THE CAUSE OF DIFFERENTIAL STICKING DEFINITION: The sticking force generated by the overbalance pressure driving the static drillstring into the thick filter cake of a permeable formation. CAUSE: pressure is exposed to a permeable Overbalance Hydrostatic overbalance pressure is formation (sand or fractured limestone). Pressure . Pipe motion is stopped with with the un-stabilized BHA section in contact with the wall of of the permeable zone. .
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High fluid loss allows loss allows a thick static filter cake to build quickly after pipe motion is stopped. .
The build-up of the static filter cake seals the hydrostatic pressure from the contact area between the pipe and filter cake. .
Fluid loss allows the filter cake in the contact area to drain down to formation pressure. pressure. .
Permeable Formation
A differen differential tial force is exerted against the pipe relative to the contact area and the overbalance pressure. .
Over time, the differential sticking force increases with increases with growth of the low pressure contact area. WARNING: High overbalance pressure; High fluid loss; BHA in permeable formations; Operation requiring static drillstring time. Overpull, set-down weight or torque surge to start pipe motion after static time.
FREEING: Most likely to occur during the drilling operation when string motion is stopped (i.e., connection connections, s, survey, etc). .
Pipe motion prior to sticking: Static. .
Circulation after sticking: Unrestricted. .
(Top Drive) Apply 80% of DP make-up torque. .
Jar down with maximum jar-trip load.
4"
PREVENTION: Minimize mud weight and fluid loss. .
Use fine calcium carbonate to plug permeable zones. .
Minimize the length drill collars, replace with HWDP. .
Use spiraled/flex drill collars in the un-stabilized BHA section. .
Maintain pipe motion when BHA is in potential sticking zones. Page 1
Low Pressure filter cake
DIFFERENTIAL STICKING MECHANICS For differential sticking to occur, five basic conditions must be present. The string must be in contact with with the wall of the hole and static and static for for some time. Overbalance pressure drives fluid into the permeable the permeable zone zone and and develops a thick static filter static filter cake. cake. Under these these conditions, conditions, a pressure differential differential develops across the string-to-f string-to-formation ormation contact area producing the differential sticking force.
Filter Cake Development No Filter Cake
Uncontrolled Fluid Loss
Formation Pressure 5000 psi
While the liquid phase of the mud can flow into the permeable zone, the much larger solid phase (gel, barite, drill solids, etc.) is filtered out at the wall of the hole forming a filtered cake of mud solids (filter (filter cake or wall cake). cake). The filter cake is necessary to control fluid loss and allow the overbalance pressure to support the wall of the hole.
Filter Cake
Controlled Fluid Loss
Formation Pressure 5000 psi Page 2
Dynamic Filter Filter Cake Under dynamic (moving) (moving) conditions, the thickness of the filter cake decreases as decreases as the mechanical forces of pipe and fluid motion washes the top layer of the cake off the wall of the hole.
Fluid loss increases as the filter cake is washed off the wall of the hole.
Static Filter Filter Cake Under static (non-moving) conditions, the thickness of the filter cake increases increases with with the loss of the mechanical forces.
Fluid loss decreases as the Static Filter Cake developes.
Page 3
String In Contact Dynamic Conditions Dynamic Filter Cake
The forces of drillstring movement and circulation prevents the development of the static filter cake.
5000 PSI
Without the seal of the static filter cake, hydrostatic pressure is exerted equally around the pipe.
at i o n o t
R
The differential sticking force cannot develop without a pressure differential across the pipe.
Hydrostatic Pressure Equal Around the Pipe
Static Conditions Static Filter Cake
After drillstring drillstring motion motion is stopped, the static filter cake developments and restricts the flow of fluid to the filter cake in the pipe-to-wa pipe-to-wallll contact area.
5000 PSI
Fluid loss drains the pressure of the filter cake in the contact area down to formation pressure.
1" 1"
Contact
A differe differential ntial sticking force is exerted against the pipe relative to the size of the low pressure contact area and the pressure differential across the pipe.
Low Pressure Filter Cake
Fluid Flow Restricted By Static Filter Cake Page 4
Time Dependent As the static filter cake thickens with time, the hydrostatic pressure connection to the low pressure filter cake is sealed off creating a pressure differential across the contact area. The thickness of the static filter cake also increases the width of t he contact area which greatly increases the differential sticking force.
