D R I L L I N G

A N D

C O M P L E T I O N

F L U I D S

DRILLING-FLUID RHEOLOGY UNDER

DEEPWATER DRILLING CONDITIONS
Previous rheological studies of
water-based muds (WBMs) and oilbased muds (OBMs) concentrated
on fluid viscosities at elevated temperatures and pressures. To evaluate
the impact of cold-temperature rheology on the equivalent-circulatingdensity (ECD) calculations in deepwater wells, three objectives were set
for the present study.
Extend the available rheological
data for the most common drillingfluid systems to low-temperature
environments.
Examine the rheograms and determine the best model to fit the data.
Evaluate the effect of low-temperature drilling-fluid rheology on
the ECD for deepwater applications.
FLUID RHEOLOGY

Fluids that have a viscosity dependent on

shear rate, such as drilling fluids, exhibit
non-Newtonian behavior that is difficult
to describe with simple models. Description of two different non-Newtonian
fluids may require use of two completely
different models.
The simplest model often used to
describe drilling-fluid flow properties is the
Bingham plastic model which describes the
behavior of a fluid that does not flow unless
submitted to a minimum stress (the yield
stress). The Bingham plastic model has been
found to be an unrealistic description of
drilling-fluid rheograms. More appropriate
models are the Herschel-Bulkley model and
Casson models. The Herschel-Bulkley modifies the power-law model by introducing a
This article is a synopsis of paper SPE
56632, Rheology of Various DrillingFluid Systems Under Deepwater Drilling
Conditions and the Importance of
Accurate Predictions of Downhole Fluid
Hydraulics, by J.M. Davison, SPE, and
S. Clary, SPE, Dowell; A. Saasen, SPE,
Statoil; and M. Allouche, SPE, D.
Bodin, SPE, and V-A. Nguyen, SPE,
Dowell, originally presented at the
1999 SPE Annual Technical Conference
and Exhibition, Houston, 36 October.
36

yield stress. The Casson model combines a

yield stress with greater shear-thinning
behavior than the Bingham plastic model.
These three models have been applied to the
data set reported in the full-length paper.
The best data fit was computed for each
rheogram. A linear least-squares method
was used for the Bingham plastic model,
and nonlinear regression was used for the
Herschel-Bulkley and Casson models.

49C at 1 bar.
90C at 344.7 bar.
Viscosity was measured first at 1C and
then the fluid heated to 90C and viscosity
measured. The temperature was reduced to
1C. Viscosity measurements were
repeated at the various temperatures and
pressures to determine whether there was
any hysteresis in the fluid behavior. This
temperature cycling models drilling-fluid
circulation in deepwater wells.

EXPERIMENTAL METHODS

Fluid Selection. Drilling fluids selected

included many of the most commonly used
fluids, including OBMs, synthetic-based
muds (SBMs), and WBMs. Two fluid densities were used for all drilling fluids:
unweighted and 1.6 g/cm3. The base oil
used for the OBMs was a low-toxicity mineral oil while the base for the SBMs was a
linear alpha olefin.
The fluids mixed and tested for each fluid
system were typical formulations using standard drilling-fluid chemicals. All fluids tested
had simulated drilled solids added. The OBM
and SBM formulations included primary and
secondary emulsifiers, calcium chloride
brine, organoclay, gilsonite, and lime, with
barite as the weighting agent. The salt-polymer-based WBM used xanthan gum as the
viscosifier and either starch or polyanionic
cellulose (PAC) for fluid-loss control. The
bentonite-based WBM used bentonite as the
viscosifier/fluid-loss additive with either
starch or PAC for additional fluid-loss control.
The fluids were formulated to achieve a
yield point of 8.6 to 12.9 Pa, measured with
a Fann 35 viscometer. After mixing, the mud
was hot-rolled at 79C for 4 hours to aid
homogenization and additive yielding. Fluid
viscosity then was measured by use of the
Fann 35 viscometer at 49C to verify that the
yield point was close to the specified yield
point. A Fann 70 viscometer was used for
rheological measurements at the various
temperature and pressure conditions.
Test Matrix. The conditions at which fluid
viscosity was measured were the following.
1C at 1 and 137.9 bar.
5C at 68.9 bar.
20C at 1 and 137.9 bar.

