Directional Drilling

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Directional Drilling

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- Overview :

Controlled directional drilling is the engineering process of planning and drilling a well

It is a drilling method in a predetermined trajectory, until reaching a target or objective situated off of the

vertical line that passes thorough the rig

Directional drilling started as corrective operations for some problems occurring during drilling such as:

Sidetracks due to fishing operation failures or

Extreme borehole tortuosity

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- Overview :

Today, however, the applications of directional wells in the industry are manifolds

Most recent advances include the drilling of horizontal wells and multilateral wells, have revealed extremely

efficient in increasing the productivity of reservoirs

with small thickness


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- Overview :

Among the most important uses of directional wells is the development of offshore fields

The cost of the development of those fields using vertical (conventional) wells would have made their

exploitation prohibitive


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- Application of Directional Drilling :

Inaccessible locations:

Natural or artificial barriers preclude the access

from the vertical of the target

Relief wells:

Technique used to combat a blowing up well

The relief well reaches an area close to where the gas is entering the well and mud with adequate

weight is pumped to kill the well


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Natural directional wells:

Use the natural tendency of certain formations

to reach the target with lower cost

(vertical too expensive)

Horizontal wells:

In thin formations, horizontal wells increase the

area exposed to flow, increasing production


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Sidetrack:

Change in the original trajectory due to pieces of metal or fishes that could not be recovered by fishing operation

Sidetracks are not planned in advance, but the equipment and techniques are the same used

in directional drilling


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Environmental pressures, as for example in Alaska and in the Amazon, have increased the use of

onshore directional drilling

Directional drilling is also used in the development of geothermal projects

Geothermal reservoirs can reach temperatures of

370oC, and are used to generate steam from water

pumped from surface


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- Well Trajectory Local Coordinate System:

The planned and drilled trajectories of a directional well are either 2D or 3D objects

Associated to any point along the trajectory (planned or actual) there are

three values that can be determined (planned

trajectory) or measured (actual trajectory)

Those values can be used to determine the coordinates of the point in the rig local coordinate

systems


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The values are :

Measured Depth

Inclination

Azimuth


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The measured depth at the point of the trajectory is the measured of the length along the trajectory,

from the rotary table to the point

Therefore, the measured depth is not, in fact, a depth in the strict sense of the word but a length

(unless for a perfectly straight vertical trajectory)


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- Planning a 2D Directional Well Trajectory :


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The location of the rig determines the rig local coordinate system

The rig and the target determine a vertical plane which contains the vertical V, and the target T

It is on this vertical plane that the directional well trajectory is planned drawn


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Point R corresponds to the rig location, and also the origin of the rig local coordinate system

Point T is the target, whose coordinates (v, n, e) are respectively the TVD, relative North/South,

and relative East/West coordinates of the target


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The distance d from the target to the vertical axis V is called the horizontal departure to target and is

calculated from the relative coordinates n and e as

follows:


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The target azimuth is the angle of the vertical plane and is given by:


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The parameters v (TVD) and horizontal departure d are the basic parameters we need to plan the 2D

trajectory of the directional well


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- Terms for Well Trajectory :

When drilled from conventional rigs, a directional well trajectory starts with a vertical hole section

drilled to a given depth where it starts to curve

This point is called kick-off point (KOP) and its determination depends on various factors,

in particular, the mechanical competence and

stability of the formation where the curved section

(and the angle) starts to build


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A too soft formation will not support the lateral force required to drive the bit into a curved path

A too hard formation will resist to the effects of the deviation tool


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The position of the KOP will also affect the final and/or the maximum inclination of the trajectory

The engineer, therefore, will have to simulate several trajectories and choose the one that best

fits the characteristics of the region to be drilled

Some restrictions on the trajectory will narrow the range of possibilities


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For example, the ideal or necessary angle to reach the target :

To drill the reservoir perpendicular to the plan of highest permeability in anisotropic rocks, or

Perpendicular to the fractures of a naturally fractured reservoir

Parallel to a blowing out well to more efficiently kill the well


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The hole section that follows the KOP is called build-up section

It is always recommended to run a casing shortly after the end of the build-up section

A cased borehole will prevent the formation of key seats which may lead pipesticking problems

Since casing projects should follow several design criteria, it is appropriate, and advisable, that

engineers work as an integrated team aiming to

end up with a small number of optimized plans

that comply with all design criteria


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The final choice will consider economics, logistics (availability of deflection tools at or close to the

location, etc)

