Hydraulic Rotary Coring

Hydraulic-rotary coring is commonly referred to as diamond drilling. The name seems to fascinate people, possibly because it was originally used to explore for gold and other valuable minerals. Even today, when coring is used to collect samples for many reasons, this fascination with coring still exists. Coring is the one method of drilling that extracts from the earth an actual piece of the earth that the hydrologist, geologist, engineer or other scientist can feel, see, and study in great detail.

Hydraulic-rotary coring as a drilling method has existed for over 100 years; during that relatively short period of time, the equipment and techniques for obtaining good core quality and recovery, particularly in hard rock, have advanced dramatically, although the basic method has changed little.

Some of the basic techniques involved in hydraulic-rotary coring parallels the techniques of hydraulic-rotary drilling. A drilling fluid is circulated to carry cuttings out of the hole, cool the bit, lubricate the rotating drill pipe, and so forth. Some major components used in hydraulic-rotary coring, such as drilling rigs and mud pumps, may be identical to those used for hydraulic-rotary drilling. However, for most coring operations, equipment can vary considerably; see figure 10 for a typical hydraulic-rotary coring rig and associated equipment. Hydraulic-rotary coring does not require the amount of rig power that is needed for standard hydraulic-rotary drilling, nor does it require a large drilling-fluid circulation pump because much less volume of drilling fluid is needed in coring operations. Drill bits used for hydraulic-rotary drilling also vary considerably from those used in hydraulic-rotary coring. Drill bits used for hydraulic-rotary drilling are designed to cut away all material that is penetrated, whereas the coring bit is designed to cut the perimeter of penetrated materials and allow the central material to remain intact and enter the core barrel.

In the next sections, a detailed description of the problems encountered and techniques used in the hydraulic-rotary coring of unconsolidated formations is discussed because this is the type of material that is most often involved with groundwater studies; most difficulties in obtaining hydraulic-rotary cores also are presented. No detailed description of diamond grades, carat weights, setting of diamonds, etc., is provided. If the reader wants information concerning these subjects, see publications from the many manufacturers of diamond-drilling products, some of which are included in the list of references. This subject is treated in detail in "Diamond Drill Handbook" (Cumming, 1969).

Equipment and Accessories

Before discussing methods of hydraulic-rotary coring, some information on equipment sizes is necessary. For years, the nomenclature used in describing various component parts of hydraulic-rotary coring equipment has been difficult to understand. Bar example, the sizes designated—EX, AX, BX, NX, and HX for bits, reaming shells, core barrels, and flush-coupled casing; E-EW, A-AW, B-BW, N-NW, and H-HW sizes designated for drill rod—are difficult to understand. Because these size designations were not systematic, they could not be retained in the mind of the casual user. In addition, each manufacturer has its own thread design for many of the various components, which further complicated the situation and confused even drill crews. As an example, a drill crew might be beginning a job that requires setting a few hundred feet of flush-joint NX casing through overburden and obtaining core using an NX-core barrel to a depth of 1,000 ft. Very likely, the drill crew might not have enough of one manufacturer's type of casing to completely case out the overburden and would have to use two or three different manufacturer's casing, all with different threads. In addition, the drill crew might have to use drill-rod diameters made by various manufacturers to core the total depth. Coupling the different diameter drill rods requires a crossover sub for each thread change, amounting to as many as six or eight subs and much time loss and extra expense.

Problems resulted from the past lack of hydraulic-rotary coring equipment size standards, and much of this mismatched equipment is still in use today and will continue to be in use. Fortunately, over a period of years, core-drill manufacturers, and users of the equipment, recognized the need for established standards in the United States and other countries, and they developed standards for the inhole components for diamond drilling. These are known as DCDMA (Diamond Core Drill Manufacturers Association) standards and were adopted by the industry in 1970 (fig. 11). Unfortunately, standards have not as yet been adopted for wire-line core barrels. However, where

