5,.Drill String/BHA Components and Design

01. Describe the purpose of the BHA.

Ans. BHA or Bottom hole Assembly is the lower portion of the drillstring. It consists of all the special instruments used while drilling.  These components include, but are not limited to: The Bit, HWDP, NMDC, MWD and LWD tools, DD tools, stabilizers, jars, and subs.

 

02. Describe the basic components that make up the BHA and drill string, including:

Bits: Used to crush or cut the rock. It’s located at the bottom of the drillstring and must be changed when it becomes excessively dull or stops making progress.

Stabilizers – (including spiral, integral, straight blade, clamp on):

It’s an important part of the BHA as they stabilize the bit and the drill collars in the hole. When the bottom hole assemble is properly stabilized, optimum

drilling weight can be applied to the bit. They have variety of designs.

Stabilizers are specifically used to:

q Control hole deviation

q Reduce the risk of differential sticking

q Ream out doglegs and key-seats

 

Spiral Stabilizers are provided with spiral blades, they may be either open or tight spiral.

Integral Stabilizers are made from one piece of material rolled and machined to provide the blades; they can have three or four blades. I.B. Stabilizers normally have tungsten carbide Inserts (TCI’s).

Straight Blades Stabilizers are those with straight blades, which might be either straight or offset.

Clamp on Stabilizers Allow more flexibility in the BHA design, they can be positioned on the NMDC’s, MWD, PDM’s, etc. at the required spacing to maintain direction control, non-magnetic clamp on stabilizers are also available.

 

 

 

 

 

 

 

 

 

 

 

Drill Collars:

The drill collar is a component of a drill-string that provides weight on bit for drilling. Drill collars are thick-walled tubular pieces machined from solid bars of steel, usually plain carbon steel but sometimes of non-magnetic nickel-copper alloy or other non-magnetic premium alloys. That is to allow the LWD and MWD tools to properly operate. Last, threaded connections, male on one end and female on the other, are cut so multiple collars can be screwed together along with other downhole tools to make a bottom hole assembly (BHA).

 

Crossovers: Used to connect different pin and box sizes and types.

 

Float subs: Float subs have a float valve to prevent the fluid from traveling up inside the drill pipe.

 

Bent subs: A small section of drill collar used in directional drilling.  The pin end of the sub is slightly offset so that when a connection is made, a bent joint is created.

 

Orienting subs (UBHO): A sub with a sleeve located inside that can direct survey equipment in the direction of the BHA bend.

 

Junk Baskets: Used in retrieving junk from downhole and to prevent the junk from settling down to the bottom when circulation is stop.

 

Jars: Transfers the potential energy stored in the stretched drill pipe to kinetic energy in the BHA above the jars in an attempt to free a stuck drill string.

 

Reamer: Used in drilling to smooth the wall of a well, enlarge the hole to the specified size, help to stabilize the bit and drill directionally.

 

Hole openers/under reamer: It is normally used to enlarge the hole when problems have been encountered and casing has been placed below the planned depth.

 

Drillpipe - Tubular steel conduit fitted with special threaded ends called tool joints. The drill pipe connects the rig surface equipment with the bottomhole assembly and the bit, both to pump drilling fluid to the bit and to be able to raise, lower and rotate the bottomhole assembly and bit. The different grades include G, S, E and the thread for the 5” is 4 ½ IF.

 

Heavy Weight Drill Pipe (HWDP): Similar to drill pipe but thicker walled.  Located above the drill collars, to allow a smooth transition between the drill collar and the drill pipe.

 

Steerable PowerPak motor: Mud motor with variable bend section.

 

Baffle plate (TOTCO ring): Landing plate for surveying tools.

 

Rotary Steerable systems: Directional-drilling systems that are designed to allow steering while the drillstring is rotating – such as the PowerDrive system.

 

 

 

 

03. Define and identify classic examples of the following BHAs:

 

Build: In build BHA, we position the BHA so that we exert positive side forces on the formation through the bit - Fulcrum effect.  This has the effect of pushing the bit up.

 

Drop: Generally opposite to build BHA’s.  By not placing a stabilizer at the bit, you create a Pendulum force which makes the bit push towards the low side of the wellbore and drop in inclination.  The stabilizers should be placed at a greater distance away from the bit in order to increase the drop.

 

Hold: Lock up BHA’s. In order to keep the hole inclination constant.

 

Steerable: Some of the same principles as for rotary BHA’s.  Objective is to kick off the well and build inclination to maximum angle along the desired hole direction

 

 

 

04. Explain why stabilizer placement and stabilizer size is critical to drill string design in both vertical and directional wells.

