Methods of Coring

Coring Procedures in Unconsolidated Sediments

Hydraulic-rotary coring of unconsolidated sediments for the purpose of obtaining undisturbed cores is extremely difficult, compared to hydraulic-rotary coring of rock. Most core drillers consider it impossible to core unconsolidated materials and, as a consequence, many will not attempt it. These unconsolidated materials are most commonly referred to as overburden and they are cased off in the exploration-drilling industry, prior to starting any coring operations. However the overburden is usually of major interest in water-resource investigations.

The USGS Water Resources Division has funded research to look into methods and techniques for coring and sampling unconsolidated formations, particularly aquifer materials. These research

STANDARD

drill rods & coupling

size

00

id

threads

vet

coupling

-id

in.

m

in.

i'm

per in.

lbs/ft.

kgAi

in.

in

•rw

1-3/32

27.8

23/32

18.3

4

1.95

2.90

13/32

10.3

•ew

1-3/8

34.9

15/16

23.8

3

3.1

4.6

7/16

11.1

•aw

1-3/4

44. 4

1-1/4

31.8

3

4.2

6.25

5/8

15.9

bw

2-1/8

54.0

1-3/4

44.5

3

4.3

6.4

3/4

19.0

nw

2-5/8

66.7

2-1/4

57.1

3

5-5

8.18

1-3/8

34.9

hw

3-1/2

88.9

3-1/16

77.8

3

7.73

11.5

2-3/8

60.3

•e

1-5/16

33.3

7/8

22.2

3

2.7

4.02

7/16

11.1

•a

1-5/8

41.3

1-1/8

28.6

3

3-7

5.51

9/16

14.3

•b

1-7/8

47.6

1-1/4

31.7

5

5.0

7.44

5/8

15.9

n

2-3/8

60.3

2

50.8

4

5.2

7.74

1

25.4

parallel wall roo length is 2', 5' 10'

"x" series flush coupled casing parallel wall

"x" series flush coupled casing

size

od

id

threads

wgt.

coupling-id

in.

in.

rti

per in.

lbs/ft.

kiial

in.

m

rx

1-7/16 36.5

1-3/16

30.2

8

1.75

2.6

1-3/16

30.2

ex

1-13/16 46.0

1-5/8

41.3

8

1.80

2.68

1-1/2

38.1

ax

2-1/4 57.2

2

50.8

8

2.90

4.32

1-29/32

48.4

bx

2-7/8 73.o

2-9/16

65.1

8

4.55

6.75

2-3/8

60.3

nx

3-1/2 88.9

3-3/16

81.0

8

5-57

8.28

3

76.2

hx

4-1/2 114.3

4-1/8

104.8

5

8.65

12.87

3-15/16

100.0

'v

series

flush joint

casing

size

00

id

threads

in. m

in.

m

per in.

lbs/ft.

kgAi

rw

1-7/16 36.5

1-3/16

30.2

5

1.75

2.6

ew

1-13/16 46.0

1-1/2

38.1

4

2.76

4.11

aw

2-1/4 57.2

1-29/32

48.4

4

3.80

5.65

bw

2-7/8 73.0

2-3/8

60.3

4

7.00

10.42

nw

3-1/2 88.9

3

76.2

4

8.69

12.93

hw

4-1/2 114.3

4

101.6

4

11.35

16.89

pw

5-1/2 139.7

5

127.0

3

15.35

22.84

sw

6-5/8 168.3

6

152.4

3

19.49

29.0

uw

7-5/8 193.7

7

177.8

2

23.47

34.92

zw

8-5/8 219.1

8

203.2

2

27.80

41.36

casing length is 2', 5' or 10'

minimum physical material strength

yield

tensile

parallel wall drill roo

65,000

ps i

80,000 psi

upset or forged end drill roo

40,000

ps i

60,000 psi

casing

65,000

ps i

(NOMINAL)

'v group core barrels

"g" design core size od diameter

0esiSre size od diameter

"t" design core size od diameter

evg 1-1/2 awg 1-7/8 bwg 2-3/8 nwg 2-15/16

2-1/8

nwt 2-15/16 2-5/16

large design core barrels

size

00

core diameter

in.

in.

