An introduction to Reverse Circulation drilling

Transcription

1 An introduction to Reverse Circulation drilling Reverse Circulation, or RC, drilling is a fast and cost efficient method of retrieving high quality samples from exploration and mine drilling. The system has been continuously developed since its inception in Australia in the early 1970s, and is now a preferred method for initial exploration, ore body development and in-pit grade control. Any method is only as good as the equipment developed around it, and Atlas Copco has called on the experience of drillers worldwide in order to perfect its offering. Rigs such as the Explorac 220RC are revolutionizing exploratory drilling, producing samples faster from deeper holes and in more difficult situations. RC methodology The RC method employs dual wall drill rods that comprise an outer drill rod, with an inner tube located inside the drill rod. The inner tubes overlap, and seal on the tube below with O rings when the drill rods are screwed together. These inner tubes provide a continuous sealed pathway for the drill cuttings to be transported from the bit face to the surface. The circulating medium, in most cases high-pressure air, enters the annulus between the rod and tube via the air swivel, which is normally part of the drill string, or sometimes mounted on top of the rotation head. The air travels down the annulus to the drilling tool, which is usually an RC hammer, or can be a blade bit or tricone roller bit. As in conventional open hole drilling, the air powers the drilling tool and the exhaust air carries the cuttings. In RC drilling the cuttings are returned to the surface through the inner tubes in the drill string and rotation head. Once through the rotation head, the air and cuttings comprising the sample change direction at the discharge blast box and are transported through the Explorac 220 in Australia. sample hose to the cyclone. The cyclone slows the sample, separates it from the air, and collects it. RC history Difficult drilling conditions in some types of soft iron ore and mineral sands using conventional open hole techniques led to the development of RC drilling in the early 1970s for sampling. A dual tube configuration, occasionally used in the US oil industry, was adopted as the basis for the RC drill rod. The first RC drill rods were made in 1972 by Bruce Metzke and John Humphries in Kalgoorlie, Western Australia. Shrouded tricone roller bits were initially employed in softer formations, returning previously unheard of sample accuracy and target depth achievement. The development of the crossover sub facilitated the use of conventional DTH hammers, and thereafter RC could be applied to almost all ground conditions. Speed and cost advantages over diamond drilling led to a boom in RC drilling, and by the late 1980s more than 2 million m/y of RC exploration drilling were being completed in Western Australia alone. The need for cleaner samples led to the development of the RC hammer in High-pressure boosters and auxiliary compressors were introduced for deeper holes and faster penetration. Air pressures up to 100 bar (1 500 psi) were available, driving necessary advances in all aspects of RC drilling and RC systems. In the late 1990s, gold processing improved, viable ore grades became lower, but mining costs were higher, so many mines looked to improve their ore selection processes. One of the easiest ways to do this was grade control drilling, and RC drilling was the most cost efficient and accurate method available.as a result, RC drilling is now being used for initial exploration, ore body development drilling, and in pit grade control drilling. 24 exploration drilling

2 RC benefits RC drilling provides virtually uncontaminated cuttings to the cyclone. As the cuttings travel directly from the drill bit through the steel inner tubes and sample hose, there is no cross contamination from other areas of the hole. Using good sample splitters and sampling procedures, RC results rival the accuracy of diamond core assays. Drilling penetration rates are similar to open hole drilling, and are often faster at greater depths. The sample velocity through the inner tubes can be up to 250 m/sec, so retrieval of the sample and hole cleaning is rapid. Production rates of up to m/ day are common at rates exceeding 10 m/h, many times faster than diamond drilling, and achieving rapid results for the customer. Unconsolidated formations can often be drilled and sampled without casing. Washing and scouring of the hole is minimized, because there is normally no fluid or cuttings flow against the walls after the drill bit has passed. Low impact bits, such as RC blade or RC roller, are ideal in these soft or loose formations. With good drilling techniques, samples can be kept dry, even several hundred metres below the water table. Dry samples are preferred as they split more accurately for assay, and are easier to handle. RC sample content ranges from dust to 25 mm chips, and is already partially processed for analysis. Wireline surveying of the hole is still possible through the drill rods. With the use of a stainless steel rod at the bottom of the string, hole azimuth readings are also possible. Drill string The components in an RC drill string have a similar arrangement to those of a conventional drill string, but are designed specifically for RC drilling, with all components having a central inner tube. As these inner tubes carry almost all of the cuttings from the hole at high velocity, they are subject to wear. The rate of wear is governed by the air volume and pressure of the rig, and the type of formation being drilled. Explorac 220 with cyclone and cone splitter. exploration drilling 25

