SHALE PROBLEMS 1. Background __________________________________________________________2 1.1 Mechanical Effects:___________________________________________________3 1.2 Chemic Chemical al Effects: Effects: _______________ ________ ______________ _______________ __________ __________ _________________ ____________ ___ 4
2. Problems in Soft/Firm Shales ___________________________ ____________________________________________ _________________ 5 2.1 2.1 Bit Bit Ball Ballin ing g ____________________________ ____________________________________ ______________________________5 ______________________5 2.1.1 Occurre Occurrence nce of bit ballin balling___ g___ ____________ ____________ _________________________ __ 5 2.1.2 Preventat Preventative ive Measures Measures of bit balling balling _____________ ____________ ____________________ 5 2.1.3 Symptom Symptomss of bit balli balling ng ___________ ____________ __________________ _____________ 5 2.1.4 Cure for bit balling __________________________________________________________5
2.2 2.2 Mud Mud Ring Ringss ______________________________ ________________________________ ___________________________6 _________________________6 2.2.1 2.2.1 Occurrenc Occurrencee of mud rings___________ ____________ __________________ _____________ 6 2.2.2 Measures Measures to prevent prevent mud mud rings rings _____________ ____________ ___________ ________ _____ 6 2.2.3 Mud rings rings symptom symptomss ____________ ____________ ____________ __________ __________ _ 6 2.2.4 2.2.4 Curing Curing mud mud rings rings _ ____________ ____________ _________________ _________________ 6
2.3 Tight Hole __________________________________________________________7 2.3.1 Tight Tight hole definiti definition. on. ____________ ____________ ____________ ___________ _________ 7 2.3.2 Occurr Occurrence ence of tight tight hole hole ___________ ____________ __________________ _____________ 7 2.3.3 2.3.3 Techto Techtonic nicss _______ ____________ ____________ _____________ ____________ ________ 7 2.3.4 Overpr Overpressu essure re ( Geo Geo-pre -pressures.) ssures.) ___________ ____________ ____________ ________ ______ 7 2.3.5 Preventi Preventing ng tight tight hole ____________ ____________ ____________ __________ __________ _ 8 2.3.6 Symptom Symptomss of tight tight hole hole ____________ _____________ ____________ ____________ ______ 8 Cure ______________________ __________________________________ _______________________ _______________________ ________________________ ____________ 8 2.3.7 Cure
2.4 Erosion/Washout_____________________________________________________9 2.4.1 Occurren Occurrence ce of erosion erosion and and washout______________ washout______________ ____________ ____________ ________ 9
3. _____________________________________________________________________9 3.1.1 Preventing hole washout. _____________________________________________________9 3.1.2 Symptoms of hole washout____________________________________________________9 3.1.3 Curing Curing hole washout____ washout____ ____________ ____________ ____________ ___________ _____ 10
4. Problems in Hard/Brittle Shales _______________________________ ________________________________ _ _________11 4.1 Tight Hole _________________________________________________________11 4.1.1 4.1.1 Occurr Occurrenc encee _______ ____________ ____________ ______________________ __________ 11 4.1.2 Prevent Preventativ ativee Measu Measures res ____________ _____________ ____________________ _________ 11 Sympto toms ms ____________ _____________ ____________ ____________ _____________ __ 11 4.1.3 Symp 4.1.4 Cure ____________________________________________________________________ 11
4.2 Sloughi Sloughing, ng, Cavings Cavings and Packing Packing--off _______ ___ ________ _______ _______ ________ _______ _______ _______ ___ _____1 ___ __12 2 4.2.1 4.2.1 Occurr Occurrenc encee _______ ____________ ____________ ______________________ __________ 12 4.2.2 Preventi Preventing ng wellbore wellbore cavings cavings ____________ ____________ ______________________ ____ 12 Symptoms of wellbor wellboree cavings cavings ___________________ _____________ _________ _______ 13 4.2.3 Symptoms 4.2.4 Curing Curing wellbore wellbore cavings cavings ___________ ____________ ____________ _________ _________ 13
1. Background Shales can be defined as clay-rich clay-rich sedimentary rocks. They are sensitiv sensitive e to both mechanical and chemical influences and so are particularly problematic to drill. A useful classification of shales shales can be made in terms of hardness. Soft shale's (claystone/mudstone ) tend to be relatively young and are usually located near to surface. Harder shales are older and usually more deeply buried. Each type can be associated with a particular set of drilling problems, as summarised in the table below. For example, soft shales are more likely likely to cause tight hole, bit balling and hole hole erosion problems. problems. Hard or brittle shales shales suffer more from sudden cavings cavings and breakout breakout due to failure at the wellbore wall. wall. It is also seen from the shale that hardness correlates with other properties such as MBT, water content and shale s hale density. density. In tackling drilling problems in shales, both mechanical and chemical aspects should always be be borne in mind. The following provides provides a brief summary of the key points:Table 1: Classification Classification of shales according to hardness. h ardness. Shale Type