4"
Contact
Low Pressure Filter Cake
Fluid Flow Sealed By Static Filter Cake
Page 5
The Differential Sticking Force Top View at Sand Depth
Dynamic Filter Cake
Formation Pressure 5000 psi
4 "
6000 PSI Static Filter Cake
Low Pressure Filter Cake
The differential pressure applied across the contact area generates a substantial differential sticking force driving the pipe against the wall of the hole.
1,600,000 lbs x 0.2 Fric = 320,000 lbs Freeing Force
1600 sq in x 1000 psi = 1,600,000 lbs Diff. Sticking Force
4"
Contact Area 400"
Efforts to pull the drillstring free would require an overpull equal to the differential sticking force times the frictional coefficient of the filter cake. In most situations, this would exceed the overpull limit of the drillstring. Page 6
SAND
WARNING, DIFFERENTIAL STICKING OCCURRENCE Differential sticking is most likely to occur during the drilling operation when drillstring motion is stopped for for connections, surveys, etc. Usually the sticking occurs in the unstabilized BHA section but section but can also occur in the HWDP or drillpipe sections if conditions are favorable. .
Also possible with slow reciprocation of reciprocation of the string (casing most likely) or with slow slide-drilling if if conditions are favorable.
WHILE DRILLING High overbalance (1.0 overbalance (1.0 ppg +). . High % sand sand content and low gravity solids. . Slow slide-drilling (little pipe movement).
High fluid loss resulting loss resulting in a thick static filter cake. BHA in a permeable formation (sand, limestone).
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CONNECTIONS / SURVEYS Long connection time with no string movement.
Difficulty sliding and orienting the tool face.
..
Increasing trend of overpull off the slips after connections, surveys, etc.
TRIPPING High overbalance (1.0 ppg +) to maintain wellbore stability. .
High mud weights to control abnormal formation pressure increases overbalance overbalance to shallower sands. .
High fluid loss. High % sand content content and low gravity solids. solids. .
Little or no circulating and mud conditioning before the trip. .
Long set-back time on stand connections. .
Increasing trend of overpull off the slips after stand connections. .
Drillstring is not rotated on connections with the BHA in a zone with high sticking potential. .
Unnecessary back reaming reaming with the BHA in a permeable permeable zone removes the filter cake. The static filter rebuilds during stand set-back time. .
Page 7
FREEING, DIFFERENTIAL STICKING Torque-up & Jar Down (first action): Immediate action is most effective as the differential sticking force increases with time. Torquing-up the string and jarring down is an effective technique that can be applied quickly with little possibility possibility of creating other problems. problems. The torque and down-jarring combined loads greatly increases the possiblity of freeing the string. Circulate at maximum rate while applying string torque. Circulation washes away the static filter cake; cleans the cuttings up and out of the hole to reduce the overbalance; while the annulus friction pressure pressurizes the filter cake. (Top Drive) Torque-up the string to +/- 80% of the drillpipe make-up torque. Carefully reciprocate the string to ensure torque limit is applied at stuck depth.
Reduce the pump speed to +/-1 bbl/min: to relieve the annulus friction pressure; to relieve pump-open force while jarring; and to k eep the jar cool. Jar down with maximum jar-trip load. Hold torque and set-down weight until the jar trips or for +/-10 minutes.
If the string does not jar free, maintain torque, slow circulation and continue jarring down +/-20 times. .
If the string does not jar free, carefully release the string torque and jar up with maximum jar-trip load +/-10 times.
If the string does not jar free, maintain slow circulation, re-apply string torque and continue jarring down while considering conside ring a second plan of action. TORQUE /TENSION COMBINED LOADS String tension should not exceed the recommended value relative to the applied torque. 600
5”
Applied torque Applied torque reduc reduces es the the tension capacity of the string. Release the string torque to allow maximum overpull when jarring up.
19.50 lbs/ft
500
) . s b l
S
0400 0 0 1 (
G
n300 o i s n e T200
X E
100
0
0
10
Reprinted with permission of T. H. Hill Associates, Inc.
20
30
Torsion (1000 ft-lbs.)
