Exploration Drilling.

 

Exploratory Explorato ry drilling techniques DR BS Choudhary ISM-dhanbad

 

Exploration

 

Introduction

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Drilling is the first unit operation in mining or excavation process.

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The evaluation of drilling started, with the keenness to find the resources present inside the earth, which lead to the discovery of methods like fire setting and wet wedged to fragment the rock. But with the sickle of time, these methods lost its vita vi taln lnes esss be beca caus use e of it itss un unpr prod oduc ucti tive ve re resu sult ltss

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Exploration drilling is an important step before development drilling should take plac pl ace. e. Ex Expl plor orat atio ion n dr dril illi ling ng is es esse sent ntia iall fo forr ob obta tain inin ing g ac actu tual al te temp mper erat atur ure e me meas asur urem emen ents ts,,

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rock sa rock samp mple les, s, an and d re retr trie ievi ving ng fl flui uid d sa samp mple less fo forr ch chem emic ical al an anal alys ysis is.. The data collected from the geologic and geophysi sic cal surveys are use sed d to formulate probable definitions and realizations of the geologic structure that may contain oil and/ an d/or or ga gas. s.

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Basically we have three principles of drilling Percussive, rotary, rotary, and rotary percussive

There are different mechanisms that have been devised within a span of the past 60 years to transfer the mechanical energy produced by the prime mover to the rock or ground being drilled. These mechanisms are known as; top-hammer, down-the-hole (DTH), or in-the-hole (ITH), and rotary. Each one has its merits, limitations and field of applications.

Rotary-percussive methods. These are the most frequently used in all types of rocks, the top hammer as well as the down-the-hole hammer. Rotary methods. These are subdivided into two groups, depending upon if the penetration is carried out by crushing, with with tricones or by-cut with with drag bits. bits. B.S.Choudhary

The first system is used in medium to hard rocks, and the second in soft rocks.

 

Rock properties that affect drilling 

Hardness : Hardness is considered to be the resistance of a surface layer to be penetrated by Hardness: another body of harder consistency. In rock, it is a function of the hardness and composition of its mineral grains. the porosity, degree of humidity, etc. The hardness of rocks is the principal type of resistance that must be overcome during drilling. because once the bit has penetrated. the rest of the operation is easier.

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Strength: Mechanical strength of a rock is the property of opposing destruction by an external Strength: force. either static or dynamic. The rocks give maximum resistance to compression. normally, as the tensile strength strength is not more than 10 or 15% of the compressive compressive strength. strength. This is due to the fragility of rocks to the large quantity of local defects and irregularities that exist and to the small cohesion between the particles of which they are constituted.

The rock strength fundamentally depends on its mineralogical composition. compositi on. Among the integrating minerals, quartz is the most solid with a strength that goes over 500 MPa, MPa, while that of the ferromagnesian ferromagn esian silicates silicates and and the aluminosilicate aluminosilicatess vary between between 200 200 and 500 Mpa Mpa and that that of calcite from 10 to 20 MPa. Therefore, the higher the quartz content, the more the strength increases. increases.

 

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Elasticity : The majority of rock minerals have an elastic-fragile behaviour, which obeys the Law of Hooke, and are destroyed when the strains exceed the limit of elasticity. Depending upon the nature of deformation, as function of the stresses produced by static charges,

three of which rocks are taken into of consideration: 1) plastic The elasticfragilegroups or those obey the Law Hooke, 2) The fragile, that have plastic deformation before destruction, 3) The highly plastic or very porous, in which the elastic deformation is insignificant. The elastic properties of rocks are characterized by the elasticity module 'E' and the Poisson coefficient 'v'. The elasticity module is the proportionality factor between the normal stress in the rock and the relative correspondent deformation, its value in most rocks varies between 0.03 x 104 MPa and 1.7 x 104 MPa basically depending upon the mineralogical composition, porosity, type of deformation and magnitude of the applied force.

Curves of stress deformation for different types of rocks.

