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Robert Schlemmer, Scomi Oiltools, GRTC, Malaysia,
gives an overview of how to minimise fluid losses and
reduce drilling costs in the South China Sea.
D
rilling in cold deep water or cold
weather situations can cause a
3 - 4 fold increase in viscosity of
conventional invert emulsion drilling
fluids. Mud losses due to induced fracture
are likely if that viscosity increase occurs
when the mud weight window is narrow.
Typical invert emulsion drilling fluids
become viscous and do not flow easily
at seabed temperatures; the resulting
increase in resistance to flow due to
frictional losses increases the apparent
fluid column hydraulic pressure when
circulating. Any increase in column
hydraulic pressure can overcome the
wellbore horizontal stress or fracture
gradient, which then results in costly loss
of drilling fluid, severe reservoir damage,
and loss of wellbore integrity.
OILFIELD TECHNOLOGY
NOVEMBER 2008
35
www.oilfieldtechnology.com
Table 1. Comparison of formulations ‘System R’ and ‘CONFI-DEEP’
Components
System ‘R’
concentration lb/bbl
CONFI-DEEP
component
CONFI-DEEP
concentration lb/bbl
Surfactants
7-8
cDEEP-MUL
7 - 11
Wetting agent
1-2
Lime
3-5
Lime
Polymer
0.5 - 1
cDEEP-MOD
Rhelogical modifier
0.3 -1
1st line organoclay
1-2
2nd line organoclay
0.4 - 0.7
Filtration control
2-4
Mud plant viscosity
As needed
Thinner
As needed
Total components
System ‘R’ 10 basic
components
not needed
8
1-2
not needed
cDEEP-TONE
0-2
mud plant and transport procedures, elimination of three
components, and training of drilling and mud engineers.
CONFI-DEEP techniques and components were
carefully developed; their application was properly
planned in detail; and the system was put to work
with surprisingly few problems. The first application
was a complete success.
The cost
CONFI-DEEP’s reduced contribution to ECD and
subsequent potential whole mud losses directly
reduces non-productive rig costs. The flat rheology
Not needed
fluid’s ability to increase clearance of cuttings allows
dependable increased ROP and reduced drilling
hours. CONFI-DEEP like other flat rheology drilling
CONFI-DEEP only 4 basic components
fluids provides higher drilling efficiency and can
reduce formation damage due to whole mud losses to
reservoir rock. CONFI-DEEP, unlike other flat rheology
drilling fluids, improves rheological properties with
fewer additives and much easier mud plant and transport
handling. Overall performance is generally superior to that of
systems introduced in the Gulf of Mexico.
Environmental considerations
Figure 1. Rheological properties of conventional and flat rheology
drilling fluids. Data collected using Fann 75.
The challenge
A fluid is needed which will reduce the likelihood of circulating
losses to induced fractures by providing improved flow
properties at low temperature. This problem occurs when mud
hydraulic pressure approaches the minimum horizontal rock
stress (fracture gradient). The problem is difficult to control with
conventional lost circulation material and may be addressed
more fundamentally by introduction of unique additives which:
l Maintain uniform maximum limit of rheological properties
over a broad temperature range extending from near zero
to bottom hole temperature approaching 250 ˚F.
l Maximise hole cleaning and minimise sag by providing
controlled minimum limit of rheological properties over that
same range of temperatures.
l Make a fluid system that is easy to mix and transport, and
stable and robust.
l Combine to form a very special fluid, which does not
increase equivalent circulating density (ECD) under extreme
deepwater drilling conditions.
An engineered solution
The simple robust CONFI-DEEP ‘flat rheology’ drilling fluid has
exceeded expectations. CONFI-DEEP, a simple formulation
applying only one unique proprietary polymer with off the shelf
components, was refined over 18 months and is now confidently
and routinely used in deep offshore waters of the South China
Sea. An extended development period allowed invention of novel
CONFI-DEEP is an extremely robust system, which is uniquely
solids tolerant. The surfactant package and special polymer
support actually improve rheological properties as drilled solids
increase. CONFI-DEEP is immune to cement contamination.
CONFI-DEEP remains stable with seawater intrusion. Disposal
losses due to contaminants are minimal. Less dilution is
required and the system can be reused across several wells.
