Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF FREEING STUCK DRILL PIPE
Field of the Invention
This invention relates to well servicing and more
particularly to a method for the auxiliary use of
ultrasonic energy in the case of differential sticking of
pipe to reduce the contact area of a filtercake prior to
applying freeing force.
Background of the Invention
During the drilling of oil and gas wells, drilling
fluid is circulated through the interior of the drill
string and then back up to the surface through the annulus
between the drill string and the wall of the borehole. The
drilling fluid serves various purposes including
lubricating the drill bit and pipe, carrying cuttings from
the bottom of the well borehole to the rig surface, and
imposing a hydrostatic head on the formation being drilled
to prevent the escape of oil, gas, or water into the well
borehole during drilling operations.
There are numerous possible causes for the drill
string to become stuck during drilling. Differential
sticking, one of the causes for stuck pipe incidents,
usually occurs when drilling permeable formations where
borehole pressures are greater than formation pressures.
Under those conditions, when the drill pipe remains at rest
against the wall of the borehole for enough time, mud
filter cake builds up around the pipe. The pressure
exerted by drilling fluid will then hold the pipe against
the cake wall.
Some warning signs that put one on notice of the
possibility of differential sticking are the presence of
prognosed low pressure along with depleted sands; long,
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unstabilized bottom-hole assembly (hereafter BHA) sections
in a deviated hole; loss of fluid loss control and
increased sand content; and increasing overpull, slack off
or torque to start string movement.
Indications of the actual presence of differential
sticking include a period of no string movement; the string
cannot be rotated or moved, but circulation is
unrestricted.
Methods of freeing differentially stuck drill string
include applying torque and jar down with maximum torque
load; using a spot pipe releasing pill if jarring is
unsuccessful; and lowering mud weight, which may have
implications with respect to hole stability. The overpull
required to release the pipe may exceed rig capacity, and
even cause collapse of the rig. It would be very
beneficial if a method were available to reduce the
required freeing force, so that the existing rig would be
adequate for overpull without possibly causing collapse.
Application of wave energy in the oil industry is
known, however the most common application of ultrasonic
energy is cleaning of electronic microchips in the
semiconductor industry and daily household cleaning of
jewelry.
In addition to the use of acoustic and ultrasonic
methods for core measurements in the laboratory, logging,
and seismic applications in the field, acoustic energy has
been shown by Tutuncu and Sharma to reduce the lift-off
pressure of mud filter cakes by a factor of five. See
Tutuncu A.N. and Sharma M.M., 1994, "Mechanisms of
Colloidal Detachment in a Sonic Field", 1st AIChE
International Particle Technology Forum, Paper No 63e, 24-
29.
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Other uses of ultrasonic energy include supplying the
energy through downhole tools into hydrocarbons to
facilitate the extraction of the oil from the well by
reducing the viscosity of the oil. See, for example, U.S.
Pat. Nos. 5,109,922 and 5,344,532. U. S. 5,727,628
discloses the use of ultrasonic to clean water wells.
Freeing pipe using vibrational energy has also been
tried in recent years. U. S. 4,913,234 discloses a system
for providing vibrational energy to effect the freeing of a
section of well pipe which comprises: a) an orbital
oscillator including a housing; b) an elongated screw
shaped stator mounted in said housing and an elongated
screw shaped rotor mounted for precessionally rolling
rotation freely in said stator; c) means for suspending
said oscillator for rotation within said drill pipe about
the longitudinal axis of the drill pipe in close proximity
to the stuck portion thereof; and d) drive means for
rotatably driving said rotor to effect orbital lateral
sonic vibration of said housing such that said housing
precesses laterally around the inner wall of said pipe,
thereby generating lateral quadrature vibrational forces in
said pipe to effect the freeing thereof from said well
bore.
