Geophysical well logging in environmental monitoring wells

• Grant number: 13/22912-1
• Support Opportunities: Regular Research Grants
• Start date: June 01, 2014
• End date: May 31, 2016
• Field of knowledge: Physical Sciences and Mathematics - Geosciences - Geophysics
• Principal Investigator: Carlos Alberto Mendonça
• Grantee: Carlos Alberto Mendonça
• Host Institution: Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated researchers: Vagner Roberto Elis

Abstract

Recent studies at a landfill site at the plains of the Tietê River in the city of São Paulo revealed that organic-rich Quaternary sediments are able to generate biogas (methane and carbon dioxide) in enough amounts to compromise the land use of affected places. The biogas tends to encroach in more permeable sediments, preferably in sandstone lenses within the organic-rich sedimentary layer. Biogas lenses usually are kept under over-pressure conditions which drive gas migration towards the ground surface where it may be lost or trapped in non ventilated buildings. Such indoors accumulations create unhealthy and risky environments since methane and oxygen mixtures can reach flammable limits. For safeness, such biogas lenses should be preventively detected and exhausted although no proved techniques have been available so far. Trapped layers of biogas create contrasting resistive targets with respect to their surroundings. In this proposal we intend to explore this property by applying well logging resistivity measurements to identify possible biogas pockets existing in the vicinity of a set of monitoring wells. Since common monitoring wells are cased, specialized geophysical probes are required, such as: electromagnetic induction to measure electrical resistivity and natural gamma radiation to identify clay horizons. To characterize horizons interpreted as gas bearing, we develop a selective exhausting system to isolate intervals along the well, enabling topic pumping tests and gas sampling. (AU)