Soft
Firm
Hard Brittle
Typical hole problems Tight hole due to swelling Hole enlargement due to washout (dispersion ) Ledges if interbedded with sandstone Bit balling, mud rings, blocked flow lines Tight hole due to swelling Possible wash out ( poorly inhibited mud ) Particularly prone to bit balling Occasional cavings Cavings Cuttings beds leading to packing off Tight hole in stressed formations Cavings Hole collapse Time delayed failure
MBT* (meq /100g)
Water content (wt%)
Clay types
20 - 40
25 - 70
smectite illite
20 - 30
1.2 - 1.5
10 - 20
15 - 25
illite mixed layer
20 - 30
1.5 - 2.2
3 - 10
5 - 15
20 - 30
2.0 - 2.3
0- 3
2-5
5 - 30
2.1 - 2.5
illite possibly smectite illite koalinite chlorite
Wt %
Density (g/cc)
MBT T = Methy *MB Met hylene lene blue test-a measure of cation exchange capacity; high MBT equates to smectite-rich scale.
1.1 Mechanical Effects: There are a large number of variables. Those which can be controlled, and have a significant influence over hole stability are: Table 2 : Controlable shales variables
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Well Trajectory
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BHA Design
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Bit Type
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Jet Velocity
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Nozzle Size
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ROP (rpm)
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Weight on Bit
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Mud Flow Rate
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Swab/Surge Pressures
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Mud Rheology
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Mud Weight
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Drillstring Vibration
Of these, mud weight is probably the most fundamental with respect to hole condition. If the mud weight is too low, tight hole or or wellbore breakout breakout can occur. Too high a mud weight can fracture the formation.
1.2 Chemical Effects: Water will hydrate the clay minerals in shale. This can lead to swelling, softening and dispersion in softer types, or rock failure (fracturing) in the harder materials. If the shale is physically constrained so that it cannot swell, significant swelling pressures can develop. It is crucial to minimise minimi se the reaction with water (i.e. create good inhibition) utilising appropriate drilling drilling fluids. For water based muds, muds, this is done using a combination of mud additives such as salts and polymers. polymers. Salts, e.g. kcl, reduce clay swelling and dispersion, and polymers may encapsulate the shale to t o hold it together (e.g. PHPA*) PHPA*) or have a chemical strengthening strengthening effect effect (e.g. glycols). The action with water can be eliminated completely by using oil based mud, but the salinity of the mud must be sufficient to prevent prevent water entering the shale by osmosis. * Partially Hydrolysed Hydrolysed Polyacrylamide Polyacrylamide
2. Problems in Soft/Firm Shales
2.1 Bit Balling 2.1.1 Occurrence 2.1.1 Occurrence of bit balling balling Some shales can adhere to the BHA causing clogging of the drill bit, stabilizers and drill collars. This is most likely with soft-firm soft-firm and especially with plastic shale shale sequences and and when poorly inhibited inhibited water based muds are used. Bit balling is extremely rare in oil based muds. 2.1.2 Preventative Measures Measures of bit balling balling Review previous previous incidence incidence of balling balling in the area and adopt similar procedures if they were successful. Use inhibiting mud package (e.g. KCl/polymer rather than lime of gyp/PAC mud, consider KCl/glycol muds). Add film-forming lubricant (consult (consult service company for specific specific product). Avoid excessive weight on bit. Ensure good hole cleaning to avoid excessive cuttings build-up at the bit. Use appropriate bit type - PDC bits are more prone to balling than tricone. •
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2.1.3 2.1 .3 Symptoms of bit balling Increased pump pressure as pathways become blocked Reduced ROP due to balled bit. Less cuttings coming over shakers (lower ROP + possible blockages). Blocked shaker screens indicative of clay with balling tendency. Overpull on trips due to restricted fluid pathway. Possible stuck pipe.
