SPE 90987
New Flat-Rheology Synthetic-Based Mud for Improved Deepwater Drilling
E. van Oort, SPE, Shell EP Americas; J. Lee, SPE, M-I SWACO; J. Friedheim, SPE, M-I SWACO and B. Toups, SPE,
M-I SWACO
Copyright 2004, Society of Petroleum Engineers Inc.
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Abstract
In deepwater drilling operations, synthetic based mud (SBM)
is the preferred drilling fluid because it delivers high drilling
rates and excellent wellbore stability.
However, lost
circulation problems caused by adverse elevation of viscosity
and increase in equivalent circulation density (ECD) as a
function of temperature and pressure often result in significant
operational cost increase. Controlling the pressure and
temperature dependence of SBM rheology thus is an important
step towards achieving successful and cost-effective
deepwater drilling operations.
This paper highlights the development and application of
an improved SBM that optimally balances the requirements
for excellent rheological properties, ECD control, and
improved barite suspension. Unlike conventional SBMs, the
new SBM exhibits a constant (“flat”) rheological profile over
a wide temperature and pressure range. For instance, certain
key rheological parameters, such as 6-rpm reading, yield
point, and gel strengths, remain virtually unchanged when
temperature and pressure are varied. This flat-rheology
characteristic allows for a higher viscosity to be maintained
without negatively affecting drilling rate or ECD. Moreover,
cuttings carrying capacity and barite suspension properties are
greatly improved.
The flat-rheology profile is achieved through the usage of
a re-designed package of emulsifiers, rheology modifiers and
viscosifiers. The emulsifier package minimizes the impact of
drill solids on the rheological properties of the new SBM.
Optimal rheology modification is achieved through minimal
use of organophilic clays. Moreover, a new rheology modifier
also reduces the key viscosity parameters at low temperatures
while raising them at high temperatures. The viscosifier is
used to provide the desired enhancement in overall viscosity
and suspension capacity.
Extensive field data from Gulf of Mexico (GoM) wells
(including drilling performance data, ECD and downhole
pressure data, etc.) shows the benefits of the new flat-rheology
SBM (FR-SBM) system, including lowering of ECD
(resulting e.g. in lower SBM mud losses), improved cuttings
evacuation from the well and prevention of barite sag
problems. The operational success of FR-SBM has led to rapid
acceptance in the field, allowing it to displace conventional
SBM in Shell’s GoM deepwater drilling operations.
Introduction
The utility and reliability of synthetic based muds (SBMs) as
drilling fluids has been well documented in the past decade1-3.
Often it is the fluid of choice in deepwater environments due
to its superiority in achieving high penetration rates and
maintaining desired wellbore stability. One drawback to the
use of SBMs is their potential for lost circulation, especially
when the mud has been static for an extended period of time.
In the deepwater environments, water temperatures easily
reach 40°F (5°C) and below.
This low-temperature
environment effectively cools down the drilling fluid,
significantly increasing fluid viscosity, which in turn impacts
equivalent circulating densities (ECDs) and surge pressures.
Narrow drilling margins (i.e. the window between fracture
gradient and pore pressure) encountered in deepwater drilling
operations often make such rheological increases intolerable,
resulting in severe losses of SBM and thus significant
increases in fluid cost and rig time.
One method of managing these critical ECDs is to manage
the fluid rheology. In general, a thinner fluid yields lower
ECD’s. However, fluid rheology considerations not only have
to address ECD control but also must take into account
cuttings removal and barite suspension for specific wells.
Thus, the main challenge to fluid design in most deepwater
drilling operations consists of effectively balancing fluid
rheology for hole cleaning, ECD and barite suspension
simultaneously. This concept is illustrated in Fig. 1. Since
fluid rheology is the common denominator of all these issues,
a re-design of SBM to minimize the temperature dependence
of rheological properties was believed to be a promising
approach to alleviating the difficulty routinely encountered in
achieving the desired balance among all these issues.
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NEW FLAT-RHEOLOGY SYNTHETIC-BASED MUD FOR IMPROVED DEEPWATER DRILLING
This paper discusses a new generation of SBM that can
achieve the desired balance of fluid rheology and drilling
performance without sacrificing ECD requirement. Field data
are provided to illustrate the benefit of this “flat-rheology”
SBM (FR-SBM) in deepwater drilling.
Design and Development
In the preliminary design phase of this new system certain
criteria were set forth to proceed in a systematic fashion to
accomplish both the desired environmental and performance
goals of the project. Since the initial target area was
deepwater Gulf of Mexico, these goals were to design an
invert fluid that:
1.
Met conditions set forth in EPA Gulf of Mexico
Region 6 GMG 290000 General Permit for syntheticbased drilling fluids.