 

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Plasticity: This begins when the stresses exceed the limit of elasticity. elasticity. In the case of an ideally plastic body, that deformation is developed with an invariable stress. Real rocks are deformed and consolidated at the same time: in order to increase the plastic deformation it is necessary to increase the effort. The plasticity depends upon the mineral composition of the rocks and diminishes with an increase in quartz content, feldspar and other hard minerals. The humid clays and some homogeneous rocks have plastic properties. The plasticity of the stony rocks (granites, schistoses, crystallines and sandstones) becomes noticeable noticeable especially at high high temperatures. temperatures.

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 Abrasiveness: Abrasiveness is the capacity of the rocks to wear away the contact surface of another body that is harder, in the rubbing or abrasive process during movement. The factors that enhance abrasive capacities of rocks are the following:  The hardness of the grains of the rock.:The rock.:The rocks that contain quartz grains grains are highly abrasive. 

The shape of the grains: Those that are angular are more more abrasive than the round round ones.

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The size of the grains.

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The porosity of the rock: It gives rough contact surfaces surfaces with local stress concentrations. concentrations.

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The heterogeneity: heterogeneity: Polym Polymineral ineral rocks, although these are equally hard, are more abrasive because theyleave theyleave rough surfaces surfaces with hard grains grains as, for example,quart example,quartz z grains in a granite. granite.

 

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Texture: The texture of a rock refers to the structure of the grains of minerals that constitute it. The size of the grains are an indication, as well as their shape, porosity etc. All these aspects have significate influence on drilling performance. When the grains have a lenticular shape, as a s in a schist, drilling is more difficult than when they are round, as in a sandstone.

The type of material that makes up the rock matrix and unites the mineral grains also has an important influence.As to porosity, porosity, those rocks that have low density and, consequently, consequently, are more porous, have low crushing strength and are easier to drill.

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Structure: The structural properties of the rock masses, such as schistosity. Structure: bedding planes, joints, and faults. As well as their dip and strike affect the alignment of the blastholes, the drilling performance and the stability of the bla blasth sthole ole wall walls. s.

 

One of the earliest diamond drilling machines is the steam-powered rig shown here in 1908, recovering cores for assessment of shallow coal deposits.

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From about 1900, shallow “dry borings” for geotechnical

exploration utilized a 3-man crew with a simple A-frame, such as that shown here, along with a gas powered engine. This could be set up and broken down by hand, in confined spaces.

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A-frame rigs • A-frame rigs are still utilized world wide for  drilling exploratory borings and advancing large diameter  caissons.

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Mud rotary drilling rigs are typically used for  deeper applications, such as holes more than 50 or 100 feet deep, below the water table Mud rotary drilling employs a dense drilling fluid under its own hydrostatic pressure to support the walls of an open borehole • The drilling fluid is usually comprised of bentonite, butBSsome Choudhary drillers will use soap or a variety of other agents.

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Rotary rigs with their masts extended. A water truck accompanies the rig, to usually provide the drilling fluid. The biggest headache is loss of circulation, most common in limestone or adjacent to broken rock along

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Truck-mounted drill rigs • Generally used on semi-level ground, if easy site access • Quick set-up • Rate of advance depends on height of mast & drill strings • Large normal force can be employed

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Tracked Rigs • A number of manufacturers offer tracked rigs of varying size • These can be extremely useful when working on sift or sloping ground

Smaller tracked rigs can be operated remotely, remotely, using radio controls

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‘Swamp hog’ hog’ all terrain vehicles use oversize floatation tires inflated ~ 4 psi tire pressure to allow access on soft, mucky ground

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Portable ‘skid rig’ placed on a fat-bottom barge problem is anchoring the craft to resist drilling induced

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This view shows a custom remote drilling rig powered by remote-source offsite hydraulics. It is working on a 30 degree slope 500+ feet above the nearest bench.

