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Next: Tasks 2, 3, and Up: Subproject 4: Borehole monitoring Previous: Geophysical logging

Task 1: Repeated logging in borehole Nefsholt

All together 7 logging campaigns took place. The locations are shown in Figure 11. Table 2 lists the logs performed during the project.


  
Figure 11: Map of Iceland showing the location of the site of repeated logging (LL-03) and of the other boreholes, where measurements have been performed. The SISZ extends between the southern ends of the WVZ and EVZ. Wells LL-03 and NG-01 are inside the SISZ, BS-11 is north, THB-13 is south of the zone.
\includegraphics[scale=0.8,keepaspectratio='true']{/net/ris/ris3/prenlab2-2001/ch3/sub4/all-wells.ps}

Altogether, the seven campaigns have contributed to the two aims of this Subproject, that are:

$\bullet$
Observation of changes in physical parameters of the rock, in the degree of fracturing, and in the orientation of principal stresses with respect to seismic activity.
$\bullet$
Indirect measurement of tectonic stress orientation to evaluate the tectonic stress field in the area of the SISZ. For that, the borehole geometry was observed for certain structures, that allow to determinate the orientation of the horizontal principal stress, as there are borehole breakouts and vertical tensile fractures. Borehole breakouts are failures of material of the borehole wall, that result from accumulation of tangential stress at the borehole wall at the azimuth of the lower horizontal principle stress ($\sigma_h$), caused by the free surface produced by drilling. In a linear elastic isotropic theory, this would be perpendicular to the maximum horizontal principal stress ($\sigma_H$). Alternatively, at this free surface, the tensile strength of the borehole wall can be exceeded at the azimuth of the maximum principal horizontal stress, what produces vertical fractures along $\sigma_H$. These vertical fractures extend in the direction of the maximum and open in the direction of the minimum horizontal principal stress. The occurrence of these tensile fractures can be enhanced by thermal stress caused by cold water pumped into the well.

Orkustofnun provided a lithology log of the well LL-03 at Nefsholt. This log is based on cuttings (rock pieces crushed while drilling), not on cores, what strongly limits its depth resolution. It shows a nearly continuous sequence of altered basalt, interrupted only by a few thin sedimentary layers and some thin layers of dolerite, hyaloclastites and fresh basalt. Different kind of layers show different characteristic sets of logs, that allow to distinguish between the lava flows and the sediments (Figure  12, Table 4). The result, i.e. the correlation with the rock physical parameters, is very reasonable: The sedimentary layers seem to contain more water (larger pore space) entailing higher travel times of elastic waves, lower resistivity and higher absorption of neutrons. The correlation is an important indicator that the log results have good quality.


  
Figure 12: Data example showing the correlation between the performed logs and the lithology based on the analysis of cuttings provided by Orkustofnun. GR = gamma-ray in API units, DT = compensated travel time in microseconds per foot, Neutron = neutron-neutron log in API units, ILD = induction log of deep penetration in $\Omega$m, ILM = induction log of medium penetration in $\Omega$m, and LL3 = latero log in $\Omega$m.
\includegraphics[width=\textwidth]{/net/ris/ris3/prenlab2-2001/ch3/sub4/figure10b5.ps}


 
Table 2: All logs performed during both PRENLAB projects. As borehole NG-01 partly collapsed between log runs, the hole was abandoned and well LL-03 was chosen for repeated logging. GR indicates gamma-ray log, SP stands for spontaneous potential, 16" for short normal resistivity tool, and 64" for long normal resistivity tool. FED means four-arm-dipmeter, which includes an oriented four-arm-caliper. BHTV and BHT mean ultrasonic borehole televiewer and borehole temperature, respectively. BCS is borehole compensated sonic log; DIL is dual induction/latero log. The deepest parts of wells LJ-08, LL-03, THB-13, and BS-11 were not accessible anymore.

Name: Location: Max. Logged Tools used: Date:
    depth: interval:    
           
NG-01 Ólafsvellir 1070 m 180-1070 m FED, GR July 1996
  (inside SISZ)        
HS-36 Reykjavík 980 m 330-980 m BHTV, BCS-GR July 1996
LPN-10 Laugaland 890 m 80-880 m BHTV, July 1996
  near Akureyri,     BCS-DIL-GR  
  North Iceland        
LJ-08 Syðra- 2740 m 120-1890 m FED, BHTV, July 1996
  Laugaland   120-1330 m BCS-DIL-GR,  
  near Akureyri   500-1980 m BHTV June 1998
TN-02 Ytri-Tjarnir 1370 m 260-1370 m BCS, GR July 1996
  near Akureyri        
LL-03 Nefsholt 1309 m 80-1100 m BHTV, July 1996
  (inside SISZ,     BCS-DIL-GR October 1996
  site of repeated       September 1997
  logging)       December 1997
          June 1998
        SGR September 1997
          June 1998
        BHT April 1999
          August 2000
        neutron-neutron April 1999
          October 1999
        X-Y-caliper September 1996
        16"- and 64"- April 1999
        resistivity October 1999
          August 2000
        SP October 1999
           
THB-13 Þykkvibær 1254 m 466-1225 m BHTV December 1997
  SW of Hella        
  near the        
  south coast        
           
BS-11 Böðmóðsstaðir 1193 m 703-1090 m BHTV December 1997
  near   500-1090 m   June 1998
  Laugarvatn,        
  north of the SISZ        
 


 
Table 3: Repeated logging by Orkustofnun in Well LL-03, files used for comparisons. + : depth adjusted to neutron-neutron log of August 2000. * : calibrated and depth adjusted to neutron-neutron log of August 2000.

Log type: Date: Depth range: File name: Remarks:
Temperature log (T) 07-07-1977 0-920 T07071977  
Temperature log 18-10-1977 0-1100 T18101977  
Temperature log 17-09-1980 0-1060 T17091980  
Temperature log 25-06-1992 0-1106 T25061992  
Temperature log 23-04-1999 4-1080 21415  
Temperature log 03-08-2000 0-1087 T03082000  
Natural gamma log (GR) 23-04-1999 0-1080 21424  
Natural gamma log 27-10-1999 10-1080 22588  
Neutron-neutron log (NN) 23-04-1999 3-1080 21421  
Neutron-neutron log 27-10-1999 10-1080 22585 +
Neutron-neutron log 03-08-2000 7-1087 N03082000  
Self potential log (SP) 27-10-1999 25-1080 22570  
16" normal resistivity log (SN) 27-10-1999 25-1080 22568 *
16" normal resistivity log 03-08-2000 27-1087 S03082000 *
64" normal resistivity log (LN) 27-10-1999 25-1080 22569 *
64" normal resistivity log 03-08-2000 27-1087 L03082000 *
LL3 of DIL 07-1996 36-1070 LL3-07/96  
LL3 of DIL 12-1997 37-1071 LL3-12/97  
 


 
Table 4: Characteristic set of log values for different kinds of layers, derived from Figure 12.

Depth GR: BCS neutron-neutron DIL, Lithology:
interval:   travel water content 16", 64"  
    time: porosity: resistivity:  
265-275 m low low low high altered basalt
275-280 m low high high low sediment
280-285 m low low low high altered basalt
285-298 m high high high low sediment
298-305 m high low low high fresh basalt
 


next up previous contents
Next: Tasks 2, 3, and Up: Subproject 4: Borehole monitoring Previous: Geophysical logging
Hjorleifur Sveinbjornsson
2001-01-08