The Method of Management Equivalent Circulation Density
in Extended Reach Drilling
1
Sun Mingxin , Yang Chunxu1,2, Feng Guangtong1,Tang Honglin1
(1.Shengli Petroleum Engineering Co. Ltd, Drilling Technology Research Institute, Donying,
Shandong, 257017 ; 2.Petroleum Engineering College of China University of Petroleum, Qingdao,
Shandong, 266580)
Abstract: Extended reach drilling (ERD) wells are defined as wells that have a
horizontal departure at least twice the true vertical depth of the well. Advantages of
ERD have been unfolded in the flied with its development in exploration. However,
management of equivalent circulation density (ECD) is one of the key issues in
shallow-type ERD and in a narrow drilling window where the pore pressure and
fracture pressure gradients are close. The conventional techniques that are used to
reduce ECD were summarized. The methods of management ECD were proposed in
Operational Phase. The new method and tools of management ECD were also
introduced.
Keywords: extended rearch drilling; equivalent circulation density; the method;
Introduction
ECD management is one of the key issues in designing and drilling ERWS. High
ECDs experienced while drilling a long highly inclined or horizontal section increase
the risk of drilling problems such as lost circulation, formation failure, and formation
damage. In some situations, the reduction in mud-flow rate to control ECD and to
eliminate circulation losses may cause insufficient hole cleaning. ECD is thought to
be a more significant issue in drilling ERWs because of the following.
(1)Long measured depth intervals relative to the vertical depth.
(2)ERWs generally are shallow by their mature, The shallow-type ERD wells are
particularly prone to ECD problems as their formation are often so shallow as to have
little integrity.
(3)ERWs generally use larger-diameter drill pipe for hydraulics reasons and
prevention of buckling.
(4) More aggressive parameters (flow rate and rpm) are generally for hole
cleaning.
(5) Longer exposure times with long intervals on ERD wells.
The Effects of ECD
High ECD’s increase the risk of lost circulation, especially while (1)drilling 81/2”
or smaller hole size, or (2) while running or circulating long casing strings. Further,
reservoir damage can be a side effect if ECD’s are not minimized.
Wellbore instability can be caused by the constant flexing and relaxing of the
wellbore when the pumps are turned on and off. This is particularly the case if the
formation is brittle formation.
Effectively, the wellbore is failed through fatigue, as would a paper clip when
bent back and forth. A paper clip can be bent back and forth once or twice without
breaking, even if it is bent quite severely. It will break due to fatigue failure if it is
bent enough times. The time to failure is dependent upon(1)how severe the bending is
(2)how many times it is bent, (3)the strength and elasticity of the material. It is the
same with the wellbore and ECD fluctuations. The wellbore can eventually fail,
depending on the lithology, and the size and frequency of ECD fluctuations.
Casing collapse can be initiated by ECD’s while running buoyancy assisted
casing strings on long, deep ERD wells, Casing collapse calculations should account
for the increased annular pressure due to ECD’s while running casing, rather than just
for a static on-bottom scenario. Long mud-over-air casing flotation jobs have
experienced collapsed casing due to the running ECD’s alone.
Surge pressure creates a “piston fore” that behaves like drag. This can be critical
for marginal casing runs.
An indirect effect will be the impact on hole cleaning if losses are encountered
due to ECD’s. In an effort to reduce losses, flow rates will be reduced. This will result
in poorer hole cleaning and a build up of cuttings in the hole. In turn, ECD’s will
become worse and a vicious cycle is started.
The reduction in flow rate will also have a negative impact on torque and drag,
and drilling performance.
The conventional techniques that are used to reduce ECD
Current conventional techniques that are used to reduce ECD include:
(1)Using low fluid rheologies to reduce frictional losses.
(2)Using drillstrings and casing strings that provide greater annular clearance.
(3)Using expandable tubulars to increase hole size.
(4)Use of drilling liners in place of casing strings.
(5)Reducing flow rates to decrease frictional losses.
(6)Reducing penetration rates to reduce the amount of cuttings in the annulus.
These techniques can solve ECD problems but can result in higher drilling costs.
The higher drilling costs could make some wells uneconomical to drill. Dualgradient
drilling and riserless drilling also can reduce ECD but can have higher capital
expenditures than an ECD reduction tool.
The method of management ECD in Operational Phase
The numerous options were presented, which were used to help reduce ECD’s on
ERD wells in planning phase. The usual field practice is to treat ECD’s as an
after-thought. So, in this papers the numerous options were proposed, which could
provided the guidance for field operation.
1.Flowrate and RPM
Reducing flow rates is generally the first option if ECD’s are an issue. It is
important that any reduction in flowrates is within the hole cleaning limitations of the
drilling system. But the minimum allowable flowrate will be dependent upon many
factors, such as mud rheology, rpm, slide frequency, hole size, ROP and other
practices.