2.
Exhibited temperature/pressure independent rheological properties focusing primarily on yield point (YP),
low end rheology (e.g. 6 rpm) and gel strengths.
3.
Attained fluid performance/properties for good hole
cleaning and minimal barite sag.
With these design criteria, ester-based fluids were ruled
out due to performance limitations (particularly at
temperatures >250oF when ester hydrolysis may occur),
toxicity under certain conditions and cost. Both an olefinic
external phase and a calcium chloride internal phase were
maintained due to overall utility, cost and compliance status.
This left all the other components such as emulsifiers, wetting
agents, viscosifiers, rheology modifiers, fluid loss additives,
and thinners open for re-design or development. With the
exception of the weight material (barite), every component in
this system is specifically developed to attain the goals set out
above.
Primary consideration was given to the emulsifier package.
Previous work showed certain interactions between
emulsifiers, drill solids and organophilic clays and their effect
on the overall rheology of an invert. It was quickly noted that
increasing the total amount of organophillic clay also
increased the dependence that rheology had on temperature
(Fig. 2). Unfortunately, subsequent work showed that the
absence of organophillic clay leads to severe barite sag and
settling. Furthermore, certain emulsifiers acted (or reacted) in
a fashion to apparently “yield” drill solids, thus resulting in
substantial increases in rheology after contamination (Figs. 2
& 4). Therefore, the initial problem was to develop an
emulsifier package that would not react with drill solids and
thus allow rheological control of the fluid by other additives.
Polymeric viscosifiers and rheological additives were
identified that met the temperature independent requirements.
Subsequent testing, however, showed that these polymeric
additives were not alone sufficient to control barite sag
tendencies in the fluid and validated earlier observations that
in some instances even promoted sag! Therefore, minimal
amounts of organophillic clay were used to provide
SPE 90987
suspension for the barite while not contributing adversely to
the thermal dependence of the rheology.
As with any fluid design, the real “trick” is in the final
formulation work where performance of individual
constituents must work in concert with the other components.
After performing hundreds of tests to evaluate various
combinations and chemistries of surfactant packages,
organophilic clays, and rheology modifiers, a new generation
of SBM with temperature and pressure independent
rheological characteristics was developed. This new SBM
showed similar values of low-end rheology, YP, and gel
strengths in the temperature range of 40°F – 150°F, thus
denoting the “flat” rheological profile moniker (Fig 3). It
should be noted that the “flat” character was not extended
towards plastic viscosity (PV), nor did it need to be. It was
found that normal variation of PV with temperature and
pressure in FR-SBM did not in any way adversely influence
ECD, hole cleaning or barite sag tendencies.
Despite the drastic contrast in rheology, the additives used
to make up the FR-SBM are of a similar type but utilizing
slightly different chemistries than those in the conventional
SBM. A comparison of the types of chemicals and additives
used in these two systems is given in Table 1. Full
environmental compliance was achieved with FR-SBM, which
uses the same base fluid as conventional olefin-based SBM
(hence, there is no change in biodegradation behavior). Mud
toxicity test results indicate no deterioration in environmental
compatibility when compared to conventional SBM. For
further details on mud development as well as in-depth
laboratory evaluation (highlights of which are given below),
the reader is referred to Ref.4.
Laboratory Evaluation
Once the base mud formulation was developed, the fluid
system was evaluated further in the laboratory. Tests such as
contamination with drill solids, seawater, and cement were
carried out to evaluate the stability of the system. In addition,
the behavior of the fluid under temperature and pressure was
measured using a Fann 70 viscometer. Moreover, the potential
for barite sag to occur with the fluid was evaluated in
dedicated inclined flow loop experiments. The evaluation
results are briefly discussed as follows.
The solids tolerance of the new FR-SBM was evaluated
and the properties measured at three temperatures, 40°F,
100°F and 150°F. Fig. 4 shows the rheological profiles of the
FR-SBM (as well as a conventional SBM) after contamination
with 10% by weight (35 lb/bbl) of simulated drill solids
(OCMA clay).
The FR-SBM showed a good response to drill solids,
illustrating the system is not particularly sensitive to solids
contamination. Thus, the interactions among drill solids,
rheology modifier and emulsifier were minimal. More
importantly, the rheological profile (flatness) of the FR-SBM
was retained after contamination with solids.
The tolerance to seawater and cement contamination was
also found to be excellent for the FR-SBM4.
SPE 90987
E. VAN OORT, J LEE, J. FRIEDHEIM AND B. TOUPS
The flat-rheology character also exists under varying
pressure conditions. Figure 5 shows a comparison of the YP
values of an ester-based SBM (I), an isomerized olefin-based
SBM (II) and the flat-rheology SBM measured by Fann 70
with temperatures up to 250°F and pressures up to 15,000 psi.
