OA12742A - Method of freeing stuck drill pipe. - Google Patents

Abstract

An auxiliary method for freeing a drill pipe stuck due to build up of filter cake, which provides a reduction in the amount of force required to free said pipe which comprises: a) Lowering an ultrasonic horn type device down the drill pipe to the point of contact between said drill pipe and mud filter cake; b) Producing ultrasonic energy at the point of contact until the contact area is sufficiently reduced such that substantially less force is required to free the pipe.

Description

1 012742

Field of the Invention

This invention relates to well servicing and moreparticularly to a method for the auxiliary use ofultrasonic energy in the case of differential sticking ofpipe to reduce the contact area of a filtercake prior toapplying freeing force.

Background of the Invention

During the drilling of oil and gas wells, drillingfluid is circulated through the interior of the drillstring and then back up to the surface through theannulus between the drill string and the wall of theborehole. The drilling fluid serves various purposesincluding lubricating the drill bit and pipe, carryingcuttings from the bottom of the well borehole to the rigsurface, and imposing a hydrostatic head on the formationbeing drilled to prevent the escape of oil, gas, or waterinto the well borehole during drilling operations.

There are numerous possible causes for the drillstring to become stuck during drilling. Differentialsticking, one of the causes for stuck pipe incidents,usually occurs when drilling permeable formations whereborehole pressures are greater than formation pressures.Under those conditions, when the drill pipe remains atrest against the wall of the borehole for enough time,mud filter cake builds up around the pipe. The pressureexerted by drilling fluid will then hold the pipe againstthe cake wall.

Some warning signs that put one on notice of thepossibility of differential sticking are the presence of 2 012742 prognosed low pressure along with depleted sands; long,unstabilized bottom-hole assexnbly (hereafter BHA)sections in a deviated hole; loss of fluid loss controland increased sand content; and increasing overpull,slack off or torque to start string movement.

Indications of the actual presence of differentialsticking include a period of no string movement; thestring cannot be rotated or moved, but circulation isunrestricted.

Methods of freeing differentially stuck drill stringinclude applying torque and jar down with maximum torqueload; using a spot pipe releasing pill if jarring isunsuccessful; and lowering mud weight, which may hâveimplications with respect to hole stability. The overpullrequired to release the pipe may exceed rig capacity, andeven cause collapse of the rig. It would be verybénéficiai if a method were available to reduce therequired freeing force so that the existing rig would beadéquate for overpull without possibly causing collapse.

Application of wave energy in the oil industry isknown, however the most common application of ultrasonicenergy is cleaning of electronic microchips in thesemiconductor industry and daily household cleaning ofjewelry.

In addition to the use of acoustic and ultrasonicmethods for core measurements in the laboratory, logging,and seismic applications in the field, acoustic energyhas been shown by Tutuncu and Sharma to reduce the lift-off pressure of mud filter cakes by a factor of five. SeeTutuncu A.N. and Sharma M.M., 1994, "Mechanisms ofColloïdal Detachment in a Sonic Field", lst AIChEInternational Particle Technology Forum, Paper No. 63e,24-29. 3 012742

Other uses of ultrasonic energy include supplying theenergy through downhole tools into hydrocarbons tofacilitate the extraction of the oil front the well byreducing the viscosity of the oil. See, for example, U.S.Pat. Nos. 5,109,922 and 5,34*, 532. U. S. 5,727, 628discloses the use of ultrasonic to clean water wells.