RESULTS

OBM/SBM. The data is best described by the

Herschel-Bulkley and Casson models. Both
models show very good correlation for OBMs
and SBMs at both densities. Low-temperature
effects are quite pronounced on OBM and
SBM viscosities. All the test fluids thicken
considerably at low temperatures. The OBM
exhibits the most pronounced increase in viscosity at low-temperature. In all cases, the
increase in pressure at various temperatures
increased OBM and SBM viscosity, especially
at higher shear rates. Pressure effects do not
appear to be very dependent on temperature.
A similar magnitude increase in viscosity
occurred at 1C and 20C for a pressure
increase from 1 to 137.9 bar.
WBM. The Herschel-Bulkley model provided the best fit for the rheograms of both
unweighted and weighted salt-polymerbased fluids. There was no significant evidence of hysteresis in the salt-polymer-based
fluids when circulated from cold to hot and
back to cold. For bentonite-based fluids, the
Herschel-Bulkley model fit the rheograms
very well, but the Casson model was a better
fit for the weighted fluids. Bentonite-based
muds showed some viscosity hysteresis.
Although the plastic viscosity wasnt greatly
different, yield stresses were greater measured on the cooling-down cycle after the
fluid had been heated to 90C. Viscosity
increases at cold temperatures were most
prominent at high shear rates. All WBMs
showed a relatively larger viscosity increase
for unweighted muds than for weighted
muds. For the salt-polymer-based fluids, the
(To Page 39)
NOVEMBER 1999

D R I L L I N G

3-cm core section was extruded and also

placed in a precleaned, prelabeled glass jar.
The remainder of each core was discarded.
Core samples for biological analysis were
treated similarly, except only the top 2 cm of
the core was used. All glass jars were placed
inside clean plastic bags and frozen. The fulllength paper describes sample handling in
detail. When sampling was complete, samples were transported to the Geochemical
and Energy Research Group analytical laboratories at Texas A&M U. for analysis.
FIELD OBSERVATIONS

Video Inspection. No cuttings piles were

observed during either field survey. There was
a thin veneer of cuttings dispersed over much
of the seafloor in a patch fashion. In some
areas, the cuttings were as thick as 2 to 25 cm.
The largest deposits of large chunk-like cuttings were along the southwest transect.
These likely would have been seafloor drilled
during the riserless-drilling phase. In the
1997 survey, the sediment appeared dark,
interspersed by whitish mats and small patches of an orange, mat-like gelatinous material
that was dislodged easily from the bottom.
During the 1998 survey, the seafloor was
more uniform in color and texture, which was

DRILLING-FLUID . . .
(From Page 36)

potassium formate and sodium silicate fluids

exhibited the most significant viscosity
increase. Fluids containing polyglycol
showed excellent tolerance of different temperatures. Increasing pressure at low temperatures does not affect the salt-polymer-based
fluids significantly except for the weighted
sodium silicate and potassium formate fluids,
which exhibit a fluid-shear-stress increase.
Bentonite-based fluids were not affected by
pressure increases at low temperatures.
DEEPWATER ECD

Reliable ECD prediction in wells with significant changes in temperature and pressure regimes requires use of pressure- and
temperature-dependent rheology and density. Rheologies measured during the experimental phase were incorporated into a
software model that can predict ECDs. An
accurate transient-temperature profile must
be computed to model heat transfer
between the wellbore and rock formation
and the riser and sea. The advanced numerical simulator incorporated in the software
model has been validated in a field application. The transient-temperature simulator
NOVEMBER 1999

