82 A new concept of karst development based on hydrogeology and geophysics and a large meadow was used for drilling (Fig. 1). The HES is animated by the IC2MP/Hydrasa team (UMR CNRS 7285) as part of the National Network of Hydrogeological Sites (SNO H+) and the ‘EAUX’ programs of the former PoitouCharentes region (CPER 2002-2006 and 2007-2013). HES description The HES is situated in the middle of the Poitou threshold, in the watershed of the Clain river (Fig. 2). The plateau is above 125 m above sea level, and a little talweg crosses the site. The first drilling campaign (2002) resulted in the completion of eight wells distributed over a wide area, intending to establish a protocol for positioning future boreholes. However, the heterogeneous nature of the formations encountered and the discharge rates observed by air-lift testing did not allow for the development of a drilling strategy based on lithology or fracturing. Consequently, a “five-spot” design was adopted for subsequent boreholes. Widely used in the oil and gas industry to maximize reservoir recovery, the five-spot scheme enhances fluid circulation and reservoir drainage by positioning one central well surrounded by four additional wells located at the corners of a square. At the beginning of summer 2003, a second drilling campaign was initiated to install the experimental system, which was completed at the end of 2004. Following these three phases, the system now provides four distinct mesh sizes: i) 13 meshes with 70-m sides, ii) 5 meshes with 100-m sides, iii) 5 meshes with 140-m sides, and iv) 1 mesh with 210-m sides (Fig. 3). At present, the experimental platform comprises 45 boreholes, including two vertical and two oblique cored holes. The vertical cores (C1 and C2) were drilled to investigate sedimentary heterogeneity and fracturing in the limestones, while the two oblique cores (C4 and C5) targeted potential fault zones (see location on Fig. 2). Figure 1 Aerian view of the HES.
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