the drillstem

7/27/2019 The Drillstem

1/12


2/12

These drill collars perform three important functions:

They provide weight on the bit while holding the drillpipe in tension.

They act as a pendant to keep the hole straight.

They maintain rigidity to drill a straight hole.

Drillpipe should never be subjected to high torque or compressional forces, since it could easily"twist off". (The exception to this rule is horizontal drilling, where the pipe is run in compression inthe lateral hole section. In situations like this, heavy weight pipe specifically designed forcompressive service is used). The heavy weight of the thick-walled drill collars provides the forcenecessary to direct the bit through the rock, while keeping the drillpipe above in tension ( Figure 3) . The collars also stabilize the drillstem and cause it to drill a continuing straight extension of the

previously drilled hole.

Included in the drillstring are a variety of shorter lengths of pipe (or "subs") thatperform a variety of tasks:

Crossover Subs: These subs are designed to link different sizes or types of drillpipe and

collars, which may have different threads and tool joint configurations.

Shock Sub: This sub is run behind the bit with a spring or rubber cushions,both of which absorb the impact of the bit bouncing on hard formations.

Bumper Sub: This sub is specially designed with a telescoping joint to help

maintain a constant weight on the bit and still keep the drillstring in tensionwhen drilling from a floating rig that is moving up and down. (The "motion

compensation" equipment on floating rigs also helps maintain a stationarydrillstring.)

2
http://figurewin%28%27../asp/graphic.asp?code=4752&order=2%27,%272%27)http://figurewin%28%27../asp/graphic.asp?code=4752&order=2%27,%272%27)


3/12

Stabilizers: A stabilizer is a sub with "blades" to keep the drill collars

centered in the hole and maintain a full gauge hole. Often the blade surfaces

have tungsten carbide edges or inserts ( Figure 4, (a) spiral and (b) straight

blades).

Bit Sub: This is a very short sub with a box on both ends. Remember, a drill collar

or drillpipe joint is always run with the pin pointing down, and the bit, of course, has

a threaded pin pointing up. This sub allows the bit to be joined to the collar or drill-

pipe above.

There are other specialized tools that may be incorporated into the entire arrangement of drill

collars and subs located at the bottom end of the drill-stem. A particular "bottomhole assembly"(BHA) will be designed by the driller and drilling engineer to provide for the most efficient drillingof the well.

Drill Bits

The drill bit is generally the most critical component of the drillstem. Bit technologyhas undergone more advancement since the early days of rotary drilling than any

other element of the drilling system. There are several types of bits, which we shalldiscuss in turn:

drag bits

rolling cutter bits

diamond bits

special purpose bits

3
http://figurewin%28%27../asp/graphic.asp?code=4752&order=3%27,%273%27)http://figurewin%28%27../asp/graphic.asp?code=4752&order=3%27,%273%27)http://figurewin%28%27../asp/graphic.asp?code=4752&order=3%27,%273%27)


4/12

DRAG BITS: The oldest of the rotary bits, the drag bit utilizes flat cutter blades toscrap away the rock. These bits, though relatively simple and inexpensive, and still

used for drilling soft, shallow formations, have been largely replaced by other types.

ROLLING CUTTER BIT: This bit, which is also called a roller cone bit, three-cone bit,or rock bit, is the most commonly used today and comes in a variety of designs

( Figure 1 and Figure 2 ).

The cones of this bit are designed to individually roll as the bit turns on the bottom of

the hole.

4
http://figurewin%28%27../asp/graphic.asp?code=4753&order=0%27,%270%27)http://figurewin%28%27../asp/graphic.asp?code=4753&order=1%27,%271%27)http://figurewin%28%27../asp/graphic.asp?code=4753&order=0%27,%270%27)http://figurewin%28%27../asp/graphic.asp?code=4753&order=1%27,%271%27)


5/12

While the cones distribute the weight of the drill collars, their teeth bite into the rock,gouging and scraping away the cuttings, which are then carried to the surface by the

circulating mud. The toothed cones "mesh" together to provide a self-cleaningaction; this is furthered by the directed flow of drilling mud from nozzles, or jets,

through which the fluid passes with high velocity. Roller cone bits vary according tothe type and configuration of their teeth and the type of bearing used to join the bit

body and cones.

