Earthquake Prediction Research in a Natural Laboratory Subproject 4: Borehole Monitoring of Fluid-Rock Interaction Contractor: F. Roth, Section "Rock Mechanics and Stress Field of the Earth's Crust", Div. "Geomechanics and Management of Drilling Projects", GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg A34, D-14473 Potsdam, Fed. Rep. of Germany Tel.: +49-331-288-1530, Fax: +49-331-288-1534, roth@gfz-potsdam.de Subcontractor: V. Stefa'nsson, Orkustofnun (OS; National Energy Authority), Grensa'svegur 9, IS-108 Reykjavi'k, Iceland Tel.: +354-569-6063, Fax: +354-568-8896, vs@os.is Report 1 In the framework of the EU-project "Earthquake Prediction Research in a Natural Laboratory", a pilot study has started to obtain a time series of logs in the South Iceland Seismic Zone (SISZ). An 1100 m deep borehole (LL-03; "Nefsholt") inside the zone (63.92 N, 20.41 W, 7 km south of the seismological station SAU) is used and provides the unique opportunity to perform measurements much nearer to earthquake sources than usual - the hypocenter depths at that location range between 6 and 9 km. Moreover, data can be obtained for a depth interval of more than 1000 m, uninfluenced by the sedimentary cover and less disturbed by surface noise. In the preparational phase of an earthquake, stress accumulation is expected to be connected with the creation of borehole breakouts, changes in the number and size of cracks, a possible variation of the stress direction etc. Therefore, the following set of geo-parameters is monitored: + p-wave travel time, + electrical conductivity, + water content and porosity, + stress information from borehole breakouts (orientation and size), + crack density, crack opening. This is achieved by repeated logging with tools as: - sonic log (BCS), - dual induction/latero log (DIL), - neutron log, - four-arm-dipmeter (FED) and - borehole televiewer (BHTV). The neutron log is run with the logging equipment of OS, the rest with the Halliburton logging truck of GFZ. Temporal changes visible in these logs will be correlated with data obtained by other methods used in the whole project, as there are: seismicity, anisotropy observed in S-waves, crustal deformation, gravity etc. During winter and spring 1996, the BCS, DIL and neutron tools were checked for azimuthal isotropy in their sensitivity, as it cannot be guaranteed that the tools follow the same spiral path through the well in each log run. Task 1: Between April and June, OS checked the condition of the borehole selected for repeated logging - at first this was well NG-01 -, organised and supervised cleaning of the drillhole, and also arranged the opportunity to log in other wells on Iceland. The data of the selected borehole are: Borehole: LL-03 at Nefsholt, South Iceland Lowlands, inside the South Iceland Seismic Zone, provided from OS Drilled: 1977 Position: 63.92 N, 20.41 W Depth: 1108 m (originally drilled down to 1309 m) Casing: 0 - 28 m, 12.5" diameter Uncased: 28 - 1108 m, 8.5" diameter Used section: 80 - 1108 m, i.e. 1028 m with basaltic lava flows and interbedded hyloclast (tuff) Mud density: 1.0 kg/dm**3 (water) Max. temperature: 105 C Task 2 to 3: Two field campaigns took place in July 1996, a third in October '96. More (Task 4) are to follow in 1997. Moreover, additional logs could be run by us in July '96 in 5 other boreholes on Iceland to add new information to previous results on the regional stress field. The following table gives an overview on the logging activities: Name Location max. Depth Logged Depth Interval Tools used about NG-01 O'lafsvellir 1070 m 180 - 1070 m FED, GR, 3-Arm-Caliper, (inside SISZ) 16"- & 64"-Resistivity, SP HS-36 Reykjavi'k 980 m 330 - 980 m BHTV, BCS, GR LPN-10 Laugaland 890 m 80 - 880 m BHTV, BCS, DIL, GR near Akureyri, North Iceland LJ-08 Sydra Laugaland 2740 m 120 - 1890 m FED, BCS, DIL, GR near Akureyri 120 - 1330 m BHTV TN-02 Ytri-Tjarnir 1370 m 260 - 1370 m BCS, GR near Akureyri LL-03 Nefsholt 1309 m 80 - 1100 m BHVT, BCS, DIL, GR, (inside SISZ, Neutron-Neutron, site of repeated X-Y-Caliper, SP, logging) 16"- & 64"-Resistivity ---- Remarks: GR indicates Gamma-Ray-Log, SP stands for Spontaneous Potential. As borehole NG-01 partly collapsed between log runs, the hole was abandoned and well LL-03 was chosen for repeated logging. For technical reasons, no FED run was possible in LL-03, no BHTV run was possible in LJ-08 below 1330 m depth. Due to the limited availability of a crane, no BHTV or FED run were possible in TN-02. The deepest parts of wells LJ-08 and LL-03 were not accessible anymore. ----------------------------------------------------------------------- Task 5: The processing of the data has begun, even though until now (Nov. '96) no qualified scientist could be hired for the project. All logs add up to about 46 km logged intervals. This is due to the number of holes, the number of tools used, the number of repetitions of logs in LL-03 and the fact that in LL-03 several logs of the same kind were performed immediately one after the other. The latter is done to get information on the scattering of data in short time periods during which no changes due to tectonic processes are expected to have occurred. The preprocessing of the data, especially merging of segments and depth matching, is done for all data. All BHTV data were converted and are being plotted. Wellbore breakouts were found in LL-03 and LJ-08 at least. In NG-01, there are sections with cavities, where the large diameter indicates a breakout, but the small diameter does not have bitsize. This has to be checked more carefully to decide whether these cavities may be considered as breakouts induced by anisotropy in tectonic stresses. Figures: Yet to be decided Publications: A report on the outline of the project was given at a German workshop on borehole geophysics and rock physics. An extended abstract of six pages on this presentation is submitted for publication in a special issue of the proceedings of the German Geophysical Society. Meetings of V. Stefa'nsson with F. Roth took place in April and September in Reykjavi'k and in September in Potsdam. Meetings of P. Einarsson with V. Stefa'nsson and F. Roth took place in April and September in Reykjavi'k. ---...