Ans: The size, position & no. of the stabilizers are critical.

v If a full gauge stabilizer is placed near to the bit, it will create a build-up assembly due by the force of the unstabilized collar.

v If an additional stabilizer is placed around 30 meters from the bit, the build-up rate will be reduce.

v The build-up effect could be completely avoided by using a third stabilizer at 40 meters from the bit, which would keep the hole straight.

 

05. Explain the differences between a rotary assembly, a motor assembly and steerable assembly.

Ans: The differences are listed below:

The Rotary assembly does not have downhole motor; the bit is turned just with the use of the rotary table or top drive.

Assembly with a downhole motor is called Motor assembly but without bend.

The Steerable assembly involves the use of a downhole motor that has an adjustable bend.  

 

06. Explain some important aspects of BHA manipulation

a) Importance of minimizing the handling of BHA.

Ans: Over-handling puts undue stress and strain on equipment, especially at the connections.

 

b) Potential problems caused by always breaking the same tool joint connections.

Ans: Breaking the drill-string in the same place every POOH could create fatigue on the same thread which could weaken it, wash it out, or break down-hole. For that reason it is always important to remove one pipe of any of the stands when POOH. Then the disconnection and connection point for a stand will always change.

4. Introduction to Directional Drilling

 1) Describe the following types of wells according to objective: Exploration, Appraisal, Development and Production.

 

Exploration wells, these types of wells are drilled after an exploration phase (Seismic Interpretation, geology studies, etc) had been done. The purpose of drilling exploration wells is to know whether or not there is hydrocarbon in the area of interest. Also, they are called wildcat, which is the well that the company drills to determine whether oil or gas exist in a subsurface rock formation.

 

Appraisal wells, Theses types of wells are drilled after the exploration wells have shown the presence of hydrocarbons. The purpose of drilling appraisal wells is to make an estimation of the size of the possible reservoir, such area and reserves. Also, they are called confirmation wells, because they try to verify whether the wildcat well tapped a rock layer with enough hydrocarbons for the company to develop it.

 

Development and production wells, these wells are drilled after the appraisal wells have shown that the reservoir is big enough to be produced and, with enough hydrocarbons reserves to exploit and have profits. The purposes of these wells are to develop and produce the hydrocarbon in situ to sell it in the world market.

 

2. List the reasons and applications of directional, Horizontal and extended reach wells. 

 

For many reasons it may not be possible or desirable to drill a vertical well. There may be constrains because of the surface location. In the subsurface, multiple targets, the shape of the structure, faults, etc. may preclude a vertical well. So directional wells are the option to avoid this constrains.

 

Main applications of directional wells

 

1. Sidetracking.

 

Sidetracking was the original directional drilling technique. Initially sidetracks were “blind”. The objective was simple to get past a fish. Oriented sidetracks are most common. They are performed when, for example, there are unexpected changes in the geological configurations

 

2. Inaccessible locations.

 

Target located beneath a city, a river or in environmentally sensitive areas makes it necessary to locate the drilling rig some distance away. A directional well is drilled to reach the target

 

3. Salt Dome Drilling.

 

Salt domes have been found to be natural traps of oil accumulating in strata beneath the overhanging hard cap. There are several drilling problems associated with drilling a well through salt formations. These can be somewhat alleviated by using a salt saturated mud. Another solution is to drill a directional well to reach the reservoir, thus avoiding the problem of drilling through the salt.

 

4. Fault Controlling.

 

Crooked hole are common when drilling nominally vertical. This is often due to faulted sub-surface formations. It is often easier to drill a directional well into such formation without crossing the fault lines.

 

5. Multiple exploration wells from a single well bore.

 

A single well bore can be plugged back a certain depth and deviated to make a new well. A single well bore is sometimes used as point of departure to drill others. It allows explorations of structural locations without drilling other complete wells.

 

6. Onshore Drilling.

 

Reservoirs located below large bodies of water, which are within drilling reach of land, are being tapped by locating the wellheads on land and drilling directionally underneath the water. This saves money and land rigs are much cheaper. 

 

7. Offshore Multi-Wells Drilling.

 

Directional drilling from a multi-well offshore platform is the most economic way to develop offshore oil fields. Onshore a similar method is used where there is space restrictions, e.g. jungle, swamp. Here the rig is skidded on a pad and the wells are drilled in “clusters”.

