Bottom Hole Assembly Notes for undrstanding By Dr. Santosh Gupte
BOTTOM HOLE ASSEMBLIES DEFINITION OF BHA A bottom hole assembly (known as BHA) is a component of a drill string. A BHA resides in the drill string above the drill bit and below the drill pipe. The primary component of the BHA is the drill collar. The following figure shows the possible components of a BHA and their typical location within a BHA. PURPOSE OF BHA 1. Protect the drill pipe in the drill string from excessive bending and torsional loads, 2. Control direction and inclination in directional holes, 3. Drill more vertical holes, 4. Drill straighter holes, 5. Reduce severities of doglegs, key seats, and ledges, 6. Assure that casing can be run into a hole, 7. Increase drill bit performance, 8. Reduce rough drilling, (rig and drill string vibrations), 9. as a tool in fishing, testing, and work over operations, 10. Not to place weight on the drill bit TYPES OF BHA'S The "SLICK" BHA is composed only of drill collars. It is least expensive and perhaps carries the least risk in regard to fishing and recovery. The "PENDULUM" BHA is designed to drill holes more vertically and to drop inclination in inclined holes. Lubinski and Woods published tables and charts to locate the lowest most stabilizers in the BHA. Most BHA theories which were intended for vertical holes apply to holes which are inclined 20 degrees or less. The "PACKED" BHA is designed to drill straight holes and to reduce the severities of doglegs, key seats, and ledges. It provides the highest assurance that casing can be run into a hole. The theory which supports the packed BHA was developed by Roch. A packed BHA can be expensive and perhaps carries the highest risk in regard to fishing and recovery. The "DIRECTIONAL" BHA is designed either to turn the hole to a chosen inclination and direction or to maintain a course selected for the hole. The directional BHA is based on the principles of levers and fulcrums. The "FISHING, TESTING, and WORKOVER" BHA is designed to assist the many tools found in these areas.
Many theories and practices apply equally to all four types of BHA's while others are very restricted to one of the types. The performance of BHA's are affected most by the rugosity (hole enlargements) of the hole. Other factors are 1. Formation dips 2. Torque while drilling 3. Drag during trips 4. Stabilizer blade wears 5. Drill collar OD wear 6. Mud filter cake COMPONENTS OF BHA Drill Bit: Technically, the drill bit is not a component of the BHA; however, it does generate and sends axial and torsional loads to the BHA. Its basic function is to cut rock at the bottom of the hole. Junk Sub: A narrow trough which exists in the junk sub is designed to collect small pieces of metal; such as, balls and rollers from bearings and broken bit teeth. y Shock Sub: 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. Shock sub are not as stiff (resistance to axial bending) as drill collars and, for this fact, often have limited application in straight hole drilling. In addition, large drill collars may be more effective in reducing bottom hole vibrations. Roller Reamers: A reamer serves two functions: (1) it cuts the wall of the hole to gauge and (2) it centers the BHA in the hole at its location. Reamers with 3 and 6 rollers are most popular. The rollers are on bearings and they contain tungsten carbide inserts which are similar to those placed in drill bits. Roller reamers are used in hard rock where blade reamers cause excessive drill string torque. Stabilizers: Stabilizers center the BHA in the hole at their location. In soft rock, a rotating blade type can effectively ream a hole to gauge. There are two basic types: rotating and non-rotating. The blades of the rotating type may be spiraled around the body of the stabilizer or aligned vertically and attached by welding, with a sleeve, or by pins. The rubber blades of the non-rotating type are aligned with the vertical axis of the body of the stabilizer. The rubber blades are popular because they are easily washed-over and do dig into the wall of the hole causing enlargements. Short stabilizers can pivot in the hole and allow bending moments to be transmitted to adjacent BHA components. They act as fulcrums. Long stabilizers or stacked short ones do not pass bending moments and align themselves in the hole. A BHA which does not transmit bending moments is called a "locked-up" BHA. Square Drill Collars: Square drill collars accomplish four goals: (1) they provide continuous centralization over their length, (2) they maximize bending resistance (stiffness), (3) they maximize torsional damping, and (4) they minimize axial vibrations. On the down side, they are expensive to buy and to maintain. They usua1ly create high rotary torque. They grind
drill bit cuttings and cavings to fines which radically increases mud consumption. They are difficult to fish. Note that an 11.2 inches round collar has the same stiffness as a 12.125 inches square collar. Spiral Drill Collar: Spiral drill collars reduce the risk of differential pressure sticking of the BHA. About 4% of the weight of the drill collar is lost because of the machining of the spirals. Thus, the linear weight per foot of an 8 by 3 inches spiral collar is 141 ppf rather than 147 ppf. Non-magnetic Drill Collars: 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 the below the magnetic compass contained in the survey tool with the earth's magnetic field. The non-magnetic collars reduce this type of interference by moving the BHA sections away from the survey compass. There are four critical factors in selecting non-magnetic collars: (1) their total length, (2) the location of the survey compass with the non-magnetic collars, (3) the type of material of which the collars are composed, and (4) distinguishing "hot spots". "Hot spots" are zones of high magnetic field strengths within the material of the collars. These zones are detected with a magnetic permeability probe. The measurements are recorded on a strip chart versus distance into the collar. Hot spots can affect compasses by as much as 4 degrees (2 degrees is common). A "cold" collar will affect a compass by less than 1/4 degree over its entire length. Non-magnetic collars may be manufactured from many types of material. The selection is based primarily on the corrosion resistance of the material. The most common nonmagnetic material is stainless steel while monel, which is 60% nickel and 30% copper, is seldom used. A practical method of ascertaining the best location for the survey compass and the requisite non-magnetic collar length is to run a BHA into the drill hole and pull a compass through the non-magnetic collars. The portion of the nonmagnetic collars which do not show the effects of the fields of the BHA may be removed from the BHA. Medium and Large Round Collars: The purposes of large round collars are to provide stiffness next to the drill bit and 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. Heavy Weight Drill pipe: Heavy weight drill pipe is small drill collars with drill pipe tool joints. They serve the same purposes as medium weight collars. They may be operated in a buckled mode while drilling or a part of a fishing BHA.
Key seat Wiper: The purpose of key seat wipers is to ream the "key" section out of the wall of the hole. Key seat wipers are run in the BHA or the drill pipe. The outside diameter of the cutting structure of the key seat wipers are commonly selected as 1/8 to 1/2 inch larger than the collars or tool joints in which they are installed. Key seat wipers are either single or double acting; that is, the blade section which slides on a mandrel reams either in the up or down position if single acting but not both. The double acting type act if the blade section is up or down, but not if the section is floating between up and down because most rigs cannot raise and rotate the drill string simultaneously, the single acting wiper which reams in the down position is required. Bumper Sub: Bumper subs are used to counter the heave of floating drilling vessels by permitting the extension and contraction of the drill string. In this use the bumper subs are placed between the BHA and the drill pipe. They are also used in fishing operations in a manner similar to that of jars. Safety Joint: Safety Joints are rarely used in a drilling BHA. Their purpose is to provide a means of easily releasing a BHA.
