Drilling core barrel

Keywords d rec methods, indirect methods, rock properties, coring, core barrel, standard core analysis, special core analysis, slabbed core, sidewall samples, direct indications, microfossils, sonde, logging unit, invasion, mudcake, formation pressure measurement, fluid sampling, measurement while drilling, formation evaluation while drilling. 

Coring is performed in between drilling operations. Once the formation for which a core is required has been identified on the mud log, the drilling assembly is pulled out of hole. For coring operations a special assembly is run on drill pipe (Fig. 5.33) comprising a core bit and a core barrel. 

Wireline Core Barrel Systems. The wireline system can be used for continuous drilling or coring operations. The inner barrel or the drill plug center of the core bit can be dropped from the surface and retrieved without pulling the entire drill string. 

Drilling jars, stabilizers and, usually, core barrels are also made from AISI 4140 or 4145H steel and sometimes AISI 4340 or 4340H steel is also used. Tbe steel is heat treated to the hardness level of 285 to 341 Bhn. 

The usual procedure is to drill the hole, remove the core barrel or other tool, seat the packer, make the test, remove the packer, drill the hole deeper, set the packer again to test the newly drilled section, and repeat the test (see Fig. 15.2a). If the hole stands without casing, a common procedure is to drill it to final depth, fill with water, surge it, and bail out. Then set two packers on pipe or drill stem as shown in Figs. 15.2c and d. The length of packer when expanded should be 5 times the diameter of the hole. The bottom of the pipe holding the packer must be plugged, and its perforated portion... 

Rock All rock types found in Hong Kong, Diamond core drilling with double or triple-tube core-barrel. N/A... 

It is also important that there be an understanding of best procedure and control features necessary to achieve the optimum results. If the bit pressure is too great, it is possible to block the bit and grind up the coal sample. Conversely, drilling too slowly in a coal seam may permit the circulating fluid to wash the coal away before it enters the core barrel. It is, therefore, necessary to drill cleanly into and through the coal seam with proper bit pressure. 

Wire-line drilling is performed using thin-walled drill rods through which an inner tube can be lowered into position within the outer core barrel to receive the core as it is cut by the bit. After completion of the coring, the inner tube (containing the core samples) is disconnected so that it may be retrieved by hoisting to the collar of the hole the drill rods, outer barrel, and bit remain in the hole until they need to be withdrawn to, say, replace a dull bit. 

Disturbance of the core is iikeiy to occur when it is removed from the core barrel. Most rock cores should be removed by hydraulic extruders while the tube is held horizontal. To reduce disturbance during extrusion the inner tube of double-core barrels can be lined with a plastic sleeve before drilling commences. On completion of the core run, the plastic sleeve containing the core is withdrawn from the barrel. 

Conventional core barrels consist of an inner and outer barrel with a bit assembly. To obtain a core at a discrete interval (1) the borehole is advanced to the top of the desired interval, (2) the drill pipe is removed, (3) the core barrel/bit is placed on the bottom of the pipe, and (4) the assembly is run back to the desired depth. The selected interval is cored and the core barrel is removed to retrieve the core. Conventional systems typically are most effective at shallow depths or in cases where only discrete samples are required. 

In contrast, wireline coring systems allow for continuous core retrieval without removal of the drill pipe/bit assembly. The wireline system has a retrievable inner core barrel that can be pulled to the surface on a wireline after each core run. 

This technique can also be applied to cohesive soils, but in such soils it is alternatively possible to use a rotary drill, equipped with a carbide-tipped bit. If undisturbed samples are required, a core barrel of the double tube type can be used. In many instances, however, a single tube core barrel will adequately serve the purpose if water flush can be dispensed with. Drilling operations are liable to be particularly difficult, even if little water is used, in clays containing minerals which swell and thus cause a narrowing of the hole. Under such conditions it is certainly necessary to case the hole directly above the drill bit. 

In especially difficult cases the drilling operations may be carried out with double tube core barrels or wire line barrels equipped with a plastic inner tube for enclosing the sample. The plastic tube is withdrawn along with the sampled material and serves also as its container for despatch to the laboratory. 

For successful exploration with the aid of core drilling the correct choice of drill bits, core barrels and flushing media is of major importance. 

The choice of a suitable drill bit will depend on the rock itself the bedding, fissuring and tectonic characteristics of the deposit, and the abrasiveness of the rock. Carbide-tipped as well as diamond drill bits are used. With large diameters and heavily fissured rock the risk that parts of the core will tilt and jam in the core barrel is greater with carbide bits besides, the core is more exposed to the action of the flushing medium than with diamond bits. In such cases the choice of the most suitable bit will depend on the foreman-driller s experience. 

If borings are carried out in very soft and shattered material (though firm enough to enable a stable hole to be drilled), it is possible to use a special double tube core barrel in which a third tube, made of plastic, can be inserted into the inner tube. The core is then removed together with the plastic tube from the barrel, so that a substantially undisturbed sample for assessment is obtained. 

It has already been noted that with a flu id medium for flushing the borehole there is a risk that clay and marl strata, as well as sand and silt inclusions, will be washed out and that soluble constituents of the limestone will likewise be lost. In principle, a distinction is to be drawn between air and liquid flushing media. In all cases air flush is preferable, because it ensures that no constituents will be removed by washing or dissolving action. With air flush it is often unnecessary to use a double tube core barrel, for in the single tube the samie is enveloped only in a stream of air, though admittedly the rate of drill bit wear is then higher. 

Burrs will be thrown up inside the bore by the drill s entry and exit into and out of the bore. Therefore, it will be necessary to lap the rifled portion after the ports are drilled by casting a lead 1 > around a steel rod as described in the barrel making chapter of Volume One of this series. The occurrence of these burrs can be greatly reduced by pouring the bore full of molten lead before the ports are drilled and driving the lead core out with a close-fitting rod after the drilling is complete. 

There are, however, certain features that are desirable in any drilling operation, including instrumentation, feed control, long feed and chucking, circulating pumps, and core bits and barrels. 

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