2-3/4 x 3-7/8

3-7/8

2-11/16

4 x 5-1/2

5-1/2

3-15/16

6 x 7-3/4

7-3/4

5-15/16

diamond core bits

si ze

hole di am

core 01 am

set dimensions (inches ± -005)

in

in

00

id

ewg-ewm

1-1/2

7/8

1.470

.845

aw;-awm

1-7/8

1-1/8

1.875

1.185

bwg-bwm

2-3/8

1-5/8

2.345

1.655

nwg-hwm

3

2-1/8

2.965

2.155

hwg

3-7/8

3

3.890

(î .0075) 3.000

diamond casing bits & casing shoe bits

set dimensions (inches t -005)

casing

bits

casing shoe bits

si ze

od

id

od

id

ew-ex

1.875

1.405

1.875

1.494

aw-ax

2.345

i.78o

2.345

1.899

bw-bx

2.965

2.215

2.965

2.370

nw-nx

3.615

2.840

3.615

2.992

hw-hx

4.625 CÎ

.0075) 3.777

4.625 (+ .0075)

3.925

not dcdma standard for reference only longyear 0 series wire line (nominal sizes-inches)

size

drill rods

outer

tube

inner

tube

core hole

barrels core

00 id

00

id

diam

diam

1-13/16

1-11/16

1-31/32

2-1/2

Figure 11 .—Diamond Core Drill Manufacturers Association standards for casing, drill rods, core barrels, and diamond bit dimensions (Acker, 1974, reprinted by permission from

Diamond Drill Bit Logo

Figure 12.—Face-discharge diamond-core bit.

Barrel Shoe

Figure 13.—Recessed bottom-discharge-type diamond-coring bit and pilot inner-barrel shoe.

Figure 12.—Face-discharge diamond-core bit.

efforts have shown that: (1) most unconsolidated materials can be cored by the hydraulic-rotary method and (2) hydraulic-rotary coring is a slow and expensive process requiring a considerable amount of operator patience and expertise. All sands, silts, clays, and combinations of these materials can be cored by the hydraulic-rotary method with very little disturbance or contamination, if the proper techniques are used. Certain materials, such as boulders, and gravels having no matrix of sand-silt-clay to hold them in place, can possibly be cored, but the core is badly disturbed and contaminated by invasion of drilling fluid. The methods and techniques that the USGS Water Resources Division uses to core unconsolidated sediments are shown in figures 14 through 16 which are photographs of hydraulic-rotary cores of unconsolidated and loosely consolidated sediments.

fbr example, assume a short section of surface casing has been installed to prevent erosion or cratering around the borehole at ground surface, the top several feet of the formation are top soil (loam, silt, sand, clay mixture), and 50-s viscosity mud has been prepared. After attaching a 5-ft long HQ core

Figure 13.—Recessed bottom-discharge-type diamond-coring bit and pilot inner-barrel shoe.

Unconsolidated Sand
Figure 14.—Hydraulic-rotary core of unconsolidated, finegrained Ogallala sand.

barrel (no longer core barrel should be used in coring unconsolidated materials) to the spindle rod or fluted kelly and setting the coring bit in the

Figure 15.—Hydraulic-rotary core of loosely consolidated Ogallala sand with solution openings.

surface casing, circulate the drilling mud at a rate of about 10 gal/min. Note that the circulation of this amount of viscous drilling mud through the coring bit ports results in a fluid pressure of about 25 lb/in.2 as shown on the pressure gage. Rotate the core barrel at about 50 r/min while, at the same time, applying a tow down pressure not to exceed 50 lb/in.2 The down pressure applied should be only enough to penetrate the material at a rate of about 1 ft every 5 min. The core must be cut and not pushed into the core barrel. As the core is being cut, the fluid pressure increases to 35-50 lb/in.2 as a result of the drill mud forcing the cuttings up a restricted annulus. However, if the fluid pressure climbs very rapidly, then the penetration rate is too fast. The pressure buildup is caused by: (1) collecting of too many cuttings in the hole, or (2) plugging of the coring-bit discharge ports. If the penetration rate is not reduced at this point, deep drilling-fluid contamination of the core or complete plugging of the coring bit will occur. After the core barrel is seated in, assuming the drill pipe is rotating true (no wobble or vibration) in the hole, the rotational speed can be increased above the recommended 50 r/min used for starting the coring operation. However, high rotational speeds used for coring hard rock cannot be employed for coring fragile, unconsolidated material. With the exception of coring in clay, rotational speed of the coring bit should not exceed 250 r/min. After the 5 ft of core has been taken, the core barrel is removed from the hole, the core taken out of the inner barrel, and the

Figure 16.—Hydraulic-rotary core of loosely consolidated gravels In a matrix of silty sand.

core barrel is returned to the bottom of the hole for the continuation of coring.

