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Abstract:
The utilization of subsurface reservoirs for geothermal or aquifer thermal energy storage applications requires the drilling of boreholes, which are part of the site infrastructure. For a successful utilization and implementation of this subsurface infrastructure, it is important to know the hydraulic characteristics of the technically connected reservoir system as reliable and early as possible. Comprehensive hydraulic tests required for this are usually carried out after well completion and first filter developing in order to minimize possible influence of the drilling mud and any alteration of the near wellbore area caused by the drilling operation. This normally requires a temporal decoupling of the first well development and the main hydraulic test which is carried out afterwards. In order to optimize this procedure, a combination of well development and hydraulic testing was performed at the High Temperature Aquifer Thermal Energy Storage (HT-ATES) site in Berlin Adlershof. It is shown that hydraulic parameters, such as transmissibility, permeability, productivity index (PI), and skin factor can also be determined already during the well development. For this purpose, a five- and a two-stage step-rate test were carried out, each with subsequent shut-in phases. The combination of analytical and numerical modelling was employed to analyze the aquifer performance. For the analysis, the concept of radially varying permeability around the borehole was developed to account for the area of influence due to drilling mud infiltration and to determine its transient course. The application of a combination of a classic Pressure Transient Analysis (PTA) together with numerical models enabled reliable characterization of the aquifer. The methodical approach developed herein delivers a permeability of 1.5 to 2.0 D and a PI of 1.1 to 1.2 L s−1 bar−1 for the Hettangian (Jurassic) HT-ATES target aquifer, which is screened between 369 and 387 m True Vertical Depth (TVD). The method enabled to determine the drilling mud influence area of approximately 0.32 m which corresponds to a skin factor of 0.7 to 1.8.
