Gas sampling and monitoring during Drilling the Ivrea-Verbano zonE (DIVE)

Abstract

<p>The ICDP project "Drilling the Ivrea Verbano zonE (DIVE)" explores the Ivrea Verbano Zone in the Southern Alps of Italy, the probably most complete pre-Permian lower crust–upper mantle transition worldwide, by deep scientific drilling. A first borehole has been completed near the city of Ornavasso in mid-December 2022, reaching a final depth of 578.5 m, with excellent drill core recovery (100%). The drilling was accompanied by various scientific experiments, including the continuous extraction, measurement and sampling of gases from the circulating drilling fluid (On-Line Gas Analysis, OLGA). The gas phase was continuously measured with two gas mass spectrometers (MiniRuedi © and Pfeiffer Omnistar ©) for Ar, H2, He, N2, O2, CH4 and CO2, a gas chromatograph for hydrocarbons (CH4, C2H6, C3H8 and i/n C4H10), and a radon detector for 222Rn. Off-line samples were taken for laboratory analysis on hydrogen and noble gas isotopes to better characterize the origin of the gases including possible artificial hydrogen from the drilling process.<br />Initial results show a correlation between formation gases in drilling mud and the drilled lithology. In addition to the unavoidable input of atmospheric gases in drilling mud, the most non-atmospheric gases extracted from drilling mud are hydrogen (up to ~1.2 vol.-%) and methane (up to ~0.3 vol.-%). Likewise, helium content was sometimes found to be higher than atmospheric. OLGA data match well with the gas composition data of bubbeling gases sampled directly from the wellhead.</p>