Freeing pipe using vibrational energy has also beentried in recent years. U. S. 4,913,234 discloses a Systemfor providing vibrational energy to effect the freeing ofa section of well pipe which comprises: a) an orbitaloscillator including a housing; b) an elongated screwshaped stator mounted in said housing and an elongatedscrew shaped rotor mounted for precessionally rollingrotation freely in said stator; c) means for suspendingsaid oscillator for rotation within said drill pipe aboutthe longitudinal axis of the drill pipe in closeproximity to the stuck portion thereof; and d) drivemeans for rotatably driving said rotor to effect orbitallatéral sonie vibration of said housing such that saidhousing precesses laterally around the inner wall of saidpipe, thereby generating latéral quadrature vibrationalforces in said pipe to effect the freeing thereof fromsaid well bore. U. S. 5,234,056 discloses a method for freeing a.drill string which comprises a) resiliently suspending amechanical oscillator from a support structure on anelastomeric support having a linear constant spring rate; b) coupling said oscillator to the top end of the drillstring, the elastomeric support creating a low impédancecondition for vibratory energy at said drill string topend; c) driving said oscillator to generate high levelsonie vibratory energy in a longitudinal vibration modeso as to effect high longitudinal vibratory displacement .,. 01274 2 of the top end of the drill string; and d) the drillstring acting as an acoustic lever which translates thehigh vibrational displacement at the top end of the drillstring into a high vibrational force at the point wherethe drill string is stuck in the bore hole, therebyfacilitating the freeing of the drill string.

Often when a drill pipe is differentially stuck theresuit is that it has to be eut and the target zonecannot be reached by the optimal route. It would beextremely désirable in the art if a method were availablewhich provided a means of reducing the amount of forcerequired for freeing a stuck drill pipe. Such a methodcould potentially save enormous amounts of time and moneyin drilling operations.

In the présent invention, it has been discovered thatthe auxiliary use of ultrasonic energy can help reducethe pipe contact area, thus reducing the required freeingforce and often permitting the existing rig to besufficient for use in the overpull. The présent inventionwill save rig time and prevent sidetracking of the well,a high cost operation especially in offshore deepwaterenvironments.

Suromary

In accordance with the foregoing the présent _invention provides a method for reducing the amount offorce necessary to free a stuck drill pipe whichcomprises : a) Lowering an ultrasonic source having preferably atleast 20 kHz central frequency down a drill string to thepoint of contact causing sticking; b) Applying ultrasonic vibrations at the point ofcontact; c) Reducing contact area; U12742 - 5 - d) Applying reduced freeing force to free pipe.

Brief Description of the Drawings

Figure 1 is a diagram of one possible position of adifferentially stuck drill pipe.

Figure 2 is a schematic diagram of the hollowcylinder filtration cell used in the experimental work.

Figure 3 is a graph showing the réduction in pull out(freeing) force as a function of sonification time for analoxite hollow cylinder sample damaged by drill-in fluid,where the filter cake was built at an elevated pressureand room température.

Figure 4 is a graph showing the réduction in pull out(freeing) force as a function of sonification time for aBerea sandstone hollow cylinder sample.

Detailed Description of the Invention

The présent invention describes a method of freeingstuck drill pipe, particularly in the case ofdifferential sticking, by the auxiliary use of ultrasonicenergy to reduce the amount of freeing force necessary.

Figure 1 is a diagram representing one example of theposition of a differentially stuck drill pipe, The drillstring, 4, becomes embedded in filter cake, 3, oppositethe permeable zone, 2, at high differential mud pressureoverbalance, leading to stuck pipe in the contact» zone.Under dynamic circulating conditions, the filter cake iseroded both by hydraulic flow and by the mechanicalaction of the drill string. When the well is left staticwith no pipe rotation, a static filter cake may build up,which increases the overall cake thickness. The stringmay now become embedded in the thick filter cake,particularly when the wellbore, 1, is at high déviationand/or the BHA is not properly stabilized. The staticfilter cake seals the wellbore pressure (at overbalance) 012742 6 from the backside of the pipe. An area of low pressure develops behind the backside of the string/BHA and starts

to equilibrate to the lower formation pressure. A differential pressure starts to build up across the 5 pipe/BHA. With time the area of pipe sealed in the filter cake increases. The overbalance pressure times thecontact area provides a drag force that may prevent thepipe from being pulled free. The build-up of the dragforce is very rapid from the start and will increase with 10 time.

Typical actions used to free the string includeapplying torque and jarring down with maximum torqueload. Circulation is usually not restricted in the caseof differential sticking. Therefore, spotting fluids can 15 be circulated across the zone causing the stuck pipe.

Spotting fluids contain additives that can dehydrate andcrack filter cakes and additives that can lubricate thedrill string. Cracking the filter cake will help totransmit the mud pressure to the backside of the string 20 and remove the differential pressure across the string, resulting in minimization of friction. The sticking forcethen is reduced by an équivalent amount as shown inEquation 1.

