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Nudging - Techniques for “nudging”

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Techniques for “nudging”
When formations are suitable (soft), jetting is the best technique to use.
The most common method is to use a mud motor of 9.5" OD or greater
with a 17.5" bit and a 1.5° bent sub. Using a 1.5° bent sub gives low build
rates and hence a low dogleg severity. The hole is then opened to the
required size after the mud motor run. Occasionally the job is performed
with a large mud motor and a 26" bit from the start. In this case either a
1.5°or 2° bent sub might be used.

Nudging

Nudging
The technique of “nudging” is used on platforms in order to “spread out”
conductors and surface casings, which minimizes the chance of a collision.
Basically, when the hole for surface casing is drilled, some angle is built at
a low rate (e.g. 1°/100') in the chosen direction.
In addition to “spreading things out”, other reasons for “nudging” are:
• to drill from a slot located on the opposite side of the platform from
the target, when there are other wells in between
• to keep wells drilled in the same general direction as far apart as
possible
• if the required horizontal displacement of a well is large compared to
the total vertical depth, then it is necessary to build angle right below
the surface conductor to avoid having to use a high build rate

Planning The Well Trajectory - Lead angle

Lead angle
In the old days (pre 1985) it was normal practice to allow a “lead angle”
when kicking off. Since roller cone bits used with rotary assemblies tend to
“walk to the right”, the wells were generally kicked off in a direction
several degrees to the left of the target direction. In extreme cases the lead
angles could be as large as 20°.
The greatly increased use of steerable motors and PDC bits for rotary
drilling have drastically reduced the need for wells to be given a “lead
angle”. Many wells today are deliberately kicked off with no lead angle
(i.e. in the target direction).

Planning The Well Trajectory - The horizontal projection

The horizontal projection
On many well plans, horizontal projection is just a straight line drawn from
the slot to the target. On multi-well platforms however, it is sometimes
necessary to start the well in a different direction to avoid other wells. Once
clear of these, the well is turned to aim at the target. This is a 3-dimensional
turn, but on the horizontal plan it would typically look like Figure 5-18.
The path of the drilled well is plotted on the horizontal projection by
plotting total North/South coordinates (Northings) versus total East/West
coordinates (Eastings). These coordinates are calculated from surveys.

Planning The Well Trajectory - Drop-off section

Drop-off section
On S-type wells, the rate of drop off is selected to ease casing problems and
avoidance of completion and production problems. It is much less critical
to drilling because there is less tension in the drill pipe that is run through
deeper doglegs and less time spent rotating below the dogleg.

Planning The Well Trajectory - Tangent Section

Tangent Section
During the eighties, a number of extended reach projects were successfully
completed. If wells are drilled at inclinations (up to 80°), the area which
can be covered from a single platform is approximately 8 times that
covered when maximum inclination of the wells is limited to 60°.
However, high inclination angles can result in excessive torque and drag on
the drill string and present hole cleaning, logging, casing, cementing and
production problems. These can generally be avoided with current
technology.
Experience over the years has shown that directional control problems are
aggravated when tangent inclinations are less than 15°. This is because
there is more tendency for the bit to walk (i.e. change in azimuth) so more
time is spent keeping the well on course. As such, most run-of-the-mill
directional wells are still planned with inclinations in the range 15° - 60°.

Planning The Well Trajectory - Kick-off Point and Build-Up Rate

Kick-off Point and Build-Up Rate
The selection of both the kick-off point and the build-up rate depends on
many factors. Several being hole pattern, casing program, mud program,
required horizontal displacement and maximum tolerable inclination.
Choice of kick-off points can be limited by requirements to keep the well
path at a safe distance from existing wells. The shallower the KOP and the
higher the build-up rate used, the lower the maximum inclination.
Build-up rates are usually in the range 1.5°/100' M.D. to 4.0°/100' M.D. for
normal directional wells. Maximum permissible dogleg severity must be
considered when choosing the appropriate rate.
In practice, well trajectory can be calculated for several KOPs and build-up
rates and the results compared. The optimum choice is one which gives a
safe clearance from all existing wells, keeps the maximum inclination
within desired limits and avoids unnecessarily high dogleg severities.

Planning The Well Trajectory - Horizontal wells and Allocation of slots to targets

Horizontal wells
For many applications, the best well profile is one in which the inclination
is built to 90° or even higher.