Steel-tooth (also called milled-tooth) bits have long, widely spaced teeth for soft

formation models, and shorter, closely set teeth for harder formation types. Theteeth of insert bits also vary in length depending on use, but are made of extremely

hard tungsten carbide, and inserted into the steel cones. The bearings may benonsealed, where the drilling fluid lubricates the rollers, or sealed, where a rubber

seal isolates a high performance lubricant within the cones ( Figure 3 ,

cutaway of nonsealed bearing bit with ball and roller bearings inside cone , Figure 4 ,

5
http://figurewin%28%27../asp/graphic.asp?code=4753&order=2%27,%272%27)http://figurewin%28%27../asp/graphic.asp?code=4753&order=3%27,%273%27)http://figurewin%28%27../asp/graphic.asp?code=4753&order=2%27,%272%27)http://figurewin%28%27../asp/graphic.asp?code=4753&order=3%27,%273%27)


6/12

cutaway of sealed bearing bit with bearings lubricated by internal grease reservoir

and Figure 5 , cutaway of high performance insert bit showing roller bearingsreplaced by journal bearing surfaces, with bearings lubricated by grease from an

internal reservoir).

In the case of journal bearings, a precision machined hard metal alloy surface

replaces the cylindrical roller bearings. Journal bearings are designed to be strongerand more wear-resistant, for maximum service life.

6


7/12


8/12

The PDC surface is self-sharpening as it wears away, continually presenting a freshedge. (Madigan and Caldwell 1981).

SPECIAL BITS: Other bit-type tools have been designed for special purposes, notablyhole openers and underreamers. These tools are run above a bit to maintain orenlarge the hole size. Under reamers have collapsible arms that are held open by the

pressure of mud circulating through the drillstem. These arms enable them to

enlarge the bottom of the hole and then be retrieved through the smaller diameterupper portion of the hole.

Whatever type is used, all bits perform their job with the help of the drilling fluid,which cools the cutting surfaces and circulates rock chips from underneath. Drilling

8


9/12

fluid technology has become increasingly sophisticated and deserves treatment as aseparate element of the drilling system.

Drilling Fluids

Most wells are drilled with clear water for faster penetration rates, until a depth is

reached where hole conditions dictate a need for a fluid with special properties. Theaddition of clay and chemicals to the water permits the adjustment of viscosity,

density, and other properties to improve hole cleaning and prevent sloughing shale,lost circulation, formation flow, and formation damage.

In most cases, the circulating fluid utilized in a rotary drilling operation is a water-

based mixture of clays, suspended solids, and chemical additives. In some cases, oilis added to the fluid or the entire system may be converted to an oil-based mixture.

A small percentage of wells are drilled with air or foam as the circulating fluid forpart of the drilling operation. In any case, the properties of the drilling fluid must

enable it to perform the following functions:

control subsurface pressures

remove cuttings from the hole

cool and lubricate the drillstem

aid formation evaluation and productivity

CONTROL SUBSURFACE PRESSURES: This function is performed by adjusting the density ofthe drilling fluid so that a balance is maintained between the hydrostatic pressure imposed by the

column of drilling fluid and the pore pressure of the formations being drilled ( Figure 1 ).

9


10/12

This balancing act can be difficult at times. If the formations are abnormally pressured, anunexpectedly high pore pressure can cause a flow of formation fluid, or "kick," into the well-bore.This further lightens the already underbalanced mud column, and can lead to a loss of control ofthe well unless checked. On the other hand, a mud column that has been "weighted up," or madeheavier (denser) by mineral additives, such as barite, may control the pressures in anoverpressured formation but impose such a force on a normally pressured formation that itfractures, allowing unobstructed flow of drilling mud into the lower pressured formation. Lostcirculation such as this can also occur when naturally fractured or cavernous formations aresuddenly encountered. The loss of drilling fluid allows a drop in the fluid column (meaning a dropin pressure) that permits other formations to flow into the well-bore. One can easily see that these

situations can quickly go from bad to worse. This chain of events must be prevented from everhaving a chance to get started.