3 From top: RC bit face, RC bit with a shroud, RC saver sub, digout sub, RC blade with skirt, Adapter sub. RC rod with inner tube. Apart from the hammer inner tubes, which may only last a few hundred metres in extreme conditions, the remaining components should last for several thousand metres of drilling. Drilling tools There are generally only three types of down hole tools used in RC drilling: hammer, roller and blade. The RC hammer is the most common method used, drilling almost all formations with few changes required. The commonest hammers are in the 4-5 in range as these meet the power, standard drill strings, and sample size requirements. The hammers work on the same principles as conventional hammers, but with a hardened, replaceable inner tube through the centre. The inner tube extends into the top of the drill bit. A conventional hammer exhausts the air through the bit, whereas a RC hammer exhausts around the outside of the bit splines and around the head of the bit, forcing the sample through the holes in the face of the bit and upward through the inner tubes. To help create a higher pressure zone above the bit face, and to force the sample up the inner tubes, a sealing ring is situated above the bit. This ring can be described as a shroud, sleeve or compensator ring and is mounted on the drive sub, or bit chuck, and is usually replaceable. The RC hammer bit is similar to conventional hammer bits, but with two large ports in the face and a large bore through the centre to accept the hammer inner tube. There are deep channels on the outside of the drill bit head to allow the exhaust air to flush the sample into the ports in the face. An RC roller setup comprises a sub onto which a bit and skirt similar to a hammer shroud are screwed directly to the drill string. The bit is normally a standard mill tooth tricone roller bit, modified to allow a shroud to be fitted. The RC roller is only suited to softer formations, but can be extremely fast and produces a very accurate sample and very little disturbance in the hole. It requires minimal air volume, and down hole costs are low, so it is a very economical method of drilling. RC blade uses a sub and skirt setup similar to RC roller, but with a drag blade as the cutting tool. Used in heavy clay formations, which can be difficult or impossible with hammer or roller, it can be very quick and produces an accurate sample. Rotating parts RC drill rods consist of an outer tube, the rod, and an inner tube. The rods are externally flush and provide the strength 26 exploration drilling

