24.11 Rotary drilling

24.11.1 General


Rotary drilling methods, in which the drill bit is rotated on the bottom of the borehole, are used to drill rocks and soils for investigation purposes. The drilling fluid, which is passed from the surface through hollow drill rods to the face of the bit, cools and lubricates the bit, transports drill cuttings to the ground surface and, when using particular types of drilling fluids, stabilizes the borehole. Common drilling fluids include clean water, air or a mixture of both. Mud, polymers or foam are also frequently used to maintain or assist borehole stability, aid the transport of drill cuttings to the surface and maximize core recovery, particularly in soils and weaker rock formations.

There are two basic types of rotary drilling: open hole (or full hole) drilling, where the drill bit cuts all the material within the diameter of the borehole; and core drilling, where an annular bit, fixed to the bottom of the outer rotating tube of a core barrel, cuts a core, which is recovered within the innermost tube of the core barrel assembly and brought to the surface for examination and testing. Rotary drilling for ground investigation is usually core drilling.

When open hole drilling or coring, temporary casing is normally used to support unstable ground or to seal off fissures or voids, which cause excessive loss of drilling fluid. Drilling fluid additives or cement grouting can sometimes be satisfactory alternatives.

The rotary drilling rig should be well maintained and should be capable both of controlling rotational speed and providing axial load and torque to suit the nature and hardness of the material penetrated, the diameter of the core barrel and drill string, drilling fluid and flushing system, weight of drill string and installation of temporary casing(s). When recirculating, the drill fluid should be cleaned as an integral part of the process so that it is suitable for continuing reuse.

NOTE Drilling is in part an art, and its success is dependent upon good practice and the skill of the lead driller, particularly when coring fractured and extremely to very weak rocks or soils.

24.11.2 Open hole drilling


Open hole drilling is sometimes used in soils and weaker rocks as a rapid and economical means of making holes in the ground for the purpose of advancing the hole to a sampling depth, progressing a hole to a specified depth or material type, for carrying out in-situ tests, for installing instruments, etc. The technique can also be used to probe for voids such as mine workings, solution cavities, etc. Systems are available whereby casings are driven simultaneously with the open hole bit. While drilling, only drill cuttings are returned in the drill fluid.

The rate of progress of drilling and observations of the flushing medium and the cuttings should be recorded. These records may be made by the lead driller or a separate logger. The cuttings constitute very low quality samples and it is usually difficult to detect a change in strata, unless there is a good contrast in properties such as colour, mineral content or hardness. Where such contrast prevails, open hole drilling can be used as a probing technique.

NOTE The use of suitable instrumentation, termed "drilling parameter recording" (DPR) or "measurements while drilling" (MWD), in order to record the progress of the drilling rig can considerably enhance the results obtained (see BS EN ISO 22476-15).

24.11.3 Core drilling


Core drilling is normally carried out using conventional or wireline double or triple tube core barrels fitted with diamond or tungsten tipped core bits. With conventional drilling, the core barrel is run on a string of rods which are rotated by the rig at the surface. This method might require a separate string of casing to support the walls of the borehole. Wireline core barrels are rotated from the surface by drill rods which are normally of the same diameter as the outer core barrel obviating the need for a borehole casing. Information about all the major components found in core barrels, rods and casings is given in BS EN ISO 22475-1.

The conventional double tube core barrel consists of two concentric barrels. The outer is rotated by the drill rods and, at its lower end, carries the coring bit. The inner barrel Is mounted on a swivel so that it does not rotate during the drilling process. The core, cut by the coring bit, passes up into the inner barrel and, at the end of the coring run, the core barrel assembly is lifted to the surface by raising and removing each drill rod individually. This becomes increasingly time consuming as the borehole becomes deeper. The core is prevented from dropping out of the core barrel by a core catcher made of spring steel and located just above the core bit. The drill fluid flows down through the annulus between the inner and outer barrels on its way to the core bit. The core itself is only in contact with the drill fluid as it passes through the core bit.