Page 8
40
50
60
Reference - Standard DS-1(TM) Drill Stem Design & Inspection
FREEING DIFFERENTIAL STICKING Spot a Pipe Releasing Pill (third action): .
If first attempts failed to free the string, a chemical agent should be placed as quickly as possible and not disturbed until the string comes free or the recommended r ecommended waiting time has expired. Refer to the literature for the chemical agent for recommended waiting time. Determine where to spot the pill . If the jar can be tripped, the stuck depth is is below the jar. jar. If the jar cannot be tripped , sticking occurred at some depth above above the jar. jar. Run a Free-Point Log or perform a Pipe Torque Calculation (see next page) to determine the stuck depth. The pill The pill density should should be 1.0 to 2.0 ppg greater than than the mud weight to keep the pill on spot. Pill volume should should be sufficient to cover the stuck zone by at least 1.5 times. Continue recommended jarring procedure procedure until the pill is ready to pump. .
Pump the pill in place at maximum rate to wash off the static filter cake. Stop the pumps when the top of the pill reaches the top of the stuck depth. Pump +/- 1 bbl every +/-30 minutes while waiting to replace the pill volume lubricating down hole. Continue recommended jarring operations while waiting on the pill. .
If pressure can be applied to the wellbore
without the threat of lost circulation, hold pressure on the well for +/-15 minutes to pressurize the filter cake and to drive spotting fluid into the filter cake. (See detailed procedure on previous page.) (Top Drive) App (Top Apply ly torque torque and cock the jar. .
Close the BOP and apply 200 to 400 psi pressure to the wellbore. Hold torque and pressure for 10 to 15 minutes. .
Quickly Bleed off the pressure through the choke and open the BOP. .
Slack off maximum jar-trip load . Hold torque and setdown weight until the jar trips or for +/-10 minutes.
If the string does not come free, consider repeating this procedure while while waiting on the the pill. If the string is not not free after waiting the recommended time, consider picking up a Heavy Down-Jaring Fishing Assembly. (See page 12)
Page 9
Spotting fluid lubricating down hole
DETERMINE THE STUCK DEPTH Note: If the jar can be tripped, the stuck point is below the jar.
Pipe Torque Calculation Preparations: 1. (Kelly drive) space-out to place the kelly drive bushing in the rotary table. 2. To ensure all tooljoints are tight, torque the string to 80% of the drillpipe make-up torque. (Make-up Torq Torq x .8) Maintain the torque and work the pipe until the string stops rotating. 3. Carefully release the string torque. Work the pipe with high set-down weights and moderate overpulls to work any trapped torque out of the drillstring. 4. Make four equally spaced vertical marks around the drillpipe or kelly with one long mark as the starting point. .
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Torque Factor For Size and Weight of Drillpipe Drillpipe Lbs/Ft Torq Fac Drillpipe Lbs/Ft Torq Fac 4-1/2” 16.60 2-3/8” 6.65 19.221 2.058 20.760 18.15 23.023 20.00 2-7/8” 8.35 3.848 28.538 5” 19.50 10.40 4.610 3-1/2” 13.30 20.50 30.401 9.002 15.50 10.232 5-1/2” 21.90 38.670 24.70 4” 14.00 43.141 12.915 . 15.70 14.314 56.284 6-5/8” 22.20
Four Marks around Pipe Starting Mark
Count the Revolutions for the Applied Torque: 5. Calculate 30% of the drillpipe make-up torque (Make-up Torq x .3 = Applied Torq). .
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6. While applying the torque, count the string revolutions from the starting mark . Adjust the torque limiter to the calculated torque and torque the string until the Applied Torq Torq is reached. .
7. Maintain the applied torque and work the pipe with high set-down weights and light overpulls overpulls to work the torque down the drillstring. The string will continue to rotate until the torque is uniformly applied applied down to the stuck point. When the string stops rotating, record record the total string revolutions (Wraps), i.e., 4.25 revolutions. .
F ac) for the size and weight of the drillpipe, i.e., 5” 19.5 lbs/ft. 8. Select the torque factor ( Torq Fac Calcul Cal culate ate the Stuc Stuck k Depth Depth (St (Stuck uckFT):
. .