The relatively low rheological properties of a flat rheology fluid
with circulation of bottoms up or when using a booster pump do
not interfere with use of fine shaker screens. Mud is not lost over
the shakers on bottoms up. It is not necessary to bypass shakers
because of excessive fluid viscosity. API 270 mesh screens have
been routinely installed when HyPR-BAR fine grind barite has
been used. The solids tolerant CONFI-DEEP combined with
HyPR-BAR fine grind barite can be well managed using primary
solids control equipment, the shale shaker. Material balance
measurements have shown that mass difference between fine
grind barite and drilled solids allows a high speed centrifuge to
more efficiently partition barite and drilled solids for additional
separation. Oil on cuttings is efficiently removed and offshore
compliance will not be compromised in many parts of the world.
History of development
Scomi Oiltools Global Research and Technology Center (GRTC)
in Shah Alam, Malaysia, has engineered a unique non-aqueous
drilling fluid. CONFI-DEEP flat rheology drilling fluid, unlike
traditional invert emulsion systems, has relatively unchanging
viscosity and controlled gelation characteristics when exposed
to the full range of temperatures from coldest deep seabed to
moderately hot reservoir. Work began on CONFI-DEEP in 2006
specifically to satisfy a customer’s request for such a fluid. A
workable formulation was quickly achieved and was tested
by the operator against design specifications in their Houston
laboratory. The fluid was also tested on what has become the
industry standard sag test flow loop using both API barite and
HyPR-BAR fine grind barite variants. Basic fluid performance
was as expected and sag performance as reported by the sag
engineering lab was surprisingly good.
36 OILFIELD TECHNOLOGY
www.oilfieldtechnology.com
NOVEMBER 2008
Despite these positive initial results, testing and
development continued. The delay in rig availability allowed the
Scomi GRTC laboratory to investigate in detail:
l Fluid performance and formulation requirements with
both reduced and higher bottom hole temperature: All the
original work had been performed assuming maximum
sustained temperature of 190 ˚F. It was quickly understood
that a 130 ˚F formulation required significantly less
emulsifier to achieve desired ideal properties. A more
economical formulation can be put to use before higher
temperature reservoir rock is reached. This practice has
been put to use in the field. The system was also tested for
sustained stability at temperatures up to 250 ˚F.
l Mud plant procedures: It was discovered in laboratory
modelling of the extended low temperature aging (as would
be experienced in a liquid mud plant, and during storage
and transport) produced a much more stable fluid for later
application in a well. This lower temperature lower shear
mixing and simulated aging regime is now included in the
CONFI-DEEP procedure manual and has resulted in more
economical formulations.
l Use of special mud plant viscosifiers: It was found that
use of special fatty acid based emulsifier/viscosifiers
typically used in competitive flat rheology systems and
in conventional invert emulsion muds causes extended
complications to CONFI-DEEP properties and presumably
all similar flat rheology muds. As simple fatty acid
emulsifiers degrade with temperature they begin to act
as thinners and increase the required concentration of
polymeric viscosifier cDEEP-MOD and cDEEP-TONE
organophyllic clay. The use of fatty acid viscosifier
materials is no longer recommended and the result is a
longer lived more stable flat rheology fluid and extended
economical and efficient performance of cDEEP-MOD and
cDEEP-TONE.
l Mix technique in absence of special viscosifiers: To
substitute for the temporary viscosifier, a modest
adjustment in the mud plant formulation was developed
and applied on the first offshore job. It worked flawlessly.
No additional materials were required; a yield point of
45 - 50 lb/100 ft2 was achieved. No barite sag was reported
at the mud plant or on the boat during transport.
l Use of system thinners: cDEEP-THIN, a proprietary
thinner, was originally developed for the CONFI-DEEP
system. This thinner is extremely potent, matching
the performance of similar material used in competing
systems. It was found again that use of such a chemical
causes ongoing depression of correct properties of a
flat rheology system which can be time consuming and
expensive to compensate. It has been eliminated from
Scomi’s procedure manual as a standard product. Thinners
such as cDEEP-THIN can be recommended only in the very
unlikely event that rheological properties get out of control
due to mistaken and extreme additions of organophyllic clay.