Fs = μ A ΔΡ _ (1) 25 where μ is the friction coefficient, A is contact area and ΔΡ is overbalance. In order to free the pipe thefreeing force needs to be equal to or greater than Fs.However sometimes it is not possible to generate enoughforce due to drill string and/or rig limitations, in 30 which case the drill string must be eut, thus causing great financial loss and making it impossible to reachthe target zone by the preferred route. Lowering mud 012742 - 7 - weight may be helpful in some cases, but may compromisehole stability.

Design of the drill string is a major considération.The strength of drill pipe limits the maximum allowableweight and hence the ability to exert overpull. Even ifthe drill pipe is designed strong enough, the overpullrequired to release the pipe may exceed rig capacity. Itis possible, particularly with small rigs in landoperations, for rigs to collapse due to forces appliedexceeding the maximum overpull. Downhole jars also allowhigh impact force to be exerted at the stuck point withrelatively lôw overpull and setdown. However, sometimesthe forces exerted are not enough to release the stuckpipe. Jar itself may become stuck as well. In the présentinvention decrease of contact area of the stuck pipereduces the amount of overpull required for application.Since A is reduced, sticking force is also reduced (seeEquation 1). Hence, the existing difficulties in therelease of stuck pipe are minimized.

In the présent invention an ultrasonic source isenclosed in a housing of a pipe that permits dispositionin the drill string. The ultrasonic source is a high-power sweeping acoustic transducer that opérâtes at - either a fixed frequency of approximately 20 KHz,_ or thefrequency can be varied between several Hz and 40 KHz.

The tool is made up of a variable number of cylindricalceramic transducers, which transmit the acoustic energyradially. The transmitter itself is a piece of solidsteel to which a piezoelectric driver(s) are attached.

The acoustic tool is connected via a normal logging cableto a high power amplifier. The power amplification isrelated to the ratio of the cross-sectional areas of the*tool. 01274 2 - 8 - Το demonstrate the invention, dynamic filtrationexperiments were conducted with fully brine-saturatedBerea sandstone and aloxite hollow cylinder cores withknown pore size distribution. Figure 2 is a schematic 5 drawing of the dynamic hollow cylinder filtration cell used in the experiments. Hollow core tests representrealistic borehole geometry. The cell is designed andbuilt to handle core samples of 4-inch outside diameter(OD) with 8.3-inch length. Variable internai diameters 10 (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 UltrasonicsCorp. of Danbury, Connecticut was used for ultrasoniccleaning purposes. The System consists of the power

15 supply unit, the Controls, the converter and a horn. A PC was used to interface with the System and to collect thedata off the System.

The hollow cylinder Berea cores were first damagedusing drilling and/or drill-in fluids of different 20 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-psipressure différence for about 12 hours. The cakethickness was varied between 2 to 3 mm. Drilling .fluid 25 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 wasinitiated at 500 psi long enough to stick a pipe andstatic filtrate was collected. Then the ultrasonic horn 30 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 température 012742 9 were monitored as a function of sonification treatment time, and the energy requirement for near-complete permeability recovery and pullout force were investigated. 5 The following exemples will serve to illustrate the invention disclosed herein. The examples are intendedonly as a means of illustration and should not beconstrued as limiting the scope of the invention in anyway. Those skilled in the art will recognize many 10 variations that may be made without departing from the spirit of the disclosed invention.

EXPERIMENTAL'STUDY

Experiments were designed to demonstrate theusefulness of ultrasonic in reducing pullout force for 15 stuck pipe. A spécial dynamic hollow cylinder circulation device, described above and shown in Figure 2 wasdesigned for conducting experiments. The cell pressure,température, flow rate, applied horn power and theamplitudes were monitored continuously using data 20 acquisition software. The distance between the damaged surface and the horn was varied to study the effect ofdistance away from the source.