4 for the assembly, and also the pin and box threads. RC drill rod threads have been developed to retain strength while maintaining a large hole through the centre for the inner tube and airway. The inner tube is installed into the rod through the box, or female, end, and usually sits on a shoulder in the rod and is retained with a circlip. Each inner tube has a male and female end, one of which has O-ring seals. Once the rods are screwed together the inner tube ends overlap, and the O-rings seal the tubes. The annulus between the rod and inner tube carries the high-pressure air to the drilling tool, while the inner tube provides a smooth bore sealed tube to carry the cuttings to the surface. Most drill rods are 3 m or 6 m long, and run pin down because of the inner tube installation. The most commonly used rod size is 4.5 in (11.43 cm), coupled with a 5 in (12.7 cm) hammer and 5.25 (13.3 cm) to 5.75 (14.6 cm) in bit, but rods are also available from 3.5 (8.9 cm) in to 5.5 in (14 cm) to suit other rig or drilling requirements. The inner tubes are a wearing, but easily replaceable, item. As with any drill string there are various subs used for adapting, reducing, and stabilizing. These are all available for RC drill strings. The air swivel feeds air into the drill rod annulus, while still retaining an inner tube to allow sample flow. They can be either in-line in the drill string immediately beneath the rotation head, or mounted on top of the head as an integral part of the head. The rotation head on an RC rig has a large bore through the spindle to allow for the replaceable sample inner tube. RC heads are usually built to provide high torque at moderate speed, with at least Nm and 100 rpm normal for larger rigs. Discharge system The discharge system is the non-rotating part of the sample path that carries the sample from the rotation head to the sample cyclone. It normally consists of a mud swivel, blowdown valve, blast box, discharge manifold and sample Standard roller bit and drag bit. hose. The mud swivel, blowdown valve, manifold and blast box all mount rigidly to the top of the rotation head. The mud swivel seals the rotating head shaft and inner tube from the stationary parts of the discharge system. The seals in the mud swivel are critical as they need to contain the pressurized flow of sample. Most RC rigs now have a blowdown valve fitted to the discharge system. This is usually a hydraulic or air driven valve that closes off the sample inner tube and redirects the downhole air flow down through the sample inner tubes. This function is used to clear blockages in the bit ports or the inner tubes, and to force all air up the outside of the drill hole, hence cleaning the hole. It is done without having to depressurize and unscrew the drill string to add a sub, so is a very useful tool in difficult drilling conditions. The blowdown is mounted on top of the mud swivel. The sample stream can be travelling at up to 250 m/sec and needs to be redirected towards the cyclone. The blast box usually turns the sam-ple flow about 90 degrees to meet the sample hose. This direction change also reduces the energy of the sample considerably, but in doing so incurs very high wear. Most systems have easily replaceable wear components in this area. The discharge manifold extends sideways from the blast box. It helps slow the sample to reduce sample hose wear and also holds the hose clear of the drill rig as the head travels up and down. There is often provision to inject small amounts of water into the manifold to mix with dry sample and reduce the dust at the cyclone. The sample hose is a heavy materials handling hose specially manufactured for RC drilling. The hose transports the sample from the discharge manifold to the sample cyclone, and is long enough to allow for the movement of the rotation head up and down the mast. Sampling The majority of RC drilling is done to obtain mineral samples for analysis, so correct sampling equipment and practices are necessary when undertaking this type of drilling. There are two main components to the sampling system: the cyclone; and the splitter.the cyclone serves to reduce the speed of the sample stream, and to separate the sample from the air, allowing it to be collected. It is important to have an efficient cyclone to remove exploration drilling 27

5 Side inlet air swivel. Tiered riffle splitter. Cone splitter. as much of the sample as possible, and also to avoid contamination of samples. A good cyclone will typically collect greater than 99% of the sample, with the remaining dust and air going to a dust collector or to atmosphere. The cyclone should be able to hold two complete sample intervals without contamination. An area of the cyclone called the dump box is equipped with a door or knife valve for this purpose. The sample interval is normally 1 m or 2 m of hole. As one sample has collected in the dump box, another is being drilled and is collecting in the cyclone. The lower dump box doors are opened to allow the sample to fall through the splitter. The lower doors are then closed, and the upper doors are opened to drop the next sample into the dump box. In this way sampling becomes a continuous process, with little or no interruption to drilling. Processing of the sample is one of the most important aspects of RC drilling. The sample from 1 m of a 5.5 in drill hole is about 30 lit, or up to 50 kg. The purpose of a splitter is to divide the sample down to a smaller size that is an accurate representation of the complete sample. This assay sample is collected in a bag and sent to a laboratory to be analysed for various minerals. Two main types of splitter are in use. Riffle splitters use several tiers of dividers that halve the sample at each level, until the assay size is reached. This usually involves 3 or 4 tiers to give 12.5% or 6.25% of the total sample. Riffle splitters are easy to use and clean with dry sample, but do not perform too well with wet samples. Tiered riffle splitter, sometimes known as a Jones riffle splitter. These essentially divide the sample in two at each tier. Half the sample goes to waste and the other half to be split at the next tier and so on. The number of tiers dictate the final assay sample size, see picture above. Cone splitters drop the entire sample over the point of an inverted cone and allow it to run down the cone. The assay sample is taken by collecting a segment of the sample as it runs off the edge of the cone. This segment size can be adjusted to collect the required percentage for assay. Cone splitters can give a more accurate split, but are more sensitive to setup than riffle splitters. The con splitter works by dropping sample through a 120 mm hole over the point of a cone in an hourglass effect. This provides an even flow of sample over the cone. Beneath the bottom of the cone are 2 segment shaped chutes that direct a percentage of the sample to the assay bags. These chutes are adjustable to take between 3 and 12% of the total sample. One is used as the assay sample, the other for a dublicate sample. The waste materials falls through a chute and can be either collected in a large bag or wheelbarrow or left as waste, see picture above. Rotating cone splitters are used for wet sampling, as they reduce or eliminate the bias that is created as a wet stream favours one portion of the splitter. Drilling guidelines RC Drilling has many similarities to conventional DTH drilling, but there are also many procedures and techniques that are required to achieve the best results. In RC drilling, the hole is of little importance, while the sample is paramount. Holes are normally set up in a similar way to a conventional hole, with a short length of collar pipe or conductor casing set at the surface. A stuffing box, Tee piece or deflector box is mounted on the collar pipe to direct any lost sample or outside circulation away from the drill rig. Rotation speeds and feed weights are similar to conventional drilling. As much sample as possible should be retrieved from the hole. It is preferable to have at least 95% inside circulation, so that most of the sample is coming through the inner tubes and only 28 exploration drilling

