Investigation of a constructive geostatistical method to assimilate unused conventional data during geothermal drilling for hydraulic conductivity profiling

Open geothermal systems, such as standing column wells, can benefit from a thorough characterization of the geological and hydrogeological properties of the fractured rock formation in which it is installed. Methods providing depth-dependent properties typically require additional, and often expensive, field investigations. Therefore, characterizing the vertical distribution of hydrogeological properties in a geothermal borehole requires alternative approaches that provide rapid and cost-effective insights. This study proposes to analyze the information collected routinely during exploratory drilling. One potential data source is flow rates measured at the borehole outlet during air-injection drilling. These measurements, which are relatively inexpensive to acquire during the investigation phase, may provide valuable information on fracture networks that can be used to infer the hydraulic conductivity profile with depth. To this end, a constructive geostatistical dataassimilation technique is used to estimate the hydraulic conductivity of fractured rock as a function of depth, using fluid flow rate measurements obtained during drilling. The method assimilates flow rate data by optimizing the distribution of fracture hydraulic conductivity. A likelihood-based objective function accounts for uncertainties in in-situ observations, enabling a probabilistic interpretation of fracture characteristics. Using the observed flow rate as input, the geostatistical algorithm estimates the depth-dependent hydraulic conductivity profile and identifies major fractures intersected during drilling. These profiles are then used to simulate transfer functions, offering insight into how fracture networks affect geothermal heat transfer. Two demonstrations are presented: a synthetic numerical model constructed from realistic geological constraints, and a real-world application in which flow rates were measured during the investigation phase of a geothermal system project in Mirabel, Quebec, Canada. Results from both cases indicate that flow rates measured at the borehole outlet during air-injection drilling provide some information on the depth-dependent hydraulic conductivity profile and help identify zones of low- and high-permeability fractures.

Mots clés

• Ground heat exchanger
• Standing column well
• Bayesian geostatistical inversion
• Transfer function
• Flow rate assimilation
• Coupled hydro-thermal simulation