| ROCK MECHANICS ANALYSIS |
| |
|
|
| |
Rock mechanics is the subject concerned
with the response of rock to an applied disturbance. The basic processes
of rock deformation as rock mechanical properties result elastic moduli,
from elastic moduli rock would hence be described by four mechanical
parameter i.e: |
|
| |
| » |
Young modulus is a measure of the property of the rock to
resist deformation. If a cylindrical rock sample is subjected
to strees parallel to its long axis, it will lengthen and the
diameter of the cylindrical becomes smaller under tension. |
| » |
Poisson`s ratio, under compressiom parallel to the axis, the
rock sample will shorten while its diameter becomes greater,
the ratio of transverse or lateral strain to axial strain. |
| » |
Modulus of rigidity, which is a measure of the resistance
of a body to change in shape. |
| » |
Bulk modulus is the ratio of change in hydrostatic pressure
(stress) to the corresponding volumetric strain. |
|
|
| |
Rock mechanics properties, such as Young`s
modulus, Poison`s ratio, shear modulus and bulk modulus can be obtained
from laboratory measurement with recovered core sample from discrete
depths and with used apparatus i.e : |
|
| |
• Pore Volume Compressibility
Apparatus |
|
| |
| |
 |
Pore volume compressibility
apparatus is subjected to the internal stress exerted by fluids
contained in the pores, and to external stress which is in part
exerted by the overlying rocks. The depletion of fluids from
the reservoir rocks results in a change in the internal (hydrostatic)
stress in the formation , thus causing the rock to be subjected
to an increased and variable overburden load, and the result
is the compaction of the rock structure due to an increase in
the effective stress. This compaction result in changes in the
volume of solid rock constituent (grains) per unit change in
pressure is defined as rock matrix compressibility. The fractional
change in the total or bulk volume of formation per unit change
in reservoir pressure is called the rock bulk compressibility.
Which is the fractional change in the pore volume per unit change
in pressure. In areas where fluids withdrawal from underground
reservoirs may induce subsidence which could result in the loss
wells, appreciable property damage , or earthquakes, the bulk
compressibility is very important. |
|
|
| |
•
Triaxial Apparatus |
|
| |
The triaxial
test cell is usually performed by increasing the axial and confining
loads simultaneously, until a prescribed hydrostatic stress level
is reached , then the confining pressure is kept constant while the
axial load is increased until failure occurs. Triaxial test is used
to determine the ultimate strength (ability of rock to resist stress
without yielding or fracturing ) of rock , i.e: the maximum value
of stress attained before failure. Triaxial test are influenced by
several factors : size and shape of the test sample , rate of loading,
amounts and types of fluids present in the rock sample, mineralogy,
grain size , grain shape, grain sorting and rate of loading.
A triaxial cell contains the loading pistons, which are pressed againts
the sample. The sample is surrounded by a sleeve, which separates
it from the confining fluid in the case of confined test. Stress measurement
are performed with strain gauges in the load cell and radial deformation
of the sample can be measured is to use LVDT`s (Linear Variable Displacement
Transducers, which operate on amagnetic induction principle). |
|
| |
• Acoustic
Velocity Apparatus |
|
| |
The digital Oscilloscope Tektronix
Test Cell is designed to measure the acoustic velocity of a
wave through a core sample, while simulating overburden pressure
on the sample.
Lead titanate Zirconate crystal tranducers are used to transmit
and receive the sound through the core sample. The transmitting
crystal is pulsed either 10 or 100 times per secong by a 450
to 500 volt pulse. The transmitting crytal for the compressional
wave is tuned to about 400 kHz, while the transmitting crystal
for the shear wave is tuned to about 150 kHz. The receiving
crystal are likewise tuned.
Signal from the receivers are amplified in instrumentation amplifier
in the Oscilloscope Panel to make them large enough for the
Oscilloscope to view.
A precision timing circuit generates the pulse to fire the transmitter
and it generates a delayed trigger pulse to be used to synchronize
the oscilloscope to the firings. The trigger delay is variable
in 1 microsecond increments , from 0 to 99 microseconds. Pulse
rates of eighter 10 pps or 100 pps are operator selectable from
the front panel. By delaying the trigger pulse, it is possible
to get an accurate measurement of the travel time of the acoustic
wave through the sample.
|
 |
|
|
|
| |
The mechanical properties of the subsurface
formations is important in connection with wellbore stability problems,
fracturing operations, subsidence problems and sand production problems. |
|
|
| |
|
|
|
|
 |
