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Drilling Engineering 2 Course (2nd Ed.)

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1. General Casing Design Criteria

2. Graphical Method for Casing Design

3. Other Casing Design considerations

1. Necessity of Directional well

2. well’s trajectory

3. Major Types of Wellbore Trajectories

4. Trajectory rule of thumbs and terms

5. Trajectory Calculation

Directional well

A well is declared as a directional one when it follows a predescribed traverse or trajectory to intersect specific targets. They can be necessary when:Reaching a target which is below

inaccessible or restricted areas such as a mountain, a highly populated area, a national park, etc.,

Multiple wells have to be drilled from one offshore platform to deplete large portions of a reservoir from one structure,

Side tracking has to be done around a fish,Fault drilling is necessary,Salt dome drilling takes place,Drilling a relief well to intersect a blowout well,Sidetracking from an old well

to explore different horizons and/or directions.

Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 5

Directional well; schematics


Directional well; schematics (Cont.)


bit walk

While drilling, many factors influence the trajectory of the bore hole.

Some of them are easy to control, others may be difficult to estimate. Amount the difficult ones to predict

is the so called “bit walk” that may occur in certain formations and does not follow a general trend.


the trajectory displacement

To plan a well’s trajectory, to follow up the drilled one as well as to correlate its deviation from the plan, the trajectory is generally displayed in

a horizontal view and a vertical view.

The horizontal view projects the trajectory to a plain that has

the N-S and E-W directions as their axis and the rotary table at its center.

The vertical or section view shows the trajectory projected to a vertical cross-section

defined by the rotary table and the target.


Definitions of trajectory


Definitions of trajectory (Cont.)


Target location

The first step of planning a directional well is to identify where the target (targets) are located in respect to the rig location. For drilling,

the origin of the trajectory is taken from the rotary table.

Thus the location of the target, which is mostly determined by the production

department in UTM or Lat/Long coordinates,

has to be re-calculated into “Northing” and “Easting” in respect to the rotary table.


Target depth

The depth of the target, which can be referenced to

ground level, mean sea level or water table, has to be referenced to the rotary table as well.

Note that the rotary table height is specific to a particular rig and

when an old well has to be re-entered or sidetracks drilled, the survey of the old well is referenced to the rotary table height of the rig it was drilled with

which can be different from the one use later on.


significant points

When these identifications and corrections are done, the trajectory is planned

so that the specified target is reached from the surface location.

The views of the planned trajectory show and contain values for the location of

the rotary table, the kick of point(s), the end of build point(s), the drop down point(s), the end of drop point(s) as well as the target in

TVD (true vertical depth), MD (measured depth), inclination, azimuth and horizontal departure.


Basic 2-D types of trajectories

Apart from these “significant points” a survey is created that lists the values mentioned above

as a sequence of points that have a constant MD difference (or are closer when needed, e.g. at turning intervals).

Following basic, 2-D types of trajectories have been established by the industry for practical realizations:Build-and-hold trajectory,

Build-and-hold-and-build (double build) trajectories,

Build-and-hold-and-drop (S) trajectories,

Build-and-partial drop-and hold (modified S) trajectory.


Different types of basic (2D) trajectories


type 1: Build-and-hold trajectory

At type 1 trajectories, the well is kicked of at a specified depth,

inclination is build up until a certain amount (end of build) and kept until the target is reached.

This type of profile is often applied when a large horizontal displacement is required

at relatively shallow target depths.

Since there are no major changes in inclination or azimuth after the build-up section is completed, there are fewer directional problems with this profile,

such as dog-leg, key seats, etc..


type 2: Build-and-hold-and-build (double build) trajectoriesAt type 2 trajectories,

the well is kicked of at a specified depth and inclination is build up until a certain amount (end of build).

Then this inclination is hold until a second kick of point is reached, inclination is built up again to a certain amount (end of build) and kept until the target is reached.

This type of trajectory is preferred for relative large horizontal displacements

which are achieved at the first holding section. This holding section is commonly designed with an inclination

angle of between 30 to 40◦ since within these values, close control over the trajectory-progress is convenient.

After the second building interval horizontal or nearly horizontal wells are often planned for.


type 3: Build-and-hold-and-drop (S) trajectoriesAt type 3 trajectories,

the well is kicked of at a specified depth,

inclination is build up until a certain amount (end of build) and

kept until the drop down point is reached.

From the drop down point until the end of drop point the inclination is reduced to zero degrees and

the well is continued until the target is hit vertically.

