Study Analysis of Stuck Pipe Incidents

Stuck pipe or sticking and lost circulation are the two main events which cause Non-productive time (NPT) in the drilling industry. A considerable amount of time and resources can be spent in efforts to free a stuck pipe. Sometimes, stuck pipe events result in breakage (either intentional or non-intentional) of the drill string leading to a lot of money being spent in fishing. Unsuccessful fishing operations have resulted in costly alternatives including side-tracking or worse still, well abandonment. Stuck pipe situations are very common around the world with most data gathered in the petroleum drilling industry.

1.CAUSES OF STUCK PIPE.

Pipe sticking Mechanism and Causes can be summarized in Table 1.

Pipe Sticking Mechanisms and Causes
Mechanism Differential Sticking Mechanical Sticking
 

 

 

 

 

Cause

Hole Pack Off Formation & BHA (Wellbore Geometry)
 

 

 

 

 

Differential Force

Settled Cuttings Key Seating
Shale Instability Mobile Formations
Fractured Rocks Under gauge Hole
Cement Blocks Micro Doglegs and Ledges
Junk Drilling Into Magma
drilling field
Figure 1: Map Showing Menengai Field and Wells (GDC, 2013)

1.1 Differential Sticking

During drilling the drilling fluid pressure is maintained at a higher value compared to the reservoir or formation pressure. When a permeable zone is reached, the difference in these pressures forces some of the fluid to seep into the permeable zone. As this happens, the solids in the drilling fluid are filtered out at the hole wall forming a layer called a filter cake. If a substantial area of the string surface comes into contact with the cake formed, then only the outer wall surface exposed to the drilling fluid “sees” the higher drilling fluid pressure and the contact surface to the cake “sees” the lower formation pressure. This pressure difference pushes the pipe to stick to hole wall and embed itself further into the filter cake with a great force (can reach more than a million pounds force). The string thus gets differentially stuck and force required to pull it exceeds the yield point of the pipe. The signs of differential sticking are:

  1. The pipe can neither be moved up and down nor rotated;
  2. Circulation is unaffected

The differential sticking force depends on the pressure differential and the area of contact with the porous formation zone among other factors。

Differential sticking may be prevented by:

    • Maintaining lowest continuous fluid loss;
    • Keeping circulating mud free of drilled solids;
    • Keeping a very low differential pressure with allowance for swab and surge;
    • Using a mud system that yields smooth mud cake (low friction co-efficient);
    • Maintaining drill string rotation at all times;
    • Using grooved or spiral drill collars;
    • Minimizing length of drill collars and Bottom Hole Assembly (BHA).

If differential sticking occurs, the following solutions are mostly used:

  • Immediate working/jarring of the string downwards;
  • Reducing drilling fluid hydrostatic pressure by gasifying with air or by diluting the fluid. Close attention must be paid to kick indicators while reducing hydrostatic pressure;
  • Oil spotting around stuck portion of string;
  • Washing over the stuck pipe.

Hole pack off causes

Settled Cuttings

This is one of the major causes of mechanical stuck pipe. It is where cuttings pack off or settle and build on the well bore and causes compaction around the BHA when the pipe is moved upwards. The compacted cuttings then prevent the string from coming up especially during trip out. Figure 2 shows settled cuttings.

 

Settled Cuttings Due to Poor Hole Cleaning
Figure 2: Settled Cuttings Due to Poor Hole Cleaning (Rabia, 2001)

The problem is more prone in highly deviated or horizontal wells since the cuttings tend to fall on the low side of the hole and are harder to clean out. These settled cuttings pile up and form beds and may compact against the BHA on trip outs. In vertical wells, good hole cleaning is achieved by selection and maintenance of suitable mud parameters and ensuring that the circulation rate chosen results in an annular velocity (around 100 -120 feet/min) which is greater than the slip velocity of the cuttings. Besides causing stuck pipe, settled cuttings can also cause:

  • Formation breakdown due to increased Equivalent Circulating Density;
  • Slow rate of penetration;
  • Excessive over pull on trips;
  • Increased torque.

