• The lower portion of the drillstring, consisting of (from the bottom up in a vertical well)
the bit, bit sub, a mud motor (in certain cases), stabilizers, drill collar, heavy-weight drillpipe,
jarring devices ("jars"), and crossovers for various thread forms.
• The bottomhole assembly must provide force for the bit to break the rock (weight on bit),
survive a hostile mechanical environment, and provide the driller with directional control of
• Oftentimes the assembly includes a mud motor, directional drilling and measuring
equipment, measurements-while-drilling tools, logging-while-drilling tools and other
• A bottom hole assembly is a component of a drilling rig.
• It is the lowest part of the drill string, extending from the bit to the drill pipe.
• The assembly can consist of drill collars, subs such as stabilisers, reamers, shocks, hole-openers, and the bit
sub and bit.
Bottom hole assembly
• A bottom hole assembly (BHA) is a component of a drilling rig.
• It is the lowest part of the drill string, extending from the bit to the drill pipe.
• The assembly can consist of drill collars, subs such as stabilisers, reamers, shocks, hole-openers, and the bit sub
• The BHA design is based upon the requirements of having enough weight transfer to the bit (WOB) to be able to
drill and achieve a sufficient Rate of Penetration (ROP), giving the Driller or Directional Driller directional control
to drill as per the planned trajectory and to also include whatever Logging While Drilling (LWD) / Measurement
While Drilling (MWD) tools for formation evaluation.
• As such BHA design can vary greatly from simple vertical wells with little or no LWD requirements to complex
directional wells which must run multicombo LWD suites.
• Prior to running a BHA most oilfield service providers have software to model the BHA behaviour such as the
maximum WOB achievable, the directional tendencies & capabilities and even the natural harmonics of the
assembly as to avoid vibration brought about by exciting natural frequencies.
Drilling String Design
• The drill string components should be designed to deliver sufficient weight to the Bit and provide sufficient
torsional and tensile strength to withstand the vigorous and dynamic conditions of drilling.
• The components should also withstand burst and collapse pressure loads and be designed to minimize hole
• There are many factors that must be considered in its design such as:
o Total depth
o Hole size
o Mud weight
o Over pull
o Bottom Hole Assembly
o Hole angle
o Pipe weights and grades
o Corrosive environment
o Ability to fish tools out of hole
• The BHA may contain the following items such as:
o i) drill collars,
o ii) stabilizers,
o iii) jars,
o iv) reamers,
o v) shock subs, and
o vi) bit sub.
• In addition, the drill string may include:
o shock absorbers,
o junk baskets,
o drilling jars,
o reamers, and
o other equipment.
• Special tools in the BHA or drill pipe, which may include monitor-while-drilling (MWD) tools, and drill stem-testing
• Drill bit at the lower end of the drill string.
• Stabilizers are placed above the bit to control the direction in which the drill bit penetrates the formation.
• Downhole motors with bent subs and rotary-steerable tools are also used for controlling the direction in which the
• There are three types of BHA configurations.
• These configurations addressed are usually concerned with the use or layout of drill collars, heavy weight drill
pipe and standard drill pipe.
o Type 1, standard simple configuration, uses only drill pipe and drill collars.
In this instance the drill collars provide the necessary weight on the bit.
o Type 2 uses heavy weight drill pipe as a transition between the drill collars and the drill pipe.
Weight on bit is achieved by the drill collars.
o Type 3 uses the drill collars to achieve directional control.
The heavy weight drill pipe applies the weight on the bit. Such a layout promotes faster rig floor BHA
It may also reduce the tendency for differential sticking.
• In most cases the above three types of configurations usually apply to straight/vertical wellbores at most low to
medium angle wellbores.
• For high angle and horizontal wellbore careful weight control of the BHA is a must.
• In this instance the weight may be applied by running the drill pipe in compression in the high angle section.
• The high angle may help to stabilise the drill pipe allowing it to carry some compression.
• Rotary assemblies are commonly used where formations are predictable and the rig economics are an issue.
• In such an assembly the weight of the drill collars gives the BHA the tendency to sag or flex to the low side of the hole, collar
stiffness length and stabiliser diameter and placement are engineered as a means of controlling the flex of the BHA.
• This will bring about the desired hold, build or drop tendency.
• The ability to vary the directional tendency of the assembly comes from varying the weight on the bit. A fixed assembly has
only one directional tendency.
• The weight on bit allows you to tune that tendency.