^Bolt and clevis

Double sheave

4-leg derrick^

Manila rope^ Wire drum hoist

Cathead hoist

Double sheave

4-leg derrick^

Cathead hoist

Controls

Variable displacement water pump

Drilling Rigs Sketch

Foot valve and strainer

  • Overburden fCP/7 " O o
  • Settling pit

Controls

Transmission / Power unit

Variable displacement water pump

<7 Drive pipe

<7 Drive pipe

Drive shoe'

Foot valve and strainer

  • Overburden fCP/7 " O o
  • Settling pit

Drive shoe'

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Diamond casing shoe'

Soil

Diamond casing shoe'

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- Drill rod coupling „Drill rod

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  • Drill rod coupling „Drill rod
  • Core barrel

Reamer „ Diamond bit

Bedrock

Figure 10.-Typical diamond-drilling rig for exploration (Acker, 1974, reprinted by permission from Acker Drill Co., Inc.).

standards do exist they should be used whenever a written drilling and coring specification contract is prepared.

Much of the equipment used for hydraulic-rotary coring of unconsolidated materials is the same as that used for hydraulic-rotary coring of rock. The core-drilling rig, drilling-mud mixing equipment, and drilling-fluid circulating pump may be the same. However, the smaller duplex pumps ordinarily used to circulate the drilling fluid for rock coring operations cannot be used for circulating the high-viscosity muds for coring unconsolidated materials. Most of the combination auger-drilling and hydraulic-rotary drilling rigs that are used by the USGS Water Resources Division for drilling and coring are equipped with positive displacement, rotor-stator pumps that can readily circulate drilling muds having high viscosities. Drill pipe and core barrels can be the same; however, use of wireline-core barrel systems insure greater hole stability while drilling and coring. The wireline-core barrel consists of an outer tube equipped with a diamond-set reaming shell and core bit at the tower end, a retrievable locking inner-barrel assembly equipped with a core retainer at the lower end, a swivel-type ball-bearing head at the upper end of the inner barrel, and a locking head and spear above the ball-bearing swivel assembly. Cutting bits used for coring unconsolidated material are different from those used for coring rock. Most rock-coring bits allow passage of the drilling fluid through the annulus between the core barrel, outer tube, and inner barrel, through the cutting edge of the bit (in direct contact with the core); and referred to as face-discharge diamond-core bits (fig. 12). These bits cannot be used to core unconsolidated or extremely friable formations because of the eroding action of the drilling fluid as it discharges directly on the core. Bits used for coring unconsolidated or friable formations (fig. 13) are of the recessed, bottom-discharge types, and because of the recessed waterway the drilling fluid does not come in contact with the core, thereby practically eliminating core-erosion problems.

The recessed-bottom-discharge bits provide another feature to help eliminate core erosion. As the drill pipe rotates, the fluid tends to be thrown outward, not downward, thereby preventing washing and contamination of the core; however this feature is more costly to the life of the diamond-coring bit because little fluid gets to the inside cutting edge of the bit, resulting in excessive diamond wear of that section of the bit. When a bottom-discharge diamond-coring bit is used, adjustment of the inner barrel is very important. The core barrel is taken apart and a skirted, special-length, pilot core-retainer inner-tube shoe (fig. 13) is attached to the end of the inner barrel. The inner barrel must be adjusted to fit closely to the bottom of the cutting bit so the drilling fluid will be directed out through the recessed ports and not out through the face of the bit. This inner-tube adjustment is made by loosening the holding nut on the inner-barrel spindle assembly (threaded rod) and turning the threaded spindle rod into or out of the swivel assembly. The amount of adjustment is made by trial and error; the adjustment, reassembly, disassembly, and adjustment may have to be done several times to accomplish the proper clearance (about 1/16 of one inch) to avoid slack in the bearings, particularly after use. To check the inner tube for proper clearance, the assembled barrel is placed horizontally and the inner tube is pushed up. The core will force the inner tube up to this position when coring is being done. The inner tube should not rest tightly against the cutting-bit shoulder, or the inner tube will turn with the outer barrel. After proper adjustment of the inner tube, the spindle nut must be retightened so the adjustment cannot change during coring operations.

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