--- New proposal: Subproject 4: Borehole Monitoring of Fluid-Rock Interaction Contractor: F. Roth, Section "Rock Mechanics and Stress Field of the Earth's Crust", Div. "Geomechanics and Management of Drilling Projects", GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg A34, D-14473 Potsdam, Fed. Rep. of Germany Tel.: +49-331-288-1530, Fax: +49-331-288-1534, roth@gfz-potsdam.de Subcontractor: V. Stefa'nsson, Orkustofnun (OS; National Energy Authority), (Assoc. Contractor) Grensa'svegur 9, IS-108 Reykjavi'k, Iceland Tel.: +354-569-6063, Fax: +354-568-8896, vs@os.is Application for a second phase, 2 years, from 3/1998 to 2/2000 Objectives Although much progress has been made during the past decades in investigating the nature of active faults, most of this progress has involved kinematics issues. These can be observed directly using numerous geological, geophysical and geodetic techniques. The dynamics of the processes at faults are far more elusive and difficult to characterise using the established methods of earth sciences. Neither the applied stresses nor the rheological response to these stresses are observable using surface-based instruments or techniques because of the depths within the lithosphere at which critical processes occur. The same applies to pore fluids, their presence and temperature, their composition, their physical and chemical behaviour, their pressure and the rock permeability in situ. Key questions in earthquake prediction research (EPR) are still unanswered, for instance - What forces, or stresses, are required to cause fault slip? - Are active fault zones weak? If so, why? - What factors determine whether a fault is seismically active or aseismic? - What is the role of fluids in fault processes and where do they originate? - How does fault zone behaviour change with depth? - How do geophysical observations relate to fault zone properties? - Are there fundamental differences between faults in oceanic versus continental settings? If so, what causes these differences? In the framework of the first phase of the EU-project "Earthquake Prediction Research in a Natural Laboratory", a pilot study has started in spring 1996 to obtain a time series of logs in the South Iceland Seismic Zone (SISZ). An 1100 m deep borehole (LL-03; "Nefsholt") inside the zone (63.92 N, 20.41 W, 7 km south of the seismological station SAU) is used and provides the unique opportunity to perform measurements in a fault zone, much nearer to earthquake sources than usual - the hypocenter depths at the location range between 6 and 9 km. Moreover, data can be obtained for a depth interval of more than 1000 m, uninfluenced by the sedimentary cover and less disturbed by surface noise. In the preparational phase of an earthquake, stress accumulation is expected to be connected with crustal deformation, the creation of borehole breakouts, changes in the number and size of cracks, movement of fluids combined with heat transport and poro-/thermo-elastic stresses, a possible variation of the stress direction etc. Therefore, the following set of geo-parameters is monitored: + p-wave and s-wave travel times, + electrical conductivity, + water content and porosity, + stress information from borehole breakouts (orientation and size), + crack density, crack opening. This is achieved by repeated logging with tools as: - sonic log (BCS), - dual induction/latero log (DIL), - neutron log, - four-arm-dipmeter (FED) and - borehole televiewer (BHTV). The neutron log is run with the logging equipment of OS, the rest with the Halliburton logging truck of GFZ. Emphasis is laid on the detection of changes in the above mentioned parameters. Nevertheless, from the logs and from combining information from several log types, further rock physical parameters can be deduced in several ways under model assumptions: density, elastic parameters of the rocks, permeability, layering, bedding planes, rock types etc. Presently, in the first phase, logs obtained in the initial logging campaigns (three up to the end of 1996) are analysed. This includes > correlation of several log runs in one campaign to obtain a value for the precision of the measurements, > correlation of logs from different campaigns to look for temporal variations, > search for anomalies via a comparison of different log types and via cross checks between the series of logs and data bases or time series obtained in other experiments as there are: seismicity, fault plane solutions, shear wave splitting, surface deformation, gravity, borehole strainmeter recordings etc. In the second phase proposed now, the sequence of logs should be continued with another nine campaigns in 1998 and 1999. In addition, emphasis will be