• The bottom-hole assembly (BHA) can be:
o slick with no stabilisers
o a pendulum bottom-hole assembly for reducing the deviation of a well
o packed with stabilisers for a straight hole / a short lock assembly
o a fulcrum for increasing the deviation of the well / a long lock assembly
• Bottom-hole assemblies are also described as
o specialized, such as articulated,
o steerable, and
o oriented or non-oriented.
o Fulcrum / Long Lock
• This assembly is used to build angle. The assembly usually incorporates a near bit stabiliser. Behind this would be a selection of
drill collars and Heavy Weight Drill Pipe (HWDP).
• The length of the section AFTER the near bit stabilizer would determine the extent of the angle build rate.
• After the appropriate length of pipe after the near bit stabiliser you would place a drill string stabiliser.
• In short the longer the gap between the near bit and the drill string stabiliser the greater the angle building rate.
• Care must be taken to not have this section too long as the pipe may sag too much and rub against the borehole wall.
• This could result in key seating and pipe digging into the borehole wall.
Stabilization /Short Lock
• This assembly is usually used to maintain borehole angle.
• This assembly is very rigid allowing little movement of the bit.
• Such an assembly would mean that the stabilisers are closely packed: a near bit stabiliser within 0–30 feet of the
drill bit, and two more spaced 30 and 60 feet beyond it.
• If a short drill collar is used, then the stabilisers can be even closer together.
• The shorter the distance between the stabilisers means that the drill collars bend less and the weight on bit
(WOB) pushes directly on the bit, hence maintaining the angle.
• A pendulum assembly is used to reduce the angle build rate or reduce well-bore angle.
• In this assembly there is no near bit assembly. The front portion of the (BHA) is allowed to hang as a result of its
• In such a case the first stabiliser is placed 30–45 feet behind the bit.
• This hanging means that there is a force acting on the low side of the hole, which causes the deviation.
• In the case of a straight hole then the bit simply continues downward.
• A stabiliser is used within a column of drill collars. They help guide the bit in the hole. They play a major part in directional drilling as
it helps determine the well-bore path and angle.
• It is used to
o equalize the load on the bit;
o prevent whirl of the lower assembly;
o minimize bit walk;
o minimize bending and vibrations that cause tool joint wear;
o prevent collar contact with the sidewall of the bore-hole;
o minimize key-seating with differential pressure;
o restrict lateral movement of the lower BHA as such they lower the strain on the drill collar and BHA connections.
• Solid stabilisers have no moving or replaceable parts. The blades and the mandrel can be one piece (integral) or welded to the
mandrel (weld-on/welded blade). The blades may be either straight or spiral. The working surface can be impregnated with tungsten
carbide or diamonds inserts.
• Replaceable blade stabilisers can maintain full gauge stabilization, but their blades can be changed with tools no machining or
• Sleeve type stabilisers have replaceable sleeves that can be changed in the field. The sleeves are either rotating or non-rotating.
• Reamers are stabilisers that have cutting elements embedded on their fins, and are used to maintain a gauged well-bore. They can be
used to drill out doglegs and key-seats in hard formations. Due to the cutting ability of the reamer the bit does less work in
maintaining well-bore gauge and more work drilling.
• An underreamer is used to enlarge the well-bore, usually beginning at some point under the surface. It does this by utilizing
expandable cutters that only deploy at the designated time or depth. It is not to be confused with hole opening which occurs from
the surface and in most cases the hole opener tool is of a fixed diameter.
• The underreamer utilises an increase in mud pressure or flow rate to deploy the expandable cutters. A corresponding pressure drop
across the tool would indicate that the tool has fully deployed. 9
Bottom Hole Assembly BHA Drilling Components:
Heavy Weight Drill pipe ( The Top most BHA Component)
• Heavy Weight Drill Pipe Specs is important to
understand as it is the entrance of the
heavyweight drill pipe design.
• In this article, we will discuss its specs and how to
select in the drill string design.
• In addition to that, we will talk about its main
• you can also download the specs sheets from
any size of heavy-weight drill pipes.
Drill Collar (a predominant component of the BHA components)
• Drill-collar (D/C) are large diameter-small bore
steel pipes that possess a great weight and great
• While designing BHA, drilling engineers place
D/Cs above the Drilling Bit as D/Cs are which is
the predominant component of the Drilling
Bottom Hole Assembly BHA components.
• To select the proper D/C you will have to have all
information about Drill collar Weights, Size, Specs,
Definition, lengths & Types.
• Drilling stabilizer types & design is the main subject of this article.
• First, we will introduce to you the definition of stabilizer as one of the bottom hole assembly BHA
• Then we will move to the stabilizer application.
• And finally, we will talk about its types.
•Shock subs, also known as vibration dampeners, are used to absorb vibrations and bit shock loads in drill collar strings.