Loss of rheological control is very unlikely in the CONFI-DEEP
system. The primary viscosifier of the CONFI-DEEP system
is polymeric cDEEP-MOD. Depending upon synthetic
or mineral oil used from 0 - 2 lb/bbl CONFI-TONE
organophyllic clay is required.
l Use of supplemental polymeric rheological modifiers:
cDEEP-RM was originally proposed to supplement and
partially replace a portion of cDEEP-MOD. This substitution
results in incremental loss of rheological ‘flatness’ and
is no longer recommended. Nor is cDEEP-RM needed to
supplement rheological properties in a mud plant mix in the
mix scheme applied for CONFI-DEEP.
Figure 2. Detailed Pressure Analysis Performed using ‘HyPR-CALC’.
l Summary of component elimination work: cDEEP-THIN and
the mud plant viscosifier as well as a proposed secondary
rheological modifier, cDEEP-RM, are not needed nor
recommended if CONFI-DEEP’s very simple preparation and
maintenance procedures are followed. In addition, because the
cDEEP-MUL emulsifier is fully reacted and does not depend
upon formation of calcium soap, lime was omitted from the
early formulation. However, lime was soon discovered to play
a primary role in temperature stability of the system as brine
phase buffer and polymer solubility regulator. Lime, 8 lb/bbl, is
now recommended for every formulation. Excess lime should be
maintained at 4 lb/bbl or more.
Rheological properties, details
Traditional invert emulsion muds are viscosified using relatively
large concentrations of oil dispersible clays and polymers
to efficiently transport cuttings from the well at elevated
temperature. A derivative of that viscosity is expressed as
‘yield point’. Invert emulsion muds which rely on dispersible
organophyllic clay with a yield point of 20 - 25 lb/100 ft2, may
exhibit very high plastic viscosity of 150 cP or more at deep
seabed temperature of 39 ˚F (4 ˚C). CONFI-DEEP does not rely
on cDEEP-TONE organophyllic clay as a primary viscosifier
and gellant at higher temperature. A very small amount, if
any, cDEEP-TONE is used as a low temperature viscosifier.
The small quantity of cDEEP-TONE recommended has little
or no impact on rheological properties above 80 ˚F. For higher
temperature range of rheological control cDEEP-MOD, 2
lb/bbl, is most typically added to the CONFI-DEEP system.
cDEEP-MOD has little or no effect on rheological properties
below approximately 60 ˚F. This small combination addition
of cDEEP-TONE organophyllic clay and cDEEP-MOD polymer
provide balanced rheological properties as the temperature of
the CONFI-DEEP system changes as it is circulated in the well,
through the cold water riser, and the pits.
This improved viscosity profile of CONFI-DEEP flat
rheology drilling fluid supports efficient improved clearance of
drilled cuttings from the deepening wellbore. Drilled cuttings
detrimental contribution to the actual average density of the
drilling fluid is thus reduced. It may seem paradoxical, but the
yield point is more easily controlled at a relatively high value
than the yield point of a conventional invert emulsion drilling
fluid, yet the fluid frictional losses calculated or measured for
the flat rheology fluid are typically significantly less.
OILFIELD TECHNOLOGY
NOVEMBER 2008
37
www.oilfieldtechnology.com
The fundamental benefit of the balanced flat rheological
profile of CONFI-DEEP is excellent hole cleaning across the
full range of temperatures experienced in deepwater wells or in
cold environments. Efficient cuttings removal can be achieved
with the maximum plastic viscosity limited to approximately
50 cP or so, approximately one half to one third of what will
be experienced with a conventional invert emulsion drilling
fluid prepared from the same base fluid. Conventional invert
emulsion muds must often sacrifice hole cleaning to reduce
the ECD component caused by severe thickening of the mud
due to cold temperature viscosity and gelation. Conventional
invert emulsion muds are run with less viscosity. Unfortunately
this can lead to cuttings accumulation in the fluid column and
increased ECD due to actual density increase of the solids
laden drilling fluid itself. Alternatively penetration rate must be
slowed and controlled to prevent overloading the annulus with
cuttings.
Rheological properties, hole cleaning and rate of
penetration
The goal in every drilling interval is to manage flow rates and
fluid characteristics so that the mass of cuttings removed by
the shakers and other surface equipment matches the mass
of the cuttings generated at the bit. However, accelerated
penetration rates in every case lead to some degree of increase
in concentration of cuttings load in the annulus (all other
factors remaining constant). This increase in cuttings load
absolutely causes an increase in mud density and ECD and
can lead to mud losses to induced fractures if not controlled
by ether adjustment of rate of penetration or rheological
properties.
