The mud is pumped down through the hollow drill strings, flows through the bit nozzles and continuous it’s way upwards in the annulus between the rock wall and the drill string. During this process, it transports the drilled rock material (cuttings) to the surface where the solids are separated from the drilling mud. The clean mud is then re-circulated into the borehole. The separated cuttings are fully recycled. Hence they serve as a resource for other industry e.g. construction industry.
The purpose of drilling mud is to cool and lubricate the bit, transport cuttings to surface and prevent formation fluids from entering the borehole. Furthermore it forms a “filter cake” at the borehole wall preventing mud from flowing into different formations.
To be able to fulfill its objectives the mud has to have the right composition as well as the right specific gravity. But the mud alone is not able to protect the borehole wall permanently. To achieve permanent wall protection casing is run into the whole at certain depths preventing the borehole from caving in. The annular space between casing and borehole wall is filled up with a special type of cement to obtain a permanent seal.
In the first section of a well (down to approx. 500 m) a natural clay-water mud (bentonit suspension) is used. In the sections below, water-based potassium carbonate (K2CO3) mud is used. Both mud types can be processed completely and reused.