American Oil and Gas Reporter - November 2016 - 72

SpecialReport: Drilling Fluids Update
FIGURE 3
TWC Testing of Cylindrical Mancos Shale Samples

10.0%

UT_MS_5V3_TWC
INCHES

8.0%
Pre Test
L: 2.141 in
D: 0.998 in
Wt: 69.435 g
L: 2.141 in
D: 0.998 in
Wt: 61.521 g

2
Post Test

Source: Eric van Oort, et. al., AADE 2016

duces the cost and liability associated
with using NAFs in many areas.
Stability, Lost Circulation
Various shale/fluid compatibility tests
have been used to help select a WBM to
stabilize the specific formation to be
drilled. Scientists at the University of
Texas at Austin have introduced a lowcost laboratory shale test for wellbore
stability that provides results with the
quality of more sophisticated and expensive protocols, such as downhole simulation cell testing.
The modified thick wall cylinder
(TWC) test exposes cylindrical shale
samples to mud formulations at overbalanced conditions under confined downhole
temperature/pressure regimes. After a
specified time, the failure strength of the
sample is assessed. The new procedure
is easier and more economical than other
test methods. Moreover, it can be used
for the comprehensive qualification of
new nanoparticle and high-salinity fluid
formulations.
Figure 3 shows photographs of cylindrical Mancos Shale samples before coring
the 0.3-inch borehole, before TWC testing,
and after TWC testing when collapse has
occurred. The samples measure one inch
in diameter.
There is significant industry interest
in understanding the material properties
of lost circulation materials (LCMs),
including size degradation studies from
mechanical compression tests and shear
72 THE AMERICAN OIL & GAS REPORTER

FIGURE 4
Average Reduction in Low-Gravity Solids

6.0%
4.0%
2.0%
0.0%

Active Mud

Centrifuge
Effluent

Centrifuge Paste

Rig Shaker
Discard

Source: Mike Morgenthaler, et. al., AADE 2016

flow. These types of tests are particularly
important in mature fields, which often
require LCMs as background material.
Total E&P U.K. and Halliburton
performed a flow loop test to study the
size degradation of LCMs, mainly ground
marble (GM) and resilient graphitic carbon
(RGC). The flow loop test used flow
rates and conditions selected on the basis
of what would be encountered in the
field. Predetermined amounts of GM and
RGC were added to an 80-barrel synthetic
fluid, and the fluid/LCM was circulated
through the flow loop at a rate of 400gallons/minute.
The flow loop testing demonstrated
that RGC had excellent resistance to size
degradation compared with GM and other
LCMs such as calcium carbonate. The
results obtained from the flow loop tests
were in good agreement with data on
size degradation of LCMs from laboratory-based testing, and show the
importance of using a resilient LCM such
as RGC as part of a background package
when drilling lost-circulation problem
zones in mature fields.
Retort Testing
In a presentation at the AADE conference, Cutpoint Inc.'s Mike Morgenthaler, Cenergy Engineering's Weston
Hinton, and Texas A&M University's
Lance Nace described a consistent and
systematic error in API retort testing.
Accepted field practice is to report
volumetric water, oil and solids based on

the sample density measured prior to the
start of the retort procedure. However,
the researchers discovered that the API
retort procedure and using volumetric
data resulted in an error of ±4 percent
for a drilling fluid. This error is caused
by the loss of mass during the retort test
run.
The missing mass is predominately
from the liquid phase of the sample, and
not the solid phase. Failure to capture
and condense this missing liquid in the
graduated cylinder causes measurement
errors and results in reporting solids contents higher than actual.
A gravimetric method is proposed as
an alternative retort procedure that can
be used as a check on the API procedure
and to correct the calculated results from
the retort test. Figure 4 shows the average
correction for retort data on different
sample types.
From the test results, the study concluded that mass was always lost when a
50-milliliter retort was run, and that qualitative evidence pointed to water as the
most likely constituent failing either to
report to or condense in the graduated
cylinder. In addition, the study found that
low-gravity solids concentrations in drilling
mud and cuttings likely were being overreported because of the missing mass
when empirical data from 50-ml retorts
were reported accurately.
Further lab work must be undertaken
to confirm the nature of the missing mass
and establish a statistical basis for math-

American Oil and Gas Reporter - November 2016

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