A simple case history.
What is the required stability of a wellbore?
Key factors of instability.
Recent progress and review 1)
Even if this does not happen on a routine basis, wellbore instability is unfortunately quite a common feature of drilling. It seems interesting to start the general report on this subject by a short description of a case of instability, without any specific worsening problem. This case is from the Norwegian North Sea, but it will look familiar to any drilling engineer. The Hordaland group is a thick formation (about 1500 m) which covers most of the reservoirs in the North Sea. Even if it is of course variable from one place to another, it is mainly composed of clayey formation, claystone at the top and more indurated shale at the bottom. In that particular case, the well was drilled with a 12" 1/4 bit. The bottom of the Hordaland group was at about 2000 m. The sequence of events is reported on the following table (Figure 1). This sequence of event is quite typical of a wellbore instability. The hole was drilled through the shale on day 1 with a mud weight of 1.13, and the well deepened into the underlying formation. The trip to the surface was smooth. While running the new bit down, it was necessary to redrill the hole down from 1950 m. This may indicate that instability started during the trip. However the mud weight was slightly raised because shale "cavings" (chips of rocks producted by hole wall caving) were coming on the shakers. A "wiper trip" was decided to control the hole stability after the well had been circulated "bottom up", i.e. for a sufficiently long time to bring all the cuttings to the surface. While pulling out, the pipe got stuck when the bit was at 2232. It was impossible neither to rotate the pipe, nor to circulate. After a while, circulation was restored, and lots of cavings came to the surface with a high viscous pill. After some time working on the stuck pipe, it was decided to disconnect the stuck part of the pipe ("back off"), to cement the hole above the "fish," and to start a new hole at 1980 m.(side-track). On day 19 the drilling reached the previously reached depth on day 11. Even for a simple instability, this means a loss of 8 days, on an offshore rig where 70 persons are working permanently. This is why wellbore stability is considered as a serious concern by most of the oil industry operators. In this particular well, caliper logs revealed that at the bottom of the shaly Hordaland group the hole was largely caved. The instability must have started during the first trip but was not causing serious problems to the drilling operation. It seems however that large "cavings" were produced, which cannot be carried up to the surface by a regular mud flow.