Again referring to Figure 2, the System comprises a- stainless steel cell, two movable pistons, and an, 25 ultrasonic horn holder. It is capable of handling in excess of 5,000 psi pressure and also can be operated atelevated température under a specified differentialpressure. Two syringe pumps (manufactured by andcommercially available from ISCO, Inc. of Nebraska) were 30 used to inject fluid and to control the differential pressure simultaneously with a précision of ± 1 psi to measure the permeability of the sample. A data acquisition System was used to record and monitor the 10 012742 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 with4" OD, 0.9" ID and 8.3" length were placed in the dynamichollow cylinder filtration device, and external filtercakes were built by circulating drilling or drill-influid under in situ stress conditions between a casingpipe and walls of the hollow cylinder as shown inFigure 2. Continuous permeability measurements made itpossible to observe when the fluid completely plugged thesample pore spaces. Then the ultrasonic horn was placedinto the pipe simulating a stuck pipe scénario in thelaboratory as shown in Figure 2. No sonification wasapplied in the first test. The application of pullingforce was initiated and applied to the stuck pipe ingradually increasing magnitude until the pipe wasreleased. The load required to free the pipe was recordedin this case. Then other identical tests were run withthe stuck pipe scénarios, but this time sonification wasapplied for 1, 3, 5, 10, 15, 20, 25, 30 and 35 minuteintervals, 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 energylevels (30% amplitude, 50% amplitude, and 70% amplitude).A summary for the aloxite cylinder at various amplitudeand sonification times is presented in Figure 3. Figure 3is a graph showing the réduction in pull out (freeing)force as a function of sonification time for an aloxitehollow cylinder sample damaged by drill-in fluid, wherethe filter cake was built at an elevated pressure androom température. The pullout force ratio is the ratio of 012742 11 freeing force after sonification to freeing force beforesonification.

The fastest réduction in the freeing force wasobserved when 70% (highest power) was applied; however, 5 any amplitude level and timing of sonification helped reduce the freeing force compared to the case of nosonification- The results for Berea hollow cylinder coresare shown in Figure 4. Different samples were used totest the effect of increasing sonification time. For ail 10 the tests except the 40-minute sonification test, a pulling force was applied to free the pipe. However, thelonger the sonification time, the smaller was themagnitude of the required force. And, finally, for40-minute sonification, no pulling force was needed; the 15 release was instantaneous after the sonification. The test results were explained by réduction in the contact ""area. Because sonification reduced the thickness of thefilter cake, it resulted in a réduction in the contactarea. Therefore, from équation (1), Fs = μ A ΔΡ, μ and ΔΡ

20 are kept constant, A is smaller, hence Fs is smaller. A summary of the pullout force ratios for aloxite and Bereahollow cylinder samples is shown in Figures 3 and 4.

Claims (12)

1. 012742 - 12 -

   1. In any method of freeing a drill pipe stuck due tobuild up of filter cake, the auxiliary method whichprovides a réduction in the amount of force required tofree said pipe, which comprises: a) Lowering an ultrasonic horn down the drill pipe tothe point of contact between said drill pipe and filtercake; b) Producing ultrasonic energy at the point of contactuntil the contact area is sufficiently reduced thatsubstantially less force is required to free the pipe.
2. 2. The method of claim 1 further comprising the pipe isdifferentially stuck. "3. The method of claim 1 further comprising theultrasonic energy is applied at the point of contact sothat at least one ultrasonic wave is directed substantially perpendicular to the filter cake.
3. 4. The method of claim 1 wherein the ultrasonic energyis varied in the range of (several) 2 kHz to 40 kHz.
4. 5. The method of claim 1 wherein the ultrasonic energy is about 20 kHz ± 5. „
5. 6. The method of claim 1 wherein the ultrasonic energyis a fixed frequency of about 20 KHz.
6. 7. The method of claim 1 further comprising the powersupply is in the range of 50 watts to 450 watts.
7. 8. The method of claim 7 wherein the power supply is inthe range of 100 watts to 250 watts.
8. 9. The method of claim 8 wherein the power supply isless than 200 watts. 13 012742
9. 10. The method of claim 1 further comprising the pressureis atmospheric to 10,000 psi.
10. 11. The method of claim 10 wherein the pressure is in therange of atmospheric to 5,000 psi.
11. 12. The method of claim 11 wherein the pressure is in the range of 100 psi to 700 psi.
12. 13. The method of claim 12 wherein the pressure is in therange of 200 psi to 600 psi.