 

8. Multiple sands for a single well bore.

 

In this application a well is drilled directionally to intersect several incline oil reservoirs. This allows completion of the well using a multiple completion system. The well may have to enter at specific angle to ensure maximum penetration of the reservoir.

 

9. Relief well.

 

The objective of a directional relief is to intersect the borehole of a well, which is blowing and allowed to be killed. The borehole causing the problem is the size of the target. To locate and intersect the blowing well at a certain depth, a carefully planned directional well must be drilled with great precision.   

 

3. List the types and designs of directional wells. Horizontal, J  type, S type.

 

 

1. Horizontal wells.

 

This type of well consists of a vertical section, a build section, a tangent section, a second build section, and a horizontal section to reach the target.

Given the lateral distribution of reservoir rock or reservoir fluids, a horizontal well may provide the optimum trajectory. The build up rate of angle is the main distinction from a drilling point of view. Mediums radius wells are preferred since they can be drilled, logged and completed with fairly standard equipment. The horizontal drilling target can be controlled within a vertical window of about 2m. To target the horizontal section with the require accuracy a pilot hole may be drilled first for depth control. Accurate directional surveys are critical for the optimum positioning of the well.

 

Main application

 

Reduced production in a field may be due to many factors, including gas and water conning or formations with good but vertical permeability. Engineers can then plan and drill a horizontal drain hole in order to increase production. Others application are in developing geothermal field and in mining.

 

2. Extended Reach Drilling (ERD),

 

An extended reach well is loosely defined as having a horizontal displacement of at least twice the vertical depth. Extended reach drilling (ERD) will be considered,

 

 

· Where surface restrictions exist

· Where marginal accumulations are located several miles from existing platforms/clusters

· Where ERD allows a reduction in the number of platform required

 

3. J type wells.

 

In this type of the well consists of a vertical section, a build section, a tangent section where the acquired angle is held constant till it reaches the target.

 

4. S type wells.

 

In this type of well, it consists of a vertical section, a build section, a tangent section and a drop section to reach the target.

 

 

4. Explain what well planning is.

Well plan is an organized process used to put together data that will be used to design a successful well. A well plan is a very important stage prior to the commencement of the drilling operations to ensure that all aspects are tailored to meet the specific objectives of that well.

 

The data provided by the client are used by the well planner to develop a geometric profile for the safest and easiest path from the surface location to the target.

 

 

5. Name and describe the following basic components of a well plan:

 

a. Location.

 

A place where the well is going to be drilled, usually includes the name of the well and field. The first thing to do is to define a local coordinate system, originating at the structure reference point. In many land wells, this will be the surface location. The target location is then converted to this local coordinate system, if necessary

 

b. Target Location and Size.

Target location is a point defined in space by geographical coordinates at given true vertical depth. A well profile could have multiples target.

 

c. Selection of Well profile;

Smoothest way to reach the target is selected with less dog leg and more easily reachable way is selected.

 

 

 

d. Anti-collision considerations.

The well planner uses three techniques to analyze the proposed trajectory of the wellbore. The techniques are: anti-collision, traveling cylinder analysis and ellipses of uncertainty. The goal of these techniques is to eliminate the possibility of a collision with a nearby well.

1.	Anti-collision spider plots
2.	Traveling cylinder analysis
3.	Ellipses of uncertainty

 

 

e. Well Profile features:

 

i. Kick off point and start of drop

 

A point in the wellbore at a given vertical depth below the surface location where the well is to be deviated away from vertical. It is deviated in a given direction up to a given inclination and at a given build rate. The selection of the kickoff point is made by considering the geometrical well path and the geological characteristics of the formation.

. Start of Drop is the location where the wellbore starts dropping inclination (applicable for S-shape well).

ii. End of build and end of drop

End of build, it is the point where the angle stop building.

End of Drop, it is the point where the angle stop dropping or decreasing.

 

1) Surface Location: Surface location is a point from where the well is started.

2) Azimuth: Azimuth is the clockwise angle of departure from a reference direction i.e. north, measured in a horizontal plane. It is also referred as “Direction”

3) Target Location: A defined area at a prescribed vertical depth and location, which will be intersected by the wellbore.

4) Vertical Section: The vertical section is the length of the projection of the horizontal displacement onto the vertical plane of projection.

5) End of Build-up: The inclination of the well bore is not in increased.

6) Kickoff Point: A point in the wellbore at a given vertical depth below the surface location where the well is to be deviated away from vertical.

7) Quadrant: The angle in the horizontal plane measured from either a N or S reference direction towards E or W, defining the direction of the wellbore.

