Abstract
We use Matlab 3D finite element fluid flow/transport modelling to simulate localized wellbore temperature events of order 0.05–0.1 °C logged in Fennoscandia basement rock at ~1.5 km depths. The temperature events are approximated as steady-state heat transport due to fluid draining from the crust into the wellbore via naturally occurring fracture-connectivity structures. Flow simulation is based on the empirics of spatially-correlated fracture-connectivity fluid flow widely attested by well-log, well-core, and well-production data. Matching model wellbore-centric radial temperature profiles to a 2D analytic expression for steady-state radial heat transport with Peclet number Pe ≡ r0φv0/D (r0 = wellbore radius, v0 = Darcy velocity at r0, φ = ambient porosity, D = rock-water thermal diffusivity), gives Pe ~ 10–15 for fracture-connectivity flow intersecting the well, and Pe ~ 0 for ambient crust. Darcy flow for model Pe ~ 10 at radius ~10 m from the wellbore gives permeability estimate κ ~ 0.02 Darcy for flow driven by differential fluid pressure between least principal crustal stress pore pressure and hydrostatic wellbore pressure. Model temperature event flow permeability κm ~ 0.02 Darcy is related to well-core ambient permeability κ ~ 1 µDarcy by empirical poroperm relation κm ~ κ exp(αmφ) for φ ~ 0.01 and αm ~ 1000. Our modelling of OTN1 wellbore temperature events helps assess the prospect of reactivating fossilized fracture-connectivity flow for EGS permeability stimulation of basement rock.
Highlights
It is not yet understood how to obtain significant quantities of heat energy from the Earth’s deep crystalline rock heat store
To explore how wellbore fluids couple to ambient basement rock crustal fluids, we focus on a series of isolated wellbore temperature-log events of order 0.05–0.1 ◦ C recorded at 1–2 km depths in Fennoscandia granites
Agreement between the model and observed profiles indicate that axial temperature diffusion by conduction combined with radial advective heat transfer via 0.6-m-thick flow structures into a 30 cm-radius wellbore provide reasonable approximations to the diffusion-advection process hypothesized for fracture-connectivity fluid percolation structures observed in Fennoscandia basement rock at depths to 2.5 km
Summary
It is not yet understood how to obtain significant quantities of heat energy from the Earth’s deep crystalline rock heat store. To explore how wellbore fluids couple to ambient basement rock crustal fluids, we focus on a series of isolated wellbore temperature-log events of order 0.05–0.1 ◦ C recorded at 1–2 km depths in Fennoscandia granites. ∝ exp(αφ(x,y,z)) flow heterogeneity do so as a consequence of a reduced crustal deformation energy budget Such fluid-rock interaction energetics may explain the persistence of rock-physical spectral scaling phenomenology S(k) ~ 1/k to the 5–9 km depth crusts recorded in the KTB scientific deep well [24]. The observed naturally occurring 1.5 km-deep wellbore temperature events are direct analogues for wellbore-centric fluid-rock interaction with in situ fracture-connectivity structures at Dm scales relevant to EGS wellbore-to-wellbore flow stimulation in basement rock. OTN1 well-log spatial fluctuation and well-core poroperm data in a wider the KTB well and the Borrowdale volcanics at the UK Sellafield nuclear facility. UK Nirex metamorphic basement data [24,25,26,27,28]
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