cause of pipe stuck

Date : 04/99 Rev : 0 draft

Clt : 04-291

DGEP/DO/FPL/MTH

CAUSE OF STUCK PIPEPage : 1/8

1.1 INTRODUCTIONThere are 20 root causes for getting drill string stuck whichcan be divided in four groups.

1.2 DIFFERENTIAL STICKINGA sticking force is developed when a differential pressure(over balance) forces the string into thick filter cake builtin front of permeable zone. Refer to here below statisticschart established from analysis of 600 well histories in Gulfof Mexico.

Warning signs: Drilling through a prognosed low pressure zone,long and non stabilised BHA, use of plain (none spiralled)drill collars. Increasing over pull, slack off weight ortorque to start string motion.Indications: Occurs after a period of stationary or very slowmoving string, (connection, surveys, steering/sliding with lowROP). The string cannot be moved (either up and down norrotate) with unrestricted circulation.

1.3 HOLE PACKING OFF - HOLE BRIDGING

DEFINITIONS:

Hole pack off: formation solids (cuttings, cavings) settle

around the drill string and packs off the annulus resulting

in stuck pipe

PROBABILITY OF DIFFERENTIAL STICKING

0102030405060708090100

0 1000 2000 3000 4000DIFFERENTIAL PRESSURE (PSI)

STUC

K PI

PE P

ROBA

BILI

TY


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Hole bridging: medium to large pieces of hard formation,

cement or junk falls into the well bore and jams the drill

string.

1.3.1. DUE TO INADEQUATE HOLE CLEANING.

1.3.1.1. SETTLED CUTTINGS IN NEAR VERTICAL WELL BORE(? < 45°)Drilled cuttings are not transported out of thehole due to low annular velocity or/and to poormud property. When circulation is stopped, thecuttings fall back down the hole and pack offthe drill string.Warning signs: High ROP, low pump rate, littleto no circulation time at connection; torque,drag and pump pressure increase, over pull whenpicking the string off slips, pump surge tobreak circulation, fill on bottom.Indications: likely to occur on connections,possible during trip. Circulation restricted orimpossible.

1.3.1.2. SETTLED CUTTINGS IN HIGH ANGLE WELL BORE(? > 45°)Drilled cuttings settle on the low side of thehole and form a cutting bed. The cuttings bedbuilds and slides down hole packing off theBHA; or/and while POOH the cuttings bed isdragged upward by the BHA and packs off thestring.Warning signs: Planned hole angle greater than45°, drilling with down hole motor (sliding orvery low rotation of string), high ROP, lowpump rate; torque, drag and pump pressureincrease, increased over pull on trip.Indications: likely to occur while POOH,possible while drilling. Circulation restrictedor impossible.

1.3.2. UNCONSOLIDATED FORMATION.Poorly bonded formation that cannot be supported byhydrostatic over balance (sand ,boulder, etc..) fallsinto the hole and packs off the string. In deep sealocation a phenomenon called mud flow could end withthe same effects (“over pressurised” shallow waterreservoir not balanced by mud hydrostatic kicks anddrag formation into the well bore).Warning signs: Likely to occur as the formation isdrilled. Torque and drag increase, pump pressure


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fluctuations, hole fill on connections, shale shakerand desander overloading.Indications: Generally occurs in surface hole, canoccur while drilling or tripping sudden pack offwithout warning, circulation impossible

1.3.3. FRACTURED – FAULTED FORMATIONNear the fracture or the fault the formation can bebroken into pieces, these pieces can be loose or becomeloose with BHA impact and fall into the well bore andjam the string. This very seldom type of sticking caneven occur while drilling.Warning signs: Prognosed fractured or faulted formation(limestone or shale). Fault damaged caving on shaleshaker, hole fill on connections and trip.Indications: Likely to occur while tripping alsopossible as the formation is drilled through. Suddenand erratic torque and drag just before sticking;circulation may be restricted.

