IADC/SPE 72290
Drilling Fluids Design and Management for Extended Reach Drilling
C. Cameron, SPE, Halliburton Energy Services
Copyright 2001, IADC/SPE Middle East Drilling Technology
This paper was prepared for presentation at the IADC/SPE Middle East Drilling Technology
held in Bahrain, 22–24 October 2001.
This paper was selected for presentation by an IADC/SPE Program Committee following
review of information contained in an abstract submitted by the author(s). Contents of the
paper, as presented, have not been reviewed by the International Association of Drilling
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Abstract
This paper focuses on the potential problems facing the
operator in extended reach exploration and development
drilling. A practical review is presented of world-wide
extended reach drilling fluid experience with examples of
successful technology application from specific projects.
Critical drilling fluid success factors in the planning and
construction of an extended reach well include borehole
stabilisation, hole cleaning, lubricity and equivalent
circulating density (ECD) management. The latest techniques
in monitoring drilling fluid and wellbore behaviour while
drilling an extended reach well are also introduced.
Introduction
Drilling extended reach wells involves some critical issues
that can pose significant challenges for the operator. From the
drilling fluids’ perspective, these include:
• Narrow Mud Weight/Fracture Gradient Window
• ECD Management
• Hole cleaning
• Torque and Drag
• Borehole stability
• Lost Circulation
• Barite Sag
The drilling fluid is a key success factor in extended reach
drilling (ERD) and, historically, oil or synthetic base muds
have tended to be the fluids of choice. However, in the current
era of increasingly stringent environmental constraints, the
industry is striving to expand the fluids technology envelope
with the development of more inhibitive water-based mud
systems, in conjunction with suitable lubricants, to replace
invert emulsion muds.
Drilling fluids for ERD wells are engineered to provide a
flatter rheological profile in order to minimise the effect of the
fluid rheology on ECD. Performance-enhancing products are
used to boost the low shear rate viscosity, a critical factor in
achieving good hole cleaning and avoiding barite sag.
Experience on some of these ERD operations has led to the
development of many torque and drag reducing products and
techniques. For example, a novel fibrous lost circulation
material was found to dramatically reduce torque and played a
major part in the successful drilling of world-record ERD
wells at Wytch Farm and Tierra del Fuego.
Unique downhole hydraulics and hole cleaning modelling
software has been used in conjunction with down-hole
pressure while drilling (PWD) tools to accurately plan and
predict fluid hydraulics and cuttings transport to great effect
on the world’s longest wells. Now incorporating the effect of
cuttings loading in the annulus, together with the effect of pipe
rotation and pipe eccentricity, this software has become an
invaluable operational tool.
Fluid Selection
Invert emulsion muds have been the key ingredient in
successful long reach drilling developments in many areas of
the world, e.g. Wytch Farm, Argentina and the Gulf of
Mexico. This is the direct result of their ability to provide high
lubricity, stabilise reactive clays, preserve hole stability, resist
contamination and produce firm, dry cuttings. Torque and
drag readings and the potential for differential sticking are
substantially lower, using an invert system, with an enhanced
ability to slide and less tendency for cuttings bed compaction.
All these factors combine to make invert emulsion muds the
fluids of choice for extended reach wells. Invert emulsion
muds, based on mineral or synthetic base fluids with low
kinematic viscosity, are well proven in the field and provide a
low ECD, excellent hole cleaning and cuttings suspension and
extremely stable mud properties.
The most suitable water-base fluids currently available for
ERD drilling, when shale inhibition is required, are potassiumbased, non-dispersed, polymer muds containing glycol or
silicates. When inhibition is not required, low solids polymer
formulations or mixed metal silicates may be used. These
systems will provide the required hole cleaning and their use,
with a suitable lubricant, can be highly effective.
