Continuous/discrete geochemical monitoring of CO_2 Natural Analogues and of Diffuse Degassing Structures (DDS): hints for CO_2 storage sites geochemical monitoring protocol

摘要

Italy is one of the most promising prone areas to study the CO_2 behavior underground, the caprock integrity to the CO_2 leakage, mostly in presence of pervious/geochemically active faults, due to a wide availability of CO_2 rich reservoirs at a depth between 1 and 10 km, as highlighted by recent literature. These deep CO_2 reservoirs generate at least 200 leakage areas at surface throughout Italy which have been defined "Diffuse Degassing Structures" (DDS) by INGV. These are widely studied by INGV institutionally by a long term convention with the Civil Protection Department (DPC) with the aim to catalog, monitor and assess the Natural Gas Hazard (NGH, namely the probability of an area to become a site of poisonous peri – volcanic gas exhalation from soils). More than 150 researcher of INGV are involved in monitoring areas affected by the CO_2 presence underground and at surface, by continuous monitoring on-line networks (around 40 stations throughout Italy, including the Etna area, Aeolian Islands, Umbria region, Piemonte region, etc...) and discretely (9 groups of research were involved in the last years to localize, define and monitor almost all the DDSs in Italy), by sampling and analyzing chemical and isotopic compounds, useful to discriminate the origin, evolution and natural gas hazards of the examined DDS. In this paper, we will discuss some DDS catalogued and studied by a Rome INGV Research Unit (UR 11) which focused its work in Central Italy, throughout different DDS, also in relation to the diverse seismotectonic settings, to discover buried faults as possible gas leakage pathways, mostly if they are "geochemically" activated. In particular we discuss, among the discrete monitoring techniques exploited by INGV, soil gas surveying, which consists in a collection of gas samples from the soil zone not saturated (dry zone) to measure the geogas gaseous species both in fluxes (CO_2, CH_4, ~(222)Rn) and in concentration (He, H_2, H_2S, helium, hydrogen, CO_2, CH_4, ~(222)Rn), that permeate the soil pores. The total CO_2 flux budget was calculated as "baseline" degassing rate of these "CO_2 analogues". A good discrete areal monitoring is prerequisite to design sound continuous monitoring network to monitor CO_2 related parameters in liquid/gas phases, to review the protocol of the Annex II of the European Directivity on CCS.