Here an extra torque and drag are expected due to the additional bend.


type 3: Build-and-hold-and-drop (S) trajectories (Cont.)This type of profile is used

when the target is deep

but the horizontal displacement is relatively small.

It also has applications when completing a well

that intersects multiple producing zones, or

in drilling relief-well where it is necessary to run parallel with the blowing well.


Build-and-partial drop-and hold (modified S) trajectory.At type 4 trajectories,

the well is kicked of at a specified depth, inclination is build up until a certain amount (end of build) and kept until the drop down point.

From the drop down point until the end of drop point the inclination is reduced but

differently to type 3 trajectories, not to zero degrees.

Then the inclination is kept until the target is intercepted.

The applications and characteristics of this well type are similar to the ones of type 3.


Other trajectories

Along with these basic trajectory types, so called catenary trajectories (designed to minimize torque and drag) and general 3D trajectories that turn in space are common practice today.

uncertainty ellipses along a directional wellSpring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 28

trajectory planning rule of thumbs

For the trajectory planning itself, following rule of thumbs should be kept in mind:The build-and-hold type is

the least expensive one of all trajectory types and

easiest to drill.For a given TVD and

horizontal departure of a target, the higher the kick of point,

the smaller the slant angles, less build up and reduced total MD is necessary to reach the target.

If reasonable, designing

the slant angle between 30◦ and 40◦is good practice.

Slant angles smaller than 15◦ are to be avoided since they are difficult to control.

A deep kick-off point has certain disadvantages: formation will probably

be harder and less responsive to deflection,

more tripping time is needed to change out BHAs during side tracking,

build-up rate is more difficult to control.


Classification of build up section according to the applied build up rate


often used terms definitions

Below definitions assume the RKB position to be at the center of the chosen coordination system.

Northing:Horizontal distance between one survey point and

the RKB, measured to the North. A distance to the South is generally denoted as being negative.

Easting:Horizontal distance between one survey point and

the RKB, measured to the East. A distance to the West is generally denoted as being negative.


often used terms definitions (Cont.)

True Vertical Depth:Vertical distance of one survey point to the RKB.

Horizontal Departure:Horizontal distance between one survey point and

the RKB, at the level of the survey point.

Azimuth:Horizontal angle (0 - 2.π), measured clockwise

from the true North to the tangent of the trajectory at this survey point.


often used terms definitions (Cont.)

Inclination:Angle between the vertical component of the tangent of

the trajectory at the survey point and the vertical axis. Thus a vertical well as an inclination of 0, a horizontal well an inclination of π/2.

Measured Depth:Actual length of the trajectory,

starting at the RKB up to the survey point.

Vertical Section:Horizontal departure of the survey point projected to

the vertical view. In this way the vertical section is always smaller or equal to the horizontal departure.


Calculations

In the following, calculations for the radius of curvature (r1),

the maximum inclination angle (θ),

the measured depth and horizontal departure for the buildup intervals as well as

the measured depth and horizontal departure for the holding intervals

are presented for different basic trajectory types.


Build-and-Hold Trajectory

This type of trajectory is most common in the industry whenever applicable. Figure shows this type of

trajectory where X3 < r1.

TVD and the horizontal departure of the target are denoted with

D3 and X3 respectively,

TVD of the kick of point is given by D1.


Calculation of r1 and θfor Build-and-Hold TrajectoryRadius of curvature:

q [◦/ft] build up rate or inclination angle buildup

r1 [ft] radius of curvature

Maximum inclination angle (θ in [◦]):


Calculation of DMi and Xifor Build-and-Hold TrajectoryMeasured depth and horizontal departure along

the buildup are computed with:

where, at the end of build

Along the holding interval, the measured depth and horizontal departure are:

Di [ft] vertical depth at point i along the buildup or holding interval


Build-Hold-and-Drop (S) Trajectory

The maximum inclination angles can be calculated by following equations: for r1 + r2 > X4

and for r1 + r2 < X4:

When replacing X4 by X5 + r2.(1 − cosθ’) and D4 by D5 + r2.sinθ’, the equations above can be used to calculate the modified S type trajectory.


1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling Engineering.” Master of Petroleum Engineering. Curtin University of Technology, 2001. Chapter 9

1. the Survey of a Well

2. Calculating the Survey of a Well

3. Deflection Tools and Techniques

4. Hydraulic Method (Jetting)

5. Mechanical Methods

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