When the drillstring becomes stuck, immediate action will prove to be most effective in most cases. Prior to taking this action, the cause of the problem must be determined. This is achieved by considering the circumstances leading up to getting stuck and relating this to well conditions and history. Pipe will be stuck, either mechanically, or differentially. Sometimes, mechanically stuck pipe will end up differentially stuck too, but even in the worst hole, this will take time.
3.3.1 Identifying the cause of stuck pipe
At the time of getting stuck we have to consider the following:
Can we pull the string? Can we loweT the string? Can we rotate the string? Can we circulate?
What operation was being carried out at the time of sticking the string? What were torque and drag trends prior to becoming stuck? What incidents occurred in nearby wells of the same depth? What is the BHA design? What formation is opposite the BHA? Are there any other salient details? junk dropped down hole loss in circulating pressure excessive cuttings over shakers no cuttings over shakers.
By considering these details, the likely cause can be determined and this may indicate whether the pipe is stuck mechanically or differentially.
If the pipe was in motion at the time of getting stuck, then the first attempt to free it should be in reverse to this motion. For example, if the string has been pulled into a tight spot, then it should be worked or jarred downwards. Conversely, if the string has been run into a tight spot it should be worked or jarred upwards.
If the mechanical sticking is due to inadequate hole cleaning, then pump rate should be increased to the maximum operationally permissible in an attempt to assist hole cleaning. High circulation rates can also encourage washing out and consequently enlarging the hole in some cases.
If a drilling jar is in the string, the jarring should be carried out in the opposite direction to which the string became stuck. If the string was pulled into a key seat, jar downwards and if the stabiliser jammed in the formation whilst reaming down, jar upwards. If a drilling jar is not in the string, then the pipe should be worked in the same direction as above to the safe working limit of the drillstring.
By looking at the formation which is sticking, an indication of which chemical approaches might be successful can be arrived at. If the string is stuck in carbonates, then a slug of 2 per cent HC1 can be used to eat into the formation and release it.
When stuck in salt, freshwater will dissolve the formation and free the pipe in most cases.
3.3.3 Freeing differentially stuck pipe
In most cases of differential sticking, the pipe will be stuck with the bit off bottom. Because of this, working pipe can be done either up or down. Working pipe and jarring is rarely effective in freeing differentially stuck pipe, but it is easy to do and worth a try in most cases.
Since the sticking force is proportional to the differential pressure existing between the mud, hydrostatic pressure and the formation pressure, then by reducing the differential pressure, the sticking force can be reduced. This reduction of hydrostatic head can be achieved by lightening the mud or circulating to a lighter fluid such as water or diesel. Alternatively, the pipe can be U-tubed. All of these methods must be done whilst still retaining full primary hydrostatic control on the formation. These methods have mixed success rates as quite often the filter cake remains in place between the pipe and formation sticking the pipe, even with reduced differential pressure.
In most cases of differential sticking, the pipe will ultimately be freed chemically (if at all). To do this a pill of chemical agent which attacks wall cake is spotted at the stuck point. This is usually mixed with diesel oil and weighted to the same weight or slightly heavier than the mud weight in use. Chemical pills take time to work and 24-36 hours should be allowed for this prior to giving up. Enough pill should be made so that the pill can be moved every hour or so. In practice, this means making a pill of approximately twice the volume required to fill the drill collar/open hole annulus and leaving half of it in the drillstring. The pill is then moved by pumping a few strokes with the mud pumps at regular intervals. If water-based mud is in use, then a very effective method of freeing differentially stuck pipe is to displace the hole to oil-based mud. As with all chemical methods, this takes time to free the pipe and 24-36 hours is a typical delay before the pipe is free.
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