Work-overs can deal with a wide variety of problems, but they come at the situation from a different perspective than the original well drilling. To being with there is a cased hole that often goes all the way down to the original pay. Further the tools that will be used are not going, in large measure, to be used to drill new segments of holes, but rather to treat the original well, replace parts that have failed, or change the layer of rock that the well is getting the oil from.

Now there is a word of caution here. To work on the well the first thing that you are likely to do is stop it pumping oil. That is known as shutting the well in or killing the well. Then you bring in the work-over rig, do what needed to be done, and it leaves, and you start the well producing again. Here is the caution. Because you stopped the well producing for a while, when it restarts, in almost every case, and regardless of whether the action or treatment that the company applied really worked, the well will begin by producing more oil than it did just before it was shut in. Because they may have paid quite a bit of money for a treatment, it is sometimes amazing to me how well educated folk will see that immediate gain and believe that a treatment that in other circumstances they would find incredible, has created an improvement in production. The well behavior has to be monitored over a period of time to validate the improvement (`nuff said).

Some of the treatments that need to be carried out are not very complicated. Perhaps when the well was first drilled it was not effectively acidized or perhaps the oil might have precipitated out some of its contents into the drill pipe as it moved from the completion zone up to the surface. Paraffin or similar waxes might, for example, have started to clog the pipe , or some of the carbonate and sulphates in the oil might form a precipitate or scale on the pipeline wall as the oil flows upwards (I have seen some of these crystals be well over an inch long). These deposits can be removed by putting a scraper onto either the drilling rod of the workover unit, or from a wireline that is run from the surface. A wireline can be either a slickline or single strand cable, or a braided line which has a number of strands and is capable of carrying a higher payload. These lines can be run into the well very quickly (and in smaller cases do not need to have the well killed to be used).

In a slightly more complicated case an electrical control cable or power cable can be added to the wire to power down-hole operations,, particularly when packers or plugs are being used. Depending on purpose these might also be fielded using a more conventional drill string.

Packers are devices that are lowered into the well to isolate the well zone in which the work is to be carried out. For example if one were going to seal off the old production zone and move to another one, one might pack off the old zone, first before pumping cement into the sealed-off segment to fill it with cement. There is a more technical article available.

You can imagine that almost every type of repair must be carried out in this fashion. If something goes wrong down hole, then because of the limits of access it is going to take some imagination to deal with the problem, and the oilwell service companies have now provided that for a number of years.

The more simple jobs, such as sealing off a zone that has stopped being productive, or lowering a new perforating system down the well to stimulate production from the layer of rock, to cleaning the screens at the bottom of the well that keep the rock in place, while allowing the oil to flow into the well, are all somewhat obvious once named, though perhaps not so obviously needed until you see the effect of the problem on well production.

The more common other workover uses, for stimulating production, will be saved until next time.

This is part of an ongoing weekend series on technical aspects of oilwell (and natural gas) drilling. Previous posts can be found at::
the drill



using mud



the derrick



the casing



pressure control



completing the well



flow to the well



working with carbonates



spacing your well

directional drilling 1



directional drilling 2


types of offshore drilling rigs

coalbed methane
As ever, if this is not clear, or if there is disagreement then please feel free to post, and I will try and respond.