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Helicopter Assist • Under conditions of  extreme remoteness, helicopters can sometimes provide vertical lift of lighter skid  rigs • This shows drilling pad set up for exploring path of the Tetsuo Harano Tunnels on Interstate H-3 through the Koolau Range in Oahu, Hawaii. BS Choudhary

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Specialty Rigs • Some operators have built rigs, on which allowtheir themown to work steep or constricted work sites • Most of these ’remote rigs’ employ hydraulic fluid, which can be transmitted to the drill rig by hoses laid out from pumps towed behind vehicles, as much as 200 feet away. BS Choudhary

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Auger drilling 

Auger drilling is done with a helical screw which is driven into the ground with rotation; the earth is lifted up the borehole by the blade of the screw.

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Hollow stem auger drilling is used for softer ground such as swamps where the hole will not stay open by itself for environmental drilling, geotechnical drilling, soil engineering and geochemistry reconnaissance work in exploration for mineral deposits.

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Solid flight augers/bucket augers are used in harder

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ground construction drilling. In some cases, mine shafts are dug with auger drills. Small augers can be mounted on the back of a utility truck, with large augers used for sinking piles for bridge foundations. Auger drilling is restricted to generally soft unconsolidated material or weak weathered rock. It is cheap and fast.

 

Percussion rotary air blast drilling (RAB) 

RAB drilling is used most frequently in the mineral exploration industry. (This tool is also known as a Downthe-hole drill.)

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The drill uses a pneumatic reciprocating piston-driven "hammer" to energetically drive a heavy drill bit into the

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rock. The drill bit is hollow, solid steel and has ~20 mm thick tungsten rods protruding from the steel matrix as "buttons". The tungsten buttons are the cutting face of the bit.

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The cuttings are blown up the outside of the rods and collected at surface. Air or a combination of air and foam lift the cuttings.

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RAB produces lower quality samples because the cuttings are blown up the outside of the rods and can be contaminated from contact with other rocks.

 

Air core drilling 

Air core drilling and related methods use hardened steel or tungsten blades to bore a hole into unconsolidated ground. The drill bit has three blades arranged around the bit head, which cut the unconsolidated ground. The rods are hollow and contain an inner tube which sits inside the hollow outer rod barrel. The drill cuttings are removed by injection of compressed air into the hole annular annular area between the innertube the drill rod. via Thethe cuttings are then blown back to surfaceand up the inner tube where they pass through the sample separating system and are collected if needed. Drilling continues with the addition of rods to the top of the drill string. Air core drilling can occasionally produce small chunks of cored rock.

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This method of drilling is used to drill the weathered regolith, as the drill rig and steel or tungsten blades cannot penetrate fresh rock. Where possible, air core drilling is preferred over RAB drilling as it provides a more representative sample. Air core drilling can achieve depths approaching 300 meters in good conditions. As the cuttings are removed inside the rods and are less prone to contamination compared to conventional drilling where the cuttings pass to the surface via outside return between the is outside of the and drillslower rod and theRAB. walls of the hole. This method more costly than

 

Cable tool drilling 

Cable tool rigs are a traditional way of drilling water wells. The majority of large diameter water supply wells, especially deep wells completed in bedrock aquifers, were completed using this drilling method. Although this drilling method has largely been supplanted in recent years by b y other, other, faster drilling drilli ng techniques, it is still the most practicable drilling method for large diameter, diameter, deep bedrock wells, and in in widespread use for small rural water supply wells. The impact of the drill bit fractures the rock and in many shale rock situations increases the water flow into a well over rotary.

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Also known as ballistic well drilling and sometimes called "spudders", these rigs raise and drop a drill string with a heavy carbide tipped drilling bit that chisels through the rock by finely pulverizing the subsurface materials. The drill string is composed of the upper drill rods, a set of "jars" (inter-locking "sliders" that help transmit additional energy to the drill bit and assist in removing the bit if it is stuck) and the drill bit. During the drilling process, the drill string p eriodically periodically removed from borehole and a bailer is lowered to collect the drill is cuttings (rock fragments, soil,the etc.). The bailer is a bucket-like tool with a trapdoor in the base. If the borehole bor ehole is dry,, water is added dry add ed so that the drill d rill cuttings will flow into the bailer bailer.. When lifted, the trapdoor closes and the cuttings are then raised and removed. Since the drill string must be raised and lowered to advance the boring, the casing (larger diameter outer piping) is typically used to hold back upper soil materials and stabilize the borehole.

