| Abstract |
For the first time, in the frame of UE-funded project DESCRAMBLE (Drilling in dEep, Super-CRitical AMBient of continentaL Europe), a Mud Logging (ML) service specifically dedicated to gas analysis was used in the geothermal area of Larderello. The aim of the project was the development of novel drilling technologies for a proof-of-concept test of tapping unconventional supercritical geothermal resources. The test site was the Venelle_2 well, a non-productive well in the southern part of the Larderello geothermal field, for the occasion deepened down to 2909 m b.g.l.. The ML unit was equipped with a FID type gas chromatograph able to measure light hydrocarbon gas components from methane to pentane, and a MicroGC TCD type chromatograph able to measure non-hydrocarbons gases such CO2, H2, H2S, N2, O2 and He. Both gas detectors were connected to a constant volume mud gas extractor (CVD) which allowed for an efficient mechanical extraction of gases from the drilling fluid. The gases continuously extracted by the CVD went through a sampling line connected to a Gas Distribution System (GDS) to both detectors for the analysis. The sampling line was also equipped with a dedicated device which allowed to collect gas samples for additional isotopic analysis in the laboratory. All data were stored in specific time/depth database for correlation purposes. Peaks of non-hydrocarbon (CO2, H2, H2S, N2, O2 and He) and hydrocarbon gases (C1, C2, C3, iC4, nC4, iC5, nC5) were detected during the drilling. Spot samples for noble gas isotopic composition were collected for mass spectrometry analysis in concomitance with significant positive anomalies of chemically reactive species. Noble gases (He, Ne, Kr and Xe) composition showed a predominant atmospheric signature, inherently associated with drilling/sampling procedures. The presence of a deep component characterized by R/Ra values up to 1.5 was found. The almost constant and extremely low He contents (at air level) suggested that during the deepening of well, no major fracture systems were intersected, able to drain fluids from rock volumes possibly acting as reservoirs for supercritical fluids. Gas logging of Venelle_2 well emerged as an operationally simple and relatively inexpensive technique to obtain important information on the composition of formation gas, and possibly predict the occurrence of absorption/permeable zones. |