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Drilling methods

Scheme

Rotary drilling

In conventional rotary drilling the driving mechanism for the entire drill string is the rotary table mounted in the substructure of the rig. The rotation is directly transmitted from the rotary table to the top of the drill string via a hexagonal shaped pipe, the so-called Kelly. The entire drill string is composed of individual pieces of drill pipes, each one of approx. 9 m length. As the well is deepened single pieces of drill pipe are continuously added and connected to the drill string until the final depth of the well is reached.


Effect of the hydraulic jet force at the bit

Directional drilling

If the target is not located directly below the drilling site, the well must be deviated. During directional drilling the drill string itself is not rotated, only the bit is driven by a hydraulically powered down hole motor.

Borehole inclination angle and drilling direction are continuously measured, recorded, and displayed on surface. In case of deviations from the planned borehole trajectory the bit can be steered to point into the correct direction.


lr - insert roller cone bit
rr - PDC bit
lf - core bit
rf - roller cone bit

Drilling bit

The drilling bit which penetrates the rock is the lowermost end of the drill string.

During drilling, rock formations of different hardness are encountered. Steel tooth bits and polycrystalline diamond compacts (PDC’s) are designed to drill soft to medium-hard rock. Insert roller bits of different hardness are used for hard rock material. Special core bits are used for coring operations.


Mud

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.