 

Reverse circulation (RC) drilling

RC drilling is similar to air core drilling, in that the drill cuttings are

returned to surface inside the rods. The drilling mechanism is a pneumatic reciprocating piston known as a "hammer" driving a tungsten-steel drill bit. RC drilling utilises much larger rigs and machinery and depths of up to 500 metress are routinely achieved. RC drilling ideally produces metre produces dry rock chips,

as large air compressors dry the rock out ahead of the advancing drill bit. RC drilling is slower and costlier but achieves better penetration than RAB or air core drilling; it is cheaper than diamond coring and is thus preferred for most mineral exploration work. The most commonly used RC drill bits are 5-8 inches (13 – 20 20 cm) in diameter.

 

Odex ex dr dril illi ling ng Od 

As much as 90% of the land surface of the earth is covered with loose, unconsolidated material such as soil, clay, silt, sand, gravel and boulders, which varies in depth from a few centimetres to hundreds of meters.

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Drilling through this so-called overburden is often problematic, duebit. to This the tendency of the earth to cavethe in behind the drill makes it difficult to retrieve drill string after the hole has been drilled. In practice, the bore hole is often lost before a casing tube can be inserted to support it.

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Other problems are caused by cavities or porous ground, which interfere with the circulation of the flushing medium and prevent the drill cuttings from being flushed out of the hole.

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In places where overburden strata are mixed, or when their drillability is unknown, unknown, it is difficult for for the driller to decide decide what tools to use in order to get the best overall results without risking the loss of equipment in the hole.

 

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ODEX equipment enables to drill and a nd case deep holes simultaneously in all types of formation, even those with large boulders. Casing diameters from 89 mm (ODEX 76) to 273 mm (ODEX 240) can be used. The method is based on a pilot bit and eccentric reamer, reamer, which together drill a hole slightly larger than the external diameter of the casing tube. This enables the casing tube to follow the drill bit down the hole.

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When using ODEX, part of the impact energy is diverted to the casing tube via a shoulder on the guide device, which in turn impacts a special casing shoe at the lower end of the casing.

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ODEX 76 for top hammers operates with impact and rotation transmitted through extension rods. To To drive the casing down the hole, this shank adapter is used to transfer part of the impact energy from the rock drill to the casing tube.

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In both DTH and top hammer drilling the casing is driven down into the hole without rotation. When the casing enters the bedrock, drilling is stopped briefly, briefly, and reverse rotation applied carefully carefully, causes reamer to turn in,the thus reducing the overall diameter of through the drill the bit assembly.,. which assembly When this ha sthe has been accomplished, entire drill string can be pulled up inside of the casing tubes, leaving the latter embedded in the bedrock. Drilling can then be continued into the bedrock using a conventional drill string.

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To improve flushing, the ODEX guide device has back-ward pointing flushing holes.

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In difficult conditions, a foaming additive can be added to the compressed air to further improve flushing performance.

 

Diamond core drilling 

Diamond core drilling (exploration diamond drilling) utilizes an annular diamondimpregnated drill bit attached to the end of hollow drill rods to cut a cylindrical core of solid rock.

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The diamonds used used are fine to microfine industrial grade diamonds. diamonds. They They are set within a matrix of varying hardness, from brass to high-grade steel. Matrix hardness, diamond size and dosing can be varied according to the rock which must be cut. Holes within the bit allow water to be delivered to the cutting face. This provides three essential functions — lubrication, cooling, and removal of drill cuttings from the hole.

