American Oil and Gas Reporter - January 2015 - 136

Few experiments have been performed
to investigate the effect of temperature
on compressive strength and other sedimentary rock properties. Some experiments have been conducted to assess the
effect of temperature on shale rock samples, but since shale oil rock samples are
quite different from common shale rock
samples, the results cannot be applied
equally to shale oil and shale gas formations.
As part of a Texas Tech University
study, the mineralogy of core samples
from the Eagle Ford play was determined.
To simulate actual well conditions, a
high-pressure, high temperature setup
was built to apply different axial and
radial confining stresses, equivalent formation pore pressures and drilling fluid
wellbore pressures. The experiments
were conducted under elevated temperatures to better mimic real drilling operations.
The behavior of source rocks with
high total organic content is strongly
temperature-dependent and predominantly
plastic at elevated temperatures. Microfracture systems are generated that
resemble natural assemblages. The fractures are tensile and related to internal
pressure built up in the pore fluid.

Study Objectives
The general objectives of these experiments were to assess the mechanical
properties of the rocks and to analyze the
data in a way that would lead to a better
understanding of those properties. More
precisely, the study sought to be able to
predict problems that might occur during
drilling, and to create effective and efficient
solutions to avoid those problems for
cost-effective drilling operations.
Saturated Eagle Ford core samples
were tested under various temperatures,
including actual reservoir temperature.
Unconfined compressive strength (UCS)
tests were performed to investigate the
effect of temperature on core sample
compressive strength. The experimental
setup was modified to accommodate five
linearly variable displacement transducers
(LVDTs) to measure Young's modulus
and Poisson's ratio. Various experiments
were run to quantify the effect of temperature on the rock compressive strength,
Young's modulus, and Poisson's ratio.
Experiments have shown a distinct change
in the mechanical properties of the rock.
The Eagle Ford formation sample selected for the study can be described
among sedimentary rocks as a shale oil
with 26 percent clay, 0.65 percent water

content, less than 1 percent absorption,
and a unit weight of 158 pounds per
cubic foot. The mineralogy of the rock
sample showed 48 percent calcite, 13
percent kaolinite, 13 percent muscovite,
13 percent pyrite, and 11.3 percent quartz
by weight percentage.
Five samples were cored and prepared
from the main sample so that there was
one sample per experiment. All samples
were prepared according to specifications
from American Society for Testing and
Materials D-2938, and were labeled as
140, 150, 160, 170 and 180 degrees Fahrenheit to indicate the temperatures under
which the five samples were tested.
All samples were vacuumed, saturated
with 30,000 parts per million brine, and
placed in a HP/HT core holder to be
located inside an oven for 12 hours, simulating wellbore conditions. The five tests
at the different temperatures were completed,
and all samples then were tested for unconfined compressive strength. The testing
parameters for all five tests included 6,000
psi of radial stress, 4,600 psi of axial stress,
3,500 psi of pore pressure, and 3,800 psi
of mud pressure. LabVIEW™ graphical
programming software was used to record
radial stress, axial stress, pore pressure,
mud pressure and temperature.

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136 THE AMERICAN OIL & GAS REPORTER

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American Oil and Gas Reporter - January 2015

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