This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
In The Name Of God
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 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.
Hydraulic Fracturing Method
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). Hydraulic Fracturing
Method
Relationships h = Ps H = 3hPc-Po+ t t = Pc-Pr H = 3hPr-Po In
calculating the in situ stresses, the shut-in pressure (Ps) is
assumed to be equal to the minor horizontal stress, h. The major
horizontal stress, H, 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. Hydraulic Fracturing
Method
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
1, 2 and 3 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: h = Ps h =
8 MPa Relationships h = Ps H = 3hPc-Po+ t t = Pc-Pr H =
3hPr-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
1, 2 and 3 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: H =
3hPc-Po+ t H = 3(8 MPa) 14 MPa + 10 MPa H = 20 MPa The vertical
stress can be estimated from the vertical overburden (assuming a
unit weight of 27 kN/m3 for granite): V = 500 m * 0.0027 MN/m3 V =
13.5 MPa 1 = H = 20 MPa 2 = v = 13.5 MPa 3 = h = 8 MPa
)Overcoring(
overcoring - Borre probe
The Borre probe with Logger connected to a portable computer
for activation and data retrieval. overcoring - Borre probe
Principle of soft, 3D pilot hole Overcoring measurements:
(1)Advance 76 mm main borehole to measurement depth; (2) Drill 36
mm pilot hole and recover core for appraisal; (3) Lower Borre Probe
in installation tool down-hole; overcoring - Borre probe
(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) overcoring - Borre probe
Displacements from stress concentrations around a borehole are
given by K1-K4 are correction factors = 1 [ + 1 2 1 2 cos 2 + 2 sin
2 2 4] = 1 [ + ] overcoring - Borre probe
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.
USBM Deformation Probe
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 Youngs modulus.
DDGS
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 23 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, Poissons 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 Caa 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 Ingenieurbro 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