Subhorizontal well architecture and geosteering navigation enhance well performance and reservoir evaluation A field validation

Complex reservoir settings, located in several European tectonised and multilayered – continental rift and sedimentary basin environments, have raised a demand among geothermal operators towards well designs capable of sustaining high productive capacities and prolonged thermal life. The problematic however become more acute when contemplating, lower than anticipated, reservoir permeabilities, which require relevant, preferably innovative, well architectures to be substituted for long prevailing conventional drilling/completion practice. Such issues are addressed by the subhorizontal well (SHW) concept, advocated as a means for mining heat from reservoir systems which would otherwise remain unchallenged, and field validated on a geothermal district heating (GDH) site south of Paris, France, in a stratified carbonate reservoir environment. The extended reach SHW trajectory aims at intercepting over a near 90° (in fact 85 to 95°, dip dependant) inclination the whole of the layered reservoir sequence, thus maximizing drainage area and well productivity. As a result, the concept may be regarded as intermediate between the horizontal and multilateral well architectures currently practiced by the oil industry. First of its kind in geothermal design engineering it however complies with the completion and flow ratings specific to geothermal production standards. The paper highlights the SHW doublet outcome with respect to directional, RSS (Rotary Steerable System) drilling, logging while drilling (LWD) and geochemically (X Ray Fluorescence, XRF and Difractometry, XRD) assisted geosteering and 1 000 m long drain stimulation logging and testing. Extension of the technology to wider lateral investigation of reservoir attributes, assessment of depositional features (diagenetis, micro fracturing) driving reservoir porosity/permeability trends are also discussed.