A more detailed explanation of penetration rate follows: In unconsolidated materials, considerable density-hardness differences occur as lithotogies change downhole; partially indurated zones may be encountered that are directly underlain by very soft and uncemented materials. These density-hardness changes are one of the most troublesome aspects when trying to core unconsolidated materials by the hydraulic-rotary method and probably account for the reason that many core drillers do not attempt coring of unconsolidated materials by this method. For example, assume a low-density material is being cored by the hydraulic-rotary method. The coring progresses satisfactorily, using 25 lb/in.2 down pressure on the bit until a high-density cemented material is encountered and, obviously from observing the spindle chuck or hydraulic traveling table, penetration stops. Penetration is resumed by increasing the weight on the bit by adjusting the hydraulic downfeed valve until the bit pressure is about 200 lb/in2. Further, assume that, after the cemented zones have been cored, for 1 or 2 ft the bit suddenly breaks through the hard zone into a very soft formation. The 200 lb/in.2 down pressure on the bit that was required to core the hard zone at a rate of 1 ft/5 min now pushes it into the softer formation at a rate of about 20 ft/min, causing complete blockage of the bit and bit ports. The only solution is to trip out of the hole and unplug the bit. The only way to prevent this situation from occurring is to pay strict attention to the variations in bit pressure and drilling-fluid pressure as coring progresses, and as soon as the coring bit breaks through the hard material the bit downfeed pressure needs to be decreased immediately.

Some core drills are equipped with a detent valve which through manipulation allows the downfeed rate to be set at a predetermined speed regardless of the weight or down pressure acting on the coring bit. This prevents fast penetration and resultant bit plugging when breakthrough of the hard material occurs. This detent device should be considered whenever hydraulic-rotary coring in unconsolidated materials is done. The same type of safety feature is accomplished by experienced drillers using standard coring machines who partially set the brake on the planetary winch and make the hydraulic downfeed overcome the applied braking holdback.

To further discuss hydraulic-rotary coring of unconsolidated materials, look at deeper coring of various lithologic units. Assume that at a depth of 100 ft, a medium sand with a fairly high permeability is encountered. Building a quick filter cake on the core as well as the hole wall is imperative, to prevent invasion of the core and fluid loss to the formation. We have previously suggested a lightweight, 75-s viscosity drilling mud for coring under these lithologic conditions. A greater pump pressure is required to move this higher viscosity drilling mud through the bit opening and up the annulus formed between the drill pipe and the borehole. Where 35-50-lb/in.2 pressure was used for the 50-s viscosity drilling mud, 50-75-lb/in.2 pressure is used as coring progresses and cuttings are generated. Fluid pressure may climb as high as 100 lb/in2. Rotational speed and penetration rate of the coring bit vary somewhat depending upon the density of the sand encountered, but they should not exceed a rotational speed of 250 r/min and penetration rate of 1 ft/5 min. If any vibration or chattering of the coring bit occurs, the rotational speed should be decreased until the operation of the drill smooths out. If the sand gets coarser or turns to a sand-gravel mixture, mud viscosity may have to be increased to as much as 100-s. Although this viscosity will probably not prevent deep mud invasion of the core, the added gel strength is needed to surround and hold the gravels in place. Whenever the viscosity of the drilling mud is increased, the pump pressure also has to be increased to circulate the fluid.