The flat-rheology SBM displayed similar yield point values
across the wide temperature and pressure range, whereas
temperature and/or pressure impacted the conventional SBM
(II). The ester-based SBM (I) in particular displayed extreme
sensitivity to temperature and pressure variations (c.f. 40°F / 0
psi vs. 40°F / 3000 psi and 40°F / 0 psi vs. 120°F / 0 psi).
Hydraulics Modeling
Temperature- and pressure-independent rheology turns out
to provide unique benefits, as indicated by the results of
hydraulics and hole cleaning modeling studies that were later
on verified in the field. Due to its unique rheology, FR-SBM
can be run at a higher viscosity than conventional SBM to
improve hole cleaning and aid in barite suspension without
adversely affecting ECD. This may at first seem counterintuitive (according to the ECD credo “thinner is better”), until
one realizes that conventional SBM requires low viscosities at
drilling depths in order to offset a subsequent viscosity
increase in shallower (equals colder) parts of the well and in
the riser. No such rheology “compensation” is required with
FR-SBM, which will maintain a constant rheology irrespective
of temperature and pressure conditions. This, as we shall
demonstrate, presents a superior advantage over conventional
SBM, especially when dealing with high-angle extended reach
drilling (ERD) wells that otherwise would be troublesome or
impossible to drill using such conventional systems.
Figure 6 shows a typical result of our hydraulics modeling
efforts. In this example, ECD was calculated for 9 7/8” hole
on an ERD well at 29,000 ft measured depth (MD) at a hole
angle of 60 degrees. In the calculation, use was made of
rheology data as a function of pressure and temperature (as
established using a Fann 70 rheometer) obtained from actual
13.0 ppg field muds. A snapshot of some of the rheology
numbers used in the calculation is shown in Table 2. Relevant
simulation parameters included: ROP = 50 ft/hr, RPM = 115,
WOB = 10 klb, flowrate = 540 gpm. Calculated ECD at TD
for the conventional SBM was 14.10 ppg, whereas for the FRSBM it was 13.85 ppg corresponding to an improvement of
0.25 ppg with the latter mud system. Note that equivalent
static density (ESD) at TD, which is primarily determined by
the compressibility of the synthetic base fluid, was identical
for the two muds at 13.2 ppg. An ECD reduction of several
tenths of ppg is very meaningful for wells drilled in narrow
drilling margins environments. It may, in fact, make the
difference in drilling the well to TD either without any
problems or with great difficulty, trouble time and cost
overruns. Many ECD simulations were run, for both shallow
sections / large diameter hole and deep sections / smalldiameter hole, and the results predominantly favored FR-SBM
over conventional SBM. As demonstrated below, our field
experience with FR-SBM has been consistent with the positive
hydraulics modeling results.
3
Field Results – General
The new FR-SBM system was developed and field tested
in a true collaboration between drilling fluid supplier and
operator. At the time this paper was compiled, 50 hole
sections were drilled with FR-SBM by Shell in the GoM on 18
wells for a total footage of 166,000 ft, representing 33,000 bbl
of excavated hole. Water depths ranged from 1300 ft to 8700
ft and hole diameters from 22” down to 6.5”. In fact, FR-SBM
has now become the system of choice and has completely
displaced the use of conventional SBM in Shell’s GoM
drilling operations. A summary of the main reasons why this
has taken place is given below.
Field Results – Rheology Profile
Fig. 7 shows the dependence of YP versus downhole
circulating temperature as measured in the field for two offset
wells drilled with conventional SBM and FR-SBM
respectively. The conventional SBM shows the effect of
thinning with temperature (YP = 32 lb/100ft2 at 75oF, YP = 20
lb/100ft2 at 150oF) typical of these muds. The FR-SBM by
comparison shows a constant YP of 25-26 lb/100ft2 in the
measured temperature range of 65oF – 150oF. This result
indicates that the flat-rheology profile of FR-SBM as
engineered in the lab can be established and easily maintained
in the field with proper mud treatment.
Field Results – Hydraulics, ECD and Hole Cleaning
In Fig. 8, the ECD profiles for conventional SBM and FRSBM are compared on offset wells. Although the wells were
drilled in the same prospect, at the same hole angle and
compared at the same depths, there was a difference in hole
diameter: 12.25” for the conventional SBM and 11.5” for the
FR-SBM. Even though the analyzed hole size for FR-SBM
was smaller while applied flow rates where higher (770 gpm
for FR-SBM versus 728 gpm for conventional SBM), ECD
was consistently lower in FR-SBM. Moreover, the range of
ECD, i.e. the difference between maximum swab and surge
pressure while drilling and making connections, was lower in
FR-SBM (0.4 ppg) compared to conventional SBM (0.5 ppg).