•They usually feature long integral elastomeric elements, which serve to transmit torque and weight to the bit simultaneously.
•When drilling is being carried out at shallow depths, intermittent hard and soft streaks, along with broken formations, can
transmit vibrations to the surface, where they are easily detected.
•With greater depths, though, these vibrations might not be detected because the drill string cushions them.
•However, they will still cause damage to the bit, as well as bottom hole assembly components and the drill string.
Drilling jar placement design is important while drilling.
Jars are one of the bottom hole assembly BHA components that generate upward or downward impact loads to free stuck
pipe or release a fish.
Drill Pipes ( Not Bottom Hole Assembly BHA Components)
Drill pipes are not one of the bottom hole assembly BHA components .
It as a drill string component.
•Drill Pipe OD
•Drill Pipe Nominal Weight
•The Pipe Grade
•The tool joint and type of upset
•Drill Pipes Tool Joints Types / Upset Types
•The type of connection
•The classification of drill pipe is based on wear.
•API Drill Pipes Classifications
Cross over Sub
• Crossover sub is a mechanical part with threaded connections. It is used to connect threaded
components that have different sizes or have different thread characteristics.
• Crossover subs are typically found in the drill string between the drill pipe and drill collars or at
other points in the drill string.
Down Hole Motors
• These motors provide additional power to the drill bit by converting the energy and flow of the drilling fluids to
create additional rotation, and torque using a cavity pump system.
• This tool improves efficiency and power as it is connected directly to the bit. The housing of the motor also
contains a fixed bend angle that can be pointed in a required direction to make the bit steer.
• This will replace conventional down hole motor directional tools to help control wellbore trajectory in directional
• There are many different designs of tools but they all sit directly behind the bit and either push or point it in the
required direction to make it steer.
• A rotary steerable tool is more expensive than a down hole motor but offers more precision and control.
MWD and LWD Tools
• These tools are drill-collar-like components containing complex electronics and sensors.
• MWD tools measure and record the physical properties of the drilling process while LWD tools use electronics
and sensors to log the properties of the rock and the drilling environment.
• These tools communicate this data to the drilling team on surface in real time so that they can adjust the drilling
process to achieve objectives.
• As mentioned earlier, the drilling parameters, and more importantly the direction of the wellbore, determine the
necessary tools that make up the bottom hole assembly.
Bit side force
• The bit side force is the most important factor affecting the drilling tendency.
• The direction and magnitude of the bit side force determine the build, drop, and turn tendencies.
o Drop assembly: When the bit side force acts toward the low side
o Build assembly: When the bit side force acts toward the high side of the hole
o Hold assembly: When the inclination side force at the bit is zero
• Bit tilt angle: The angle between the bit axis and the hole axis and affects the drilling direction because a drill
bit is designed to drill parallel to its axis.
Factors that determine drilling tendency of the bottom hole assembly (BHA)
• Bit side force
• Bit tilt
• Formation dip
• Formation Rock Properties
• The BHA design objective for directional control is to provide the directional tendency that will match the planned trajectory of the
• A conventional rotary drilling assembly is normally used when drilling a vertical well, or the vertical or tangent
sections of a deviated well.
• When using a Rss Rotary steerable assembly or Non-Rotating Steerable Downhole Mud Motor Assembly in a
deviated well it is of course possible to drill the tangent sections of the well with the the steering action.
• But In this article we introduce how to use the Rotary Directional Drilling Bottom Hole Assembly BHA to Build (
Fulcrum Assembly), Drop (Pendulum BHA) or Hold (Packed BHA).
• The BHA of the conventional assembly can also be designed in such a way as to result in an increase or decrease
in the inclination of the wellbore but it is very difficult to predict the rate at which the angle will increase or
• The tendency of a Conventional Bottom Hole Assembly BHA Components & Design to result in an increase or
decrease in hole angle is a function of the flexibility of the BHA & WOB .
• Since all the Drill String Design Components are flexible to some degree the BHA will bend when weight is applied
to the bit.
• It is difficult to predict the impact of the above variables on the rate at which the angle will increase or decrease.
Three types of directional drilling BHA assemblies
There are three basic types of assemblies used in directional drilling.
o Building Assemblies, intended to increase hole inclination.
o Dropping Assemblies, intended to decrease hole inclination
o Holding Assemblies. intended to maintain hole inclination.
• A building assembly may not always build angle.
• Formation tendencies may cause the assembly to drop or hold angle.
Fulcrum Assembly As A Building Directional Drilling Bottom Hole Assembly BHA
• The principles behind a Fulcrum Assembly is to place a reamer near the bit (check also Drilling Bits Types) and apply a high
• When WOB is applied, the Drill collars above the reamer will tend to bend against the low side of hole, making the reamer
act as a fulcrum forcing the bit upwards.