Hole cleaning is one of the main solutions to preventing this stuck pipe problem and can be controlled by:

  • Good mud rheology especially yield point and gel strength;
  • Controlling drill rate to ensure hole is clean;
  • Checking volume of cuttings coming over to shale shaker;
  • Controlling annular velocities;
  • Recognizing increased over pull;
  • Reciprocating and rotating pipe while circulating;
  • Using viscous sweeps;
  • Recognizing low side section of deviated holes;
  • Regular Wiper trip If sticking occurs, then:
  • Attempt to establish circulation;
  • Simultaneously apply downwards force gradually until circulation starts;
  • Once circulation starts, rotate the string;
  • In low angles holes, a weighted viscous pill should be used to ‘float out’ the cuttings;
  • In high angle holes, a low viscous pill should be used to disturb the cuttings bed followed by weighted pills to carry cuttings out of hole.

Formation Instability

This is as a result of tensile and compressive failure on the borehole wall. The borehole will fail in tension while drilling mud hydrostatic pressure induces stresses in the hole wall that exceeds the tensile strength of the rock. The borehole will fail in compression when the pressure of the drilling mud is insufficient to keep the shear stresses in the borehole wall below the shear strength of the formation (Rabia, 2001). This problem can be solved by applying rock mechanics principle to define working limits for mud weights to avoid tensile or compressive failure; here, the equations and methods applied in rock mechanics are quite complex and can be found in most geo-mechanics and rock mechanics literature. The result of formation instability is either borehole widening or contraction depending on the failure mode of the rock inside the well. The Figure 3 shown shows the Inner Drucker-Prager criterion for predicting safe mud weights.

 Safe Mud Weights Envelope
Figure 3: Safe Mud Weights Envelope (Rabia, 2001)

Sticking to the fluids program can prevent effects of formation instability.

Other solutions include making use of a well program that isolates a potential troublesome formation and speeding up the drilling process to cut down time of drilling sensitive formations.

Formation instability can be identified by the following:

  • Large amounts of angular or splintery cuttings when circulating;
  • Drag on trips;
  • Large amounts of hole f

Formation instability will cause material to fall inside the hole creating caves or contract the wellbore and might cause sticking. Sloughing and caving are also due to formation instability. If these occur, then the solution is establishing circulation, then working the drill string preferably downwards; when the string is freed, circulate all material out before changing the mud properties to continue drilling.

Unconsolidated Formations

Usually encountered near surface and include loose sands, gravel and silts. These collapse due to low cohesive strength; they can collapse and jam the drill string.

Signs of sticking due to unconsolidated formation include:

  • Increased torque;
  • Drag and pump pressure increase when drilling;
  • Increased Rate of Penetration;
  • Large fill on bottom.

This problem can be prevented by using a mud system with impermeable filter cake to reduce fluid invasion into rock. Reducing annular velocity by reducing mud flow rate will also reduce erosion of hole wall and also reduce removal of filter cake.

Fractured and faulted formation

Symptoms of fractured and faulted formation include:

  • Large and irregular rock fragments at shale shakers;
  • Increased torque, drag and rate of penetration;
  • A small amount of lost circulation.

The fractured formation falls into the well due to stresses originally holding the formation together being relieved by drilling of the hole. Excessive vibration might also cause the drill string to whip down hole and dislodge the fractured rocks. The problem can be prevented by:

  • Reducing drill string vibration;
  • Minimizing surge pressures;
  • Sufficient hole cleaning to reduce hole pack off.

If sticking occurs, jar the string. If this is not successful, an inhibited Hydrochloric acid pill may be spotted around the stuck zone to break down the material surrounding the pipe.

Cement blocks

Cement blocks from the rat hole might fall into the well bore and cause sticking. This can be prevented by minimizing the rat hole to a maximum of 5 feet and ensuring good tail cement at the casing shoe. If sticking occurs due to cement blocks, jar the string or inject acid solution down hole to dissolve the cement.