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C. B. CAMERON
Current inhibitive water-based mud systems containing
glycols, etc., while achieving excellent results, still do not
match the performance of oil or synthetic base muds. They
produce aqueous filtrates that can result in the onset of timedependent hole instability effects over an extended drilling
period. This increases the potential for packing off and
mechanically stuck pipe. However, these problems occur to a
much lesser degree with silicate muds.
Silicate drilling fluids exhibit remarkable shale stabilising
properties, resulting in gauge hole and the formation of firm,
discrete cuttings when drilling reactive shales. Silicate muds
are now replacing inverts in certain applications. Silicate muds
are low solids polymer systems formulated in seawater or
monovalent brines with the addition of a soluble silicate
complex for inhibition. The mechanism of inhibition is due
principally to a precipitation reaction, which occurs on contact
with divalent ions present at the surface of the shale, rapidly
sealing or partially sealing the pore spaces. A silicate skin or
pressure barrier forms which allows mud hydrostatic pressure
to support the borehole wall in a similar manner to an invert
emulsion mud. The transmission of mud pressure to the shale
and time-dependent increase in near-wellbore pore pressure
are thus significantly reduced compared with conventional
water-based muds. Where the required mud weight is not a
constraint, mud salinity can be raised to control osmotic
pressure and further restrict water migration.
Glycol-enhanced water-based mud systems offer the
advantages of enhanced shale inhibition, improved filter cake
quality, reduced fluid loss and reduced dilution rates. Shale
destabilisation, leading to borehole breakout, is reduced when
using a glycol enhanced mud. Glycol solubility with
temperature can be adjusted by altering the glycol
concentration or the salinity of the mud system. As the
temperature of the mud system increases, under conditions of
constant salinity and glycol concentration, a micro-emulsion
begins to form in the water phase above the cloud point.
Eventually, the glycol is completely ‘clouded-out’ and is
totally immiscible. Research and field experience suggests that
maximum inhibition is provided by the glycol within this
temperature window. For most glycols to be effective, an
inhibiting ion (preferably potassium) needs to be present. In
environmentally sensitive areas, the use of potassium acetate
or potassium formate may be preferred to potassium chloride.
Whether an oil-based, a synthetic based or a water-based
drilling fluid is selected, solids control efficiency is paramount
for the control of low gravity solids and minimisation of
dilution volume. Optimal solids control equipment for an ERD
well would include four linear motion shale shakers, ideally
preceded by four scalping shakers, plus two high speed
centrifuges.
Downhole Modelling
To successfully drill an ERD well, it is crucial to be able to
accurately predict the following parameters under actual down
hole conditions:
• Static and dynamic temperature profile in the well
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IADC/SPE 72290
Hydraulic pressures
Annular pressure loss and ECD
Mud rheology
Pressure to break gels
Specially designed proprietary software is used to predict
down-hole rheology and mud density under static and
dynamic conditions, taking into account downhole pressure
and temperature effects. Down-hole pressure drops, surge and
swab pressures and pressure required to break mud gel
strength in the hole may be accurately predicted.
This software may be used during pre-well planning and
subsequently at the well site in real-time to calculate
hydraulics and ECD under down-hole conditions. Field
validation by comparison with downhole PWD tools has
provided invaluable feedback and demonstrated excellent
agreement between the model and real-time data.
Fluid volume changes in the well due to expansion or
contraction are calculated for shutdown periods without
circulation, when surface mud cools down and mud down-hole
heats up. The software is used to perform look ahead, “what
if” scenarios for ECD, swab/surge, etc. and has proven useful
in extended reach, HPHT, deepwater and slimhole drilling.
Drilling fluid rheology and hydraulics are optimised for the
anticipated drill rates.
By establishing guidelines for drilling and tripping, use of
the downhole rheology/density model, during the planning
phase of the well, can reduce the lost circulation risk. The
model is subsequently used at the rig-site to manage ECD
while drilling. The software predicts the cuttings loading in
the annulus, cuttings bed height, the effect of drill string rpm
and pipe eccentricity and the maximum recommended drill
rate for the given conditions. Use of the software assists in
optimising hole cleaning efficiency while minimising ECD.