With triple tube core barrels, the non-rotating inner barrel contains a removable liner. The liners are usually steel tubes (split or seamless) or plastic (rigid or semi-rigid). They do not increase core recovery or sample quality, but are much more likely to preserve the core in an intact condition, thus enabling the logging geologist to extract more information than would otherwise be the case. Triple tube barrels can be purpose designed, but it is common practice to use conventional double tube barrels with a semi-rigid plastic liner inserted inside the inner barrel; in which case the core diameters in BS EN ISO 22475-1:2006, Annex C reduce because different core bits are required to accommodate the liner.

The wireline system differs from the conventional in as much as the core is brought to the surface within the inner core barrel on a wire rope or line attached to an "overshot" recovery tool. Larger diameter wireline systems are particularly suitable where soils and weaker rocks are anticipated, as any vibration created from the drilling action on the surface is minimized due to the close fitting nature of the rods within the borehole. The smaller wireline sizes do not show this characteristic and are normally used for hard rock drilling at speed. The borehole wall is constantly supported during the drilling process and when recovering the inner core barrel to the surface, which makes core retrieval quicker and improves production. The main disadvantage with large diameter wireline drilling in weak materials is that it is necessary to remove the string to change the drill bit. In these circumstance in unstable holes not supported by drilling mud or in gravels, collapse of the hole might occur and redrilling of the collapsed material is then necessary.

Detailed information of core barrel sizes and the core sizes they produce is listed in BS EN ISO 22475-1:2006, Annex C.

The drilling rig and compatible in-hole and surface equipment together with the flushing medium should be selected to meet the objective of achieving optimum core recovery and core quality consistent with cost. General guidance on the available techniques and their suitability to various ground conditions is given in BS EN ISO 22475-1; however, for detailed guidance on the suitability of various techniques in different types of soil and rock, advice should be sought from a drilling specialist. Similarly detailed guidance on the selection of core bits for particular formations and on inter-related matter of the selection suitable the flushing medium for the anticipated geology to be drilled should be sought from a drilling specialist.

NOTE 1 A very wide range of coring bits is available and the type which gives the best results in any given ground conditions might have to be determined by trial. The factors affecting bit selection for optimum rate of drilling might be different from those that determine quality of core. Thin-walled bits produce fewer cuttings, which permits the use of a lower flushing rate with diminished disturbance. On the other hand, thin-walled bits normally do not provide for face discharge of the flushing fluid and some disturbance might occur as the fluid passes between the inside of the bit and the core, although this can be obviated by the use of protruding or retractor type core barrels. Stepped bits offer advantages of drilling rates and quality in some circumstances; they also tend to produce straighter holes. For strong abrasive rocks, diamond quality and matrix selection are important considerations.

NOTE 2 Water and air are the simplest and most commonly used flushing media. Drilling muds consisting of water with clay (bentonite), water with an additive such as sodium chloride, foam, polymer mixtures and air/water mist are also used as flushing media. Polymer mixtures and water with additives have an advantage over water and air in that the cuttings can be removed at a lower flushing velocity, thereby reducing disturbance and washing out at the cutting bit or within the borehole in weaker rocks or soils. Using air as the flushing medium might be more advantageous in some geological formations or where limitations are imposed on the use of other fluids in respect of aquifer or groundwater contamination. Air or water flush can also cause mobilization and mixing of contaminants with related health and safety implications. Whereas water and polymer can tend to increase the natural water content of the core, the use of air tends to reduce it. Air can fracture weaker rocks and create paths of weakness, possibly causing leakage of the air flush and a permanent change to groundwater paths. The use of air flush is undesirable in situations where ground gases are likely to be present because these gases might be displaced outwards along existing pathways or new pathways created.

Where core samples are needed for laboratory testing, they should be preserved as quickly as possible in order to maintain sample quality, e.g. water content. (Core extrusion and preservation together with the taking of sub-samples for testing is discussed in 25.7.)