The search for minerals beneath the earth's surface often requires physical “samples” of the rock to be taken. Drill rigs are used to drill holes and retrieve the drilled material from the hole. This material is called a “drill sample”. The reverse circulation (RC) method of drilling is commonly used to drill and retrieve the sample, because it is relatively fast and produces good quality samples. RC uses large volumes of high pressure air to power the downhole drilling tool; the exhaust air then conveys the sample to the surface through inner tubes located within the drill rods. The sample then continues through a large hose to the drill sampling system.
Most drill sampling systems consist of a cyclone to slow down and separate the cuttings from the airstream, a drop box to collect the sample, and a sample splitter. A sample splitter is a device that is designed to consistently and accurately divide a bulk quantity of material into smaller portions that are truly representative of the bulk. In the case of drill sampling, it is usual to “split” the bulk material from a drilled interval into one or two small “laboratory samples” and the remainder as “waste”. The small samples are generally known as the ‘assay’ and ‘duplicate’ samples. These samples are usually required to be a consistent percentage (normally between 5 and 10%) of the bulk material and both of the same size.
There are various types of splitter used, but there is a tendency now toward “cone” splitters as being more accurate in this application. The cone splitter consists of a cone oriented with the point up. This would be enclosed in a body with an inlet or funnel at the top which is centrally located over, and just above, the point of the cone. Under the lower edge of the cone are one or more radial “cutters” or chutes. The bulk material to be split falls through the inlet, over the point of the cone, and then flows in an even spread down the slope of the cone. The cutters or chutes under the lower edge of the cone will catch a portion of the bulk material and direct it away to be collected as the assay and/or duplicate. The remainder or ‘waste’ is usually directed into a bulk bag or wheelbarrow.
For a cone splitter to split correctly the sample must be distributed evenly around the circumference of the base of the cone where the cutters/chutes are. The cutters/chutes must also be of a correct segment shape and have knife ‘cutting’ edges. It follows that for an even distribution at the bottom of the cone, there must be an even or uniform distribution over the point of the cone. The cone must also be level and evenly formed. So for a cone splitter to work correctly, the bulk material must be distributed uniformly onto the point of the cone. To spread evenly over the cone the cuttings must be dropped through a circular inlet, positioned centrally over the point of the cone.
Ideally this inlet should be as small as possible to produce a slow and consistent flow and to funnel the cuttings over the cone (like an hour glass). When drilling dry material, the cuttings are slowed by the cyclone and collected in the drop box. Usually the complete interval is collected before being dropped as one onto the splitter. This fills the inlet, and the cuttings generally flow quite consistently onto the cone, producing an even spread and hence an accurate split.
An inlet that is too small will tend to block because of varying particle size and moisture content of the cuttings. Time taken to process each sample also becomes too long. These factors have dictated that the minimum practical inlet size for dry cuttings is approximately 120 mm. If water is encountered in the drilling process, or if water needs to be injected into the drilling air, the sample then becomes wet. When wet drilling, there often are huge rapid variations in the flow rate of cuttings into the cyclone. This is due to changing water flow rates in the formation, and also the dynamics of using compressed air to power downhole hammers and lift the cuttings. Flow can vary from little or nothing for the majority of the drilled interval, to a large rush of cuttings at the end of the interval when the hammer is ‘lifted off bottom’. Even with average water flows, the volume of sample and water can often exceed the capacity of the drop box. For this reason the drop box door usually has to remain open, allowing the cuttings to flow directly from the cyclone, through the drop box, and into the splitter.
This changing flow rate produces uncontrolled streams into the splitter that often favour or bias one side of the cone. This bias can produce large variations in sample size and accuracy. For example, if all the flow is down one side of the cone, directly above a cutter, then there will be a vastly oversize sample from that cutter, whilst the other cutter may well produce an undersize sample. Wet sample will flow through a much smaller hole, but again variations in flow rates and changes from dry/wet/dry sampling make it impractical to reduce inlet size.
Rotating type cone splitters have been developed to try and counteract this bias. These either rotate the cone and cutters and redirect the sample through a convoluted system of funnels and chutes to the collection bags, or they rotate the entire collection system under the cone. Rotary cone splitters assume that there is a biased flow over the cone, and attempt to pass the cutters through that flow wherever that flow may be around the base of the cone. Doing this many times per sample interval should produce a reasonably representative sample, but in practice this does not always happen.
Accepted sampling practice dictates cutter speed through the sample stream to be no more than 500 mm/sec, which translates to only about 20-25 rpm for current size cone splitters. Current rotary type cone splitters or rotary distributors on the market rotate at about than 50-60 rpm, which is beyond accepted speeds and introduces delimitation errors with the sample.
In wet drilling of a softer formation it often occurs that almost the entire sample comes into the system within a few seconds as the hammer is ‘lifted off bottom’ at the end of the interval. This is a normal result during drilling and little can be done to modify it. As there is currently no way of throttling the flow of wet sample and distributing it over the cone, it often occurs that the entire sample can pass over the cone within a few seconds. This flow is also often heavily biased to one or more areas of the cone. Even at the higher than recommended rotating speeds, the rotating cutters or collectors are only passing any given part of the cone at a rate of no more than once per second each, so they may only take a few small increments of the entire sample.
A drilled sample generally comes into the splitter in the order or sequence that it is drilled, and hence falls over the splitter in the same sequence that it occurs in situ. If the formation being drilled is very stratified, then it is probable that much of the interval will effectively not be sampled, as there will only be a few increments taken. So it is accepted that the flow of wet sample over a cone is often biased and therefore produces inconsistent and biased samples. Corrections need to be made to produce a more representative sample.
Prior art attempts to address this problem have done so in several ways:
1. Rotate the collection points beneath a stationary cone; or
2. Rotate the cone and sample cutters, and direct the sample to fixed collection funnels.
3. Channel the sample to the cutters through a rotating chute or funnel (as with the Progradex “Andis” sampler).
From a theoretical sampling point of view, rotating cutters, whilst not perfect are a fairly accurate way to take a representative sample, but this also assumes a relatively homogeneous sample stream and a relatively steady and slow flow rate. Neither of these occurs reliably in practice. All the above methods take an increment of sample each revolution, but as described above, there can often be only a few increments taken throughout each interval. This is due to physical limitations on the rotation speed of the funnel, the cone or the cutters and sample extraction errors incurred with higher cutter speed. At higher rotational speeds, centrifugal forces also begin to have a major detrimental effect on the flow and distribution of the sample.
Until now there has been little or no control over the way the cuttings are distributed as they enter the splitter. The present invention was developed with a view to providing a drill sample distributor for more uniformly distributing the particles of a drill sample at the inlet of a cone splitter. This means that a stationary cone can be used and there are none of the inherent constraints and limitations of prior rotary cone splitters or distributors. However it will be appreciated that the particle distributor may have other applications where particles are required to be distributed more uniformly.
References to prior art documents in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.