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
Introduction
Understanding subsurface fluid flow and identification and characterization of permeable zones are paramount for a safe utilization of the subsurface for e.g. injection and storage of CO2 (e.g. Bergmann et al., 2017) or production and subsequent re-injection of fluids for exploiting geothermal energy (e.g. Grant, 2015). Statements on permeability in the subsurface can be made on the basis of borehole investigations. Since the significance of a single investigation method is limited, a combination of different, mainly geophysical investigation methods is usually applied in boreholes (see Fig. 9).
Davatzes and Hickmann (2010) compare acoustic and electrical image logs from a borehole in the Coso geothermal field (US) to identify both open and closed (healed) natural fractures. Massiot et al., (2017) characterize fracture permeability in two geothermal boreholes in New Zealand by integration of acoustic and resistivity image logs as well as data from borehole testing, where pressure, temperature and spinner are measured various water injection rates. High permeability is found for low-resistivity fractures with high-resistivity haloes. A combined approach of geophysical borehole measurements (acoustic and standard geophysical logging), geochemical studies (drilling mud gas) and drilling engineering data (mud losses) was successfully applied in two geothermal boreholes in the upper Rhine Graben (France) to better characterize permeable fracture zones in hard rock (Vidal et al., 2017).
In this study, we present results from drill mud gas monitoring and compare them with resistivity data from dual laterolog (DLLd and DLLd/DLLs ratio), borehole televiewer (BHTV) images and optical drill core analysis to identify and characterize gas and fluid bearing open fractures in the COSC-1 borehole in Jämtland, central Sweden.
Geophysical borehole logging is an established technique for extracting information from the underground. Online monitoring of drilling mud gas (OLGA) is also increasingly used in scientific drilling operations, but a combined interpretation of the data sets obtained with these methods has rarely been carried out in the past (e.g. Wiersberg et al., 2015).
A first attempt for evaluating drilling mud gas for scientific purposes in a hard rock setting was conducted at the Kontinentale Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) Main Hole (Zimmer and Erzinger, 1995). Besides hydrocarbons, the range of measured gases at KTB was extended to noble gases (helium, argon, and radon) and other gases of generally little interest for conventional mud gas logging (nitrogen, hydrogen, oxygen, carbon dioxide). The continuous gas and fluid analysis in the KTB borehole indicated several fluid inflow zones with increased concentrations of methane and/or helium, some of which were accompanied by formation brines.
After the KTB experience, on-line monitoring of fluids and gases from circulating drilling mud has been successfully conducted in several continental scientific drilling projects in crystalline and sedimentary strata, most of which are funded and supported by the International Continental Scientific Drilling Program (ICDP). These drilling projects were focussing on fault zones (Corinth, Chelungpu, SAFOD, New Zealand (Erzinger et al., 2004; Wiersberg and Erzinger, 2007, 2008, 2011)), volcanic systems (Unzen, Long Valley (Tretner et al., 2008)), collision zones (Donghai, COSC (Luo et al., 2004; Lorenz et al., 2015)), and gas hydrates (Mallik (Wiersberg et al., 2005)). On-line drilling mud gas monitoring has also been applied in non-ICDP scientific drilling projects, e.g. the Wenchuan fault zone drilling (Gong et al., 2014). After its successful introduction on board the drilling vessel D/V Chikyu in 2009 (Wiersberg et al., 2015), it became a standard shipboard technique for IODP riser drilling operations and demonstrated its great value during Nankai Trough fault zone drilling (Hammerschmidt et al., 2014; Wiersberg et al., 2015, 2018) and expeditions targeting the deep biosphere (Inagaki et al., 2015).
OLGA does neither interfere with drilling operations nor does it require any rig time. However, until drilling of COSC-1, on-line mud gas monitoring was only applied in standard rotary oil-field drilling. The characteristics of oil-field drilling, namely large borehole diameters and drilling for cuttings rather than extracting drill core, promise detectable amounts of formation gases in the drilling mud gas if gas is present in the formation.
The presence of six inflow zones of saline fluids in the COSC-1 borehole was demonstrated by Fluid Flow Electrical Conductiviy (FFEC) logging (Tsang et al., 2016, Doughty et al., 2017). COSC-1 drilling was performed using a wireline core-drilling rig typically used in mining exploration (“Riksriggen”, operated by Lund University), resulting in a fully cored borehole with a diameter of 96 mm between 103 m and 1616 m and 75.7 mm between 1616 m and 2496 m (Lorenz et al., 2019). Due to the smaller borehole diameter when compared to oil or gas wells and the much higher volume to surface ratio of drill core, compared to cuttings, the release of gases into the drilling mud from the formation and from the drill core during core drilling and core uplift is limited.
Section snippets
Drilling the collisional orogeny in the Scandinavian Caledonides (COSC-1)
The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project aims to improve our understanding of Himalayan-type mountain building processes by drilling two deep scientific boreholes in Jämtland, central Sweden. The COSC project is part of the Swedish Scientific Drilling Program (SSDP) and the first borehole (COSC-1) received funding and support from the ICDP, the Swedish Research Council and the Geological Survey of Sweden.
COSC-1 provided detailed insights into
Online gas monitoring OLGA
Drilling mud circulation is applied during deep drilling operations in order to stabilize the borehole and prevent it from collapsing, to control gas kicks, to cool the drill bit, and to drag out the rock chips (cuttings) produced by the drilling process. In addition, formation fluids and gases that enter the borehole are carried to the surface with the returning drilling mud. For common drilling with bentonite or polymer-based drilling mud, the almost instantaneous formation of a low-permeable
Results and discussion
The high content of nitrogen, oxygen and argon in air-loaded drilling fluids, relative to their abundance in formation fluids, hampers identification of these gases in formation fluids. We therefore do not discuss these gases in the following. Oxygen consumption at the drill bit and the drill pipe decreases the oxygen concentration in drilling mud gas and in turn increases the relative concentration of all other gases. To avoid misinterpretations of apparent increasing gas concentrations caused
Summary and conclusions
The COSC-1 borehole drilling is the very first project where on-line mud gas monitoring was applied during continuous wireline coring. Nearly complete gas composition records have been obtained between 662 m (beginning of the experiment) and 2490 m. Gas was extracted from the surrounding formation during drill core tripping and accumulated in the borehole, where it was circulated out with the drilling mud. Core tripping could therefore be regarded as a borehole testing experiment to gain
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
COSC-1 was supported and funded by the International Continental Scientific Drilling Program (ICDP) and the Swedish Research Council (VR – Grant 2013–94). Drilling was performed using the national scientific drilling infrastructure (“Riksriggen”), operated by Lund University. Special thanks to the drilling and logging teams of Lund University and the ICDP Operational Support Group who performed the downhole logging operations. The authors are also grateful to Tina Kollaske (BGR Germany) for
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