| Abstract |
A growing demand is raising among geothermal operators towards innovative well architectures, addressing complex, tectonised and multilayred, reservoir settings and thermochemically sensitive fluid environments, capable of sustaining high productive capacities and prolonged thermal life. These issues, which become particularly acute when contemplating lower than anticipated reservoir performance and hostile fluids thermochemistries, are being addressed by recently implemented well designs in the areas of subhorizontal and multiradial well trajectories, anti corrosion fiberglass lined wells and dual completions are presented and discussed in this paper. The subhorizontal well (SHW) concept has been field validated on a geothermal district heating (GDH) site South of Paris, France, in a stratified, carbonate platform, reservoir structure. The extended reach SHW trajectories intercepted, over a near 90° (in fact 85 to 95°, dip dependant) inclination, the whole of the layered reservoir sequence. First of its kind in geothermal design engineering, the concept may be regarded as intermediate between the horizontal and multilateral well architectures currently practiced by the oil industry. The paper highlights the SHW doublet outcome with respect to directional, RSS (Rotary Steerable System) drilling, logging while drilling (LWD), geochemically (X Ray Fluorescence, XRF and Difractometry, XRD) assisted geosteering and, 1 000 m long, drain stimulation, logging and testing. Results are discussed in the light of upgraded well/reservoir performance issues, extended to a comprehensive review of a carbonate platform lithofacies, diagenetic and microfracturing trends. Initially designed as a fall back, sidetracked substitute, to a SHW failure, the multiradial well (MRW) concept was further developed as a candidate architecture in areas where space restrictions would constrain the implementation of extended reach (sub)horizontal drains. As a result, it should be regarded, in the well architecture typology, as the multilateral equivalent of (sub)horizontal wells. Fibreglass reinforced epoxy resin tubulars, long regarded as a (composite) material solution to corrosion damage, a sensitive issue in the Paris Basin extensively developed carbonate reservoir and corrosive aqueous CO2/H2S thermochemistry, have been poorly developed in spite of its structural advantage. The design advocated in this paper, which combines both mixed steel cased propping assembly and a non cemented (annulus free) production liner accommodating an, ESP sustained, artificial lift pumping chamber, is an extension of a former well completed in the mid 1980s limited to self flowing (artesian lift) production. The new well architecture has been successfully completed in the fall of 2018, South of Paris. Last but not least, two dual completion well candidates are presented, which combine either a single mixed, bi-aquifer, production, applying single drift expandable liner technology in a deep seated reservoir or separate production of two, non mixable fluids, medium depth sandy aquifers. Economic aspects are analysed with a view to standardizing the process in geothermal engineering and future undertakings. |