A N D

C O M P L E T I O N

not surprising because the last well had been

drilled just days before the survey.
In 1997, the videotransects covered 360
m2, representing 90 m in each of the four
directions. A total of 53 fish were seen, representing six taxa. Estimated fish densities
along each transect ranged from 1,333 to
1,888 fish/ha. Crabs and eels also were seen.
In 1998, the videotransects covered a
418-m2 area representing between 80 and
122 m in each of the four directions. A total
of 62 fish were seen, representing seven different taxa. Estimated fish density ranged
from 875 fish/ha on the southwest transect to
2,037 fish/ha on the northeast transect. Other
megafauna observed in the 1998 survey
included several unidentified shrimp and 9
crabs. Eight of the nine crabs were found on
the southwest transect where seafloor topography was the most rugose.
Sediment Analysis. The 1997 data indicated that, regardless of the deepwater currents
running toward the southwest, most of the
SBF was observed on the northeast transect.
This is explained by the stronger surface and
midlevel currents that flow primarily toward
the northeast. As the SBF was dispersed
through 565 m of water, it was affected by

can be used to compute changing ECD

with time for a given fluid and to predict
fluid temperature at any point in the well
during circulation and noncirculation.
Three cases were examined. For Case 1,
measured mud rheology representing temperature and pressure regimes in the well
profile were used as model input. For Case
2, mud rheology varied with temperature
and pressure but was based on laboratory
measurements at 49C and 1 bar. In Case 3,
mud rheology was independent of temperature and pressure.
DEEPWATER APPLIC ATIONS

Solids Control. The temperature simulator

can predict fluid temperature as the drilling
fluid exits the riser. This is an important
requirement if OBMs or SBMs are to be
processed correctly through shale shakers
and other solids-control equipment.
Hydrate Stability. A temperature simulation was run with the salt/polyacrylamide/
polyglycol fluid as an example. After 1,440
minutes of circulation, a period of noncirculation was simulated to determine fluid
cooling at the riser base. After a 3.5-day
period, temperature was 3C, very close to
seabed temperature. Predicted temperatures
can be used with mudline pressures to per-

F L U I D S

these higher-velocity, higher-level currents

causing the depositional pattern observed.
The highest SBF concentration appears to be
along the northeast transect. Lower SBF concentrations are seen along the other three
transects. Numbers and densities of both
mega- and macrofauna do not appear to be
affected negatively by the elevated SBF concentration along the northeast transect.
In the 1998 survey, average SBF concentration ranged from 49,000 ppm in the 2-cm
core sections of the northeast transect to
2,000 ppm in the southwest transect samples.
Average SBF concentration in the 2- to 5-cm
core sections ranged from 30,000 ppm in the
northeast transect to 1,000 ppm in the southwest-transect samples. There were 1,761
macrofauna representing 8 taxa counted in
the northeast-transect core samples and 339
macrofauna representing 12 taxa counted in
the southwest-transect core samples. Detailed
data analysis was not complete at the time the
full-length paper was written.
Please read the full-length paper for
additional detail, illustrations, and references. The paper from which the
synopsis has been taken has not been
peer reviewed.

mit correct fluid-chemistry engineering to

inhibit gas-hydrate formation.
CONCLUSIONS

1. The Herschel-Bulkley and Casson

models both accurately describe OBM and
SBM rheology for temperature and pressure
conditions used in this study.
2. The Casson model fit the weightedWBM rheograms best.
3. In simulations of a deepwater-well
profile for an SBM, ECD determined by a
model where the mud rheology is independent of temperature and pressure can be
underestimated by as much as 6.1% when
compared with the ECD determined by a
hydraulic model that varies mud viscosity
with pressure and temperature.
4. For a WBM, ECD determined by a
model where the mud rheology is independent of temperature and pressure can be
overestimated by as much as 3.1% when
compared with the ECD determined by a
hydraulic model that allows mud viscosity to
vary with pressure and temperature.
Please read the full-length paper for
additional detail, illustrations, and references. The paper from which the
synopsis has been taken has not been
peer reviewed.
39