If ECD’s are critical, prior to drilling out the shoe, it may prove beneficial to
measure the magnitude of ECD variations with a range of flowrates and RPM’s. This
should be done by completing an ECD matrix with a PWD tool. Note that the mud
will need to be circulated for an adequate length of time to shear and warm it, and
therefore obtain good quality data.
2.ROP
Downhole annular pressure is directly related to the amount of cuttings that are
in the hole. The weight of these cuttings adds weight to the fluid, just as barite does.
Hence, the more cutting that are in the hole, the higher the effective bottom hole
pressure. Therefore, there is merit in controlling ROP if ECD’s are critical.
3.Slide Drilling Practices
Slide drilling results in the build-up of a cuttings dune immediately above the
BHA. Down hole MWD pressure tools have shown that bottom hole pressure can
increase sharply when pipe rotation is initiated after a long slide interval. This is
because of the instantaneous lifting of this cutting dune into the flow regime, there is
also increased risk of packing off during this time.
If ECD’s are an issue, then slide intervals should be broken up with pipe rotation
so as to redistribute the cuttings more evenly up the hole.
4. Back reaming
As with slide drilling, back reaming can cause a significant cuttings dune to form,
which can increase bottom-hole pressure if the cuttings dune is disturbed suddenly.
This is particularly true if the well has been circulated out prior to back reaming.
Circulating out of the hole cleans around the BHA, but the absence of rotation allows
cuttings to fall out and build up directly above the BHA. The initiation of back
reaming, at this stage, must be done with care.
The New techniques that are used to reduce ECD
1. Continuous Circulation System
The continuous circulation system (CCS) is a new technology that enables a
driller to make connections without stopping fluid circulation. A CCS enables a driller
to maintain a constant ECD when making connections. In normal drilling operations,
a driller must turn the pumps off when making a connection. Numerous problems can
occur as pumps start and stop in a drilling operation.
Fig. 1.Change in equivalent mud weight during connections
Fig. 1 shows the pressure spikes that occur when making a connection. When the
pumps stop, the pressure in the well decreases. This decrease in pressure can cause a
kick, formation fluids enter the wellbore. The formation could also relax and the
formation could collapse on the hole, resulting in stuck pipe. The differential pressure
between the reservoir and the wellbore can also stick the pipe. The drilling fluid starts
to form a gel when the pumps are turned off as the fluids stop circulating. When the
pumps are restarted, pressure increases to break the gel, causing a pressure spike
which could cause lost circulation, where fluids enter the formation, and ballooning of
the wellbore. Before a connection is made, the rig has downtime associated with
circulating the cuttings out of the bottom-hole assembly. This is required so that the
cuttings do not settle at the bottom-hole assembly.
A CCS could solve these problems when drilling. It would enable a driller to
have improved control of the ECD and reduce these problems that can result from
shutting down the pumps during a connection.
2.The ECD reduction tool
The ECD reduction tool is designed to reduce the bottom-hole pressure increase
caused by friction in the annulus by providing a pressure boost up annulus.
The tool has three basic parts. The top section of the tool has a turbine motor that
is powered by the circulating fluid. The middle section consists of a mixed flow pump
that is partly axial and partly centrifugal. This section pumps the fluid up the annulus.
The bottom section consists of the bearing and seals. Two nonrotating packer-cup
seals in the lower section of the tool provide the seal between the tool and the casing.
This causes all the return fluid to flow through the pump.
A potential disadvantage of the ECD reduction tool is the surge and swab effects
that could occur during tripping. Surge refers to the down hole pressure increase due
to the downward movement of the drill string in the well. Swab refers to a decrease in
down hole pressure when the drill string is being pulled out of the hole.
The ECD reduction tool can be used in onshore and offshore environments to
help prevent problems associated with drilling wells that have narrow pressure
windows. It can help alleviate high ECD that could result in formation damage and
mud loss. It may be useful as a low cost alternative to other ECD-reduction
techniques.
Conclusions
1. Managed ECD can improve the economic drill ability of wells. It can help solve
many of the problems that result from pressure variations in the formations.
2. The method of management ECD ought to been carried out an in-depth study in
Operational Phase, in order to scientifically guide the drilling ERWs.
3. The Continuous Circulation System allows the pressure profile to remain
consistent when making connections. It prevents pressure spikes that can occur when
turning the pumps on and off.
4. The ECD reduction tool reduces the dynamic pressure profile of a well from the
point where the tool is installed on the drill string to the bottom of the hole. If the tool
passes the narrow pressure margin while drilling, the tool would cease to have any
effect on the pressure at that point. The pressure could exceed the fracture pressure
and cause lost circulation.
Acknowledgements
This work was financially supported by 863 Project which the name was development
and integration of extended well offshore drilling and completion technology
(2012AA091501). And the project was undertaken by Shengli Petroleum Engineering Co.
Ltd, Drilling Technology Research Institute.
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