The bottomhole assembly (BHA) is the component of the drill string located directly above the drill bit and below the drill pipe. The primary component of the BHA is the drill collar. Therefore, it has a signifi cant effect on drill bit performance. The other components of BHA are stabilizers, jars, reamers, crossovers, shocks, hole-openers, and various subs such as bit subs, shock subs (Figure-1). In addition to these main components, the BHA typically consists of a down hole motor, rotary steerable system (RSS), and measurement and logging while drilling tools (MWD and LWD respectively. However, some classify the drill bit as a part of the BHA. It hangs below the drill pipe and provides weight to the drill bit to induce the teeth to penetrate the formation. The functions of BHA are i) to protect the drill pipe in the drill string from excessive bending and torsional loads, ii) to control direction and inclination in directional holes, iii) to drill more vertical and straighter holes, iv) to reduce severities of doglegs, key seats, and ledges, v) to assure that casing can be run into a hole, vi) to reduce rough drilling (rig and drill string vibrations), and xi) as a tool in fi shing, testing, and work over operations.
1. Drill Collars: Drill collars (DC) are heavy, stiff steel tubulars, which have a much larger outer diameter and generally smaller inner diameter than a drill pipe. They are used at the bottom of a BHA to provide weight on bit and rigidity. The primary function of the drill collar is to provide suffi cient weight on bit. The weight of the collar also ensures that the drill pipe is kept in tension to prevent buckling. Figure-2 shows a typical short drill collar and non-magnetic drill collar. It is a pipe with thick walls that are attached to the bottom of the drill string. Therefore it has a signifi cantly thicker wall
Figure-1: Schematics of bottomhole assembly [These bottomhole assemblies are designed for drilling a straight hole (left ) and a directional hole (right)].
Figure-2: Drill Collar
than a drill pipe. Since the drill collars have such a large wall thickness, tool joints are not necessary and the connection threads can be machined directly onto the body of the collar. Drill collars add weight to the bit and make the bit cutters bite into the rock. Normally multiple drill collars are used to add weight. The purposes of drill collars are i) to put extra weight on bit, so they are usually larger in diameter than drill pipe and have thicker walls, ii) to keep the drill string in tension, thereby reducing bending stresses and failures due to fatigue, iii) to provide stiff ness in the BHA for directional control, iv) to stabilize the bit. The weakest point in the drill collar is the joint, therefore the correct make up torque must be applied to prevent failure. The external surface of a regular collar is round (slick), although other profiles are available. The drill collars are normally supplied in ranges two lengths (30–32 ft .). Drill pipe and drill collar come in sections, or joints, about 30 feet long. There are several types of drill collars that are explained below.
Figure-3: Spiral drill collar.
Square Drill Collars: Square drill collars provide the ability to maximize the available weight on the bit when drilling in challenging formations (Figure-3). The square design has a larger cross sectional area than round drill collars, which increases its stiff ness and rigidity to prevent deviation while drilling. The square shape also provides four point stabilization to prevent buckling. Square geometry makes for a stable and stiff BHA ideal for drilling in hard formations requiring all available weight on the bit. The square drill collar achieves four objectives – i) it provides continuous centralization over their length, ii) it maximizes bending resistance (stiff ness), iii) it maximizes torsional damping, and iv) it minimizes axial vibrations. These collars are usually 1/16 less than bit size, and are run to provide maximum stabilization of the BHA.
Spiral Drill Collars: Spiral drill collars decrease the risk of diff erential pressure sticking of the BHA (Figure-3). The spiral drill collars usually have slip and elevator recesses. Stress-relief groove pins and bore back boxes are optional. In directional drilling, spiral drill collars are preferable. The spiral grooves machined in the collar reduce the wall contact area by 40% for a reduction in weight of only 4%, thus reducing the chances of differential sticking. This is likely to happen when the formation is highly porous, a large overbalance of mud pressure is being used and the well is highly deviated. The problem can be overcome by reducing the contact area of the collar against the wellbore.