Allocation of slots to targets
Even this is not always a simple task. From a directional driller's
viewpoint, slots on the North East side of the platform or pad should be
used for wells whose targets are in a North Easterly direction.
Unfortunately there are other considerations (e.g. water injection wells
may have to be grouped together for manifolding requirements). Also, as
more wells are drilled and the reservoir model is upgraded, targets can be
changed or modified.
Inner slots are used to drill to the innermost targets (i.e. targets with the
smallest horizontal distances from the platform) and these wells will be
given slightly deeper kick-off points. The outer slots are used to drill to
targets which are furthest from the platform. These wells will be given
shallow kick-off points and higher build-up rates to keep the maximum
inclination as low as possible.

Planning The Well Trajectory - Catenary Curve Well Plan

Catenary Curve Well Plan
One suggestion for an efficient well path for directional wells would be to
plan the well as a continuous smooth curve, all the way from KOP to
target. This is the catenary method. A catenary curve is the natural curve
that a cable, chain or any other line of uniform weight assumes when
suspended between two points. A similar suspension of drill string would
also form a catenary curve.
Proponents of the catenary method argue that it results in a smoother
drilled wellbore, that drag and torque are reduced and that there is less
chance of key seating and differential sticking. However, in practice it is
hard to pick BHAs which will continuously give the required gradual rate
of build. It is in reality no easier to follow a catenary curve well plan than a
traditional well plan. Also, the catenary curve method produces a higher
maximum inclination than would result from the build and hold or S type
patterns.
Although the catenary method has been tried, with some success, it is not
widely used and it IS NOT Baker Hughes INTEQ policy to recommend
this type of well profile.

Planning The Well Trajectory - Types of Directional Patterns

Types of Directional Patterns
The advent of steerable systems has resulted in wells that are planned and
drilled with complex paths involving 3-dimensional turns. This is
particularly true in the case of re-drills, where old wells are sidetracked and
drilled to new targets.
These complex well paths are harder to drill and the old adage that “the
simplest method is usually the best” holds true. Therefore, most directional
wells are still planned using traditional patterns which have been in use for
many years. Common patterns for vertical projections are shown on the following pages:

Features:·
Shallow kick-off point (KOP)
Build-up section (which may have more than one build up rate)
Tangent section
Applications:
Deep wells with large horizontal displacements
Moderately deep wells with moderate horizontal displacement, where intermediate casing
is not required

Features: There are several variations:
Shallow KOP - Build, hold & drop back to vertical
Build-up section - Build, hold, drop & hold (illustrated above)
Tangent section - Build, hold & continuous drop through reservoir
Drop-off section
Applications: Disadvantages:
Multiple pay zones - Increased torque & drag
Reduces final angle in reservoir - Risk of keyseating
Lease or target limitations - Logging problems due to inclination
Well spacing requirements
Deep wells with small horizontal displacements

Features:
Deep KOP
Build-up section
Short tangent section (optional)
Applications:
Appraisal wells to assess the extent of a newly discovered reservoir
Repositioning of the bottom part of the hole or re-drilling
Salt dome drilling
Disadvantages:
Formations are harder so the initial deflection may be more difficult to achieve
Harder to achieve desired tool face orientation with downhole motor deflection assemblies
(more reactive torque)
Longer trip time for any BHA changes required
On multi-well platforms, only a few wells are given deep kick-off points,
because of the small slot separation and the difficulty of keeping wells
vertical in firmer formation. Most wells are given shallow kick-off points
to reduce congestion below the platform and to minimize the risk of
collisions.

Planning The Well Trajectory - The Target

The Target
The target is usually specified by the geologist, who will not merely define
a certain point as the target but also specify the acceptable tolerance (e.g. a
circle of radius 100 feet having the exact target as its center). Target zones
should be selected as large as possible to achieve the objective. If multiple
zones are to be penetrated, they should be selected so that the planned
pattern is reasonable and can be achieved without causing drilling
problems.

Planning The Well Trajectory

Planning The Well Trajectory
One area of well planning in which directional companies are closely
involved is the planning of the well trajectory. Again, this is not as simple a
task as it might seem at first glance, particularly on a congested multi-well
platform. There area number of aspects that must be carefully considered
before calculating the final well path.