The downhole model incorporates Bingham Plastic, Power
Law and the Yield Power Law or Herschel Bulkley model. Of
these, the Herschel Bulkley model is typically used for annular
hydraulics calculations since it most closely simulates actual
mud behaviour over the shear rate range encountered. This
model is seeing increasing application, especially in the realm
of high angle wells, as it allows more accurate modelling of
fluid flow in the area under the drillpipe, i.e. on the low side of
hole, where very low shear rates occur.
The software allows input of surface rheology, measured
using a six speed rheometer, to model downhole rheology at
actual annular shear rates at any depth in the well, taking into
account the temperature and pressure regime at that depth.
This has been accomplished by building a matrix database of
Fann 75 rheometer data to characterise the fluid behaviour. Oil
and synthetic base muds were tested under a series of varying
pressure and temperature conditions, at different base
fluid/water ratios and mud densities. The model will also
accept real-time Fann 75 data, derived during the course of the
well, to improve the accuracy of the predictions.
Gels strengths play a major role in downhole pressure
changes. In wells with a small mud weight/fracture gradient,
drilling window, the pressure required to break circulation can
IADC/SPE 72290
DRILLING FLUID DESIGN AND MANAGEMENT FOR EXTENDED REACH DRILLING
easily result in fracturing the formation if simple precautions
are not taken. High gels can also cause problems of excessive
swab and surge and, during a well-testing phase, can hinder
the transfer of pressure required to operate down-hole test
tools. The in-built routine, designed to calculate the pressure
required to break gels, can be used to minimise the potential
for losses when breaking circulation and to predict the
requirement for treatment of the mud to reduce gels prior to
running casing.
ECD Management
ERD wells are characterised by a high ratio of horizontal
departure to total vertical depth (TVD) and this gives rise to a
fundamental problem. The increasing length of the annulus
and the associated increase in annular pressure loss (APL)
with depth, for a given circulation rate, is not matched by an
equivalent increase in formation strength. This reduces the
mud weight/fracture gradient window and can limit pump rate
to the extent that achieving adequate solids transport can be
difficult, particularly in enlarged hole sections.
Thus, on ERD wells, minimising ECD can be critical to
the success of the project. A balance has to be established
between running the fluid with a minimum plastic viscosity
and the requirement to clean the hole and suspend solids. At
the same time, sufficient flow rate is required to minimise
cuttings bed formation without producing excessive ECD.
An invert emulsion drilling fluid should ideally be built
around a base fluid having a low kinematic viscosity, coupled
with effective emulsifier and fluid loss additives and a top
grade organophilic clay. The quality of the weighting agent
will also play an important role in optimising rheology.
Specialised organoclays have been developed as suspending
agents to enhance yield stress while avoiding significant
impact on plastic viscosity. A base fluid with a low kinematic
viscosity will aid in achieving a flatter rheological profile,
enhancing hole cleaning in large diameter hole sections, ratholes, riser etc., while contributing to lower ECD.
Hole Cleaning
Hole cleaning is critical in the high angle sections of ERD
wells, particularly in long 12¼” tangent sections, and the
provision of three mud pumps is considered essential in
extended reach drilling. Cleaning the hole without inducing
excessive ECD, resulting in down-hole losses, is a major
issue.
Experience has shown that deviated holes with hole angles
in the 40 to 65 degree range are the most difficult to clean.
This is due to the tendency of cuttings to form beds and to
slide back down the hole. Formation of cuttings beds can be
suppressed to a degree by using good suspension
characteristics, i.e. by boosting the low-end rheology.
Continuous monitoring of standpipe pressure/pump output
ratios, cuttings’ volumes and torque and drag will help in
detecting cuttings bed formation.
