DESCRAMBLE Project: Gas Logging While Drilling the Venelle_2 Geothermal Well (Larderello, Italy)

We present the first application of a mud logging service specifically dedicated to gas analysis in the Larderello geothermal area. In the frame of UE-funded project DESCRAMBLE (Drilling in dEep, Super-CRitical AMBient of continentaL Europe), continuous gas logging techniques were applied while deepening down to 2909 m b.g.l. the Venelle_2 well, a non-productive well located in the southern part of the currently exploited geothermal field. Peaks of major (CO2 and H2S) and hydrocarbon gases were detected during the drilling. The presence of a deep crustal component was inferred on the basis of a maximum 3He/4He value of 1.5 Ra. Overall, gas data suggested that no major fracture systems were intersected during the drilling, able to drain fluids from rock volumes possibly acting as reservoirs for supercritical fluids. The gas logging of the Venelle_2 well emerged as an operationally simple and relatively inexpensive technique to obtain important information on the composition of formation gases, and to possibly predict the occurrence of absorption/permeable zones during drilling.