U.S.5,234,056 discloses a method for freeing a drill
string which comprises a) resiliently suspending a
mechanical oscillator from a support structure on an
elastomeric support having a linear constant spring rate;
b) coupling said oscillator to the top end of the drill
string, the elastomeric support creating a low impedance
condition for vibratory energy at said drill string top
end; c) driving said oscillator to generate high level
sonic vibratory energy in a longitudinal vibration mode so
as to effect high longitudinal vibratory displacement of
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the top end of the drill string; and d) the drill string acting as an acoustic
lever
which translates the high vibrational displacement at the top end of the drill
string
into a high vibrational force at the point where the drill string is stuck in
the bore
hole, thereby facilitating the freeing of the drill string.
Often when a drill pipe is differentially stuck the result is that it has to
be cut and the target zone cannot be reached by the optimal route. It would be
extremely desirable in the art if a method were available which provided a
means
of reducing the amount of force required for freeing a stuck drill pipe. Such
a
method could potentially save enormous amounts of time and money in drilling
operations.
Summary
According to one broad aspect of the present invention, there is
provided in any method of freeing a drill pipe stuck due to build up of filter
cake,
the auxiliary method which provides a reduction in the amount of force
required to
free said pipe, which comprises: a) lowering an ultrasonic horn down the drill
pipe
to the point of contact between said drill pipe and filter cake; b) producing
ultrasonic energy at the point of contact until the contact area is
sufficiently
reduced that substantially less force is required to free the pipe.
In some embodiments of the present invention, the auxiliary use of
ultrasonic energy may help reduce the pipe contact area, thus reducing the
required freeing force and often permitting the existing rig to be sufficient
for use in
the overpull. Some embodiments of the present invention may save rig time and
prevent sidetracking of the well, a high cost operation especially in offshore
deepwater environments.
In accordance with an embodiment of the present invention there is
provided a method for reducing the amount of force necessary to free a stuck
drill
pipe which comprises:
a) Lowering an ultrasonic source having preferably at least 20 kHz
central frequency down a drill string to the point of contact causing
sticking;
b) Applying ultrasonic vibrations at the point of contact;
c) Reducing contact area;
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d) Applying reduced freeing force to free pipe.
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Brief Description of the Drawings
Figure 1 is a diagram of one possible position of a
differentially stuck drill pipe.
Figure 2 is a schematic diagram of the hollow cylinder
filtration cell used in the experimental work.
Figure 3 is a graph showing the reduction in pull out
(freeing) force as a function of sonification time for an
aloxite hollow cylinder sample damaged by drill-in fluid,
where the filter cake was built at an elevated pressure and
room temperature.
Figure 4 is a graph showing the reduction in pull out
(freeing) force as a function of sonification time for a
Berea sandstone hollow cylinder sample.
Detailed Description of the Invention
The present invention describes a method of freeing
stuck drill pipe, particularly in the case of differential
sticking, by the auxiliary use of ultrasonic energy to
reduce the amount of freeing force necessary.
Figure 1 is a diagram representing one example of the
position of a differentially stuck drill pipe. The drill
string, 4, becomes embedded in filter cake, 3, opposite the
permeable zone, 2, at high differential mud pressure
overbalance, leading to stuck pipe in the contact zone.
Under dynamic circulating conditions, the filter cake is
eroded both by hydraulic flow and by the mechanical action
of the drill string. When the well is left static with no
pipe rotation, a static filter cake may build up, which
increases the overall cake thickness. The string may now
become embedded in the thick filter cake, particularly when
the wellbore, 1, is at high deviation and/or the BHA is not
properly stabilized. The static filter cake seals the
wellbore pressure (at overbalance) from the backside of the
pipe. An area of low pressure develops behind the backside
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of the string/BHA and starts to equilibrate to the lower
formation pressure. A differential pressure starts to
build up across the pipe/BHA. With time the area of pipe
sealed in the filter cake increases. The overbalance
pressure times the contact area provides a drag force that
may prevent the pipe from being pulled free. The build-up
of the drag force is very rapid from the start and will
increase with time.