Flow rate is the primary hole-cleaning parameter and
maximum pump rate, within the constraints of maximum ECD
and the potential for downhole losses, will provide optimal
3
hole cleaning. On one, high profile ERD project, a flow rate of
1000-1100 gpm is used in the 12¼” hole section. Use of 6 5/8”
and 5½” drill pipe allows a 1,000 gpm flow rate to be
maintained to a depth in excess of 5,000 metres. Relatively
high rheology mud is used in this section, keeping Fann sixspeed rheometer 6 and 3 rpm readings around 20 and yield
point around 30 lb/100 ft2. As a result, neither pills nor backreaming are generally required.
One option, when ECD is a constraint, is the use of
circulation subs with bypass flow to boost annular velocity.
These help achieve a more uniform fluid velocity when placed
in the string near diameter changes in the annulus, e.g. above
the top of a liner. Their use can improve hole cleaning without
exceeding ECD limits.
Controlled drilling (where practicable), sweeps, short trips
and optimised flow rates can all have applicability. By coordinating flow rates and rates of penetration, the annular
cuttings concentration can be minimised and the potential for
losses reduced.
In deep deviated wells, the pump pressure can be a limiting
factor in achieving the required flow rate for hole cleaning and
consideration is frequently given to the use of larger diameter
drill pipe (5½” or 6 5/8”) to allow higher flow rates. Of course,
annular pressure loss and ECD limits have to be taken into
consideration to avoid losses and this may preclude the
resulting reduction in the annular clearance.
Torque and drag monitoring, recording drill string pick-up
weight and slack-off weight, will provide a good indication of
hole cleaning efficiency and borehole stability. Inadequate
hole cleaning may require the use of high density or tandem
pills, increased rpm while circulating off-bottom or backreaming. Monitoring of cuttings volume and weight at the
shakers, e.g. by using a trough which fills and automatically
dumps the cuttings at a pre-set weight, provides continuous
real-time feedback on hole cleaning and cuttings transport.
As a result of cuttings regrinding by the drill string, there
is a tendency for fines to build up on the low side of the hole
as a compacted bed and cleaning this out can be difficult. Use
of a novel synthetic fibre material, in hole cleaning sweeps,
helps to minimise and clean out fine solids build up on the low
side of the hole. The fibres tend to form a mat that carries
solids to the surface.
Flow Regime. Field and laboratory experience has shown
that horizontal hole cleaning can be optimised by pumping in
turbulent flow, as the ability of laminar flow to move cuttings
off the low side of the hole is limited. On the other hand,
turbulent flow can result in hole erosion, with increased levels
of filtrate invasion as filter cake is removed or fails to form,
and drilled solids are not held in suspension during
connections. The use of a laminar flow regime, at maximum
flow rate, coupled with sweeps and mechanical methods for
moving cuttings’ beds, is usually the preferred option. In a
reservoir with a low fracture gradient, hole cleaning
calculations suggest that a drilling fluid with a low yield point
will provide minimum ECD. However, in practice, a
compromise must be reached to minimise the potential for sag.
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C. B. CAMERON
Mechanical Methods. Pipe rotation and reciprocation will
assist in mechanically disturbing a cuttings bed, allowing
improved cuttings removal. Field studies indicate that pipe
rotation while drilling enhances the hole cleaning efficiency
by up to 25%. If extensive sliding for directional work is being
performed, occasional high-speed rotation of the drilling
assembly at 150-180 rpm should stir up and help clean out any
cuttings beds that have accumulated.
Ideally, the drillstring should be rotated at all times while
drilling, if at all possible, even when running downhole
motors. This action will force the cuttings up-wards to the
high side of the hole into the faster moving mud. During
periods of sliding or orientation, it is recommended that
sufficient time is allocated to rotating the drillstring while
circulating prior to making a connection. This will assist in
lifting any cuttings from the low side of the hole and transport
them away from the BHA, reducing the risk that they may
pack off around the drillstring.