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2.1.4 2.1 .4 Cure for bit balling balling Reduce weight a bit. Add drilling detergent to WBM to disperse the balled clay. If this works, maintain detergent in the mud. If it fails, consider freshwater pill to disperse the clay. Addition of glycol (e.g. BP Chemicals Chemicals DCP208 or DCP101), DCP101), has has also been known to cure balling. If still unsuccessful, POOH slowly whilst rotating, clean BHA and adopt preventative measures. In OBM (rarely a problem) increase mud salinity to draw more moisture from the shale and increase its hardness.
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2.2 Mud Rings
2.2.1 Occurrence 2.2.1 Occurrence of mud rings Soft cuttings may agglomerate in the annulus to form a “doughnut” ring which impedes mud flow and cuttings transport. The ring of cuttings can migrate up or down depending depending upon mud flow, and may lead to blocked bl ocked flow lines. Restriction Restrict ion of cuttings flow could ultimately lead to pack-off. Tends to occur only with WBM’s. 2.2.2 Measures to prevent mud rings Use sufficient mud inhibition to avoid sticky cuttings (e.g. KCl/polymer rather than lime or gyp/PAC mud, consider high KCl or KCl/glycol muds). Circulate annulus clean before trips. Use large diameter flowlines. Avoid excessive ROP’s. Ensure good hole cleaning to prevent cuttings cuttings build- up. •
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2.2.3 2.2 .3 Mud rings symptoms Increase in pump pressures as the annulus partially blocks. Reduction in cuttings flow rate, seen at the shakers. Increase in torque due to braking effect of mud ring. Decrease in ROP and hook load as mud ring acts as piston to lift the drillpipe. Blocked flow line/plugged shaker screens.
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2.2.4 Curing mud rings Circulate annulus clean by increasing increasing flow rate (if ECD’s permit). Increase size of flowlines if possible to prevent blockage. blockage. If necessary, use viscous pill pill to circulate clean. If mud ring is severe, adding detergent may help to disperse the aggregated clay making it easier to remove. remove. If detergent is successful, successful, maintain this in the mud system to prevent recurrence. •
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2.3 Tight Hole
2.3.1 Tight hole definition. 2.3.1 definition. “Tight hole” is the result of a reduction in the wellbore by mechanical and/or chemical means . It must be appreciated to be completely different to “excess drag.”
2.3.2 Occurrence of tight hole Reactive shales can swell, causing a reduction in the diameter of the wellbore. This is common with soft shales due to their frequent high swelling clay content. There may be some time delay (the shale takes some time to react), Thus, the bit may be free at the time of drilling, but subsequently subsequently could be difficult to pull and/or pump out of the hole. It can be argued that tight hole becomes more severe in deviated wells, due to increased drillpipe friction. It must be remembered however that in this instance due to the natural increased gravitational drags that are present it is much more difficult to immediately realise realise if “Tight hole” or simply “excess drags” are at play e.g due to poor hole cleaning for instance. 2.3.3 Techtonics Techtonic movement movement can also contribute to tight hole, e.g. Salt formations ( Zechstien in the southern North sea. Techtonic effects are generally more significant significant in the harder more deeply buried shales .