Typical actions used to free the string include
applying torque and jarring down with maximum torque load.
Circulation is usually not restricted in the case of
differential sticking. Therefore, spotting fluids can be
circulated across the zone causing the stuck pipe. Spotting
fluids contain additives that can dehydrate and crack
filter cakes and additives that can lubricate the drill
string. Cracking the filter cake will help to transmit the
mud pressure to the backside of the string and remove the
differential pressure across the string, resulting in
minimization of friction. The sticking force then is
reduced by an equivalent amount as shown in Equation 1.
FS = AZ~ P (1)
where is the friction coefficient, A is contact area and
AP is overbalance. In order to free the pipe the freeing
force needs to be equal to or greater than Fs. However
sometimes it is not possible to generate enough force due
to drill string and/or rig limitations, in which case the
drill string must be cut, thus causing great financial loss
and making it impossible to reach the target zone by the
preferred route. Lowering mud weight may be helpful in some
cases, but may compromise hole stability.
Design of the drill string is a major consideration.
The strength of drill pipe limits the maximum allowable
weight and hence the ability to exert overpull. Even if
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the drill pipe is designed strong enough, the overpull
required to release the pipe may exceed rig capacity. It
is possible, particularly with small rigs in land
operations, for rigs to collapse due to forces applied
exceeding the maximum overpull. Downhole jars also allow
high impact force to be exerted at the stuck point with
relatively low overpull and setdown. However, sometimes
the forces exerted are not enough to release the stuck
pipe. Jar itself may become stuck as well. In the present
invention decrease of contact area of the stuck pipe
reduces the amount of overpull required for application.
Since A is reduced, sticking force is also reduced (see
Equation 1). Hence, the existing difficulties in the
release of stuck pipe are minimized.
In the present invention an ultrasonic source is
enclosed in a housing of a pipe that permits disposition in
the drill string. The ultrasonic source is a high-power
sweeping acoustic transducer that operates at either a
fixed frequency of approximately 20 KHz, or the frequency
can be varied between several Hz and 40 KHz. The tool is
made up of a variable number of cylindrical ceramic
transducers, which transmit the acoustic energy radially.
The transmitter itself is a piece of solid steel to which a
piezoelectric driver(s) are attached. The acoustic tool is
connected via a normal logging cable to a high power
amplifier. The power amplification is related to the ratio
of the cross-sectional areas of the tool.
To demonstrate the invention, dynamic filtration
experiments were conducted with fully brine-saturated Berea
sandstone and aloxite hollow cylinder cores with known pore
size distribution. Figure 2 is a schematic drawing of the
dynamic hollow cylinder filtration cell used in the
experiments. Hollow core tests represent realistic
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borehole geometry. The cell is designed and built to
handle core samples of 4-inch outside diameter (OD) with
8.3-inch length. Variable internal diameters (ID) for
hollow cylinder cores can be used in the cell. For this
invention, 0.9-inch ID samples were used.
A Digital Sonifier 450 Model by Branson Ultrasonics
Corp. of Danbury, Connecticut was used for ultrasonic
cleaning purposes. The system consists of the power supply
unit, the controls, the converter and a horn. A PC was
used to interface with the system and to collect the data
off the system.
The hollow cylinder Berea cores were first damaged
using drilling and/or drill-in fluids of different
formulations under various differential pressures. The
drill-in fluid was used to conduct the static filtration.
The filtration was performed in the cell at 600-psi
pressure difference for about 12 hours. The cake thickness
was varied between 2 to 3 mm. Drilling fluid was circulated
into the hollow cylinder core and out from an annulus at
500-psi circulation pressure and 50 cc/min. Then the pump
was stopped and static filtration was initiated at 500 psi
long enough to stick a pipe and static filtrate was
collected. Then the ultrasonic horn with 20 KHz central
frequency was used to apply sonification from the interior
of the pipe that stuck to the wall of the core. The
permeability, differential pressure, sonification
amplitude, power, and temperature were monitored as a
function of sonification treatment time, and the energy
requirement for near-complete permeability recovery and
pullout force were investigated.