Rotation can have a very positive effect in the high angle
sections of the hole and is essential for improved hole
cleaning. The directional equipment should satisfy that
requirement. However, it is important that care is taken in the
use of high drill-string rotary speeds as this can cause hole
damage in certain circumstances. Rotary continuous
technology is being investigated by the industry to avoid
stopping rotation and circulation during connections and
prevent cuttings’ settlement. In near vertical hole sections, one
must rely on mud properties and flow rate
Hole Cleaning Sweeps. If cuttings returns do not appear to
be sufficient for the drilling rate, perhaps because of pump rate
limitations, one option is to pump a high-density pill at
maximum pump rate, while rotating the drillstring at
maximum permissible rpm. The pill should be of such volume
as to extend for 100 to 150 metres along the annulus. Care
should be exercised to ensure that excessive amounts of highdensity pills are not pumped due to their effect on hydrostatic
head.
A new coarse grind barite temporary weighting material
has been developed for weighted hole cleaning sweeps and
trip slugs so as to maintain a uniform mud weight, a critical
requirement in ERD wells and in any well with a narrow mud
weight/fracture gradient window. This temporary weighting
agent is easily removed at the shakers, ensuring that the active
system mud weight is not affected. This procedure also avoids
the requirement to handle weighted sweeps on surface and
avoids base oil treatment to counter increases in active system
mud weight, thus reducing dilution costs and avoiding downtime required to stabilise the mud weight.
If high rpm cannot be used, e.g. because a bent motor
housing is in use or a short radius well is being drilled, high
density pills are less effective and a tandem pill might be
considered. Tandem pills can be very effective in extended
reach and horizontal drilling. They typically consist of a low
viscosity pill (water, base oil or mud premix) followed by a
IADC/SPE 72290
weighted viscous pill. The low viscosity pill should be pumped
in transitional or turbulent flow for maximum benefit and to
prevent high-side channelling.
A tandem pill might consist of 30-50 bbl of unweighted, low
viscosity premix followed by a 16-18 ppg weighted pill, sized to
balance out at the circulating system mud weight. The low
viscosity pill in turbulent flow will scour cuttings from the
cuttings’ bed into the main annular flow path. The weighted pill,
with its increased buoyancy, can help in lifting the disturbed
cuttings out of the hole as they fall out of the base oil pill. Prior
to pumping such a pill, the effect on the hydrostatic head and
borehole stability, as the pills are pumped out of the drill string,
should be carefully examined.
Tandem pills should be treated with care as their use has
occasionally caused packing-off and stuck pipe with loss of
the hole section. These problems have usually arisen because
of too low a pump rate resulting in the base oil channelling up
the high side of the annulus, causing the low side cuttings bed
to avalanche downhole and bury the BHA
Pills containing fibrous lost circulation materials have
proved to be excellent hole cleaning sweeps on high angle and
extended reach projects and can help to reduce problems
associated with the accumulation of fine drilled solids in the
tangent section.
The return of all pills at the surface should be monitored at
the shakers to gauge their effectiveness. In the case of all pills
do not stop or reduce the circulation rate before the pills have
been evacuated from the hole. To do so could result in
material dropping out of the pill and possibly avalanching
downhole.
Circulating Clean. Long, high angle 12 1/4” holes, reaching
over 80 degrees, often require up to four times bottoms up to
clean up, possibly more, even with 1100 gpm flow rates.
Circulating time may be long but it is usually worth it, as
subsequent trips can prove trouble-free as a result.
Increased rpm will assist in cleaning a cuttings bed, should
one have formed. A minimum of 120-150 rpm has been found
to be beneficial while drilling ERD wells at high angle. When
circulating clean, the drillstring should ideally be rotated and
reciprocated at least one single or one stand off bottom to
avoid under-cutting the hole and inadvertently side-tracking.
It should be noted that high rotation can induce cavings
and a close watch should be kept at the shakers during this
procedure to determine both the upside and the downside to
high rotation. If the downside is greater than the upside, the
rotation procedure should be stopped or different rotary speeds
tried.