2.3.4 Overpressure ( Geo-pressures Geo-pressures.) .) High pore pressures can can also contribute to tight hole. However, their role again is generally more significant in the harder more deeply buried shales.
2.3.5 Preventing tight hole Use inhibiting inhibiti ng mud systems systems in soft reactive formations. Use sufficient mud weight to balance downhole stresses. Maintain around around 20-30ppb KCl (or equivalent) in WBM when drilling highly swelling shales. Use sufficient salinity when OBM is used. Make regular wiper trips. Use proprietary lubricant additive in deviated wells. Minimise open hole exposure time (casing design). • • •
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2.3.6 Symptoms of tight hole Increased torque. Higher pump pressures. Difficult to POOH, high Overpull/drag. • • •
2.3.7 Cure Backream ( Note ; only as a last resort ) through tight sections. Note: Backreaming in itself can create further instability problems, remove mud filter cake i.e. primary combatant to ensuring a stable wellbore. Increase mud weight to force wellbore back (provided fracture gradient allows), Check mud salinity: • •
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WBM’s: Ensure sufficient salt is present (salt is the single most important additive for reducing swelling). KCl is the most effective salt; levels of around 50ppb have been used, although although 20-30ppb 20-30ppb is usually sufficient for North No rth Sea type shales. NaCl is less effective. Lime (CaCO3) is limited by low solubility; CaC1 2 is very soluble but not recommended due to incompatibility with other additives e.g. PHPA, WC, polymer, CO2 influxes). Acetate or formate salts (e.g. Potassium acetate) can be used where use of chlorides is restricted. OBM’s: Ensure mud salinity is high enough to prevent shale swelling by osmosis. Mud salinity must must be equal to or higher than the pore pore fluid salinity. •
Add lubricant to mud.
2.4 Erosion/Washout 2.4.1 Occurrence of erosion and washout This is the result of the dispersive nature of soft reactive shales, leading to enlarged hole. The effect is mainly a associated ssociated with WBM’s WBM’s and is rare in OBM. Often localised, it is possible to experience washed out sections and tight hole simultaneously. simultaneously. Washout can lead to problems problems of hole cleaning and difficulty difficulty running into hole, plus poor cementing jobs, etc. Figure 1 ; Hole washout
2.4.2 2.4 .2 Preventing Preventing hole washout. Establish shale reactivity prior to drilling (e.g. by reviewing r eviewing offset data, and/or running lab tests). Select appropriate level level of mud inhibition. Use glycol mud in soft reactive reactive shales, if WBM is to be used. Use mud with good hole cleaning properties (see “cure” below). If there is a high probability of washout, and particularly in deviated holes, consider well-inhibited high XC polymer system. (MMH (mixed metal hydroxide) and MMS (mixed metal silicate) systems give excellent hole cleaning, but are not recommended for highly reactive shales as with current technology shale inhibition with these muds tends to be poor.
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2.4.3 Symptoms of hole washout Increase in cuttings volumes. Difficulty running in hole due to ledges. • •
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Poor hole cleaning leading leading to packing packing-off -off of o f the th e BHA. Packing off increased Overpull and drag. Directional problems. Bottoms up time increases.
2.4.4 2.4 .4 Curing hole washout •
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Increase mud inhibition inhibition (WBM’s), by using appropriate appropriate salt/raising salt level see previous section and, if available, available, by adding glycol (e.g. BP Chemicals DCP208 or DCP101. Glycols harden soft shales, so reducing dispersion/erosion. Improve low shear rheology of mud (higher YP and higher gels) to ensure good hole cleaning. This will help to clean washed-out sections sectio ns and prevent cuttings slip. Alternative regime is to use turbulent flow conditions (low viscosity mud) - consult Sunbury on this. Hole cleaning is particularly particularly important in deviated holes. Maintain mud circulation to prevent cuttings slip. When making making trips, POOH slowly to minimise swab/surge pressures.