While the drilling fluid density allows it to control pressures, other properties ofdrilling mud allow it to form a protective filter cake of clay particles on the wall of the

hole, preventing excess fluid loss (filtrate) into permeable formations and preventing

sloughing, or caving-in, of the sides of the hole. Hard rock formations that do nothave a tendency to cave in can be drilled with air, foam, or water as a drilling fluid.

Mud density is measured by means of a mud balance; a simple scale commonly

graduated in pounds per gallon (ppg) or pounds per cubic foot (ppcf) increments. Afilter press is used to force drilling mud against a filter paper, forming a sample filter

cake and measuring the amount of filtrate (in cubic centimeters) that is squeezedout of the mud during a measured period of time.

REMOVE CUTTINGS FROM THE HOLE: Viscosity is the drilling fluid property that isimportant to this function. Mud must have the proper viscosity to lift the rock

cuttings out from underneath the bit and carry them up the annulus to the surface

( Figure 2 ).

10


11/12

In addition, a drilling fluid must exhibit sufficient gel strength to hold the cuttings insuspension when circulation stops, and prevent them from settling to the bottom of

the hole, collecting around the bit, and making the pipe stick in the hole. However,the mud must also liquify upon resumption of pumping, and it must release the

cuttings easily at the surface. Viscosity is generally determined with a Marsh funnel,which measures the time it takes for a certain volume of mud to flow through an

orifice. Gel strength is measured with a viscometer, which shears the mud betweenmetal cylindrical surfaces.

While not a property of the drilling fluid itself, the velocity at which the fluid iscirculated is also important to the proper performance of the hole-cleaning function.

Annular velocities between 100 and 200 ft/min (.5-1.0 m/s) are usual. That's about

normal walking speed for most people. Annular velocities can be used to calculatethe time at which a given sample of cuttings was removed from the bottom of the

hole, which can be correlated to depth.

COOL AND LUBRICATE THE DRILL-STEM: This function is primarily per-formed at thebottom of the drillstem, where the bit is forced against the bottom of the hole and

rotated. Force applied to the bit may range from 10,000 to 100,000 Ib (45 to 445kN), and rotating speed may range from 50 to 200 rpm! This combination of weight

and speed creates frictional heat within the bit that must be removed by the

circulating fluid to prevent rapid wear. Lubricants added to the mud system can helpreduce friction at the bit, between the drillstring and hole, and within the drillstringitself, where frictional pressure losses can require high pump pressures. Air or foam

drilling fluids are particularly efficient at performing this cooling function.

AID FORMATION EVALUATION AND PRODUCTIVITY: Drilling fluid properties should

be monitored to insure that the interaction between mud and formation does notprevent the formation from being easily evaluated or produced. For example, oil-

based muds make it difficult to evaluate potential producing horizons through

11


12/12

sidewall samples and cuttings analyses, and with mud-gas-detecting instruments.Both oil-based and salt water-based drilling fluids can make some electrical logging

operations difficult.

Some formations can be irreparably damagedby the invasion of mud and mudfiltrate. Oil-based mud in gas zones and fresh water-muds in zones containing water-

sensitive clays, are examples of permeability-damaging situations. When necessary,the drilling engineers and geologist must decide what adjustments to the mudsystem are important to the overall success of the drilling operation.

Density, viscosity, gel strength, lubricity, filter cake formation; all of these properties are important to the proper functioning of the drilling fluid. A wide variety of

chemical additives are available to help control these properties. Some commonexamples are

Bentonite: clay added to fresh water to improve the properties of a natural mud resulting

from native clays;

Attapulgite: clay added to salt-water-based muds;

Barite : barium sulfate mineral with high specific gravity added as weight

material; lead and iron compounds are also used for this purpose;

Chrome lignosulfonates: modern chemical thinners used to decrease

viscosity;

Polymers: long chain molecules that act to increase viscosity;

Lost circulation materials: any of a variety of items that act to plug porous,

permeable zones, including walnut hulls, shredded cellophane, mica flakes,and vegetable fibers.

12

the drillstem

Documents