The improved hole cleaning qualities offered by FR-SBM
are more dramatically illustrated by Figs 9 and 10. In these
17.5” hole sections for offsets wells, distinct pressure spikes
are observed after connections using conventional SBM.
These pressure surges are caused by pack-off. By comparison,
a much smoother ECD profile with absence of surge spikes is
observed for FR-SBM, which also did not show the
pronounced tendency for pack-off after connections (see
close-ups in Fig.10).
Clearly, FR-SBM offers better
suspension of cuttings with a reduced risk for pack-off, while
overall improving hole cleaning efficiency.
A visual illustration of the hole cleaning behavior of FRSBM is shown in Fig.11, showing a photograph of PDC
cuttings circulated out of an ERD well which utilized FRSBM at varying YP. Also included in Fig. 11 is the ECD trace
for the section from which the cuttings were obtained. When a
relatively high YP (28-30 lb/100ft2) was maintained, the PDC
cuttings retained the characteristic markings of the PDC
4
NEW FLAT-RHEOLOGY SYNTHETIC-BASED MUD FOR IMPROVED DEEPWATER DRILLING
cutters, indicative of a relatively short residence time in the
well. For lower YP (15-18 lb/100ft2), the cuttings became
featureless and rounded due to a much longer residence in
cuttings beds. The ECD trace shows distinct correlation
between YP values and ECD, i.e. the lower the YP the higher
the ECD, presumably due to a higher degree of annular
loading at reduced YP. Note that YP was reduced (to ~15
lb/100ft2) prior to running casing and cementing.
Field Results – Mud Losses
Severe mud losses are a regular feature when drilling
deepwater wells with narrow drilling margins using SBM. The
mud loss mechanism has been investigated in detail during the
DEA 13 investigation that took place in the early 1990’s5-7. In
this investigation and subsequent work8, it was demonstrated
that OBMs (and therefore SBMs, which are equivalent invert
systems but with a more environmentally friendly base fluid)
yield inefficient tip screen-out of solids during fracture
initiation. This promotes relatively easy fracture propagation
at relatively lower propagation pressures. Moreover, after
fractures have been initiated they do not “heal” well due to the
fact that synthetic fluids prevent any occurrence of beneficial
clay swelling that may reconnect fracture surfaces. None of
these concerns are addressed by FR-SBM, which in this
respect acts like any regular SBM. We have found, however,
that the fracture propagation characteristics of any SBM,
including FR-SBM, can be improved by incorporation of
particles such as graphites of suitable size, characteristics and
concentration. The incorporation of such materials has been
particularly helpful in preventing / minimizing losses in
shallow hole sections with highly depressed drilling margins.
Despite the above-mentioned concerns, we have observed
considerable reductions in mud loss volumes and mud
consumption with the adoption of FR-SBM. Synthetic mud
consumption due to mud losses (including losses while
drilling, running casing and cementing) has gone down from a
historical 1.95 bbl/bbl of hole with conventional SBM to 1.45
bbl/bbl of hole with FR-SBM, a 26% improvement. In
addition, mud losses were characterized as a function of a
proprietary measure for well complexity (MWC). A 39%
improvement has been observed here, down from a historical
0.85 bbl/MWC with conventional SBM to 0.52 bbl/MWC
with FR-SBM. In Table 3, an example is shown of mud losses
experienced on two offset wells with slightly different casing
programs drilled with conventional SBM and FR-SBM
respectively. Despite the significant losses that were incurred
with the FR-SBM upon running and cementing the 11 3/4”
expandable string (largely due to procedural issues), overall a
greater than 50% reduction in mud losses was achieved with
FR-SBM.
The mechanism for reducing mud losses in FR-SBM
involves the following factors:
1.
Reduction of ECD due to FR-SBM’s temperature- and
pressure-independent rheology, as discussed in the
previous paragraph;
SPE 90987
2.
Better hole cleaning and cuttings suspension in FRSBM, resulting in the reduction of pack-off events that
are prime initiators of mud loss incidents;
3.
Easier annular displacement of FR-SBM during
cement jobs. A common occurrence with FR-SBM is
that strings are cemented with full or at least partial
returns, whereas on offset wells cemented in SBM no
returns during cementing was observed. Although gels
are in general maintained at higher values in FR-SBM,
breaking those gels actually appears to be easier,
resulting in lower surge pressures and frictional
pressure losses in the annulus to be cemented. Note
that this assessment is consistent with the ECD and
surge pressure considerations mentioned in the
previous paragraph. In addition, as noted below, FRSBM yields reduced sag tendency, which is also
believed to be a prime cause of induced losses during
running of casing and cementing.
