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.

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.

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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.

 

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