The experience of the team will also play an important role, reducing substantially the number

of interactions required to obtain the optimal plans


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- Geometric Elements of 2D Well Trajectories:

The simplest curved segment used in trajectory planning is the sector of circle

An advantage is that only one parameter is required to define the curve

One such parameter is the radius of the circle R

Instead of specifying the radius R, it is possible to specify its inverse c, or curvature


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The radius and the curvature are related by


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The curvature can be expressed as:


A length L of a circle of radius R subtend an angle

such that L = R for in radians

C: rad/min

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The radius can be calculated by:


for c in degrees per length

Common units of curvature are degrees per 100 ft, and

degrees per 30 m

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It is common also to refer to curvature as :

buildup rate (BUR for building angle curves)

dropoff rate (DOR for dropping angle curvatures

doglegseverity (DLS for curves in general, and in particular to measure the degree of tortuosity of a trajectory)


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The TVD and the horizontal departure are the basic parameters needed to plan a 2D trajectory for a

directional well

All other parameters are set by the engineer, or are calculated

The degree of freedom of a trajectory type is the number of independent parameters we need to set

in order to calculate the remaining parameters


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2D Trajectory Geometries

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- Classifying 2D Well Trajectories:

Build and Hold

Build, Hold, and Partial Drop (Modified S type)

Build, Hold, and Drop ( S type)

Continuous Build


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Build and Hold (Profile A):

Drilling starts vertical to the KOP

From this point angle starts to build (build up segment) until a maximum inclination is reached

(end of build)

Drilling continues with a straight segment (slant) until reach the target


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The Build and Hold type has three parameters and two degrees of freedom:

KOP

BUR

Maximum Inclination


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Determine KOP given BUR and max (Max.Inclination)


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Determine KOP given BUR and Max. Inclination


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Determine max given KOP and BUR


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Determine max given KOP BUR and


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Determine BUR given KOP and max


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Build, Hold, and Drop (profile C):



(end of build)

Drilling continues straight (slant segment) until a point called DOP (drop-off point) is reached

From this point, angle decreases (drop-off segment) until it becomes vertical (end of drop)

From this point, drilling continues vertical until reach the target


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The Build and Hold type has five parameters and four degrees of freedom:

KOP

BUR

Maximum Inclination

DOR

EOD (vertical depth of the endof-drop)


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Determine max given KOP, BUR (c1), DOR (c2), and EOD


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Determine max given KOP, BUR (c1), DOR (c2), and EOD


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The measured depth, true vertical depth, and

horizontal departure of the endofbuild (EOB) section is given by


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Build, Hold, and Partial Drop (profile B):



(end of build)

Drilling continues straight (slant segment) until a point called DOP (drop-off point) is reached


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From this point, angle decreases (drop-off segment) until a given inclination is obtained

(end of drop)

From this point, drilling continues straight until each the target


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The Build, Hold, and Partial Drop type has six parameters and five degrees of freedom:

KOP BUR maximum inclination DOR hang length - s (sometimes the vertical depth of the EOD)

hang inclination (the inclination to reach the target)


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This is very similar to the BuildHoldDrop type

The difference is that the trajectory reaches the target with a given angle (hang inclination)


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Continuous Build (profile D):


From this point angle starts to build in a relative smaller rate (build up segment) until the drilling

reaches the target


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In addition to these basic types, there exist other special profiles

These profiles are more suitable to deep drilling projects, extended reach wells (ERW), and

horizontal wells


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Catenary:

The catenary type was proposed with the purpose of minimizing the torque along the drillstring

It is a special case of the continuous build type, but with a controlled variable curvature

It would represent the natural shape of a drillstringsuspended at the rotary table and at the

target

Several operational problems put a lot of difficultiesin this type of trajectory


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Double Build:

This type is very common in horizontal drilling

It is an analogous of the build, hold, and drop, but, instead of dropping to the vertical, the second

curved segment is another build, ending at the

inclination of 90o


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Reverse Double Build:

To extend the horizontal section for pay zones not too far from the rig position, a reversed double

build type is used

In this kind of trajectory, the first curved segment is

built in a direction opposite to the target, so to

increase the departure of the target

Then a second build segment starts, dropping back

to vertical and continuing building angle, now in the

direction of the target


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Directional Drilling