A gravel with a sand-silt-clay matrix is considered by many people to be impossible to core without causing considerable disturbance. This type of material is commonly found in glacial till and sometimes occurs in alluvium. It is usually considerably less permeable than sands, so a less-viscous drilling mud can be used (possibly in the 40-s to 50-s range) to core it. This thinner mud will results in less fluid pressure required to circulate the cuttings to the surface; even at a depth of 300 ft, the pump pressure probably is less than that needed at a depth of 100 ft using a 75-s mud. The technique used to core this type of formation is cutting rather than pushing core in unconsolidated sediments. The hardest material in the formation, the gravel, must be cut. If penetration is too rapid, the gravels will be pushed or torn loose from the matrix, completely disturbing and contaminating the core. At the assumed depth of 300 ft, probably enough weight is on the coring bit from the weight of the drill pipe; in fact, if this weight is enough to dislodge the gravel, the holdback should be adjusted to hold up some of the weight of the drill pipe. Rotational speed of the drill should not exceed 200 r/min; if chatter or tearing out the gravels is indicated, the drill should be slowed until the drill pipe is running smoothly. Using the low down pressure on the core bit and relatively low rotational speeds for coring this type of material results in a slow penetration rate (in the range of 1 ft every 15-20 min), because, even though the drill is coring in an unconsolidated formation, it is also coring rock (the gravel particles).

Photographs of cores of the type material just described are shown in figures 14-16; these photographs show that the gravels can be cored by the hydraulic-rotary coring method without tearing up the matrix. As previously mentioned, to obtain good cores of these type materials, the gravels must be cut and not merely pushed up into the core barrel. For example, drive-core samples of this formation were attempted using a 3-in.-diameter drive-core sampling barrel requiring 25 to 30 hammer blows per foot. The cores obtained using the drive-coring method were badly disturbed and consequently of such poor quality that they could not be used for analytical purposes.

The last type of lithology to be discussed under hydraulic-rotary drilling methods of unconsolidated sediments is clay. Clay is the easiest of the unconsolidated sediments to core; it is not readily invaded by drilling fluids, but it poses a problem of drillability using diamond bits. This drillability problem results from lubrication of the mud and the clay particles, causing the diamond coring bit to slide on the surface of the clay instead of cutting it. Techniques for coring clay follow. Because of the ease of forming a filter cake on low-permeability material such as clays, a lower viscosity drilling mud can be used (35-s to 40-s); with clay, use of the thinnest drilling mud possible to clean and lubricate the bit allows faster penetration rates. The coring bit may be rotated as high as 400-500 r/min when coring thick clay beds if the drill pipe rotates smoothly. This higher rotational speed, used in conjunction with a compatible downfeed pressure on the coring bit, also aids in faster coring penetration rates when coring clay. Although use of a thinner drilling mud and a higher rotational speed of the coring bit accomplishes fairly fast penetration rate, one phenomenon occurs in coring clay to impose a restriction on the penetration rate: when the diamond coring bit cuts through a clay, particularly if the clay is somewhat dense, a very close tolerance hole is cut because little or no material erodes. This results in the need for high pump pressures to push the cuttings through this tight restriction; and, even though a thin drilling mud is used, a fluid pressure as high as 100 lb/in.2 might be needed to accomplish this. Although forming a filter cake on clays is easy, and clays do not invade easily, a too-high fluid pressure eventually results in drilling mud invasion of the core; therefore, the penetration rate must be governed by the fluid-pressure buildup. If considerable thickness of clay is cored, drilling-mud viscosity increases, and the drilling mud may have to be thinned.

Carbide-type bits are advertised as useful for coring soft formations. We have experimented with carbide-type bits in all types of unconsolidated formations, and have found them to be unsuitable for coring unconsolidated granular materials other than clay. The carbide-type soft-formation bit with its large carbide inserts rips the grains out, it doesn't cut them. The carbide-type soft-formation bit is useful in coring clays and cutting some soft rock cores such as some sandstones and shales By using the discussed techniques and carefully using the diamond bit, the granular materials can be cut. Diamond bits are expensive, and cutting unconsolidated sediment cores with them is harder on these bits than cutting rock; however, they are the only bits that can be used for this purpose.

One problem occurs in hydraulic-rotary coring of unconsolidated sediments where high-viscosity drilling muds must be used, and the various techniques needed for this type of coring also must be used: no diamond bits are manufactured that specifically meet requirements for this type of coring. The bottom-discharge bit ports are too small to discharge viscous drilling muds that must be used without a resultant undesirable high pump pressure; because as much fluid as possible must be kept away from the inside cutting edge of the diamond core bit, excessive diamond wear occurs. Engineering design is the only way to overcome these problems, particularly the diamond setting and grade of the inside cutting face. Design engineers in the drilling industry are not concerned about this problem probably because few people attempt this type of coring, and the product market is limited. The authors have made adaptations to existing coring bits, such as enlarging discharge port sizes, recessing discharge outlets, and so forth. The diamond setting, however, cannot be changed.