1.3.4. STRESSED FORMATION.1.3.4.1. OVER PRESSURED FORMATION

a) Naturally over pressured formation:(undercompacted shale). Pore pressure exceedingmud hydrostatic head induce stress crack andfracture that will cause the formation tocollapse.Warning signs: The sign begin to occur asthe shale is drilled. Mud logging indicatesincrease in pore pressure, D exponent, gaslevel, decrease in shale density. ROPincreases, torque increases, drag onconnection, hole fill on connection.Splintery shale caving on shaker.

Indications: Likely to occur while tripping,possible while drilling; circulationrestricted or impossible.

b) Artificially over pressured shale: over timemud overbalance is supercharging shale pore,a steady overbalance or an overbalancereduction (mud weight reduction or swabbingwhile tripping) and/or drill string impactwill induce stress crack and fracturefollowed by formation collapse.Warning signs: Occurs with water based mud,generally follows a mud weight reduction,torque and drag increase, shale cavingsplintery on shakers.


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Indications: Likely to occur while drillingor tripping after formation exposed for“long time” to mud, hole bridging orplugging, circulation restricted orimpossible.

1.3.4.2. TECTONICALLY STRESSED FORMATION.Highly stressed formation will react whendrilled through: stressed shale fractures andfalls into the well bore, sandstone squeezesinto the well bore leading to well under gauge.Warning signs: Mountain location, prognosedtectonics; erratic torque and drag; Splinterycaving on shaker, increased volume of cuttingson shaker.Indications: Possible while drilling ortripping, circulation restricted or impossible.

1.3.4.3. OVERBURDEN STRESSED FORMATION.Overburden stress is counter balanced with mudweight in vertical and sub vertical hole; whendeviation angle increases with mud weight notadjusted the stressed shale fractures and fallsinto the well bore (refer to schematic herebelow).

Warning signs: hole cleaning problems,increased torque and drag, shale caving onshaker.


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Indications: can occur while drilling ortripping; hole bridging or packing off;circulation restricted or impossible.

1.3.5. REACTIVE FORMATIONWater sensitive shale drilled with low or noneinhibited water based mud will hydrate and swell intothe well bore. This chemical reaction is timedependant: it takes time to occur (from hours to days).Warning signs: Funnel viscosity (March viscosity), PV,YP, CEC or MBT increase, torque, drag, pump pressureincrease, clay balls and/or soft cuttings on shaker,over pull and swabbing on trip, BHA balling.Indications: Generally occurs while POOH, possiblewhile drilling, circulation impossible or highlyrestricted.

1.4 MECHANICAL AND WELL BORE GEOMETRY.

1.4.1. KEY SEAT.Drill pipes rotating against the bore hole in front ofsharp direction change (high dog leg severity on onespot) are wearing a groove (or a slot) in the mediumsoft to medium hard formation. When the string ispulled out the large diameter part of the string (DP-HWDP tool joints or drill collars) jams into the slot.Warning signs: High angle dog leg in upper holesection, long drilling hours without wiper trip throughthe dog leg section, cyclic over pull at tool jointintervals on trip.Indications: Occurs while tripping put only, suddenover pull as BHA reach dog leg depth, unrestrictedcirculation, free string movement below key seat depthpossible if not yet stuck.

1.4.2. UNDER GAUGE HOLE.Drilling in hard abrasive formation wears the bit andstabilisers resulting in an under gauge section ofhole, cored sections are often under gauge. New drillbits and BHA run in such hole will jam in the reduceddiameter section.Warning signs: Prognosed abrasive sands, pulled bit andstabilisers under gauge, running in hole after coringIndications: Occurs only when running in, near thebottom of the hole or at top of cored section, suddenset down weight, circulation unrestricted.


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1.4.3. LEDGE AND DOG LEGS1.4.3.1. LEDGES: Drilling through interbeddedformations with different hardness gives ledges at theinterfaces of formation. (soft rock are washed out orover gauge and hard rock is in gauge).Warning signs: Prognosed hard-soft inter beddedformation, sudden erratic over pull.Indications: Generally occurs when pulling out,possible when picking up for a connection, generallyassociated with micro dog leg, circulationunrestricted.

