Online Gas Monitoring of Drilling Mud (OLGA)
Continuous mud gas loggings during drilling as well as offline mud gas sampling are standard procedures in oil and gas exploration, where they are used to test reservoir rocks for hydrocarbons while drilling.
OLGA extended this technique for scientific drilling in hydrocarbon and non-hydrocarbon formations, mainly to sample and study the composition of crustal gases. Hydrocarbons, helium, radon and with limitations carbon dioxide and hydrogen are the most suitable gases for the detection of fluid-bearing horizons, shear zones, open fractures, sections of enhanced permeability or permafrost methane hydrate occurrences. Off-site isotope studies on mud gas samples help to reveal the origin and evolution of deep-seated crustal fluids. The method had been successfully applied on several continental scientific drilling projects of the ICDP (Mallik, SAFOD, Corinth Rift, Unzen Volcano, Great Valley Caldera) and IODP Exp. 319.
Drilling mud gas that circulates in the borehole comprises air and components that are mechanically released as the drill bit, including components present in the pore space of the crushed rock and gas entering the borehole through permeable strata, either as free gas or, more likely, dissolved in liquids. Continuous inflow of fluids in the borehole along the entire borehole wall is hampered through the rapid formation of mud-cake that covers the borehole wall and acts as a seal.
Back at the surface, a portion of the circulating mud is admitted to a mud gas separator and gas dissolved in the drilling mud is extracted mechanically under slight vacuum. The separator is composed of a steel cylinder with an explosion-proof electrical motor on top that drives a stirring impeller mounted inside the cylinder. The gas separator is normally installed in the "Possum belly" above the shaker gas separator is normally installed in the "Possum belly" above the shaker screens as close as possible to the outlet of the mudflow line to minimize air contamination and degassing of the drill mud immediately before gas extraction. A small membrane pump is used to build up vacuum and to pump the extracted gas into a laboratory trailer, which should be installed not more than a few tens of meters away from the gas separator.
N2, O2, Ar, CO2, CH4, He, and H2 are determined by a quadrupole mass spectrometer (QMS) of the type OmniStarTM (Pfeiffer Vacuum, Germany). A complete QMS analysis with detection limits between 1 and 20 ppmv (parts per million by volume) is achieved with this setup after an integration time of less than 20s. However, a sampling interval of
1 min is chosen to reduce the amount of data produced. Hydrocarbons (CH4, C2H6, C3H8, i-C4H10, and n-C4H10) are analyzed at 10-min intervals with a automated standard field GC, which is equipped with a flame ionization detector. Detection limits for the hydrocarbons are at about 1 ppmv. Gas samples for further studies e.g. of isotopes are taken automatically when a given threshold level at the QMS is exceeded.
OLGA has been proven being a reliable and inexpensive source of information on the composition and spatial distribution of gases at depth-in real time. It is suitable to detect fluid-bearing horizons, shear zones, open fractures, sections of enhanced permeability and methane hydrate occurrences in the subsurface of fault zones [3, 4, 6], volcanoes and geothermal areas , permafrost regions , and others. Off-site isotope studies on mud gas samples help reveal the origin, evolution, and migration mechanisms of deep-seated fluids . It also has important application to aiding decisions if and at what depth rock or fluid samples should be taken or formation testing should be performed.
 Erzinger, J., Wiersberg, T. and Zimmer M.(2006) Real-time mud gas logging and sampling during drilling, Geofluids 6, 225-233
 Wiersberg, T., Erzinger, J., Zimmer, M., Schicks, J., and Dahms, E. (2004) Real-time gas analysis at the Mallik 2002 Gas Hydrate Production Research Well; in Scientific Results from Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585 pp. 15.
 Erzinger J., Wiersberg T. and Dahms E. (2004) Real-time mud gas logging during drilling of the SAFOD Pilot Hole in Parkfield, CA, Geophys. Res. Lett. 31, L15S18, doi:10.1029/2003GL019395
 Wiersberg T. and Erzinger J. (2007) A helium isotope cross-section study through the San Andreas Fault at seismogenic depths, G-cubed 8, No. 1, doi: 10.1029/2006GC001388
 Tretner, A., Zimmer, M., Erzinger, J., Nakada, S., Saito, M. (2008)
Real-time drill mud gas logging at the USDP-4 drilling, Unzen volcano, Japan. Journal of Volcanology and Geothermal Research, 175(1-2):28-34
 Wiersberg T. and Erzinger J. (2008) On the origin and spatial distribution of gas at seismogenic depths of the San Andreas Fault from drill-mud gas analysis, Applied Geochemistry 23, 1675-1690, doi:10.1016/j.apgeochem.2008.01.012
For more information or paper offprints please contact
Scientific Drilling ICDP
GFZ German Research Centre for Geoscience
Phone +49 331 288 1081
Fax +49 331 288 1088
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