Field Results – Barite Sag
Barite sag is the undesirable variation in drilling fluid
density due to downhole settling of weighting material, mostly
at mud densities larger than 12 ppg9-10. It is a serious cause of
drilling problems on HPHT wells and wells at higher deviation
(larger than 30o), particularly ERD wells. There are basically
two forms of sag: static sag or settling which correlates with
gel strength of the mud, and dynamic sag which correlates
with low-end rheology of the mud. In this paper, we are
concerned with dynamic sag in wells at higher deviations.
Development of mature prospects in deepwater
environments requires drilling wells deeper and with a larger
step-out to produce less accessible hydrocarbon accumulations. Currently, world-record deepwater wells are being
drilled in Shell’s Ursa and Princess fields that are in the range
of 20,000 ft TVD, 20,000 ft horizontal displacement and
larger than 30,000 ft MD. Such deepwater ERD wells push the
limits on available drilling systems, including drilling fluids.
In particular, severe hole cleaning and barite sag problems
plagued early ERD wells drilled with conventional SBM, to
the extent that they interfered with reaching well objectives.
Fig. 12 shows recorded pressure-while-drilling data for an
ERD well suffering from severe barite sag. The data was
recorded upon tripping in the hole after a change-out of
drilling assemblies. The mud weight on surface for this
conventional SBM system was 13.4 ppg. A constant mud
gradient was observed in the riser and the vertical part of the
hole. However, at the point where the well started to deviate
from vertical a gradual reduction of the hydrostatic gradient
was observed. This gradient reduction, which reaches a
minimum of ~12.5 ppg, is attributed to the effects of Boycott
settling of barite in deviated hole. Towards the bottom of the
hole a sudden and rapid increase in mud weight gradient was
observed at the point where barite started to accumulate. It
was established (see also below) that the effective mud density
of the fluid on the bottom of the hole was in the range of 18.5
– 19.5 ppg due to this accumulation of weighting material.
SPE 90987
E. VAN OORT, J LEE, J. FRIEDHEIM AND B. TOUPS
An interesting observation is that the hydrostatic head
measured at the bottom of the hole actually exceeds the mud
weight on surface plus any usual contribution from mud
compressibility. This increase in hydrostatic head is caused by
the telescopic nature of ERD wells, with barite weighting
material settling out of large diameter, near-vertical sections of
the hole into small-diameter, deviated sections of the hole.
Note that this apparent increase in density at the bottom of the
hole, in combination with the fact that the mud at this location
is highly viscous, increases the risk of induced fracturing and
associated mud losses. This, however, is not the only risk
associated with barite sag, as shown schematically in Fig.13.
Other problems include loss of primary well control and
borehole instability in sections where the hydrostatic head has
been reduced due to sagging, hole cleaning problems due to
adversely affected mud properties and non-uniform mud
density, etc. Such problems may greatly interfere with the
successful delivery of high-complexity deepwater wells.
Additional observed indications of severe barite sag are
shown in Fig. 14. Here, annular pressures were quantified
using a formation pore-pressure tester for an ERD well drilled
with conventional SBM at a mud weight of ~13.0 ppg upon
reaching TD. The measured annular pressures indicate a mud
of very high effective density (~19.5 ppg effective gradient) at
the bottom part of the hole due to the effects of severe barite
sag. Although this result is rather stunning, we still believe
that such instances of sag are a common occurrence on highdeviation / ERD wells and regularly go unnoticed. For
instance, on floating rigs the effects of sag on surface, i.e.
fluctuation of the return mud weight after trips (usually a light
fraction followed by a heavy slug of mud), may be masked by
the effect of the riser boost. Nevertheless, the unsuspected
impact of sag may still be the true underlying cause of
significant wellbore problems experienced in the well.
Fig.14 also shows the annular pressure measurement for
offset wells in the same field that were drilled subsequently
with FR-SBM. In this case, a constant mud weight gradient (of
13.2 ppg) was observed without any indication of barite sag.
Since the adoption of FR-SBM, slight occurrences of barite
sag have occurred only when muds were not properly treated
with rheology modifiers. With proper treatment of FR-SBM,
we have been successful in eliminating mud sagging and
associated drilling problems on high-deviation wells.
Conclusions
1) FR-SBM yields a unique, flat rheology profile
(including low-end rheology, YP and 10 min gel strength) that
is not significantly affected by temperature and pressure. This
result has now been achieved on many field applications and
has been beneficial in deepwater drilling applications, where
temperature and pressure may vary significantly as a function
of water depth and total well depth.