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Diamond drilling is much slower than reverse circulation (RC) drilling due to the hardness of the ground ground being drilled. drilled. Drilling of 1200 to 1800 metres is common and at these depths, ground is mainly hard rock. Diamond rigs need to drill slowly to lengthen the life of drill bits and rods, which are very expensive.

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Core samples are retrieved via the use of a "lifter tube", a hollow tube lowered inside the rod string by a winch cable until it stops inside the core barrel. As the core is drilled, the core barrel slides over the core as it is cut. An "overshot" attached to the end of the winch cable is lowered inside the rod string and locks on to the "backend", located on the top end of the core barrel. The winch is retracted, pulling the core barrel to the surface. The core does not drop out of the inside of the core barrel

when lifted because either a split ring core lifter or basket retainer allow the core to move into, but not back out of the tube.

 

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A medium hard stratum is drilled by Double Tube Core Barrel using a diamond bit. The main object of core drilling is to obtain a core sample of a formation for laboratory to determine its content and characteristics. characterist ics. If a portion of sample is lost or its surface is eroded the analysis may not necessarily be accurate. Therefore, to ensure reliability of the analysis, as much of the sample as possible must be recovered in a condition that is representative of the formation by using a T2 series.

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The T2 Series are thin wall double tube core barrel, used for surface exploration, especially in mining applications. Due to their thin kerf, these core barrels provide rapid penetration with proper core protection. Available in diameters 46, 56, 66, 76, 86, 101mm

 

Table for the core size available in X-Series X-SERIES

HOLE DIAMETER (mm) CORE DIAMETER (mm)

ZX UX SX PX HX

199 174 145 120 99

165 140 113 92 76

NX BX AX EX

75 60 47 38

54 40 28 21

RX

29.5

17.5

 

Wire line core barrel (tube in tube system)

 

Directional drilling Direction drilling to ainclination drilling practice wheredirection the well is is deviated from therefers vertical to a specific When the reservoir is located in an inaccessible area, for example on bad terrain or below a mountain, then directional drilling will really help to reach the reservoir easier and at a lower cost. Directional drilling allows locating several wells in one well site, thus the road construction cost, water supply cost, and steam gathering system cost is combined. Directional drilling enables the intersection of several vertical

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structures in one well.

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•  The rocks through which the hole is drilled can influence the direction.

 Any rock layered (especially thin alternating soft and hard laye la yers rs) ) or which hass a isfi ha fiss ssil ilit ity y (br (brea eaks ks pr pref efer eren enti tial ally ly alo along ng cl clos osel ely y sp space aced d parallel planes) will cause deviation if the hole is at an angle to the layering or fissility. •  If this layering or fissility has a constant attitude in the drill area, the direction of the hole deviation can be predictable and therefore useful in plan pl anni ning ng dr dril illl se setu tups ps th that at wi will ll na natu tura rally lly de devi viat atee to towa ward rd th thee de desi sire red d

target. •   If the layering is folded, the direction of deviation may change as the angle between the hole and the folded layering changes.

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drilling can creat createe forces which can amplify the rate of deviation. deviation. Worn and •   The physics of drilling loose loo se dri drill ll rod thr thread eads, s, non non-ri -rigid gid cor coree bar barrel rels, s, and ina inappr ppropr opriat iatee bit des design ign or cut cuttin ting g characteristics have long been known to contribute to crooked holes. Altern ernate ately, ly, usi using ng rig rigid id cor coree bar barrel rels, s, wel welll des design igned ed and pro proper perly ly pla placed ced sta stabil bilize izers rs and •   Alt suitable bits diminish the influence of rock imposed deviatory forces. •   The driller can influence the rate of deviation deviation by using improper drilling forces. forces. Large dir direct ection ional al dev deviat iatio ions ns can be cau caused sed by buc buckle kled d dri drill ll rod trains trains bro brough ughtt abo about ut by  •   Large excessive force on the bit. •   The desire for rapid hole advance often leads to poor drilling practices, practices, but advance rates

that are too slow may also produce more than necessary deviation. It is not unusual for each drilling shift to have their own specific deviation rate on a borehole- one crew consistently  producing higher deviations.

