Rock stress orientation from borehole breakouts and its correlation to drill parameters and geology: results from boreholes KFM01A and KFM05A of the Forsmark site investigation, Sweden

Abstract: Knowledge of stress orientation is crucial for the understanding of many
processes in the Earth's crust such as tectonic development, earthquake
occurrence, and fluid transport along faults. In the Forsmark site
investigation: this knowledge plays an important role for engineering
decisions with respect to borehole stability and the siting of the future
repository for nuclear waste. Breakouts are zones of failure of the
borehole wall in response to high compressive tangential stresses. The
failures elongate the borehole cross-section from the original circular
shape and are observed by geometrical logging tools, for example borehole
televiewer (BHTV). Borehole breakouts are reliable indicators of the
orientation of the maximum horizontal stress [e.g. Bell and Gough, 1979:
Zoback et al., 1985].

The borehole breakout method is a stress method that provides continuous
information on stress in intervals where borehole breakouts occur, which
makes this method unique in comparison to other existing methods. In other
words, this method provides information on the continuation of the stress
field along a borehole.

The orientation of borehole breakouts can be measured using mechanical
(three-, four- and six-arm caliper), acoustic (BHTV) or electrical
resistivity (e.g. Formation MicroScanner (FMS) and Formation MicroImager
(FMI)). Optical logging tools such as BIPS and borehole cameras can be used
to view borehole breakouts. While BHTV and FMS/FMI tools provide excellent
data for breakout analysis, borehole cameras, BIPS, and caliper tools are
known to have poorer quality. At the Forsmark Site Investigation, the
borehole geometries have been logged using BHTV and BIPS.

This work presents stress orientation data derived from a set of BHTV logs
for the core-drilled, approximately 1 km deep, boreholes KFM01A and KFM05A.
The two boreholes were drilled by the Swedish Nuclear Fuel and Waste
Management Co. (SKB) as part of their site investigation program for
storage of nuclear waste in hard rock in Forsmark, Southeast Sweden. The
study has the following objectives: (1) determine the downhole orientation
of horizontal stresses: (2) correlate borehole breakouts with Measurement
While Drilling (MWD) parameters: (3) study the influence of lithology and
structures on rock stress orientation: and (4) identify the zones of rock
continuum.

Two different types of breakouts were recognized from the amplitude log
with both shallow and deep failure depths. The most dominant occurrence is
of the shallow failure type which is observed in both boreholes KFM01A and
KFM05A. This study cannot categorically state the full characterization of
identified breakouts but previous studies have inferred that they could
originate as a result of the differential rock strength, the depth of the
borehole and the state of stress [Plumb, 1989].

In borehole KFM01A, 33 borehole breakouts with a combined length of 253 m
have been observed from 111 to 1001 meters borehole length (mbl). They
suggest a mean orientation of the maximum horizontal stress of about
141„b8„a
N. In borehole KFM05A, 14 borehole breakouts have been observed between 188
and 996 mbl. They have a combined length of 32 m and a mean maximum
horizontal stress orientation of about 159„b33„aN. According to the World
Stress Map quality ranking scheme, boreholes KFM01A and KFM05A are of B-
and D quality [Zoback, 1992].

MWD parameters that were used for this analysis were the rotation pressure
vs. depth, water pressure vs. depth and the feed speed vs. depth. It was
observed that MWD parameters are sensitive to fractures and any kind of
opening in the rock as they reveal changes in rock¡¦s mechanical and
physical properties of the borehole wall. The variations in the strength of
the rock and fracture occurrence in the borehole show changes in the water
pressure and also rotation pressure values. Borehole breakouts are
associated with changes in the rock quality or rock strength, fractures,
mechanical properties and change in lithology. Sections marked as regions
without breakouts gradually show minimal changes in MWD parameters and in
the rock¡¦s mechanical and physical properties within the depth interval as
compared to sections with borehole breakouts that show significant
variations in their parameters. Therefore, MWD parameters and borehole
breakouts are dependent on the mechanical properties of the rock mass,
lithology and the physical properties as well. MWD systems are not absolute
rock recognition systems: however, with proper interpretation, changes in
rock formations and properties can be inferred.

The observed breakouts in both boreholes KFM01A and KFM05A start within the
first 100 m of the core drilled part and this has a significant implication
that the horizontal stresses are even high at shallow depth intervals. The
fact that the orientation has been very much uniform in borehole KFM01A
predicts that the rock domain here have not been influenced much by the
lithology and structures which are prominent in the borehole. For borehole
KFM05A, the influence of lithology and structures in the stress orientation
was observed at depth intervals between 350 ¡V 450 m, 600 ¡V 750 m and 900
¡V
1000 m. The prominent fractures at these depth intervals result from the
gradual changes from one rock units to another and deformation zones that
have been detected. It validates that the stress field is not continuous
but with influence of previously existing structures on the prevailing
stress field in the rock mass.

Borehole KFM01A shows consistency in the minimum stress orientation values
along the entire borehole length and this signifies zones of rock continuum
without much variance in the rock¡¦s properties: lithology and structures.
Borehole KFM05A shows inconsistency in the horizontal stress orientation
and intervals with stress re-orientation suggests that there are zones of
rock discontinuum which infers that the lithology and structures have a
role to play in the changes in stress orientation.