2) FR-SBM allows elevation of overall rheology to
improve hole cleaning and barite suspension without affecting
ECD or inducing lost circulation. Simulated and measured
ECD data have demonstrated that elevated but flat rheology,
5
when compared to conventional SBM, actually lowers ECD.
This effect derives from the improved rheological profile as
well as improved hole cleaning efficiency in FR-SBM.
3) Flat rheology can reduce synthetic mud losses during
drilling and running /cementing of casing. Analysis of the 50
hole sections drilled with FR-SBM, most of which had
subsequent casing or liner runs, indicated that overall mud
losses were reduced significantly. The mechanisms believed
to contribute to this improvement are temperature/pressure
independent rheology, improved hole cleaning and prevention
of pack-off, barite suspension with “elevated” rheology, and
fragile gel structure.
4) Elevated rheology and high-but-fragile gels provide
excellent suspension of cuttings and weighting material that
accelerates cuttings evacuation from the well and prevents
adverse barite settling. Annular pressure data indicate that
barite sag control can be highly optimized when applying FRSBM on ERD wells.
5) Full environmental compliance was achieved with FRSBM, which uses the same base fluid as conventional olefinbased SBM (hence, there is no change in biodegradation
behavior). Mud toxicity test results indicate no deterioration
in environmental compatibility when compared to
conventional SBM.
6) FR-SBM is highly suitable for deepwater drilling where
delicate balancing of rheology for hole cleaning, ECD and
barite sag control is required. Its implementation has met with
considerable success in Shell’s GoM deepwater drilling
operations, to the extent that it has now completely displaced
conventional SBM.
Acknowledgments
The authors wish to thank Shell E&P Americas and M-I
SWACO for allowing the publication of this paper. Many
thanks are extended to project engineers, drilling engineers,
mud engineers, laboratory personnel and technical services
engineers (in particular Lee Conn and David Power) who
helped and were involved in lab testing and field trials.
References
1. Zamora, M; Broussard, P. N., and Stephens, M. P.: “The Top 10
Mud-Related Concerns in Deepwater Drilling Operations”, SPE
59019, SPE International Petroleum Conference and Exhibition
in Mexico, Villahermosa, Tabasco, Mexico, Feb. 1-3, 2000.
2. Davison, J.M., et al.: "Rheology of Various Drilling Fluid
Systems Under Deepwater Drilling Conditions and the
Importance of Accurate Predictions of Downhole Fluid
Hydraulics," SPE 56632, SPE Annual Technical Conference,
Houston, 3-6 Oct, 1999.
3. Wood, T. and Billon, B.: "Synthetics Reduce Trouble Time in
Ultra-Deepwater Borehole," Offshore, March 1998, p.85.
4. Lee, J., Friedheim, J., Toups, B. and van Oort, E.: “A New
Approach to Deepwater Drilling Using SBM with Flat
Rheology” paper AADE-04-DF-HO-37.
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NEW FLAT-RHEOLOGY SYNTHETIC-BASED MUD FOR IMPROVED DEEPWATER DRILLING
5. Morita, N., Fuh, G-F., Black, A.: “Theory of lost circulation
pressure”, SPE 20409 presented at the 65th Annual SPE
Technical Conference, New Orleans, September 23-26, 1990.
6. Onyia, E.C. “Experimental data analysis of lost-circulation
problems during drilling with oil-based mud”, SPE Drilling &
Completion, March 1994, p. 25-31.
7. Fuh, G-F, Morita, N., Boyd, P.A. and McGoffin, S.J. “A new
approach to preventing lost circulation while drilling”, paper
SPE 24599 presented at the 67th Annual Technical Conference
SPE 90987
8. van Oort, E., et al.: “Accessing Deep Reservoirs by Drilling
Severely Depleted Formations”, SPE 79861, SPE/IADC Drilling
Conference , Amsterdam, The Netherlands, 19–21 Feb. 2003.
9. Hanson, P.M., et al.: "Investigation of Barite Sag in Weighted
Drilling Fluids in Highly Deviated Wells," SPE 20423, SPE
Annual Technical Conference, New Orleans, Louisiana, 23-26
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Tables
Table 1. A comparison of the key components of conventional SBM and flat rheology SBM used for deepwater drilling
Components
Conventional SBM
Flat Rheology SBM
Organophilic Clay
Organophilic Bentonite, typically 4-6 ppb
Similar but with ~ 50% lower concentration, typically 1-3 ppb
Blend of surfactants
Modified to improve performance
Surfactant
Different chemistry with no adverse solids interactions
Fluid Loss Control
Polymer
Polymer
Rheology Modifier
Fatty Acid Based
Fatty Acid Based
Polyamide
Polyamide
Emulsifier
Wetting Agent
Viscosifier
Table 2. Properties of conventional SBM and flat rheology SBM used for hydraulics modeling.