StuckFT =
Wraps x Torq Fac x 501782 Applied Torq
4.25 x 28.538 x 501782 = 10,143 ft 6000 ft lbs
Verifying the Stuck Depth: Verifying 9. Repeat the procedure from step #3 using 50% of the drillstring make-up torque. The two calculated stuck depths should be within 3% of the same value. .
7 x 28.538 x 501782 10,000 ft lbs
= 10,024 ft
10. If Differential Sticking is the sticking mechanism, a permeable zone must be present at the calculate stuck depth. If a sand or limestone formation is not present, repeat the procedure or consider the possibility of Wellbore Geometry as the sticking mechanism . Page 10
FREEING DIFFERENTIAL STICKING O Heavy Down-Jaring Fishing Assembly: (Hol (Hole e angle angle abov above e fish, fish, 70 70 or less) less) .
If all other attempts failed to free the string, back-off the string below the drilling jar and pick up a down-jaring fishing assembly with all the BHA weight available up to a maximum of 80% of the hydraulic jar’s trip-load capacity. Back-off the string below below the jar. If possible, back-off at the top of the drill collar section to avoid heavy jarring loads on the more flexible HWDP. BHA Design:
Pick-up a Heavy Down-Jarring Fishing Assemble with all the BHA weight available up to 70% of the hydraulic jar’s max trip-load capacity. Max trip loads: (6-1/2” jar, +/-170,000 lbs); (8” jar, +/-300,000 lbs).
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1 - Stab-in sub 1 - 8” Hydraulic jar 14 - 8” Collars 10 - 6-1/2” Collars 30 - 5” HWDP
.
Trip Weight = (Cosine of Hole Angle) X BHA Weight (56 COS) X 138,840 lbs = 77,638 lbs
.
BHA weight: 138,840 lbs 0
56 Average hole angle above fish.
Wash the last stand to the top of the fish and fish and circulate the hole clean with rotation to avoid differential sticking. Spot a high-concentration lubrication pill in the BHA section. Rotate the string to ensure good lubrication around the collars. Stab into the fish and torque-up the connection.
Torque-up the string to +/- 80% of the drillpipe make-up torque. Carefully reciprocate the string to ensure torque limit is applied at the stuck depth.
Cock the jar and Jar down with maximum jar-trip load. Hold torque and set-down weight until the jar trips or for +/-10 minutes. If the string does not jar free, maintain torque, slow circulation (+/-1 bbl/min) and continue jarring down until down until the string jars free.
Page 11
PREVENTIVE ACTION, DIFFERENTIAL STICKING To prevent differenti differential al sticking, steps must be taken to interrupt as many of the five conditions that favor the development of the area of low pressure filter cake in contact with the drillstring.
4"
Contact
Minimize Overbalance Pressure: Plan the casing setting depths to minimize high overbalance pressure at permeable zone depths. .
Maintain the minimum mud weight required for well pressure control and wellbore stability. Control the rate of penetration to minimize mud weight and use proper hole cleaning practices. .
Take action to avoid kicks. Use recommended kick killing procedures to avoid unnecessary pressure on the wellbore and to minimize total shut-in time for the kill operation. .
Minimize Formation Permeability: Use fine calcium carbonate to isolate wellbore pressure from low pressure permeable zones until casing can be set. Add fine LCM to the mud system system if seepage loss occurs. occurs. .
Minimize Filter Cake Thickness: Minimize fluid loss to decrease the rate of static filter cake build-up and the filter cake thickness. Minimize drill solids and low gravity solids. For information on a closely related sticking sticking cause, see Wellbore Geometry, Thick Filter Cake, page 18. Page 12
PREVENTIVE PREVEN TIVE ACTION, DIFFERENTIAL STICKING In most cases, the size of the contact area (square inches) plays the greatest role in producing the differential sticking force. There are several recommended recommended actions that can be taken to to reduce the drillstring contact area.
The Contact Area Area - The thickness of the permeable zone times the width of contact determines square inches of contact area. Smooth Collar 100% Contact
Spiral Collar 66% Contact
Spiral-Flex Collar 23% Contact
Minimize The Contact Area: Reduce the length of un-stabilized drill collars by increasing the length of HWDP. .
Use spiral-flex collars in the unstabilized BHA section. Use spiral HWDP. (when possible) to minimize static filter cake build-up. Maintain string movement (when Page 13