Trips. When tripping out of the hole care must again be taken
to avoid dragging cuttings bed accumulations up the well bore
where they may pack-off around the BHA. Close attention
should be paid to the torque and drag plots. These will give a
good indication of the rate of cuttings bed development.
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DRILLING FLUID DESIGN AND MANAGEMENT FOR EXTENDED REACH DRILLING
When pumping out of the hole, use of maximum allowable
pump rate is encouraged as less than this and the assembly
will only be lubricated over any cuttings bed, effectively
packing the cuttings and increasing the drag during the next
trip in the hole.
Prior to tripping, if any concern exists in regard to hole
cleanliness, consideration should be given to pumping a high
weight or tandem pill while on bottom. This is especially
important if no pills have been pumped during the section
drilled.
If it is necessary to back-ream all the way out, it may be
prudent to stop and circulate the hole clean half way out and
again at the casing shoe. This may help avoid overloading the
annulus. The drill string should be reciprocated over a stand
while circulating.
When back-reaming out, it is again important that flow
rate is maintained at drilling gpm, not lower. If this is not
done, the bed is merely moved back up the hole by the top
stabiliser until a point is reached where the bed packs off
around it.
Torque Reduction
In minimising torque and drag levels, a smooth well profile is
important. Low dogleg severity and well tortuosity all assist in
reducing torque and drag. Many operators have adopted a
well path with a modified catenary profile to reduce frictional
losses along the drill string. Rotary steerable systems control
borehole inclination and azimuth while continuously rotating
and result in a smoother wellbore with the added benefit of
improved hole cleaning. Remedial torque reduction techniques
include the use of drill-pipe protectors and torque reduction
bearing subs.
Substantial torque reduction on ultra-extended reach wells,
at Wytch Farm and Tierra del Fuego, has been achieved as the
result of additions of a proprietary fibrous lost circulation
material to the active system. A reduction approaching 25%
has been observed on these wells and the use of this material
proved a major factor, along with the advent of rotary
steerable systems, in achieving 11 kilometre offset, worldrecord wells. Controlled continuous additions are required
while drilling to maintain the concentration above a threshold
level that must be reached before a significant reduction in
torque and drag becomes apparent.
Water-based muds have been used to drill ERD wells with
horizontal departures up to 7.6 km. The horizontal departure
of the current world record ERD well, drilled by BP at Wytch
Farm using oil-based mud, is 10.7 km. When long tangent
sections are drilled with water-based fluids, considerable
dilution and centrifugation is often required to control the
drilled solids content of the mud. Oxygen corrosion can also
be a concern when drilling long sections with polymer muds.
The use of a lubricant cocktail has achieved dramatic
reductions in the torque inside casing in high departure wells
in Alaska, with reductions of up to 50-60% at two fields on the
North Slope. Friction factors were reduced to around 0.120.20 from 0.28-0.38 for the untreated mud system. The
lubricant cocktail has the ability to endow a water-based fluid
5
with a lubricity approaching that of an oil or synthetic base
mud and substantial cost savings have been made, together
with reduced environmental impact. In one case, their use
eliminated the requirement for oil-based mud or a rig upgrade
to complete an ERD project.
For reduction in torque whilst running casing, the use of
solid glass beads has been developed, which act as “ballbearings” between the casing or open hole and the drill pipe or
casing. These beads can withstand temperatures up to 1400°F
and down-hole pressures up to 48,000 psi. The glass beads are
chemically inert and do not affect the base mud system.
Procedures have been developed for ensuring the maximum
benefit is derived from their application.
Bore-Hole Stability
Hole Stability plays a major role in extended reach drilling. In
ERD wells, well construction time increases almost
exponentially with horizontal departure, resulting in increased
trip times and increased open hole exposure at high angle. An
in-house proprietary borehole stability model takes into
account both physical and chemical aspects of the interaction
of drilling fluid with the shale and can accurately predict a
safe mud density window and optimum salinity as a function
of hole inclination and azimuth.