Rheological
Conventional SBM
Properties
40 deg.F
80 deg.F
150 deg.F
40 deg.F
PV (cP)
114
68
36
56
2
YP (lb/100ft )
78
40
24
27
6 rpm
38
20
12
15
3 rpm
35
18
11
14
10”/10’
35/57
21/37
16/22
35/36
ECD @ TD (ppg)
14.10
Flat Rheology SBM
80 deg.F
29
26
17
15
35/36
13.85
150 deg.F
23
26
16
15
31/33
Table 3. A comparison of mud losses on two offset wells drilled with conventional and flat rheology SBM. Losses with the latter are typically
less than 50% than with the former mud system
Hole Size
Casing Size
Losses Running Casing
Losses while cementing
Losses to Formations
(bbl)
(bbl)
(bbl)
Conventional SBM
17”
13 5/8”
288
20
35
14”
11 3/4”
88
441
840
12 1/4”
8 5/8”
133
128
1362
8 3/8”
6 5/8”
194
944
173
Flat-Rheology SBM
*
14”
11 3/4” expandable
746
306*
0
12 1/4”
10 3/4”
10
276
0
11 3/8”
8 5/8”
0
55
0
8 3/8”
6 5/8”
0
170
334
* Losses were not preventable due to the operational circumstances associated with running and cementing the expandable liner
SPE 90987
E. VAN OORT, J LEE, J. FRIEDHEIM AND B. TOUPS
7
Figures
Fig. 1 – (left) The concept of mud rheology that is independent of deepwater pressure and temperature behavior, therefore changing minimally
with depth; (right) concept of a balanced rheology to achieve desired drilling fluid goals.
50
45
6-rpm
YP
10' Gel
45
40
40
35
35
30
30
lb/100ft2
lb/100ft2
50
6-rpm
YP
10' Gel
25
25
20
20
15
15
10
10
5
5
0
0
40 F
100 F
Temperature
40 F
150 F
100 F
Temperature
150 F
Fig. 2 – A demonstration of the effect of organophillic clay on the rheology profile of a conventional SBM at various temperatures; (left) no clay
added; (right) 2 ppb organophilic clay added
80
80
70
YP
6-rpm
70
10' Gel
60
60
50
50
40
40
30
30
20
YP
6-rpm
10' Gel
20
10
10
0
40F
70F
100F
Conventional SBM
120F
150F
0
40F
70F
100F
120F
150F
Flat Rheology SBM
Fig. 3 – Profile of yield point, 10-minute gel strength and 6-rpm reading of a 13.8-lb/gal conventional SBM (left) and the new “Flat” Rheology SBM
(right) in the temperature range between 40°F and 150°F.
8
SPE 90987
70
70
60
6-rpm
YP
60
10' Gel
6-rpm
50
50
40
40
30
30
20
20
10
10
YP
10' Gel
0
0
40F
100F
150F
Conventional SBM
40F
100F
40F
150F
100F
Flat Rheology SBM
Conventional SBM + 35 ppb OCMA Clay
150F
40F
100F
Flat Rheology SBM + 35 ppb OCMA Clay
150F
Fig. 4 – Effect of drill solids (35 ppb OCMA clay) on the rheology profile of (left) a conventional SBM at various temperatures; (right) a flatrheology SBM at various temperatures.
40
Conventional SBM I
Conventional SBM II
35
Flat Rheology SBM
30
YP (lb/100ft2)
25
20
15
10
5
0
40°F & 0 psi
40°F & 3000 psi
80°F & 0 psi
120°F & 0 psi
250°F & 3000 psi
250°F & 15000 psi
Fig. 5 – Comparison of the yield point of two conventional SBMs and the new flat rheology SBM measured under temperature and pressure
using Fann 70 viscometer. The conventional SBM I and II show the distinct effects of temperature and pressure on rheology, whereas the flat
rheology SBM shows almost constant yield point over the wide range of temperature and pressure conditions.