Green cement is improperly set cement. Green cement can occur after setting a cement plug inside casing or open hole. If the drill string is run too fast into top of cement and the cement is still green, then the cement can flash set around the pipe and cause permanent sticking. Flash setting is phenomenon that is not very well understood but a possible explanation is that the energy release while circulating and rotating could be sufficient to cause it. A good practice to prevent this is starting circulation 2 or 3 stands above expected top of cement and also keeping a low weight on bit.

1.2 Drilling into magma

It has been shown that the 2011m depth is particularly troublesome to drill through in Menengai field, Kenya (Makuk, 2013). It has been observed that fresh glass was present in cuttings at 2082 m and 2174 m at MW04 and MW06 respectively (Mibei, 2012). It is believed that magma intrusions at these depths are rapidly chilled by the drilling fluid producing glassy cuttings. Sticking problems were recorded at these depths and are believed to be related to the occurrence of glass (Mibei, 2012). The exact mechanism of sticking due to drilling into magma is not really known. The Iceland Deep Drilling project 1 was halted after having drilled into magma and gotten stuck, however the bit came up intact (Hólmgeirsson et al., 2010). It is reported that the magma pushed up on the drill string, lowering the hook load value. It is believed that explosive chilling of the magma (steam flashing) by drilling fluid downhole could however be related to this sticking problem.

A summary of signs of sticking and parameters to watch for stuck pipe problems can be tabulated well using the Baker Hughes INTEQ, 1995 workbook as shown in the Table 2.

INDICATOR PROBLEM TORQUE PRESSURE DRILL RATE
Poor Hole Cleaning Increase Increase Gradual Increase
High Overbalance Gradual Increase No Change Gradual Decrease
Mobile Formations Gradual Increase Increase Gradual Decrease
Fractured         &         Faulted Sudden Erratic Increase May Be Unaffected Sudden Increase
Formations
Geo-pressured Formations Increase Increase Initial     Increase With a
Reactive Formations Gradual Decrease
Unconsolidated Formations Gradual Increase Increase Gradual Decrease
Junk Increase Increase Decrease
Cement Blocks
Sudden Increase No Change Sudden Decrease
Sudden Increase No Change Sudden Decrease

TABLE 2: Stuck Pipe Problems and Indicators (Baker Hughes, INTEQ, 1995)

2. MENENGAI STUCK PIPE DATA ANALYSIS

Tables 3 and 4 shows how stuck pipe events are distributed over the drilling activities in 10 Menengai wells.  This table will assist in identifying operations during which stuck pipe mostly occurs.

TABLE 3: Drilling activities during Stuck Pipe/ in Menengai

Well Depth  Of Sticking (m) Activity                        During Sticking No.             Of

Hours Stuck

Freed Hours spent Fishing Total drilling days (spud in to capping) Total              Depth (m)
MW01 114 Drilling 1 YES 0  

 

 

 

 

 

 

 

79

 

 

 

 

 

 

 

 

2206

125 Drilling 3 YES 0
380 POOH 3 YES 0
378 Casing-Stuck Casing YES 0
2206 Drilling 8 YES 0
MW02 109 POOH 72 yes 0  

 

 

 

 

 

 

 

 

 

 

 

125

 

 

 

 

 

 

 

 

 

 

 

 

3200

133 RIH <12 yes 0
135 Ream <12 yes 0
165 POOH 1 yes 0
207 Drilling 3 Yes 0
213 Drilling 17 Yes
218 Drilling/Reaming 77 yes
MW03 113 Drilling 0.75 Yes 0  

 

 

 

 

 

100

 

 

 

 

 

 

2112

167 Drilling 0.5 Yes 0
1187 Drilling 0.5 Yes 0
2093 Drilling/Reaming 216 No-back off 648
MW04 2117 Drilling 216 No-parted string 0 83 2117
MW06 2202 Drilling 268 No-parted string 0 96 2202
MW07 59 Drilling 27 Yes 0  

 

 

 

 

 

 

 

 

 

132

 

 

 

 

 

 

 

 

 

 

2136

105 Drilling <6 Yes 0
149 Drilling/Reaming 20 Yes 0
151 Drilling 5 Yes 0
1184 POOH 37 Yes 0
2135 Drilling 9 No-parted string 0
MW08 58 Drilling 1 yes 0 126 2355
MW09 1950 Drilling 1 yes 0 107 2088
MW13 1648 RIH 2 yes 0 161 2012
MW21 326 Drilling 4 yes 0 2730

TABLE 4: Total Hours Stuck (10 Wells)

Operation Hours
Drilling

 

POOH RIH

883.75

 

37

 

14

Total 934.75

Table 5 shows the history during selected stuck pipe events. The recommended pumping rates are calculated based on recommended good practice for geothermal drilling.

