In situ stresses

outline
Hydraulic Testing on Pre-existing Fractures
&Hydraulic fracturing
overcoring - Borre probe
USBM Deformation Probe
Conical Strain Cell
deep doorstopper gauge system (DDGS)
Core Discing

Hydraulic Testing on Pre-existing Fractures & Hydraulic fracturing
HTPF & HF

Hydraulic Testing on Pre-existing Fractures& Hydraulic fracturing
There exist two stress measurement methods that use hydraulics as an active method
to stimulate the rock surrounding a borehole and hence to determine the stress field.
These methods are hydraulic
fracturing and HTPF.
methods use the same type of equipment, including straddle packers, impression
packers and
high-pressure pumps to generate high-pressure water during either the formation of
new fractures or reopening of pre-existing fractures
HTPF & HF

Hydraulic Fracturing Method
A section, normally less than 1m in length, of a borehole is sealed off
with a straddle packer. The sealed-off section is then slowly pressurised
with a fluid, usually water. This generates tensile stresses at the borehole
wall. Pressurisation continues until the borehole wall ruptures through
tensile failure and a hydrofracture is initiated. The fracture plane is
normally parallel to the borehole axis, and two fractures are initiated
simultaneously in diametrically opposite positions on the borehole
periphery. The hydrofracture will initiate at the point, and propagate in
the direction, offering the least resistance.

The orientation of the fracture is obtained from the fracture traces on the
borehole wall – it coincides with the orientation of the maximum horizontal
stress, in a vertical or sub-vertical hole where it is assumed that one principal
stress is parallel to the borehole. The fracture orientation may be determined
either by use of an impression packer and a compass or by use of geophysical
methods such as a formation micro-scanner or a borehole televiewer.

The two measurements taken are the water pressure when the
fracture occurs and the subsequent pressure required to hold
the fracture open. These are referred to as the
breakdown pressure(Pc’ or PB)
and the shut-in pressure (Ps).

Relationships
σh = Ps
σH = 3σh–Pc’-Po+ σt
σt = Pc’-Pr
σH = 3σh–Pr-Po
In calculating the in situ stresses, the shut-in pressure (Ps) is
assumed to be equal to the minor horizontal
stress, σh.
The major horizontal stress, σH, is then found from the breakdown
pressure (Pc ’ or PB). In this calculation, the breakdown pressure
has to overcome the minor horizontal principal stress (concentrated
three times by the presence of the borehole) and overcome the in situ
tensile strength of the rock; it is assisted by the tensile component of
the major horizontal principal stress.

The following points should be noted with respect to HF:
There is no theoretical limit to the depth of measurement, provided a
stable borehole can access the zone of interest and the rock is elastic and
brittle.
Principal stress directions are derived from the fracture delineation on the
borehole wall under the assumption that fracture attitude persists away
from the hole.
Evaluation of the maximum principal stress in the plane perpendicular to
the borehole axis assumes that the rock mass is linearly elastic,
homogeneous, and isotropic
Hydraulic fracturing is an efficient method for determining the 2D stress field,
normally in the horizontal plane, and is therefore suitable at the early stages of
projects when no underground access exists. Due to its efficiency, it is especially
advantageous for measurements at great depth. . The method is also not
significantly affected by the drilling processes. Hydraulic fracturing normally
includes large equipment, which requires space. Furthermore, the theoretical
limitations normally imply that the measurements should be done in vertical
holes. Hence, the method is most suited for surface measurements in vertical or
sub-vertical boreholes.

Hydraulic Fracturing Method - HTPF
The HTPF method (Hydraulic Testing on Pre-
existing Fractures), consists of reopening an
existing fracture of known
orientation that has previously been isolated in
between two packers. By using a low fluid injection
rate, the fluid pressure
which balances exactly the normal stress across the
fracture is measured.

The following points should be noted with respect to HTPF:
There is no theoretical limit to the depth of measurement,
provided a stable borehole can
access the zone of interest.
The method assumes that isolated pre-existing fractures, or weakness planes,
are present in the rock mass, that they are not all aligned within a narrow
range of directions and inclinations, and that they can be mechanically opened
by hydraulic tests.
Fractures used in stress computations are delineated on the borehole wall under
the
assumption that their orientation persists away from the hole.
The method is valid for all borehole orientations. It is independent of pore pressure
effects and does not require any material property determination.
It assumes that the rock mass is homogeneous within the volume of interest.
The method is more time consuming than hydraulic fracturing as
the down-hole equipment must be positioned at the exact location
of each discrete fracture to be tested.