12
12.5
13
13.5
14
14.5
35
5000
Conventional SBM
ECD Conventional SBM
Flat Rheology SBM
ECD Flat Rheology SBM
30
ESD Conventional & Flat Rheology SBM
Yield Point (lb/100ft2)
10000
15000
20000
25000
0.25 ppg
30000
25
20
15
10
60
70
80
90
100
110
120
130
140
150
160
Temperature (degrees F)
Fig. 6 – Results of hydraulics modeling for 9 7/8” hole on an ERD
well at 29,000 ft, showing lower ECD (by 0.25 ppg) for FR-SBM
Fig. 7 – Behavior of yield point of conventional and flat-rheology SBM
as measured during actual field applications
SPE 90987
9
Equivalent Mud Weight (ppg)
Equivalent Mud Weight (ppg)
12.3
14000
12.8
13.3
13.8
14.3
12.3
14000
12.8
13.3
14.3
ECD
ECD
MW in
ECD
15000
MW
15000
13.8
16000
16000
17000
17000
18000
Depth (ft)
Depth (ft)
18000
0.40 ppg
19000
19000
0.50 ppg
20000
20000
21000
21000
22000
22000
Fig. 8 – Comparison of ECD profile on offset wells for (left) conventional SBM in 12.25” at 728 gpm; (right) FR-SBM in 11.5” hole at 770 gpm
Equivalent Mud Weight (ppg)
Equivalent Mud Weight (ppg)
10.6
6500
11
11.4
11.8
12.2
7000
12.6
10.6
6500
11
11.4
11.8
12.2
12.6
7000
7500
7500
8000
ECD
8000
MW in
8500
8500
9000
9500
ECD
MW in
9000
Packoff
9500
10000
10000
10500
10500
11000
11000
Fig. 9 – Comparison of ECD profile on 17.5” offset wells drilled at the same flowrate and slightly different mud weight with (left) conventional
SBM; (right) FR-SBM. Notice the pronounced pressure spikes observed for conventional SBM, while these are absent on the ECD trace for
FR-SBM
10
SPE 90987
12.6
1500
Flow
1200
11.5
1400
12.4
1200
ECD
11.45
12.2
600
11.6
11.4
11.4
800
ECD (ppg)
11.8
Pump Output (gpm)
12
900
ECD (ppg)
Flow (gpm)
1000
600
11.35
400
300
11.3
11.2
0
7:40:48
11
7:55:12
8:09:36
8:24:00
8:38:24
Time
200
Flow rate
ECD
0
4:07:41
4:10:34
4:13:26
4:16:19
4:19:12
4:22:05
11.25
4:24:58
Time
Fig. 10 – Close-up of ECD behavior during connections in 17.5” hole (see Fig.9) for (left) conventional SBM, with the characteristics signs of
pack-off induced pressure surge; (right) FR-SBM, with absence of any pressure surge upon bringing back on the pumps
Equivalent Mud Weight (ppg)
10.9
6900
11.1
11.3
11.5
11.7
Lower YP,
Higher ECD
7400
Higher YP,
Lower ECD
PWD data
7900
ECD Model
Mud Wt
YP
Depth (ft)
8400
Low YP,
Higher ECD
8900
Short residence
tim e, YP = 28-30
Long residence
tim e, YP = 15-18
9400
9900
Higher YP,
Lower ECD
10400
15
20
25
30
35
40
YP (lb/100ft2)
Fig. 11 – Hole cleaning–related information from an ERD well drilled with FR-SBM showing (left) the effect of yield point and associated
residence time in the well on cuttings definition; (right) behavior of ECD as a function of yield point, with higher yield points corresponding
to better hole cleaning, lower annular loading and therefore lower ECD
SPE 90987
11
Equivalent Mud Weight (ppg)
12
12.5
13
13.5
14
14.5
0
2000
13.4 ppg
4000
Depth TVD (ft)
6000
8000
10000
Pressure Data TIH
12.0 ppg
Simulated Data
12000
14000
19.0 ppg
16000
18000
Fig. 12 – Recorded ECD data upon tripping in the hole for an ERD well drilled with conventional SBM, showing the tell-tale effects of severe
barite sag in deviated hole sections. The simulated data was obtained for the simplified mud density distribution shown in the Figure.
Fig. 13 – A schematic representation of how various hole problems
(poor hole cleaning, borehole instability, lost circulation) are related to
barite sag
Mud Column Pressure (psi)
16500
17000
Depth TVD (ft)
17500
18000
18500
19000
19500
11700
11800
11900
12000
12100
12200
Mud Column Pressure (psi)
12300
12400
12500
12600
8000
12000
9000
10000
11000
12000
13000
14000
13000
Well 1 Hydrostatics
Nominal MW 19.5 ppg
14000
15000
Depth (ft TVD)
11600
16000
16000
17000
18000
Well 2 Hydrostatics
Well 3 Hydrostatics
19000
Well 4 Hydrostatics
Nominal MW 13.2 ppg
20000
20000
Fig. 14 - A comparison of annular hydrostatic pressure for (left) a well drilled with conventional SBM experiencing barite sag; pressure data was
fitted to a linear curve representing a nominal mud weight of 19.5 ppg; (right) several offset well drilled subsequently with FR-SBM without any
signs of barite sag; pressure data was fitted to a linear curve representing a nominal mud weight of 13.2 ppg. Mud weight on surface was ~13.0
ppg in all instances.