TABLE 5: Operational activities during Sticking (from GDC well completion reports)

 

Well Depth Stuck

(m) and Date

Operational activities in a span of 24-48 Hours Pumping Rates and Returns
MW07 59

 

 

 

08-02-12

Drilled 26″ well from 43.19 m to 59.06 m with brine-Returns OK.  The drill string got

stuck while making a connection.  Worked the string and applied pull up to 200 klbf. Pumped hi vis mud at every 15 minutes while working stuck pipe up and down.  No returns. Pumped L.C.M in mud into the hole while working the string. Received returns to the surface.  Pumped aerated water into the hole. Received returns on surface. The string was freed.

Approx.  1717 l/m

 

Full Returns

 

*Recommended pumping=7295 l/m of water

MW07 149 Drilled 17-1/2″ hole from 133 m to 149.25 m with intermittent partial & full returns.

Experienced drilling break from 134 m-138 m with cuttings fully flowing out.

Approx.  2720 l/m
Circulated the hole at 149 m. While lifting the string off bottom, it got stuck with high

torque and high Stand pipe pressure experienced. Worked the string up and down

Full Returns
20-02-12 while pumping brine, then introduced air. Applied tension to 160 kN.  Gained 1 foot.
Thereafter, rotation and circulation was lost.  Poured 2 drums drilling detergent into
the active mud tank and pumped through the string.
Not much progress observed.  Pumped hi-vis mud into the hole.  Very little gain
observed.  Worked the pipe up and down without gain. There was no pressure in the
Stand pipe pressure.  All fluids pumped got lost into formation.  Decided to cure loss
zone by pumping hi-vis mud mixed with wall nut shell and mica flakes. This was
repeated severally.  Worked the string and introduced air.  Leakage on the stand pipe
observed.  Continued pumping brine at 200 strokes per minute while repairing the
leakage.  Introduced air and pressure indicated 800 psi. The string got free and full

returns observed on the surface.  POOH and prepared the string for plug job.  Pumped 4 m3 of cement slurry at 112 m. POOH to shoe and WOC to cure.

Recommended pumping=3107 l/m of mud
MW07 1184 Drilled 12-1/4″ hole from 1200M to 1205M with mud- Returns ok.  Circulated the hole 1717 l/m
with 10,000 strokes of hi-vis mud to clean hole.  POOH to shoe and did a wiper trip.
Encountered a tight spot at 1070m.   Circulated the hole and spotted hi-vis mud at Full Returns
bottom.   POOH from 1205m to 1184m while back reaming.   Pump went off while
15-04-12 attempting to connect back the stand that had been removed.   Saver sub damaged.
Installed circulating head and circulated the hole-returns received on surface.   On
connecting the saver sub to string, it was noted that the string got stuck.  Circulating Recommended
with mud returns ok. Pumped high vis mud while working the string. pumping=1413 l/m
of mud
MW07 2135 Drilled 8-1/2″ hole from 2134.09 m to 2135.93 m with aerated water and foam-No 2040 l/m
returns.  The string got stuck at 2135.93 m at 0200 hrs.  Worked the string by applying
torque. Circulated with aerated water and foam while working the string.  Applied pull No Returns
310 klb and torque of 28 kNm.  The string was freed at 1100 hrs and regained rotation.
25-05-12 Circulated and POOH.  POOH experiencing high drag from the bottom to 965 where
there was no drag.  Part of the BHA left in the hole.  Two 6-1/2″ drill collars, and 8-
1/2″ bit left in the hole.  Waited for instructions from management.  Decision made to Recommended
RIH liners. pumping=1100 l/m
of water

 

MW09 1948

 

22-10-12

Drilling 8½″ hole with aerated water and foam. Partial returns.

