induced microseismic event locations within the OTN-III stimulation clusters are observed to be spatially correlated according to a power-law scaling function, G(r) ~ 1/r^1/2. The observed microseismic event spatial correlation function can be derived from the spatial correlation empirics of crustal rock well-log fluctuation power-spectral scaling and well-core relation between permeability κ and porosity φ, κ ~ exp(αφ), that leads to lognormal well productivity distributions. The logical interpretation of the spatio-temporal distribution of OTN-III induced seismicity is that the embedding basement rock is pervaded by fossilised long-range fracture-connectivity percolation structures that are re-activated by injected fluids. Such re-activated fluid transmission pathways connect all sections of the OTN-III open-hole injection sequence to all parts of the stimulation event clusters. The stimulation data did not, however, provide evidence that re-activated fracture-connectivity percolation permanently enhanced crustal permeability as required for EGS stimulation. In 2019, the present 3.3km deep companion well, OTN-II, will be drilled in relation to OTN-III with intent to, first, stimulate OTN-II well as per the 2018 OTN-III stimulation, and second, to generate zones of stable interwell permeability enhancement at suitable points along the well-pair. We here numerically simulate the observed fluid injection process to explore how to create stable permeability enhancement flow structures between wellbore doublets. The computational framework is defined by crustal flow property empirics: (i) spatially-correlated porosity constrained by well-log spectral scaling, S(k) ~ 1/k; (ii) well-core poroperm spatial correlation κ ~ exp(αφ) for appropriate empirical values of parameter α; and (iii) induced seismicity spatial correlation function G(r) ~ 1/r^1/2. We compute the degree and method by which doublet well-pairs can be pressurised in order for the fluid pressure to focus effectively on the interwell crustal volume. The combined stress action of two wellbore pressures can re-activate interwell flow channels preferentially to all other wellbore-centric radial flow directions. Attending and responding to the details of wellbore-centric flow coupling to observed strongly flow-heterogeneous basement rock represents a major component of the Deep Heat EGS project. |