Drilling Fluids

The utilization of a strictly controlled drilling-fluid program whenever hydraulic-rotary coring of unconsolidated material is done is very important. When coring unconsolidated formations, water or other thin drilling- fluid mixtures cannot be used. When coring unconsolidated materials, form a quick, thin filter cake on the hole wall, as well as on the exterior of the core, so that little or no filtrate invasion or erosion of the core occurs. Viscosity of the drilling fluid must be high, and weight must be low (fig. 17). Although some variation of the drilling fluid for unconsolidated formations is permissible, it is only in the high-viscosity ranges. When coring medium sand, we use drilling fluid in the range of 50-s to

MUD WEIGHT, POUNDS/GALLON 9.4 9.8

MUD WEIGHT, POUNDS/GALLON 9.4 9.8

Mud Density Depth

100 200

CLAY, POUNDS/100 GALLONS

Figure 17.—Mud weight versus water loss.

100 200

CLAY, POUNDS/100 GALLONS

Figure 17.—Mud weight versus water loss.

greater than 100-s funnel viscosity, with 75-s being an average. In these viscosity ranges, the drilling fluid does not readily circulate through the bit openings and annulus between the drill pipe and the hole wall; it oozes through. Even with these high viscosities, the weight of the drilling fluid including cuttings weight should not exceed 9 lb/gal in order to prevent invasion of the core. In coring a medium sand, using a 75-s viscosity drilling fluid, the mud weight should not exceed 8.8 lb/gal. These high-viscosity and lightweight drilling fluid restrictions are necessary to obtain uncontaminated core from unconsolidated formations. Drilling muds having these restrictive properties are made using very high-yield (tow solids) bentonites, such as Quick Gel. Low-solid polymers can be added to the bento-nite mixture to hold the weight down, while still increasing viscosity. We have used Revert (fig. 18) for coring unconsolidated materials; its low solids, light weight, high viscosity, and very good lubricating qualities make it a useful drilling fluid. However, the chemical properties of Revert could possibly result in nonpathogenic bacterial contamination of the cores.

PERCENT SOLIDS BY WEIGHT

PERCENT SOLIDS BY WEIGHT

Figure 18.—Comparison between Marsh-funnel viscosity and percent solids, by weight for Revert and bentonite (Universal, 1966, reprinted by permission from Johnson Division, Universal Oil Predicts, Inc.).

Coring Procedures in Rock

Drilling and Casing Through Overburden

In most hydraulic-rotary coring programs for rock coring, the overburden must be cased out or supported so coring can be accomplished without danger of the unconsolidated material caving or falling into the hole (fig. 10 for a schematic view). Casing out or supporting the overburden is usually accomplished in one of the following ways:

  1. A heavy-wall drive casing with drive shoe attached is driven to refusal by means of a heavy drive hammer. If refusal is reached prior to encountering bedrock because of boulders, gravel, stiff clay, and so forth, a drill rod with a chopping bit or cutting bit attached is used to chop or drill the materia] out of the inside of the drive casing, and the cuttings are carried out of the drive pipe with the circulated drilling fluid. This chopping or drilling out may proceed some distance ahead of the drive casing, depending on the types of material drilled. The drill rod is removed from inside the drive casing, and driving of t&e casing proceeds as above by the addition of necessary sections of drive casing; alternate washing out and driving continue until the drive shoe is seated in bedrock.
  2. Some drillers use the drilling-in method of installing casing, particularly if the overburden is fine-grained material. A diamond cutting bit on the bottom section of drill casing, and circulating drilling fluid is used to drill the casing through the overburden and down to consolidated material. If this method is used, a chopping bit may be needed to clean out accumulated debris from inside the drill casing. This drilling-in of the casing is preferred when alternate hard and soft formations are anticipated. An example of this type of coring environment would occur in basalts, where cinder zones or interbedded sediments could be encountered and result in lost circulation or caving, or in limestone, where cavernous conditions or running sands may be encountered. If the drilled-in casing method is used in these situations, the diamond cutting bit on the casing could be used to ream and advance the casing through that section of the rock cored and coring could proceed without the problems of drilling-fluid circulation loss or caving occurring.
  3. A popular method for installation of casing through overburden is to mud-rotary drill through the overburden using a drag bit or roller-cone bit, and a viscous drilling mud to build a filter cake and support the wall of the hole so casing may be installed after the drill pipe is removed. The driller may install any type of casing in this method of casing installation.
  4. This method for supporting the overburden is the same as that described in method 3, except no casing is installed in the hole. The filter cake and hydrostatic head of the drilling fluid in the hole is relied on to hold the overburden in place. This is the least- preferred method for supporting the overburden because of concern about hole caving and the possibility of pebbles or gravel falling to the bottom of the hole. These pebbles or gravels can result in considerable damage to the diamonds in the coring bit.