1.4.3.2. DOGLEGS: When drilling through interbeddedformations with different hardness the bit can deflectits course when hitting the interface resulting in asharp change in direction (so called dog leg) orfrequent hole angle correction can make the same suiteof micro doglegs. When the drill string is tippedthrough, the BHA can jam into the successive microdoglegs.Warning signs: Prognosed hard-soft inter beddedformation, frequent angle and/or direction changes,drilling with down hole motor and bent sub, erratictorque and drag on connection.Indications: Likely to occur when picking up for aconnection or on trips, circulation unrestricted.

1.4.4. BHA CHANGESRunning in hole a BHA stiffer than the previous one andthat cannot negotiate the hole angle or directionchanges can end with a BHA jammed.Warning signs: Dog leg(s) present, pulled stabilisersout of gauge, new and stiffer BHA picked up and run.Indications: Most likely to occur when running in,possible when tripping out, BHA at direction-anglechange depth, sudden set down weight, unrestrictedcirculation.

1.4.5. MOBILE FORMATION.Overburden on plastic salt or shale squeeze them intothe well bore, the BHA becomes stuck in under gaugedhole.Warning signs: Prognosed salt or plastic shale, suddenincrease in over pull or slack off weight, suddentorque increase with fast moving plastic formation.Indications: generally occurs while pulling out ,possible when running in after long period out of hole,also possible while drilling if formation moves


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quickly. Circulation unrestricted or slight restrictionpossible.

1.4.6. JUNK IN HOLE.Debris falling into the well bore from surface or fromdown hole equipment can jam the string in the hole.Warning signs: Junk sticking can occur at any time andduring any operation, metal shavings on shaker.Indications: Generally occurs when the BHA is in hardformation or inside casing, missing hand tool on rigfloor, missing rig equipment part (i.e., slips or tongdies), unrestricted circulation.

1.4.7. COLLAPSED CASING.Collapsed casing occurs when the external pressure orforces exceed casing collapse strength:a) Casing is too light duty.b) Casing is old. (corroded and/or worn)c) Casing is landed with too high tension consequently

reducing its collapse rating.d) A leak develops and external pressure exceeds casing

collapse strength.( Upon a pressure test or alongwell life pressure build up in the annulus due to“bad or poor” cementing job).

e) Casing can collapse when well is worked over: ondepleted reservoir when unsetting the packer orcutting the completion, milling out the packer onecan record total losses and in tectonic area orfaulted formation the “pressure” from the formationcannot be sustained by an empty or partially emptycasing.

1.4.8. CEMENT BLOCKCement block from shoe track, rat hole, around thecasing, open hole squeeze plug or kick off plug becomesunstable and fall around the string.Warning signs: Excessive casing rat hole, large casingcleaned with small diameter bit (i.e., 24” casingcleaned with 16” bit) cement squeeze job, cement kickoff plug; cement caving on shakers and /or in cuttingssamples.Indications: Can occur at anytime, sudden erratictorque and drag just before sticking, circulationpossible.

1.4.9. GREEN CEMENTDrill string run inadvertently into soft cement (top ofcement may be higher than expected) high to very highpenetration rate when drill out /clean out recentlyspot cement in which unset cement flash set. Running


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surge or friction in annulus while pumping increase thepressure on cement leading the cement to flash set.Warning signs: Tripping in hole after setting an openhole cement plug or after a liner cement job, set downweight occurs above theoretical top of cement.Indications: an occur as pump pressure is applied,circulation highly restricted or impossible.

1.5 ADDITIONAL WAY OF GETTING STUCK.These additional ways are not accounted for in the 20 previousones as they are seldom. However their consequences and thecost of their remedial could be much higher, most of timesthere is no other way than retrieving the fish: stuck pointbeing far too high to side track the well or to save the sloton the platform.- Liner hanger, packer, whipstock premature setting.- Junk at sea bed hidden below cuttings from surrounding

wells.

cause of pipe stuck

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