3. Problems in Hard/Brittle Shales 3.1 Tight Hole
3.1.1 Occurrence Tight hole due to chemical swelling is less common in harder shales, due to lower swelling clay (s (smectite) mectite) content. However, high downhole downhole stresses stresses can lead to tight hole, especially especially if accompanied accompanied by high pore pressures. pressures. Ovalshaped (elliptical) holes may occur in some cases. 3.1.2 Preventative Measures Establish mud weight window using available data/rock mechanics models. Utilise resistivity resistivity and a nd gamma log data if available. Strength tests will give better handle on maximum permissible mud weights. Screen shale samples, if available, to determine chemical reactivity, and use results to select appropriate drilling fluid. Consider optimum direction for deviated and horizontal wells, to minimise hole closure due to stress effects. • •
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3.1.3 Symptoms Increased torque. High pump pressures. Increased drag/overpull. • • • • • •
3.1.4 Cure If chemical reactivity is suspected, reduce swelling by improving shale inhibition inhibition (see sec sec tion on soft shales), If mechanical (i.e. due to downhole stresses), stresses), raise the mud weight. Do not raise mud weight if the formation is naturally fractured as this could destabilise the wellbore (fluid will enter the fractures and “lever” rock away). Add lubricant. Run 4-arm caliper to check for elliptical hole, which is indicative indicati ve of unsymmetrical rock stresses. •
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3.2 Sloughing, Cavings and Packing-off Packing-off
3.2.1 Occurrence Many hard/brittle shales tend to fragment into cavings at the wellbore wall, leading to sloughing and sometimes sometimes causing causing packing-off packing-off.. This is particularly likely in naturally fractured formations and in very brittle shale types. The hole enlargement which occurs can be sudden (even catastrophic) and can be difficult to predict or control. Highly stressed formations formations are the most problematic.
Figure 2 . wellbore cavings
3.2.2 Preventing wellbore cavings •
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Establish mud weight window using available data/rock mechanics models. Strength tests will give better handle on maximum permissible mud weights. Consider optimum direction for deviated and horizontal wells to minimise hole instability due to stress effects. Choose casing points carefully to reduce exposure time of unstable sections to drilling fluid. Screen shale shale samples, if available, to determine chemical reactivity and use results to select appropriate drilling fluid. Use mud with well-defined and understood rheology profile. Consider high XC polymer system, or MMH (mixed metal hydroxide), or MMS (mixed metal silicate) system for superior hole cleaning. Add particles (e.g. gilsonites, asphaltenes, asphaltenes, micas, calcium carbonates) to seal fractures in naturally fractured shales. Selection of appropriate appropriate particles requires screening tests as particle size distribution and shape s hape are important. Take steps to minimise/reduce drillstring vibration in brittle brit tle shales.
3.2.3 Symptoms of wellbore cavings Production of characteristically-shaped characteristically-shaped cavings at the t he shakers - appearance appearance of smooth curved surfaces indicative of pieces once at the wellbore wall. Sudden increase in cuttings volumes, possibly large hard chunks. Difficulty running in hole due to jamming on ledges. Cuttings fall back due to poor hole cleaning, causing pack-off. Torque/pump pressures increase. High torque and drag due to trapped cavings. Bottoms up time increased. Directional problems. •
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3.2.4 3.2 .4 Curing wellbore cavings Raise mud weight. Do not raise mud weight if the formation is naturally fractured as this could destabilise the wellbore (fluid will enter the fractures and level rock away), Improve the low shear rheology of mud (higher YP and higher gels) to ensure good hole cleaning. cleaning. This will help to clean clean washed-out sections and prevent cuttings slip. Alternative regime is to use turbulent flow conditions )low viscosity mud). Hole cleaning is particularly important in deviated deviated holes. Maintain mud circulation at all times to prevent cavings settlement. When making trips, POOH slowly to minimise swab/surge pressures.
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