Key success factors in the prevention of well-bore
instability include:
• Drilling fluid design in line with geo-chemical and geomechanical properties
• Design of the fluid with a flat rheological profile to avoid
high ECD and minimise pressure transients
• Optimisation of wellbore azimuth and inclination relative
to in-situ rock stresses and bedding planes
• Design of the fluid to eliminate flow regimes that may
promote formation washout
• Optimised procedures for hole cleaning, tripping,
surge/swab reduction and mud loss avoidance
Well-bore Breathing
During well-bore breathing or ballooning, losses occur while
circulating but fluid is regained when the pumps are turned
off. The fundamental mechanism for ballooning, the opening
and closing of induced or in-situ micro fractures while
drilling, can be explained by reference to the fracture
propagation pressure.
When the bottom hole pressure or ECD equals or exceeds
the Fracture Propagation Pressure (FPP), a stable radial
fracture is propagated. When the pumps are turned off, the
ECD falls below the FPP and the in situ horizontal stress
causes the fracture to close. The closing of the fracture pushes
the mud back into the wellbore. Thus some volume of mud is
lost every time the ECD exceeds the FPP and then regained
when the pressure drops below the FPP, as long as there is a
stable radial fracture propagation. If the ECD exceeds the FPP
sufficiently, fracture propagation becomes unstable and the
resultant large fracture extension causes massive mud losses.
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C. B. CAMERON
Research has shown that both water and invert emulsion
muds produce the same fracture initiation pressures. However,
one of the drawbacks to using an invert system is that these
muds rsult in a lower fracture propagation pressure than water
based systems. This means that in deepwater drilling or on
extended reach wells, losses can be more difficult to cure
when using an invert system. Laboratory work has confirmed
the field observation that breathing is more predominant with
OBM and SBM systems. Accurate interpretation of the leakoff test (LOT) provides valuable insight into estimation of the
formation closure stress, which is the minimum stress that has
to be exceeded to propagate a fracture in the formation.
Losses that occur during wellbore breathing appear to take
place to fractures in non-permeable formations, i.e.
predominantly shale. There is no loss of mud or filtrate to
matrix permeability, only whole mud into the fracture
network. Pressure while drilling (PWD) data clearly shows
this phenomenon on connections. The normal method of
stopping these losses is to ensure that the ECD is lower than
the FPP.
Use of granular lost circulation materials such as calcium
carbonate or ground marble will do little to stop breathing.
However, use of a novel resilient carbon lost circulation
material has proven effective, on numerous deepwater wells in
the Gulf of Mexico, in reducing the occurrence of wellbore
breathing. This material works by increasing the fracture
propagation presssure of the formation when drilled with
invert mud systems. Independent laboratory tests have shown
improvement in FPP of up to 90% when used in diesel oilbased mud.
This material is added as a pre-treatment to the mud
system to help reduce the risk of losses and increase fracture
propagation pressure. The product is composed of highly
resilient, dual composition carbon particles. When packed
under compression in a fracture, these particles expand or
contract and maintain the seal without being dislodged or
collapsing, when fracture width changes as a result of pressure
changes in the wellbore.
Barite Sag
Barite settlement or sag is a key issue in high angle or
extended reach wells. It is a problem that we can probably
never eliminate but can successfully manage by a combination
of drilling fluid design and good operational practices. In a
deviated well, barite sag can lead to fluctuations in mud
weight, downhole mud losses, well control problems, borehole
instability and stuck pipe. Critical hole angles lie in the 50°75° range. Barite sag is essentially a dynamic phenomenon but
often becomes apparent after a period when the mud has
remained static in the hole for some time.