Drilling Fluids

The drilling fluid used for hydraulic-rotary coring in rock is the most variable component of the system, ranging from water to a prepared viscous, high-gel strength drilling mud. Early literature refers only to the use of water as a drilling fluid; however, modern core-drilling operations include a mud program. The reasons for drilling-fluid variation are:

  1. If casing has been installed through the overburden and the rock to be cored is very dense with low permeability, water loss would not be a problem; so, considering the economics of the situation, the driller may choose not to use a drilling mud. However, a larger- capacity fluid pump would be required to remove cuttings from the hole when a low-viscosity drilling fluid, such as water is used.
  2. If the cored rock is soft, the high-velocity emission of the drilling fluid at the bit will erode the core resulting in poor core recovery. If this occurs, some drilling mud must be added to the drilling fluid to remove the cuttings from the hole after the fluid velocity is lessened to prevent core erosion.
  3. If the rock being cored is extremely permeable, too much water is lost to the formation; and, if water supply is a problem, then enough drilling mud is mixed with the fluid to build a sufficient filter cake on the hole wall to stop or slow down fluid loss. This may result in the mixing of a drilling mud very similar to that used for standard mud-rotary drilling for water wells: 40-50-s funnel velocity, and 8.5-9 lb/gal in weight. If the hole is being cored into formations containing hydrostatic heads greater than that of the fluid column in the hole, then drilling-mud weight will have to be increased to overcome this hydrostatic-hcad diffcrcncc. If added drilling-mud weight is required, it should be accomplished by adding barite and not by simply letting the sand content build to a high level. A high sand content is not only abrasive to the many drill components, but it also causes considerable eroding of the core.
  4. If coring is to progress satisfactorily in so-called heaving or sluffing shales, a designed drilling-mud program is necessary. Fluid enters the shale, causing hydrous swelling or disintegration of the shale, and results in sloughing or heaving. This problem can be prevented by using a low-weight, highly colloidal and viscous drilling mud or polymer that will build a quick, thin, filter cake on the shale, and thereby prevent fluid invasion, which will inhibit swelling and hydrous disintegration.
  5. Design of a drilling-mud program for hydraulic-rotary coring of rock is for the benefit of the hydrologist (or other scientist) requiring core. If the cores are to be used for determinations of any chemical or other waste materials that may be contained in the pore spaces of the rock, water or thin drilling fluid cannot be used because of the danger of flushing out the constituents of interest. This problem is more pronounced as permeability of the rock increases; a designed drilling-mud program may need to be written into the drilling-contract specifications to build a filter cake on the exterior of the core as well as on the interior of the borehole. A photograph of a rotary core of an unconsolidated medium sand obtained by the hydraulic-rotary coring method is shown in figure 19; the filter cake on the exterior of the core prevented or lessened mud invasion of the core. The amount of fluid to circulate for hydraulic-rotary coring is a variable that can only be discussed in general terms. Too little fluid will not permit proper cooling of the bit and lubrication of the tool string; it does not carry cuttings away from the face of the bit fast enough, resulting in bit blockage and high pump pressures. Too much fluid can result in abrasive damage to drilling tools and components, particularly erosion of the waterways and matrix of the bit, and will result in erosional damage to the core. The correct amount of fluid is enough to
Filter Cake Downhole

Figure 19.—Rotary core of an unconsolidated medium sand showing filter cake.

