INTRODUCTION:
The
aim of this chapter is to introduce some of the design principles required in
planning horizontal and multilateral wells. The term designer wells were
introduced in this industry primarily to emphasis the complex directional
profiles that can be implemented with today’s technology. It is not intended to
detail all the equipment and procedures in current use as this subject is
developing rapidly. Indeed, some of the equipment which were invented in the
early 1990’s are now regarded as old technology. The emphasis will be placed on
the more general procedures and equipment which will are in current use.
HORIZONTAL
WELLS
By
oilfield convention, a horizontal well is defined as a well with an inclination
angle of 90o from the vertical. A vertical well is one with zero
inclination angle. In fact, the first horizontal well was drilled by Russians
in 1950s.
TYPES
OF HORIZONTAL WELLS:
There
are three types of horizontal wells:
1. Short radius.
2. Medium radius.
3. Long radius.
SHORT
RADIUS:
The
main features of this type are the very high build-up rate of 60 – 150 degrees
/100 ft with a radius range of 40-100 ft. This type requires specialized
articulated motors to affect the high build angles.
Advantages:
1. Enables sharp turns into thin reservoirs.
2. Both motors driven and drill pipe driven.
3. Laterals can be completed and tied back
using special liners.
Disadvantages:
1. Poor directional control.
2. Special tools and equipment required.
MEDIUM
RADIUS:
The
build-up rate for this type is usually 8-30o/100ft with a radius
range of 200 to 700 ft. The horizontal drain is usually Between 1000 – 3500 ft.
A
typical well profile consists of build-tangent section and a build-horizontal
section. Two different BHA’s will therefore be required for this type of well.
The
second build-up section should ideally start at the top of the "marker
zone" and should reach a maximum of 85-100o on entry into the
reservoir. An angle hold assembly should be used to drill the horizontal
section.
LONG
RADIUS:
This
is the most common type of horizontal wells especially offshore. The build-up
rate is usually from 2o to 60/100ft. The most common BHA
used is a steerable system containing a single bent sub with a downhole motor.
Two profiles are in common use:
• A single build-up section terminating in the
horizontal section.
• A
build-tangent and then a higher build-lateral section.
WELL
PROFILE DESIGN CONSIDERATIONS:
The
following factors should be considered when designing a horizontal well:
• Target definition.
• Single curve Design.
• Double curve design.
TARGET
DEFINITION:
A
horizontal well is usually a development well with well-defined geological and
reservoir objectives.
The following information is required to define
the horizontal target accurately:
1. target co-ordinates.
2. entry point into the reservoir.
3. length of horizontal drain.
4. azimuth range of target.
5. vertical depth range of target.
6. tolerance in vertical depth and displacement.
7. dip of target.
SINGLE
CURVE DESIGN:
In
this design, the hole angle is built up from zero at the KOP to 90o
at the entry point into the reservoir. If this design is used the buildup
tendencies of both the formation and the rotary or steerable BHA should be
known in order to avoid missing the target due to excessive or insufficient
build up rates. Also, the build-up rate should be selected to land exactly on
the target. If the buildup rate is too low the well path will fall below the
target and if the buildup rate is too high the well path falls above the
target. In both cases, expensive well correction is required.
DOUBLE
CURVE DESIGN:
If the
buildup rate is too high the well path will be above the reservoir and the well
will require redrilling. Similarly, if the build-up rate is too low, the well
path will be below the reservoir and the drilling objectives will not be met.
The above problems can be solved by having a tangent section below the initial
build up curve and then build up to the required angle when reaching a
reservoir marker. In some cases, the final angle is actually built up inside
the reservoir.
MULTILATERAL
WELLS:
A
multilateral well is a well that has two or more drainage holes drilled from a
primary well bore. Either trunk or branches can be horizontal, vertical or
deviated.
Laterals into horizontal hole:
A lateral drilled from a horizontal lateral in the horizontal plane.
Laterals into vertical hole:
Multiple boreholes drilled from a single wellbore. These can be horizontal or
deviated.
Forked:
A lateral can be a horizontal portion of a well drilled from the top of the
reservoir or an entire well deviated from a given point above the top of the
reservoir.
Dual-opposing laterals:
A multi-lateral well with two laterals, usually the two laterals are opposed at
180o emerging from the same wellbore.
Stacked-lateral:
Two or more laterals departing from the same wellbore at different depths.
Multi-branched:
Two or more laterals emanating from a single point.
ADVANTAGES
OF MULTI-LATERALS:
• Increased production from a single well due
to increased reservoir exposure.
• Accelerated production.
• Reduction of surface well equipment and
surface facility costs.
• Multi-laterals provide flexible selectivity
and easy monitoring of oil and gas wells.
• Future plug backs are laid out now avoiding
expensive future re-drills.
MAIN
APPLICATIONS OF MULTI-LATERAL WELLS:
• Tight reservoirs.
• EOR tools.
• Slot recovery.
• Injection/Production from same well.
• Complex drainage reservoirs.
• Structural delineation from first few wells.
• Exploration wells keepers, if main well was
dry.
MULTILATERAL
WELL PLANNING CONSIDERATIONS:
The following is a partial list of some of the
most important considerations in planning a multilateral well:
1. Drilling methods.
2. Junction design
3. Well control issues.
4. Drilling issues.
5. Milling problems.
6. Completion requirements.
7. Multi-lateral requirements.
8. Abandonment.
REFERENCES:
Well engineering and construction (Rabia Hussain)
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