Any operation that induces low shear rates, e.g. slow
circulation rates, running casing, wireline logging or even gas
bubble migration, has the potential to accelerate sag. Flow
loop tests have shown that settling is minimal while mud is
static in the hole but barite beds begin to form as flow is
initiated. Increasing the low shear rate viscosity of the fluid
generally helps to reduce the potential for sag but may not
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eliminate it. Drillpipe rotation also helps to minimise the
problem and can virtually eliminate sag at 150 rpm or more.
Similarly, sag tends not to occur with annular velocities over
100 ft/min.
Drilling the Reservoir
In recent years, the practice of completing wells with a
deviated or horizontal open hole section has become
established as a means of achieving improved productivity.
The improved reservoir drainage and productivity that can be
achieved from a horizontal well has had a major impact on the
economics of some developments.
Along with the increased use of open-hole technology,
there has been a significant development in the field of
specialised drill-in fluids. This has followed from a
recognition that the fluid used to drill the upper hole sections
may not be appropriate for the payzone. It was realised that
the reservoir drill-in fluid should be designed to meet the
specific characteristics of the reservoir in order to be
minimally damaging. This last requirement is particularly
important as, in open-hole completions, there is no perforation
stage to bypass any permeability damage. Use of a dedicated
reservoir drill-in fluid means that specific aspects of the
payzone, such as clay stabilisation and compatibility with
formation water, can be addressed directly. It avoids any
requirement to reach a compromise with the different
requirements of the upper hole drilling fluid.
It is important to maintain a tight HPHT filtrate and a mud
density sufficient to maintain adequate hydraulic support for
the formation. The filtration characteristics of the mud should
be such that there is a rapid deposition of a tough,
impermeable, erodible filter cake. Fann 90 and particle
plugging apparatus (PPA) are used to determine the optimum
concentration and particle size distribution of bridging
materials.
Incorporation of sized ground marble in the drilling fluid
can reduce the risk of losses and differential sticking while
drilling permeable formations. Selected to match the pore
throat size distribution of the formation, the ground marble
reduces filtrate invasion and formation damage by quickly
bridging on the face of the well-bore and promoting the
formation of a thin, tough filter cake. As marble is a
metamorphic rock, it is more resistant to attrition than
bridging agents based on limestone that degrade more quickly,
resulting in failure to bridge. The use of ground marble,
combined with suitable lubricants, has been instrumental in
the successful drilling of a number of wells with high
differential pressures, up to several thousand psi, without
losses.
Well Characterisation
Well characterisation procedures can make it possible to
rapidly identify a fluid influx and to distinguish it from other
sources of flow-back (e.g. wellbore breathing), providing early
kick detection. The rapid identification of a kick, or the onset
of lost circulation, can result in significant time and cost
savings and contribute to improved safety.
IADC/SPE 72290
DRILLING FLUID DESIGN AND MANAGEMENT FOR EXTENDED REACH DRILLING
The fluid flow-back profile observed when the mud pumps
are shut down on a connection can be the result of a number of
sources: surface equipment drainage, fluid compressibility,
thermal expansion, wellbore breathing or a fluid influx. This
flow-back is compared in real-time with a base-line flow-back
profile that has been recorded with the drillstring inside a
cased and cemented wellbore prior to drilling out of the shoe.
Baroid and Sperry-Sun have worked together in developing
this procedure which involves automatic monitoring of surface
flow-back and use of the downhole modelling software. The
ability to accurately identify downhole events and make the
correct response to prevailing drilling conditions helps to
reduce non-productive time and drilling cost.
The Future
Improvements to existing mud systems to meet new
environmental regulations and the development of highly
inhibitive water-based drilling fluids, that offer performance to
match invert emulsion muds, have a high priority in the
industry. Development of new lubricants for these high
performance water-based muds, drag reducers to lower drill
string or coiled tubing pressure losses and drilling fluids with
inherently lower ECD will represent a major step forward in
our extended reach drilling capabilities.
Acknowledgements
The author would like to thank the management of the Baroid
PSL of Halliburton Energy Services for permission to publish
this paper.
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