Identification of gas inflow zones in the COSC-1 borehole (Jämtland, central Sweden) by drilling mud gas monitoring, downhole geophysical logging and drill core analysis

Abstract

On-line monitoring of drilling mud gas was for the first time applied during continuous wireline coring of the COSC-1 borehole (Jämtland, central Sweden) to analyse formation gases and to identify inflow gas zones. Nearly complete gas records were obtained with 3 m depth resolution from 662 m (installation of the separator for gas extraction) to 1709 m and 6 m resolution from 1709 m to 2490 m depth (COSC-1 final depth: 2496 m) for H2, CH4, CO2, and He. Between 662 m and 1400 m, both He and CH4 form broad peaks superimposed by several spike-like features. Zones with gas spikes coincide with high resistivity intervals from dual laterolog (DLL) geophysical borehole logging and show fractures in borehole televiewer (BHTV) images, drill core scans, and visual core inspection. Therefore, we assume gas inflow through open fractures where DLLd/DLLs ratios >1.5 imply the presence of free gas. The correlation between helium and DLLd/DLLs ratios no longer appears at depths greater than ~1550 m, probably because the formation gases are dissolved in formation fluids at higher pressure. Below 1550 m depth, the He concentration drops significantly, whereas the CH4 concentration remains relatively high and H2 and CO2 reach maximum values. The high amount of H2 and CH4 at depths below 1616 m, from where friction between the casing and the drill string was reported, imply that these gases are most certainly artificially generated at depths below 1616 m and at least partly of artificial origin at shallower depths.