The following examples will serve to illustrate the
invention disclosed herein. The examples are intended only
as a means of illustration and should not be construed as
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limiting the scope of the invention in any way. Those
skilled in the art will recognize many variations that may
be made without departing from the spirit of the disclosed
invention.
EXPERIMENTAL STUDY
Experiments were designed to demonstrate the
usefulness of ultrasonic in reducing pullout force for
stuck pipe. A special dynamic hollow cylinder circulation
device, described above and shown in Figure 2 was designed
for conducting experiments. The cell pressure,
temperature, flow rate, applied horn power and the
amplitudes were monitored continuously using data
acquisition software. The distance between the damaged
surface and the horn was varied to study the effect of
distance away from the source.
Again referring to Figure 2, the system comprises a
stainless steel cell, two movable pistons, and an
.ultrasonic horn holder. It is capable of handling in
excess of 5,000 psi pressure and also can be operated at
elevated temperature under a specified differential
pressure. Two syringe pumps (manufactured by and
commercially available from ISCO, Inc. of Nebraska) were
used to inject fluid and to control the differential
pressure simultaneously with a precision of 1 psi to
measure the permeability of the sample. A data
acquisition system was used to record and monitor the real-
time pressure, flow rate, and volume of fluid injected.
During sonification, the real-time amplitude, power, and
time were also recorded and monitored.
Hollow cylinder Berea and aloxite core samples with 4"
OD, 0.9" ID and 8.3" length were placed in the dynamic
hollow cylinder filtration device, and external filter
cakes were built by circulating drilling or drill-in fluid
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under in situ stress conditions between a casing pipe and
walls of the hollow cylinder as shown in Figure 2.
Continuous permeability measurements made it possible to
observe when the fluid completely plugged the sample pore
spaces. Then the ultrasonic horn was placed into the pipe
simulating a stuck pipe scenario in the laboratory as shown
in Figure 2. No sonification was applied in the first
test. The application of pulling force was initiated and
applied to the stuck pipe in gradually increasing magnitude
until the pipe was released. The load required to free the
pipe was recorded in this case. Then other identical tests
were run with the stuck pipe scenarios, but this time
sonification was applied for 1, 3, 5, 10, 15, 20, 25, 30
and 35 minute intervals, respectively. After various-time
sonifications, a small pulling force was applied and then
the force was gradually increased until the pipe was
released. The sonifications were repeated at three energy
levels (30% amplitude, 50% amplitude, and 70% amplitude).
A summary for the aloxite cylinder at various amplitude and
sonification times is presented in Figure 3. Figure 3 is a
graph showing the reduction in pull out (freeing) force as
a function of sonification time for an aloxite hollow
cylinder sample damaged by drill-in fluid, where the filter
cake was built at an elevated pressure and room
temperature. The pullout force ratio is the ratio of
freeing force after sonification to freeing force before
sonification.
The fastest reduction in the freeing force was
observed when 70% (highest power) was applied; however, any
amplitude level and timing of sonification helped reduce
the freeing force compared to the case of no sonification.
The results for Berea hollow cylinder cores are shown in
Figure 4. Different samples were used to test the effect
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of increasing sonification time. For all the tests except
the 40-minute sonification test, a pulling force was
applied to free the pipe. However, the longer the
sonification time, the smaller was the magnitude of the
required force. And, finally, for 40-minute sonification,
no pulling force was needed; the release was instantaneous
after the sonification. The test results were explained by
reduction in the contact area. Because sonification
reduced the thickness of the filter cake, it resulted in a
reduction in the contact area. Therefore, from equation
(1), FS = A Z~P, g and LP are kept constant, A is smaller,
hence FS is smaller. A summary of the pullout force ratios
for aloxite and Berea hollow cylinder samples is shown in
Figures 3 and 4.
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