Non-Magnetic Drill Collar: This type of collar is also called a monel drill collars. Non-magnetic drill collars are usually non-spiral. They are made of a special nonmagnetic steel alloy (Figure-2). They are manufactured from high-quality, corrosionresistant, austenitic stainless steel. Magnetic survey instruments (MWD/Magnetic Single Shots/Multi Shots) run in the hole and need to be located in a non-magnetic drill collar of sufficient length to allow the measurement of the earth’s magnetic field without magnetic interference. Survey instruments are isolated from magnetic disturbance caused by steel components in the BHA and drill pipe. The primary purpose of non-magnetic drill collars is to reduce the interference of the magnetic fields associated with those sections of the BHA, which are both above and below the magnetic compass contained in the survey tool with the earth’s magnetic field. Four critical factors play an important role in selecting non-magnetic collars. These factors are their total length, the location of the survey compass with the non-magnetic collars, the type of material of which the collars are composed, and distinguishing hot spots.
Medium and Large Round Collars: The purposes of large round collars are to provide stiff ness next to the drill bit and to add weight to the BHA. The medium collars add weight to the BHA and reduce ever-present flexure stresses between large collars and drill pipe or other tools of less rigidity than the large collars. Both may be used for jarring weight.
Short Drill Collars: Short drill collars (SDC’s) are also called pony collars. They are simply shortened versions of a steel drill collar. Short drill collars may be manufactured or a steel drill collar may be cut to make two or more short collars. For a directional driller, the SDC and the short non-magnetic drill collar have their widest application in the make-up of locked BHAs. SDCs of various lengths (e.g. 5’, 10’, 15’) are normally provided by the manufacturers.
2. Stabilizers: A stabilizer consists of a length of pipe with blades on the external surface and located above the bit. These blades may be either straight or spiral and there are numerous designs of stabilizers (Figure-4). The blades can either be fi xed on to the body of the pipe, or mounted on a rubber sleeve (sleeve stabilizer), which allows the drill string to rotate within it. According to the blades, stabilizers can be categorized. The function of the stabilizer depends on the type of hole being drilled. However, the functions of stabilizers are i) to control hole deviation, ii) to reduce buckling and bending stresses on the drill collars, iii) to prevent wall thickening, iv) to improve performance of the bit, v) to allow higher WOB since the string remains concentric even in compression, vi) to centralize drill collars in hole and increase stiff ness, vi) increase ability of drill collars to drill smooth straight hole, and vii) to wipe wall of hole to ensure full gage.
3. Jars: This type of tool is used to generate upward or downward loads to free stuck pipes or release fish (Figure-5). There are two types of drilling jars based in its operation: i) hydraulic, and ii) mechanical. Hydraulic jars are stimulated by a straight pull and give an upword blow. Mechanical jars are present at surface to operate when a given compression load is applied and given a downward blow. Jars are usually positioned at the top of the drill collars. Jars are needed when there are sloughing formations, there are sensitive shales, the mud system does not have good suspension properties, and there is expensive equipment in BHA.
4. Roller Reamers: Roller reamers are also called drilling reamers. They consist of stabilizer blades with rollers embedded into surface of the blade. The rollers may be made from high grade carburized steel or have tungsten carbide inserts (Figure-6). The reamer acts as a stabilizer and is especially useful in maintaining gauge hole. It is used to drill a bigger hole. It is also used for any potential hole problems such as doglegs, ledges, and key seats. A roller reamer is a very useful tool for drilling operation, especially used for the function of stabilization in drilling of the abrasive formation.
Figure-6: Roller reamers.
5. Various Subs: There are various subs used at BHA for diff erent purposes. The word “sub” refers to any short length of pipe, collar, casing, etc., with a defi nite function for drilling operations. The following are some of the subs used during drilling string design.
Figure-7: Images of various subs.
Crossovers: A crossover sub is used between the drill string and drill collars. Short joints of pipe to connect two pipes of different sizes or thread types are called crossovers (Figure-7).
Shock Subs: Shock subs are also called vibration dampeners (Figure-7). Th ey are normally located above the bit to reduce the stress due to bouncing when the bit passes through hard rock. The shock sub absorbs the vertical vibration either by using a strong steel spring, or a resilient rubber element. The purpose of shock subs is to dampen the vibration produced by the drill bit and the drill string. It is reasonable to surmise that shock subs prolong the life of drill bits and drill strings and in some cases the rig. They are not as stiff (resistance to axial bending) as drill collars and, because of this, often have limited application in straight hole drilling. In addition, large drill collars may be more eff ective in reducing bottomhole vibrations. Sometimes we use shock absorbers based on application. The functions of shock absorbers are almost same as shock subs, which are to: i) reduce or eliminate vertical oscillations, ii) maintain uniform bit load, iii) increase bit life, iv) increase ROP, and iv) reduce drill collar failures.