laid on forward modelling of effects observed. Data on rock types around the borehole and neighbouring wells are gathered and will be compared to published laboratory data on physical properties of the rocks (constants of elasticity, specific resistivity, strength, density, porosity etc.; e.g. Hellwege and Hellwege in Landolt-Boernstein, 1982, Carmichael in CRC-Handbook of Phys. Prop. of Rock, 1989, Lockner in AGU-Handbook of Rock Physics, 1995 etc.). These data will be used with source parameters of earthquakes below the drillhole and information on pumping in other wells of the area to calculate effects of natural and man-made influences (changes in temperature, load, stress, crack density) on the site of the borehole. Results will be compared to those obtained from the logs. The project will provide information on the state of stress of the rock near the borehole and about varying water content in cracks. As part of the multi-method approach to monitor pre-, co- and postseismic stages in the SISZ, these experiments are thought to provide essential additional information on the critical state of processes in the Earth's crust in a seismic cycle. Contractor: Frank Roth, Section "Rock Mechanics and Stress Field of the Earth's Crust", Div. "Rock Mechanics and Management of Drilling Projects", GeoForschungsZentrum Potsdam, Germany; Subcontractor: Valgardur Stefa'nsson, Orkustofnun (National Energy Authority of Iceland), Reykjavi'k, Iceland. Keywords: stress, cracks, fluids, failure, physical properties of rocks, in-situ monitoring, repeated logging ------------------------------------------------------------------------------- Work content The data of the selected borehole are: Borehole: LL-03 at Nefsholt, South Iceland Lowlands, inside the South Iceland Seismic Zone, provided from OS Drilled: 1977 Position: 63.92 N, 20.41 W Depth: 1108 m (originally drilled down to 1309 m) Casing: 0 - 28 m, 12.5" diameter Uncased: 28 - 1108 m, 8.5" diameter Used section: 1028 m, i.e. 80 - 1108 m with basaltic lava flows and interbedded hyaloclast (tuff) Mud density: 1.0 kg/dm**3 (water) Max. temperature: 105 C To monitor changes in physical rock parameters and the migration of fluids due to tectonic activities, it is of crucial importance that other changes are as small as possible or can be discriminated from the interesting ones. As the borehole is already 19 years old, drilling induced changes in the formation will have strongly diminished by all experience. Further, without tectonic activity, one would assume that the borehole will have reached a rather stationary state with the surrounding rock, concerning for example temperature or diffusion of the drilling mud into the formation and diffusion of formation fluids into the borehole, respectively. Logging tools whose signals penetrate to some depth into the formation, as induction, sonic and neutron log, will permit to discriminate between deeply reaching effects and those confined to the borehole wall. Concerning the tools, it is important to achieve a high resolution of 1) the measured signals, 2) the logging depth, and 3) the azimuth from where the signals originate. The logging tools available are: GFZ: Logging truck, winch with 7000 m cable, Halliburton DDL-1 logging system + Borehole Compensated Sonic tools (BCS) + Dual Induction/Latero Log tool (DIL, including Spontaneous Potential tool) + Borehole Televiewer (BHTV) + Four Electrode Dipmeters (FED: Dipmeter and 4-Arm-Caliper) OS: Logging truck, winch with 5800 m cable, domestic logging system + Neutron-neutron logging tools + Short Normal (16") and Medium Normal (64") Resistivity logging tools Both logging systems are different, so that only the BHTV with data acquisition on a PC can be operated with both logging trucks and cables. With these tools, we intend to monitor the following geo-parameters (tool to be used) - p- and s-wave travel time, porosity (BCS) - resistivity at different distances from the borehole (DIL and 16"/64" Resistivity Log) - stress information from borehole breakouts (BHTV, FED) - porosity / water content (NNL) - crack density & crack closing/opening (BHTV, FED), and Concerning the azimuthal sensitivity, two tools, BHTV and FED, are supplied with a navigation subunit that determines azimuth and inclination of the tools. The sensitivity of the other tools, DIL, BCS and the Normal Resistivity tools, is only slightly anisotropic. The time schedule is as follows: Cam- Task Tools Duration Scheduled paign [days] in month # 7-11 checks for changes all tools 5x3 3,4,5,6,7 12-15 checks for changes all tools 4x3 15,16,17,19 Task 1: Logging according time schedule above. Task 2: Cross correlation of logs from different campaigns. Task 3: Comparison of changes in logs of different type. Task 4: Comparison of changes in logs with changes in seismicity, fault plane solutions, shear wave splitting, gravity, borehole strain meter readings, crustal deformation etc. Task 5: Forward modelling of effects of pumping hot water from a neighbouring well. Task 6: Forward modelling of effects of stress increase on rock around the borehole. ------------------------------------------------------------------------ "Borehole Monitoring of Fluid - Rock Interaction", including "Modeling of the Earthquake Related Space-Time Behaviour of the Stress Field in the Fault System of Southern Iceland" Period: 3/1998 - 2/2000, i.e. 2 years Contractor: Dr. Frank Roth, Section 5.3, Division 5, GFZ Potsdam, F.R.Germany