 

Drill string sticking from 2300 hrs to 0000 hrs.

2210 l/m. Partial

Returns.

 

Recommended pumping=1100 l/m of water

MW21 326 Drilling 17-1/2″ hole with water and mud sweeps till 326m.  Got stuck at 0200 hours 3060 l/m- Full
and lost circulation.  Circulated hi-vis mud while working the string for 4 hours when Returns
28-12-13 string was freed .POOH after circulating to remove collapsed debris.  POOH to 291 m
and reaming the section between 291m and 326 m. *Recommended
pumping=6213 l/m
of water

*these water pumping rates are practically difficult to achieve and therefore the problem is mitigated by using high viscosity mud
sweeps at regular intervals to ensure sufficient hole cleaning. We also note that the upper sections of a well are usually drilled with slower ROP, and therefore the fluid annular velocity necessary for sufficient hole cleaning is lower. Cuttings also reach the surface faster since the well is still shallow.

Easy View Diagrams and Analysis

Drilling parameters during selected stuck pipe events were analyzed using Easy View Software to easily recognize trends at the moment of sticking and before the stuck pipe event. The drilling data was recorded using data loggers at the rig site. This data was then downloaded in excel files in 10 second intervals and has been the input into Easy View software. This analysis will assist in identifying the causes of the stuck pipe and possible solutions. The diagrams have been displayed and described in the subsequent pages. Other conditions not captured by the data loggers during the stuck pipe events have also been listed (these other conditions include pumping rates and amount of returns at shale shakers)

MW07: Stuck at 2135m at 0145 h during drilling

The trend in Figure 4 shows that the string got stuck at 0145 hrs. We see a sudden drop in WOB from 5.92 to 0 kN, the rpm also drops to 0 from 70. The pump rate, bit location and ROP remain constant. It can be observed that the driller then tries to pick up the string and it is stuck as it has to be pulled to over 84.35 tonnes. There were no circulation returns at the moment of sticking and pumping rate was 2040 l/m of aerated foam and water.

Parameters at sticking pipe
Figure 4: Parameters at sticking of MW07 at 2135 m

The trends prior to sticking are displayed in Figure 5. From 0100 hrs to sticking time at 0145 hrs, stand pipe pressure varies by 3 bars (between 5,27 MPa & 4.97 MPa). The other parameters appear to be unchanging. Pumping rate was 2040 l/m of aerated foam & water and there were no returns prior to the sticking.

Parameters prior to sticking of MW07
Figure 5: Parameters prior to sticking of MW07 at 2135 m

MW09: Stuck at 1948 m at 2229 hrs during drilling

The trend in Figure 6 shows the parameters at the moment of sticking of MW09 at 1948 m. The trends prior to sticking are shown in Figure 7.

Parameters at sticking of MW09 at 1948 m
Figure 6: Parameters at sticking of MW09 at 1948 m

We see that the string gets stuck at 2229 hrs when the rotation speed and rotary torque suddenly drop to zero. This occurs at the end of the drill pipe joint evidenced by the value of hook height i.e. the hook height is constant at about 0.41 m which implies it is the end of the current drill pipe joint. WOB is also observed to dip to zero. Pump rate and pressure do not change. There were partial returns during this stuck pipe event and pumping rate was 2210 l/m of aerated water and foam. The trends prior to sticking show that the rotation speed, pipe pressure and torque are quite regular through the drilling of this joint of drill pipe just until sticking point.

Parameters prior to sticking of MW09 at 1948 m
Figure 7: Parameters prior to sticking of MW09 at 1948 m

The results seen in the above diagrams imply that the possible reasons and types of sticking are as shown in table 6. Parameters not captured in Easy View have been obtained from Menengai well completion reports (see table 5). The possible causes are inferred from notes on stuck pipe that had been discussed earlier in this paper. The data necessary to carry out the Easy View analysis for most of the stuck pipe incidents, especially for the earlier wells were unavailable.