6 5% or less is being lost to outside. It is not uncommon to retain 100% inside. Achieving high inside circulation sample return is achieved by having the correct clearance between the bit shroud and the hole wall, thus creating a seal and forcing all sample up the inside. It is also common to allow the hole to collar off above the hammer to help with sealing. This collar will often breach when the water table is reached in the hole, or it can easily be blown out using the blowback sub. If there is water in the hole, allowing the hole to collar off helps in keeping the samples dry. Dry samples split far more accurately, and are much easier to store, transport and process. Auxiliary compressors and highpressure boosters up to 100 bar are now commonly used with larger RC rigs. These are necessary to keep the hole dry, producing a dry sample and also achieving far greater hole depths. It is almost inevitable that material will fall in behind the hammer or bit, causing the drill string to become bogged. This is normal for RC drilling, and it is common to run a short, stabilized dig-out sub above the hammer for protection when digging through fallback. Blowdown valves are fitted to almost all large rigs now to help clean holes, and are usually used at each rod change to ensure the hole remains clean, or to remove excess water from the hole. Blowdown subs are available for insertion into the drill string, but these have to be removed to continue drilling. Safety with reverse Circulation Drilling There is a wide range of safety regulations and requirements that vary from site to site, but some general rules will apply almost everywhere. While there are some potential hazards associated with the RC system, the normal safety requirements for the Explorac still need to be observed. RC drilling requires one or more Samplers or Offsiders who process the sample from the cyclone and they are also often required to manually handle some of the downhole equipment. Explorac 220 under testdrilling in Australia. They work close to the cyclone and the rig mast, so they can be exposed to hazards not usually encountered during standard production drilling. While engineering solutions have been made for most hazards, the proximity of people to the machine in this type of drilling demands vigilance and management. The customer should have a safety management system in place and all hazards should be assessed most can be managed with procedures. Manual handling: Strains and injuries can be received with handling of heavy samples, hammers, bits etc. Falling objects: Due to the vibrations involved in percussion drilling there is potential for objects to shake loose from the rig mast. Correct maintenance and regular inspections are required. High pressure air: Can be extremely dangerous. All HP air hoses, including the sample hose should have sock type restraints on each end. Sample hose clamps should be correctly fitted and couplings tightened. Pinch points: There are many areas around the rig with potential for pinch or crush injuries. Mast boom and rotation head movement, rod loader movement, cyclone tilt and rotate, and the cyclone doors can all present some hazard. Procedures, and communication between the driller and sampler are essential here. Personal protective equipment (PPE): Appropriate personal protective equipment (PPE) should be in use at all times; ear plugs, dust masks, safety boots, hard hat and gloves. These items are considered as statutory requirements in most mining environments. Summary RC drilling is a well-respected method within the exploration industry. It is fast and efficient, providing accurate samples for evaluation by the geologists using tried and tested techniques. The method is undergoing continuous technical development that will result in RC drilling being applied to deeper holes and more difficult geological conditions. RC drilling is frequently used in conjunction with core drilling for a better result in certain circumstances. The RC drilling method uses high torques, high pressures, big lifting capacities and rapid collection of samples, so safety is a major factor in the ongoing design process. Jan Jönsson exploration drilling 29