Hydraulic Fracturing Method - Example
Q. A hydraulic fracture test in a granite
rock mass yield the following results:
Given that the tensile strength of the rock is 10 MPa, estimate the values
of σ1, σ2 and σ3 assuming that one principal stress is vertical and that the
pressure values were adjusted to account for the formation pressures (i.e.
Po=0 for calculation purposes).
A. Assuming that the rock mass was behaving as an elastic material,
The minimum horizontal stress can
be calculated from the expression:
σh = Ps
σh = 8 MPa Relationships
σh = Ps
σt = Pc’-Pr
σH = 3σh–Pr-Po

Hydraulic Fracturing Method - Example
Q. A hydraulic fracture test in a granite
rock mass yield the following results:
Given that the tensile strength of the rock is 10 MPa, estimate the values
of σ1, σ2 and σ3 assuming that one principal stress is vertical and that the
pressure values were adjusted to account for the formation pressures (i.e.
Po=0 for calculation purposes).
A. The maximum horizontal stress can be
calculated from the expression:
σH = 3(8 MPa) – 14 MPa + 10 MPa
σH = 20 MPa
The vertical stress can be estimated from the vertical overburden
(assuming a unit weight of 27 kN/m3 for granite):
σV = 500 m * 0.0027 MN/m3
σV = 13.5 MPa
σ1 = σH = 20 MPa
σ2 = σv = 13.5 MPa
σ3 = σh = 8 MPa

The Borre probe with Logger connected to a
portable computer for activation and data retrieval.

Principle of soft, 3D pilot hole Overcoring measurements:
(1)Advance φ 76 mm main borehole to
measurement depth;
(2) Drill φ 36 mm pilot hole and recover core for
appraisal;
(3) Lower Borre Probe in installation tool down-hole;

(4) Release Probe from installation tool. Strain gauges bond to
pilothole
wall under pressure from the cone;
(5) Raise installation tool. Probe/gauges bonded in place;
(6) Overcore the Borre Probe and recover to surface in core barrel
(After Ljunggren & Klasson)

Displacements from stress concentrations around a borehole are given by
K1-K4 are correction factors
𝜀 𝜃 =
1
𝐸
[ 𝜎𝑥 + 𝜎 𝑦 𝐾1 − 2 1 − 𝜇2 𝜎𝑥 − 𝜎 𝑦 cos 2𝜃 + 2𝜏 𝑥𝑦 sin 2𝜃 𝐾2 − 𝜇𝜎𝑧 𝐾4]
𝜀 𝑧 =
1
𝐸
[𝜎𝑧 − 𝜇 𝜎𝑥 + 𝜎 𝑦 ]

USBM Deformation Probe
When the probe is inserted in a borehole, six
‘buttons’ press against the borehole wall and their
diametral position is measured by strain gauges
bonded to steel cantilevers supporting the
buttons.
When the borehole is overcored by a larger diameter
borehole, the stress state in the resulting hollow
cylinder is reduced to zero, the diameter of the hole
changes, the buttons move, and hence different
strains are induced in the strain gauges.

Conical Strain Cell
The hemi-spherical or conical
strain cell is attached to the
hemi-spherical or conical
bottom of the borehole. It
also do not require a pilot
hole. After the cell has been
positioned properly at the end
of the borehole and readings
of the strain gauges have
been performed, the
instrument is overcored.
During overcoring, the
changes in strain/deformation
are recorded

Using a hemispherical or conical strain
cell for measuring rock stresses, a
borehole is first drilled. Its bottom surface
is then reshaped into a hemispherical
or conical shape using special drill bits.
Thereafter, the strain cell is bonded to the
rock surface at the bottom of the borehole.
Conical Strain Cell

The Doorstopper cell is attached at the polished flat bottom of a
borehole. Hence, it does not require a pilot hole. After the cell has
been positioned properly at the end of the borehole and readings
of the strain gauges have been performed, the instrument is over
cored. During Overcoring, the changes in strain/deformation
are recorded.
Doorstopper

Leeman indicates that a doorstopper technique was
used as early as 1932 to determine stresses in a rock
tunnel below the Hoover Dam in the United
States, and also in Russia in 1935.
The cell is pushed forward by compressed air and
glued at the base of a drill hole. Reading of the strain
gauges is taken before and after overcoring of the
cell. Hence, they do not require a pilot hole.
Doorstopper

DDGS
A modified doorstopper cell called the
Deep Doorstopper Gauge System (DDGS)
has been developed lately.
The DDGS was designed to allow
Overcoring measurements at depths as
great as 1000m in sub vertical boreholes.
Installation of the DDGS:
(1) After attaining and cleaning of the bottom, the instruments
are lowered down the hole with the wire line cables.
(2) When the DDGS is at the bottom the orientation of the
measurement is noted in the orientation device and the strain
sensor is glued.
(3) The IAM and Doorstopper gauge are removed from the
installation equipment.
(4) The installation assembly is retrieved with the wire line
system.
(5) The monitoring and over drilling start, the strain change in
the bottom is measured by the time.
(6) When over drilling is completed, the core is taken up and a
bi-axial pressure test done to estimate the Young’s modulus.