TABLE 6: Possible Causes of Stuck Pipe in Menengai Wells

Well Depth Stuck Torque WOB ROP Returns Other Possible Cause
MW07

 

 

 

 

MW07

 

 

MW07 MW07

2135 m

 

 

 

 

 

59 m

 

 

 

149 m

 

 

1184 m

Increase                  Unchanged         Increase          No

 

 

 

 

 

Freed by switching from drilling with brine to aerated fluid.

 

 

 

Sudden                        _                    Sudden                 Full

Increase                                            Increase

Freed by pumping water instead of mud

Well             Pack-off   caused    by

inclined        poor hole cleaning or

24                 a new lost circulation

degrees         zone.

 

 

Settled cuttings due to poor hole cleaning.

 

Pipe Pressure     Fractured & Increase              faulted

formation. Differential Sticking

MW09 1948 m Sudden Drop       Unchanged           Unchanged           Partial Rotation             Fractured &

speed                  faulted suddenly 0.        formation.

Cement

Block/Junk

MW21 326 m Unchanged           Unchanged           Sudden                 PARTIAL

Decrease

Circulated           Poor Hole

till free                Cleaning

3.DISCUSSION AND CONCLUSION

The results show that the causes of stuck pipe are several in Menengai. Unconsolidated formation is problematic but has been mitigated through using cement plugs. As earlier mentioned sticking due to fracture and faulted formation can be controlled by reducing drill string vibration, minimizing surge pressures and sufficient hole cleaning to reduce hole pack off. Sticking due to poor hole cleaning can be reduced by ensuring the hole is clean of cuttings.  There are several ways to ensure good hole cleaning including ensuring good mud

Rheology especially yield point and gel strength, controlling drill rate to ensure hole is clean, checking volume of cuttings coming over to shale shaker and controlling annular velocities. Sticking caused by drilling through micro doglegs and ledges can be prevented by running slowly when tripping at alternating formation points; these areas should be noted and reamed through during trips.

  • Loss of Returns While Drilling With Water and Air

Drilling the production zone is quite a challenge especially when there are no returns & cuttings cannot be carried to surface. The drilling program in Menengai usually recommends blind drilling in the production section when there are no returns. The choice of drilling fluid is restricted to water, aerated fluid and foam.  Mud improves hole cleaning but cannot be used in the production zone since it will block the sensitive feeder zones. This problem can be solved by use of liquid drilling fluid polymer. This compound increases water viscosity thereby helping a lot with cutting carrying capacity; it does not affect formation permeability adversely. Polymer however does not improve gel strength and will therefore not suspend cuttings if pumping is stopped. Sweeping the hole with polymer can however still be introduced in Menengai to assist in hole cleaning as it has been successfully used in Iceland.

 

  • Drill String snapping/ tubular washout

An incident of snapped string was encountered after one day of working stuck pipe at MW07 at 2135 m. Pull applied was 310,000 lb. force and 28 kNm torque which was still not exceeding the yield point (378,605 lb. force tensile yield strength and 53 kNm torsional yield strength) of the 5’’ OD drill pipe (considered the weakest member of the drill string). The BHA snapped at a collar connection. Weakening of drill string members could be caused by drilling fluid wash out or corrosion by acidic water that is used as drilling fluid. These two problems can be solved by use of corrosion inhibitor compounds and by use of caustic soda in drilling fluid. Caustic soda is used to maintain alkalinity so that acidic fluids do not attack metal. Corrosion inhibitors work by various mechanisms to inhibit oxygen present in drilling/production fluids from corroding pipes or equipment (Schlumberger Drilling, 2014). It has also been shown that aerated fluids erode drill pipes at a higher rate than non-aerated fluids (Budi Kesuma Adi Putra, 2008).

  • Deviation Surveys

These can greatly assist in correcting trajectories and avoiding sticking related to hole geometry. The Totco survey tool and the Electronic Multi Shot tool can be dropped in the drill string during trip outs to quickly measure inclination. These tools are popular since they eliminate the non-productive time associated with setting up a conventional deviation survey.

 

One Reply to “Study Analysis of Stuck Pipe Incidents”

Leave a Reply

Your email address will not be published. Required fields are marked *