• The rate of build up depends on WOB, size of collars, position of reamer and stabilization above the reamer.
• This building fulcrum assembly uses a stabilizer acting as a fulcrum in the directional drilling bottom hole assembly BHA to
apply side forces to the bit.
• The magnitude of that force is a function of the distance from the bit to the tangency point.
• An increase in bit weight and/or decrease in drill collar stiffness will increase the side force at the bit increasing the rate of
• The strongest building fulcrum assembly consists of one stabilizer placed 3 to 6 feet above the bit face with collars and drill
pipe above the stabilizer (a second stabilizer can be placed 90 feet above the bit).
• This assembly will build under the majority of conditions.
• Of course, the rate of build will be controlled by formation tendencies, bit and stabilizer types, lithology, bit weights (at lower
inclinations), drill collar stiffness, drill string rpm’s, penetration rate, and hole geometry.
• Another strong to moderate building fulcrum assembly consists of a bottom hole stabilizer placed 3 to 6 feet from the bit
face, 60 feet of collars, stabilizer, collars, and any Drill Pipe Description & Specs.
• This is the most common assembly used to build angle.
• The second stabilizer tends to dampen the building tendency.
• This assembly can be used when the previous assembly builds at an excessive rate.
• Other important factors for the fulcrum assemblies are:
o Rotary speed
• The build rate of a fulcrum assembly increases as inclination increases because the larger component of the
collar’s own weight causes them the bend. Increasing the WOB will bend the drill collars behind the near-bit
stabilizer even more, increasing the build rate.
• A higher rotary speed tends to straighten out the drill collars, thus reducing the build rate.
• Therefore, low rotary speeds (70 to 100 rev/min) are generally used with fulcrum assemblies.
• Sometimes, in soft formations, a high flow rate can lead to formation washout, resulting in decreased stabilizer
contacts and, thus, a reduced build tendency.
• This type of configuration is a very stiff assembly, consisting of drill collars and stabilizers positioned to reduce
bending and keep the bit on course.
• This type of assembly is often used in the tangential section of a directional hole.
• We can say that, holding the inclination in a hole is much more difficult than building or dropping angle. Under
ideal conditions, most assemblies either have a building or dropping tendency.
• Most straight hole sections of a directional well will have alternating build and drop sections.
• When holding inclination, these build and drop sections should be minimized and spread out over a large interval
to reduce dogleg severity.
• Amoco had statistically analyzed various holding assemblies comparing their performance.
• Assembly “A” proved to be the most successful even though it maintained inclination only 60 percent of the time.
Assembly “B” maintained inclination less than 50 percent of the time, and assembly “C” even less.
Packed Hole Assembly As a Holding BHA For Directional Drilling
Pendulum Assembly or Dropping BHA
• Unsupported weight of drill collars will force the bit against the low side of the hole.
• The resulting decrease or drop off in angle depends on WOB, RPM, stabilization and the distance between the
bit and the first reamer.
• A stabilizer is placed at 30, 45, or 60 feet from the bit.
• Stabilizer to prevent the collar from touching the wall of the hole causing a tangency point between the bit and
• An increase in the effective length of the pendulum bottom hole assembly (the length below the tangency
point) results in an increase in the weight.
• Since the force FP is determined by that weight, the force FP is also increased exceeding the force FB due to
• Result: side force on the bit causing the hole to drop angle.
• Bit weight slightly decrease the dropping tendency of this assembly because it increases the force due to
• Section of the assembly below the tangency point affects the bit side force.
• If an increase in dropping tendency is required, larger diameter or denser collars should be used below the
• Increases the weight of the assembly results in an increase in dropping tendency.
• A dropping assembly with 7 inch OD collars used in a 12¼ inch hole, changing to 9 inch OD collars results in
increase in dropping tendency.
• Dropping assemblies will have a higher
rate of drop as hole inclination increases.
• The force FP (which causes the dropping
tendency) is calculated :
• Fp = 0.5 × W × Sin(I )
• FP=Side force at the bit caused by the
weight of the unsupported section of the
W=bottom hole assembly, lbs.
• Buoyant weight of the unsupported
section of the bottom hole assembly, lbs.
• I = Hole inclination, degrees.
Basic drop off BHA
Bit – Monel DC – reamer – DC – stab – DC – stab – 90ʹ DC – stab
To increase the tendency to drop angle :
•Apply less WOB (lower penetration rate)
•Apply more RPM and pump pressure in soft formations where jetting
and Reaming down is possible
•Use bigger size Monel DC below the reamer, small DCs above.