Successful measurements have been performed in Canada – borehole depths as great as
518m (943m depth from surface), where both hydraulic fracturing and triaxial strain cells
were not applicable at depths deeper than 360m because of the high stress situation. An
advantage for the Doorstopper, as well as the conical or spherical methods, is that they do
not require long overcoring lengths, i.e. only some 5 cm, as compared to the pilot hole
methods (at least 30 cm).
Compared to triaxial cells, a Doorstopper measurement requires less time, and
2–3 tests can be conducted per day.
Furthermore, the end of the borehole must be flat which require polishing
of the hole bottom.
Another limitation is their poor success in water-filled boreholes.
The disadvantage with the doorstopper is, however, that measurement at
one point only enables the stresses in the plane perpendicular to the
borehole to be determined.
ADVANTAGES & Disadvantages

Core Discing
Fig. 5 Discing between 1,920 and 1,931.2 m depth. The core is
oriented with depth increasing from top to bottom and from left to
right

The pre-loaded nature of rock masses has
consequences in rock stress observation.
The process of boring of holes to obtain cores results in stress
concentrations directly at the coring bit/rock interface. As
the core is formed, the annular groove causes the in situ stresses to be
redistributed, creating high-induced stresses across the core. This can
result in damage (irrecoverable strains and microcracks) to the core. If the
in situ stresses are high, and the rock brittle, this
can result in ‘core discing’— the core is
produced in the form of thin ‘poker chips’.
The thickness of the chips decreases as
the stress intensity increases; in extreme
cases, the discs can become so thin that
they have the appearance of milles feuilles,
or flaky pastry. Observation of discing in
cores is often taken as evidence of high
stress zones in the rock.
Core Discing

The following minimum information is needed for the interpretation:
• the tensile strength of the rock,
• Poisson’s ratio of the rock,
• the UCS of the rock,
• the mean disc spacing,
• the shape of the fracture (morphology)
• the extent of the fracture in the core.
The confidence of the interpretation can be increased considerably if the
same information can be achieved from both normal coring and
overcoring at the same depth level.
In practice, core discing can only be used as an indicator for estimation of
rock stresses. When core discing occurs, one can of course also conclude
that rock stress concentrations are higher than the rock strength. Such
information, obtained already during the drilling stage, is of
course valuable and a guide for further decision.
Core Discing

In brittle rocks it has been observed
that discing and breakouts usually
occur over the corresponding lengths
of core and borehole. The thinner the
discs the higher the stress level.
However, the formation of discs
depends significantly on the
properties of the rock and the
magnitude of the stress in the
borehole axial direction. In addition,
the type and technique of drilling,
including the drill thrust, can
significantly affect the occurrence of
discing. It is therefore unlikely that
observation and measurements of
discing will be successful in
quantifying the magnitudes of in situ
stresses
If the discs are symmetrical about the
core axis, as shown in figure above,
then it is probable that the hole
has been drilled approximately along
the orientation of one of the principal
stresses.
Core discs symmetrical with respect to the core axis
Core Discing

Nevertheless, the shape and symmetry of the discs
can give a good indication of in situ stress
orientations (Dyke, 1989). A measure of the
inclination of a principal stress to the borehole
axis can be gauged from the relative asymmetry of
the disc. For unequal stresses normal to
the core axis, the core circumference will peak and
trough as shown in figure next to text. The
direction defined by a line drawn between the
peaks of the disc surfaces facing in the
original drilling direction indicates
the orientation of the minor
secondary principal stresses.
Core discs resulting with
unequal stresses normal to the
core axis
Core Discing

Non-symmetrical core discing,
indicating that the core axis is
not a principal stress direction.
Lack of symmetry of the discing, as shown in figure
above, indicates that there is a shear stress acting
across the borehole axis and that the axis is not in a
principal stress direction.
Core Discing

Refrence
In situ stress Marek Cała – Katedra Geomechaniki,
Budownictwa i Geotechniki
In Situ Stresses &
Stress Measurement
Dr. Erik Eberhardt
Insitu Stress Measurements
U.Siva Sankar
Sr. Under Manager
Project Planning
Singareni Collieries Company Ltd
Stress measurements in
deep boreholes using the
Borre (SSPB) probe
J. Sj .oberg*, H. Klasson
SwedPower AB, Lule ( a, Sweden
Accepted10 July 2003
Geotechnisches Ingenieurbüro
Prof. Fecker & Partner GmbHStress-relief Methods
In situ rock stress determinations
in deep boreholes at the
Underground Research
Laboratory
P.M. Thompson, N.A. Chandler

In situ stresses

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