8) Build-up Rate: It is the rate at which the angle of inclination of the well bore is increased.

9) Polar Co-ordinates: Distance projected in a horizontal plane from a fixed reference point and the angle of this line from a reference direction, usually True North, e.g. 500 feet @150°

10) Drop off rate: It is the rate at which the inclination drops in well bore.

 

11) Turn Rate: It is the rate at which the azimuth of the well bore is changed.

12) Rectangular Coordinates: Defined as the projected distance along two reference axes, which are at right angles, the distance being measured from the intersection of the axes along the axes.  These axes are in the horizontal plane and typically are NS and EW.

13) Hold Angle: Drilling continues without changing the angle and holding The same angle.

14) Start of Drop: The point along the wellbore trajectory at which we   start to drop inclination.

15) End of Drop: The point along the wellbore trajectory at which we end to drop inclination.

16) TVD (True Vertical Depth): The vertical depth measured from surface to the end of the wellbore profile.

Tangent Section: Occurs after the build up section and is a section where the inclination is kept constant for a certain distance.

3. Subject: Basic Drill Floor Measurements

01. Identify all and describe the following gauges used by the driller’s console:

a) Rotary Speed (RPM)    b) Surface Weight on Bit (WOB) (string weight)

c) Pump stroke (SPM)   d) Pump Pressure (PP)    e) Surface Torque

f) Depth (Depth Encoder or Geolograph)

 

Ans. a) Rotary Speed (RPM):

It is rotational speed of the drillstring with the help of either Top drive or the rotary table in case of Kelly System.

It is displayed on a gauge in the driller’s console expressed in revolutions per minute (RPM).

 

b) Surface Weight on Bit (WOB) (string weight):

The amount of downward force placed on the bit by the weight of the drill collar. It is displayed by a gauge, which shows the string weight and the applied weight on the bits (Units: Klbs/Tons).

 

c) Pump stroke (SPM):

Pump stroke is measured by the Pump stroke counter, which is installed on the Pump frame.

The display of the SPM can be seen at the driller’s console.

 

 

 

d) Pump Pressure (PP):

It is the Standpipe pressure, which is measured using the SPT. It is measured in kPa or PSI.

 

 

e) Surface Torque:

It is the amount of torque the drilling string requires to rotate, while rotary mode drilling. It is measured in ft-lb.

 

f) Depth (Depth Encoder or Geolograph):

It is the distance of the bit from the surface. It can be measured with the help of either Depth Encoder or Geolograph. It is measured in meters (m).

 

02. Describe how the gauges in the driller’s console are calibrated.

Ans. The sensors are based on a two-point straight-line calibration. By measuring a low point and a high point of the desired system the tool can be calibrated.  The drilling company electrician typically calibrates the rig systems.

 

03. State the typical range for each of the above measurements in English (SI) & metric units.

Ans. Rotary speed (RPM):  0-200 rpm.

 

Surface Weight on Bit (WOB) (String Weight):

The deeper we go the higher the pressure. We must apply more pressure than bottom hole pressure 200 – 40000 lb.

            

Pump stroke (PS): Depends on the number of pumps if there is one could be 0-120 strokes, if more then less strokes depending on how many pumps.

 

Pump pressure (PP): Can be between 2-2000 PSI.

 

Surface Torque: 0 – 25000 ft-lb

 

Depth: Can go from 0 to 12000 m (no real limit).

 

 

 

 

 

4. Describe the measurement of ROP and list several sources of ROP data.

a) Describe the applications of ROP data.

b) List the factors, which affect ROP and describe their effect.

Ans (a).  The applications of ROP are as follows:

v ROP is a very important parameter in correlation.

v A change in ROP is a good indicator of formation change.

v It may be also an indicator for the bit condition.

v It is an indicator of the drilling performance.

 

Ans (b).  Factors, which affect ROP, are as follows:

 

Lithology

ROP is directly proportional to the hardness of the drilled formation.

Hard lithology  = Slow ROP

Soft lithology   = High ROP

 

Differential pressure

ROP increases with the increase in the differential pressure, as it makes the formation more drillable.

 

Surface & Down Hole Weight on bit

With roller cone bits ROP is directly proportional to the WOB, while PDC & Diamond bits are more efficient with less weight.

 

RPM

ROP is directly proportional to RPM.

 

Bit condition

The ROP decreases as the bit starts wearing

 

Bit type

Some bits give high ROP with certain formations (depends on bit specification).