Casing drilling

In the countryside, in shallow water or at great depths – our engineers start drilling at very different locations. All sites have several things in common: boreholes begin with an average diameter of some 70 cm, are mostly drilled to depths of several kilometres, and get narrower the deeper they go down. At the bottom they are little more than 10 cm in diameter. But how is a borehole actually constructed?

The drill string with the bit at the bottom end is made up of individual pipes, around nine metres long, with special threaded ends known as tool joints. Pre-assembled stands made up of three pipes ("trebles") are stood upright in the derrick. For the fitters on a rig assembly, their work means removing the protective caps, oiling the threads, screwing on a stand and then tightening everything up. A great deal of manual work is involved before a bit finally reaches its target formation thousands of metres underground. And that is not all. When a bit becomes blunt or something goes wrong, the whole string has to be brought up to the surface and the necessary action taken before it can be returned to the borehole.

Once the bit has drilled through the first massive, load-bearing rock formations, we bring in the first anchor pipe – a strong steel pipe with a slightly smaller diameter than the borehole. Then we carefully fill the gap between the pipe and the walls of the borehole with cement. Once it has dried, the pipe is firmly anchored in the rock. This stable connection is crucial if the anchor pipe is to safely bear all the additional loads. Our laboratory specialists are closely involved in the development of these cements, which not only form an effective seal between the reservoir and the surface and between the various rock formations, but also protect freshwater aquifers and ensure that the aggressive brines found in the pores of the rocks do not corrode the pipes.

In view of the complex and laborious steps the drilling process involves, we want, above all, to prevent the borehole from collapsing once drilling has been completed. This is why steel casing is cemented into place at various places en route to the reservoir – particularly in difficult-to-drill rock formations. These so-called casing strings are made up of casing and a liner. When the casing is in place, the gap between the pipe and the borehole wall is again largely filled with cement. And once all this work has been completed, theoretically a well can go on stream.

However, before production commences, the well needs to be completed. A production string made up of joined-up pipes is lowered into the borehole. The pipes at the bottom end of the string are perforated with numerous holes or slits. This perforation allows the oil or gas to flow into the production string, which is not cemented in place but is embedded in a special kind of sand.

In casing drilling technology, which RWE Dea has used for several wells, there is no conventional drill string. For the drilling operation, the drilling crew use the casing string needed for production purposes. The drilling fluid (or mud) is pumped down through the casing, transports the cuttings to the surface, cools the bit and drives its motor. The mud flows through the annulus between the rock and the casing back up to the rig, where it is treated for re-use. While flowing back to the surface, the mud has an extremely positive lubricating effect – and that is the reason why longer casing strings can be used in these casing drilling operations. Another advantage of this technology is that casing can be continually run, which saves a great deal of time.

Another specialised drilling technique, which RWE Dea only uses in certain cases for workover or production purposes, involves long steel tubes replacing the individual threaded pipes. As these tubes are stored on huge coils, they are known as coiled tubing. Though this tubing avoids the time-consuming business of joining up the threaded pipes, its diameter is limited and this in turn restricts its hydraulic properties during drilling and production operations.