Since whilst drilling, the hydrostatic head exerted by the mud column is greater than the pressure of the formation fluids in permeable formations, mud filtrate flows from the well into the formation leaving a filter cake on the wall of the bore hole. A pressure differential will exist across the filter cake, equal to the difference in the hydrostatic head of the mud column and the formation. The ECD created whilst pumping, increases this pressure differential.
When the drillstring comes in contact with the filter cake, any part of the pipe which becomes embedded in the cake, will be subject to a lower pressure than the part which remains wholly in the well. If the pressure difference is high enough and acts over a sufficiently large area, the pipe may become stuck. The force required to pull differentially stuck pipe free, depends upon:
Most junk in holes comes in from the surface. Pipe wrenches, slip dies and tong jaws are commonly dropped downhole inadvertently. This type of junk will generally stick the drillstring as it is being pulled up, either on trips, or whilst making connections. Junk, can however, be created downhole by tool failure in such tools as rebel tools, under-reamers and Tricone rock bits.
If cuttings are not effectively removed from the well, then when circulation stops, they can settle around the drillstring, causing the hole to pack off and the pipe to become stuck.
The stability of formations encountered downhole varies greatly. Some formations are inert and are no problem to drill through, whilst others such as shales and salts, can cause extensive problems. Troublesome shales fall into two groups, swelling or sloughing.
Swelling shales react to water and as their name indicates will swell with time. The speed at which swelling takes place will depend on the type of drilling fluid in use and the shale type itself. Swelling shales can be detected during drilling as overpulls, as larger diameter drilling components are pulled through swelled sections. The cuttings circulated up from drilling through these sections will also give an indication of the type of shale at the bottom of the hole, since they will quite often yield on the way out of the hole to create clayballs at surface.
Sloughing shales degrade downhole by having fragments falling off into the well bore in the form of cavings. Cavings have a totally different appearance from cuttings and therefore can be spotted at the shale shaker. As the mud weight is increased, more filtrate can be forced into the shales which can increase the amount of cavings. It is also possible that increasing the mud weight can reduce the problem by bringing hydrostatic mud pressure closer to formation pressures in the case of overpressured shales. Field experience alone will indicate how the local sloughing shales will react.
Most salt sections can be drilled without problems, providing that super saturated salt or oil-based mud is used. However, the plastic nature of some salts will inevitably cause hole problems. As the hole is drilled through, the salt section stresses are relieved and the salt flows plastically into the borehole. In some areas, this requires special drilling practices, such as regular reaming, to allow the well to be drilled ahead.
This is caused by the rotation of the tool joints and drillstring against the formation. It typically only manifests itself at abrupt angle changes in the well. The extent of key seating is dependent upon the abruptness of the hole dog-leg, the softness of the formation there, the amount of tension in the drillstring which acts to force the string into the wall of the hole and the rotating hours.
Key seating is detected initially on trips as a blip on the drilling recorder chart. This blip indicates a slight overpull as the BHA is pulled into the key seat. At the first sign of this, the key seat should be wiped either using a proprietary key seat wiper or a stiff stabilised BHA. Once detected, the key seat area should be tripped through with extreme care, as even when a key seat has been effectively wiped out, a new key seat can develop during subsequent drilling operations.
Whenever anything changes in a well, the Drilling Supervisor must consider the effects that these changes might have. When changing from a limber BHA to a stiff BHA, care must be taken, as the string is tripped into the well, to prevent it from sticking. If reaming is being carried out, it should be done gingerly so that the BHA is not 'screwed into' the formation. Similarly, if an undergauge BHA has been pulled, care must be taken when running a full gauge BHA on the next trip in.
When pulling out of the hole, the driller should be advised as to what overpulls are permitted, prior to lowering the string again, or if necessary back reaming out of the hole.
3.2 Prevention of stuck pipe
The prevention process begins as the well is programmed and continues whilst the well is being drilled.
3.2.1 Prevention of stuck pipe at the programming stage
When writing the Drilling Programme, each formation and situation must be assessed with its potential as a source of stuck pipe problems. The reaction of formations in offset wells must be studied and responded to in the Drilling Programme.
Correct mud system selection is paramount for a successful well. The use of oil-based muds in troublesome shales has, in some cases, eliminated the problems. In other formations, oil-based muds can create as many problems as they solve (see chapter 7). Keep mud weights to a working minimum. Programme the use of Hi "Vis or Lo Vis pills for hole cleaning, if problems are anticipated.
More and more rigs are coming equipped with top drive systems. These have taken a lot of the excitement out of back reaming and should be specified if other methods of combating squeezing salts or heaving shales have proved to be ineffective in the past. Troublesome formations sometimes only rear their ugly heads after a finite open hole time. This is especially true in directional wells. In this type of formation, typically shales, the likelihood of expected hole problems can be thought of as being proportional to the square of the time in days that the hole is open.
With these formations, it simply means that if you can put the well in fast, then you will see few problems. There are limits to how fast wells can be drilled, however, and in some cases it may be necessary to plan an extra casing string just to consolidate the hole prior to its beginning to cause problems. Do not programme mud motor kickoffs or any other activity which involves keeping the drillstring stationary in hole sections which are known or suspected to be potentially troublesome. In extreme conditions, consider using MDW rather than electric logs for some evaluation purposes, since they are less likely to get stuck.
The Drillers, Mud Engineers and Mud Loggers must be kept informed of potential stuck pipe problems and how to respond to given situations in order to preserve the hole's integrity.
Drilling instrumentation must be in perfect working order and the mud logging unit running according to specifications.
Be aware of and note on reports any drags, torque fill on trips, and cavings.
The Drillers, Mud Engineers and Mud Loggers must report any and all detected applicable trends to the Drilling Supervisor, who in turn must consider the appropriate response.
When running in the hole do so in such a manner that at no point is more than the BHA weight slacked off at a tight spot, without calling the Drilling Supervisor.
Perform wiper trips in response to hole conditions, not according to any pre-set levels. The author has, on occasions, wiped troublesome holes every four hours to keep the string free. On other occasions a wiper trip does nothing.
Keep the BHA as small as is practical to supply effective weight on bit.
Keep the BHA well stabilised and utilise an undergauge stabiliser on the top of the BHA to facilitate back reaming.
Keep the drillstring moving as much as possible when in open hole.
Overlay the trip charts on successive trips to highlight potential key seats and squeezing formations.
Integrate this overlay with known dog-legs in the well and highlight these depths on each trip.
Record all depths of sticky hole when drilling. Study the mud log as cuttings are returned to surface and trouble-shoot potential trip problems.
The operator's Drilling Supervisor should be on the rig floor for every trip out through newly drilled hole and known hydrocarbon bearing formations.
Sufficient circulation rate should be used in all hole sections to clean the hole.
Prior to trips out, the mud must be conditioned to bring solids content down to designed levels and to ensure that all drilled solids are out of the well. Check at all times that the hole is being cleared effectively by the mud.
The BHA should be inspected as it comes through the rotary for clay deposition, especially on stabiliser blades. In most water-based muds, this deposition can be eliminated by dowsing the system with a couple of barrels of soap or detergent during the pre-trip circulation.
An effective drilling jar should be used in every drilling BHA situation such that it will not be run at neutral weight during any envisaged drilling scenario.
A pipe wiper should be used at all times to cover the hole to prevent junk falling down through the rotary table.
Always measure and check all equipment being run in the hole. Always use the correct gauge ring for each tool.
After pulling a worn BHA always ream carefully back to bottom.
When running a stiff BHA following a flexible BHA run, always ream carefully back to bottom.
Was this article helpful?
What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.