Well Planning - Field Coordinates

Field Coordinates
Although the coordinates of points on a wellpath could be expressed as
UTM coordinates, it is not normal practice. Instead, a reference point on
the platform or rig is chosen as the local origin and given the coordinates
0,0. On offshore platforms this point is usually the center of the platform.
The Northings and Eastings points on the wells drilled from the platform
are referenced to this single origin. This is important when comparing
positions of wells, in particular for anti-collision analysis.
Direction Measurements
Survey tools measure the direction of the wellbore on the horizontal plane
with respect to North reference, whether it is True or Grid North. There are
two systems:
Azimuth.
In the azimuth system, directions are expressed as a clockwise angle from
0° to 359.99°, with North being 0°.

Quadrant Bearings
In the quadrant system , the directions are expressed as angles
from 0°-90° measured from North in the two Northern quadrants and from
South in the Southern quadrants. The diagram in Figure 5-14 illustrates
how to convert from the quadrant system to azimuth, and vice versa.

Well Planning - Reference Systems and Coordinates, (3) Azimuth Reference Systems

Azimuth Reference Systems
For directional surveying there are three azimuth reference systems:
• Magnetic North
• True (Geographic) North
• Grid North
All “magnetic-type” tools give an azimuth (hole direction) referenced to
Magnetic North. However, the final calculated coordinates are always
referenced to either True North or Grid North.
True (Geographic) North
This is the direction of the geographic North Pole which lies on the Earth’s
axis of rotation. Direction is shown on maps using meridians of longitude.
Grid North
Drilling operations occur on a curved surface (i.e, the surface of the Earth)
but when calculating horizontal plane coordinates a flat surface is assumed.
Since it is not possible to exactly represent part of the surface of a sphere
on a flat well plan, corrections must be applied to the measurements. To do
this, different projection systems which can be used.
UTM System
One example of a grid system is the Universal Transverse Mercator (UTM)
System. In transverse mercator projection, the surface of the spheroid
chosen to represent the Earth is wrapped in a cylinder which touches the
spheroid along a chosen meridian. (A meridian is a circle running around
the Earth passing through both North and South geographic poles.)
These meridians of longitude converge towards the North Pole and do not
produce a rectangular grid system. The grid lines on a map form the
rectangular grid system, the Northerly direction of which is determined by
one specified meridian of longitude. This “Grid North” direction will only
be identical to “True North” on a specified meridian.

Well Planning - Reference Systems and Coordinates, (2) Inclination References

Inclination References
The inclination of a well-bore is the angle (in degrees) between the vertical
and the well bore axis at a particular point. The vertical reference is the
direction of the local gravity vector and could be indicated by a plumb bob.

Well Planning - Reference Systems and Coordinates.- (1) Depth References

Depth References
During the course of a directional well, there are two kinds of depths:
• Measured Depth (MD) is the distance measured along the actual
course of the borehole from the surface reference point to the survey
point. This depth is always measured in some way, for example, pipe
tally, wireline depth counter, or mud loggers depth counter.
• True Vertical Depth (TVD) is the vertical distance from the depth
reference level to a point on the borehole course. This depth is always
calculated from the deviation survey data.
In most drilling operations the rotary table elevation is used as the working
depth reference. The abbreviation BRT (below rotary table) and RKB
(rotary kelly bushing) are used to indicate depths measured from the rotary
table. This can also be referred to as derrick floor elevation. For floating
drilling rigs the rotary table elevation is not fixed and hence a mean rotary
table elevation has to be used.
In order to compare individual wells within the same field, a common
depth reference must be defined and referred to (e.g. When drilling a relief
well into a blow-out well, the difference in elevation between the
wellheads has to be accurately known). Offshore, mean sea level (MSL) is
sometimes used. Variations in actual sea level from MSL can be read from
tide tables or can be measured.

Well Planning - Reference Systems and Coordinates.

Reference Systems and Coordinates
With the exception of Inertial Navigation Systems, all survey systems
measure inclination and azimuth at a particular measured depth (depths
measured “along hole”). These measurements are tied to fixed reference
systems so that the course of the borehole can be calculated and recorded.
These reference systems include:

• Depth references
• Inclination references

• Azimuth references

Well Planning - Introduction

Well Planning
Introduction

There are many aspects involved in well planning, and many individuals
from various companies and disciplines are involved in designing various
programs for the well (mud program, casing program, drill string design,
bit program, etc). A novel approach to well planning is one where the
service contractors become equally involved in their area of expertise.
This section will concentrate on those aspects of well planning which have
always been the province of directional drilling companies.

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