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Core storage 

A core sample is a cylindrical section of (usually) a naturally occurring substance. Most core samples by drilling with special drills intoare theobtained substance, for example sediment or rock, with a hollow steel tube called a core core   drill.. The hole made for the core sample is called drill cal led the "core bowling". A variety of core samplers exist to sample different media under different conditions. More continue to be invented on a regular basis. In the coring process, the sample is pushed more or less intact into the tube. Removed from the tube in the laboratory, it is inspected and analyzed by different different techniques and equipment depending on the type of data desired.

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Coring has come to be recognized as an important source of data, and more attention and care is being put on preventing damage to the core during various stages of it transportation and analysis. The usual way to do this is to freeze the core completely using liquid nitrogen, which is cheaply sourced. In some cases, special polymers are also used to preserve and seat/cushion the core from damage.

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Equally, a core sample which cannot be related to its context (where it was before it became a core sample) has lost much of its benefit. The identification of the borehole, and the position and orientation ("way up") of the core in the borehole is critical, even if the borehole is in a tree trunk dendrochronologists always try to include a bark surface in their samples so that the date of most-recent growth of the tree can be unambiguously determined.

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If these data become separated from core samples, it is generally impossible to regain that data. The cost of a coring operation can vary from a few currency units (for a hand-caught core from a soft soil section) to tens of millions of currency units (for sidewall cores from a remote-area offshore borehole many kilometres deep). Inadequate recording of such basic data has ruined the utility of both types of core.

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Different disciplines have different local conventions of recording these data, and the user should familiarize themselves with their area's conventions. For example, in the oil industry, orientation of the core is typically recorded by marking the core with two longitudinal colour streaks, with the red one on the right when the core is being retrieved and marked at surface. Cores cut for mineral mining may have their own, different, conventions. Civil engineering or soil studies may have their own, different, conventions as their materials are often not competent enough to make permanent marks on.

 

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It is becoming increasingly common to retain core samples in cylindrical packaging which forms part of the core-cutting equipment, and to make the marks of record on these "inner barrels" in the field prior to further processing and analysis in the laboratory. Sometimes Sometimes core is shipped form the field to the laboratory in as long a length as it comes out of the ground; other times it is cut into standard lengths (5mcapable or 1m orof3 being ft) forreversed shipping,onthen laboratory laboratory. . Some ofthe thesample "inner barrel" barr systems are the reassembled core sample,insothe that in the laboratory goesel" "wrong way up" when the core is reassembled. This can complicate interpretation.

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Goniometers are used to measure angles of fractures and other features in a core sample relative to its standard orientation.

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If the borehole has petrophysical measurements made of the wall rocks, and these measurements measurements are repeated along the length of the core then the two data sets correlated, one will almost universally find that the depth Which "of record" a particular piece of core differs between the two methods measurement. set offor measurements to believe then becomes a matter of policy forof the client (in an industrial setting) or of great controversy (in a context without an overriding authority. Recording Recording that there are discrepancies, for whatever reason, retains the possibility of correcting an incorrect decision at a later date ; destroying the "incorrect" depth data makes it impossible to correct a mistake later.. Any system for retaining and archiving data and core samples needs to be designed so that later dissenting opinion like this can be retained.

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If core samples from a campaign are competent, competent, it is common practice to "slab" them them - cut the sample into two or more samples longitudinally - quite early in laboratory processing so that one set of samples can be archived early in the analysis sequence as a protection against errors in processing. "Slabbing" the core into a 2/3 and a 1/3 set is common. It is also common for one set to be retained by the main customer while the second set goes to the government (who often impose a condition for such donation as a condition of exploration/ exploitation licensing). "Slabbing" also has the benefit of preparing a flat, smooth surface for examination and testing of profile permeability, permeability, which is very much easier to work with than the typically rough, curved surface of core samples when they're fresh from the coring equipment. Photography of raw and "slabbed" core surfaces is routine, often under both natural and ultra-violet light.