Figure 19.—Rotary core of an unconsolidated medium sand showing filter cake.

perform the needed functions but not so much that it will cause abrasive damage to both the component parts and the core. The driller will learn through experience the proper amount of fluid to use.

Coring Rock

After the overburden has been stabilized using casing or drilling fluid, a diamond core barrel (fig. 20) consisting of a double-tube barrel, a diamondset reaming shell, a nonrotating inner barrel equipped with a core catcher and shoe, and a diamond cutting bit that meets the requirements for cutting the material, is attached to the drill pipe and lowered to a point just off the bottom of the hole. The kelly or spindle rod is connected to the top of the drill pipe by means of a mechanical or hydraulically tightened fastening chuck. Fluid is pumped through the drill pipe to establish circulation. Starting or seating in of the core barrel is accomplished using minimal down pressure and turning slowly. After the core barrel has been started, the down pressure and rotational speed are increased for obtaining optimum penetration rate in the formation being drilled. Down pressure and the rotational speed are variable, depending on the hardness of the rock and the diameter of the core barrel. Although down pressure and rotational speed are primarily dictated by the experience of the drillei; the following observations can be made:

  1. The down pressure and rotational speed working together control penetration rate and bit life. In the case of hard-rock coring, fairly high down pressure can be applied, if the rotational speed is compatible. When coring hard rock using an NW-size core barrel, the core barrel can safely be rotated at 600 r/min using a down pressure of as much as 2,000 lb, assuming the drill pipe is straight and no undue vibrations or chattering of the drill pipe result. If, because of vibration, the rotational speed must be decreased, the weight on the bit must also be decreased accordingly or polishing and dulling of the diamonds will occur. The rotational speed must vary accordingly with the diameter of the core barrel used since this is controlling peripheral speeds of the diamonds. For instance, an NW core barrel rotating at 600 r/min has a peripheral speed of about 460 ft/min, but if coring with an AW core barrel, it would have to be rotated at about 1,000 r/min to reach a peripheral speed of 460 ft/min. In practice, under ideal conditions using straight drill pipe in a straight hole, maintaining proper drilling-fluid conditions, rotational speeds as much as 2,500 r/min for AW core barrels, and 1,500 r/min for NW core barrels, can be achieved in coring hard, competent rock. These speeds can be determined by the competent drillei; who recognizes that any vibration of the drill pipe will cause vibration and chattering of the bit, resulting in bit blockage, broken core, and poor core recovery.
  2. If coring is performed in abrasive, fractured, or friable rock, the rotational speed and down pressure must be decreased accordingly to maintain smooth running of the drill pipe. If the coring bit penetrates these materials too fast, it overdrills, which breaks out pieces of rock and results in bit blockage and poor core recovery. Considering the many variable combinations of rotational speeds and down pressures that are applicable to the coring bit, correct techniques must be practiced by the core drillei; because successful coring projects depend a lot upon the applied experience and intuition of the core driller. After the increment of core has been cut that corresponds to the core barrel length, the core must be broken off at the bottom of the core bit. This is accomplished by steadily retracting the drill pipe and core barrel about 1 ft (we prefer to retract the drill pipe without any rotation). The core

Core barrel (single tube)

Core barrel (single tube)

Upset Tubing

G Design single tube core barrel

G Design double G Design double M Design tube (rigid type) tube (swivel type) core barrel

Figure 20.—Typical diamond core barrels (Acker, 1974, reprinted by permission from Acker Drill Co., Inc.).

G Design single tube core barrel

G Design double G Design double M Design tube (rigid type) tube (swivel type) core barrel

Figure 20.—Typical diamond core barrels (Acker, 1974, reprinted by permission from Acker Drill Co., Inc.).

retainer will slide down slightly in the beveled shoe, imparting an ever-increasing grip on the core, and the core will almost always break off in the hole at or very near the bottom of the core bit. Often, the snap can be felt through the drill pipe as the core breaks. If the core does not break off after retracting the drill pipe the recommended 1 ft, slowly lower the drill pipe again to within about 2 in. of the hole bottom and again retract it about 1 ft. This procedure is recommended only if the core retainer does not catch the core on the first retraction attempt. After the core has been broken loose and prior to pulling the drill pipe for removal of the core, circulate the drilling fluid for several minutes to clear the cuttings from the hole.

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