Aggressive rates of penetration can lead to actual
accumulation of cuttings at rheological transition regions or in
washouts in the wellbore. During connections, the cuttings will
fall collecting around the bottom hole assembly or drill string
tool joints. At the very least, pressure spikes will result when
circulation is restarted. Fractures can be initiated or extended
by those pressure spikes. A worst case result of collection of
cuttings, which are not uniformly removed, is packoff of the
hole, stuck pipe, and total and often intractable induced losses.
The very consistent rheological properties of CONFI-DEEP
supports use of increased drill rate. The easily maintained yield
point and 6 rpm values of the CONFI-DEEP system assure
consistent and more reliable hole cleaning. A dependable
non-progressive gel is sufficient to prevent settling of solids
when the drilling fluid is static during connections.
Dynamic rheological measurements are most critical while
drilling but CONFI-DEEP gel strength must also be considered.
CONFI-DEEP’s low clay content and high surfactant
concentration guarantees low and non-progressive gel
strengths. Gels are sufficient to maintain suspension of barite
and cuttings when the mud is static but the pressure spikes
on connections seen with conventional invert emulsion drilling
fluids are essentially eliminated.
development and performance comparisons with conventional
invert emulsion muds. ECD predictions by hydraulics models
constructed last year have been within 0.1 lb/gal. of PWD data
later available.
Hydraulics modelling first called attention to the potential
for reduction in standpipe pressure. The reduction in use of
organophyllic clay and low temperature rheological properties
leads to a sometimes dramatic reduction in standpipe pressure
resulting from CONFI-DEEP’s flat rheolgical properties.
Studies have shown reductions of up to 600 psi depending
on pump rate and ID of the tubulars. With reduced pressure
drop in the drillstring more hydraulic horsepower is available
to the mud motor and bit. The higher potential pump rate and
fluid volume can also contribute to hole cleaning without a
significant change in overall hydraulic pressure and ECD. These
advantages are dependent on booster pump rate and resulting
pump suction temperature.
Dramatically improved flow properties thus reduce the ECD
and subsequent losses to induced fractures in three ways:
l Yield point is easily maintained contributing to efficient
cuttings clearance.
l Potentially increased pump output can also improve hole
cleaning and reduce cuttings load.
l A flat viscosity profile assures lowest possible frictional
losses in the annulus and minimum hydraulic contribution
to ECD.
By reducing hydraulic contribution and cuttings load
contribution to ECD, CONFI-DEEP delivers higher penetration
rate (ROP); as much as 30 ft/hr higher than a conventional
invert emulsion drilling mud prepared from the same synthetic
base fluid. The drilling rate can be increased without
necessarily increasing the ECD beyond designated limits. In the
initial trial of a flat rheology system, the 17.5 in. section was
reportedly drilled with penetration rates as high as 150 ft/hr
with no mud losses.
Conclusion
CONFI-DEEP is a simple, more stable, and better designed system
than competing flat rheology drilling fluids. The basic characteristics
of the flat rheology drilling fluid and CONFI-DEEP significantly
decrease the chance of losses due to fracture induction.
CONFI-DEEP provides stable rheological properties; yield point,
6 rpm viscosity, and gel strengths that are nearly temperature
independent. CONFI-DEEP improves hole cleaning with its high
yield point without accompanying higher plastic viscosity and
gels. CONFI-DEEP supports use of higher penetration rates
than conventional invert emulsion muds. CONFI-DEEP reduces
the chance of whole mud loss and formation damage to
productive reservoir rock. CONFI-DEEP is an environmentally
acceptable drilling fluid and can meet stringent US Gulf Coast
environmental criteria. CONFI-DEEP is the superior member of
the new generation of flat rheology drilling fluids. O T
Rheological properties, hydraulics
Scomi Oiltools performed detailed hydraulic modelling of
the drilling fluids using Fann 75 data collected over the full
range of temperatures and pressures expected in the South
China Sea well. Both Presmod and Scomi Oiltools proprietary
‘HyPR-CALC’ modelling software were used for fluid design
38 OILFIELD TECHNOLOGY
www.oilfieldtechnology.com
NOVEMBER 2008