Directional Drilling

Monday, September 21, 2009 by ShoXee

Introduction

Directional drilling began with the use of devices such as whipstocks or
techniques such as jetting to kick off, rotary assemblies to control inclination
in tangent sections, and wireline steering tools to orient and survey. These
tools possessed limited directional control capabilities, required frequent
tripping of the drillstring, and made directional drilling an expensive,
difficult, and sometimes risky proposition. Directional well planning was
more an art than a science, and capabilities and boundaries were based
largely on empirical observations and historical tool performance.
Recently, technological advances have contributed to a significant increase in
the use and scale of directional drilling. Perhaps the technologies with the
highest impact have been steerable mud motors, measurement-while-drilling
(MWD) tools, and logging-while-drilling (LWD) tools. These tools in
combination have provided the ability to follow complex, 3D well profiles
without changing bottomhole assemblies (BHAs), and to measure where the
bit has drilled without having to run a wireline to survey or log. Equally
important, engineering models have provided the fundamental tools for
evaluating drillstrings, hydraulics, BHAs, and the drilled formations
themselves. These advances have enabled the drilling of extended-reach,
horizontal, and multiple-target well profiles once thought impractical,
uneconomical, or impossible.
Most books currently available either discuss deviation control—the attempt
to keep vertical wells truly vertical—or discuss older directional-drilling
practices, such as whipstocks, jetting, or the use of straight mud motors
with bent subs. These practices, while still in use, are now the exception
rather than the rule. This chapter, then, primarily discusses directional
drilling as it is performed today. Emphasis will be placed on identifying the
principles and mechanics that define the capabilities and limitations of
current directional-drilling technology.

Well Planning

Planning even the simplest vertical well is a task that involves multiple
disciplines. A casual observer might think that planning a directional well
would require only a few geometry calculations in addition to the usual
tasks. On the contrary, almost every aspect of well planning is affected when
a directional well is planned. Various software systems are available to assist
in these engineering efforts, but effective application of such software
requires a good understanding of the underlying engineering principles. The
fundamental variables that dictate the planned wellpath are the surface
location for the rig and wellhead and the location(s) of the target(s)
downhole. However, many other variables also impact the final wellpath
chosen.

Well Profiles and Terminology

A simple build/hold/drop well profile, known as an "S" well, is shown in
Figure 2-1. The kickoff point (KOP) is the beginning of the build section. A
build section is frequently designed at a constant buildup rate (BUR) until
the desired hole angle or end-of-build (EOB) target location is achieved. BUR
is normally expressed in terms of degrees per hundred feet (°/100 ft), which
is simply the measured change in angle divided by the measured depth (MD)
drilled.

Hole angle, or inclination, is always expressed in terms of the angle of the
wellbore from vertical. The direction, or azimuth of the well is expressed
with respect to some reference plane, usually true north. The location of a
point in the well is generally expressed in Cartesian coordinates with the
wellhead or the rig's rotary kelly bushing (RKB) as the reference location.
True vertical depth (TVD) is usually expressed as the vertical distance below
RKB. Departure is the distance between two survey points as projected onto
the horizontal plane. The EOB is defined in terms of its location in space as
expressed by coordinates and TVD. The EOB specification also contains
another important requirement, which is the angle and direction of the well
at that point. The correct angle and direction are critical in allowing the next
target to be achieved; also, it may be necessary to penetrate the payzone at
some optimum angle for production purposes.
A tangent section is shown after the build section. The purpose of the
tangent is to maintain angle and direction until the next target is reached. In
the example well, a drop section is shown at the end of the tangent. The
purpose of a drop is usually to place the wellbore in the reservoir in the
optimum orientation with respect to formation permeability or in-situ
formation stress; alternatively, a horizontal extension may be the preferred
orientation in the case of a payzone that contains multiple vertical fractures
or that has potential for gas or water coning.

Factors in Wellpath Design
Completion and reservoir drainage considerations are key factors in wellpath
design. For fracturing, gravel-packing, completion in weak formations, or
depletion-induced compaction, it may be desirable to limit the inclination of
the well through the reservoir or even to require a vertical or near-vertical
trajectory. These conditions are also true in laminated or layered reservoirs.
Often, it may be desirable for the wellpath in the reservoir to be horizontal
to provide as much reservoir drainage and production rate as possible. In
horizontal wells, correct TVD placement will minimize gas coning or water
production. In vertically fractured formations in which the fractures may aid
in the flow of hydrocarbons, the direction of the wellpath in the reservoir
may be chosen to intersect multiple fractures. Alternatively, it may be
desirable to place the wellbore in a given direction to avoid faults that are
expected to allow water migration. Optimal placement of the wellbore in the
reservoir will result in maximum production and should actually be the
starting point for wellpath design.

Additional considerations will influence the design of the trajectory from the
surface location to the reservoir-target entry point. Some shallow formations
in sedimentary geologies are weak, and, as a result, building inclination is
difficult because of the lack of reactive forces against the BHA. If this
condition is anticipated, the KOP should be designed deeper, where
Frmations are more competent.

The interrelationship of the wellpath design and the casing/hole program
must also be recognized. The casing/hole program for the well is generally
designed on the basis of the desired completion, the pore pressure regimes
for the well, the presence of trouble zones, and regulatory requirements.
The casing program influences the planned trajectory in several ways. For a
given casing design, the trajectory plan should be optimized for operational
efficiency. For example, required builds and turns should be executed fully
within a single hole section. When this method is used, the well will be "lined
out" towards the reservoir target, and the remaining hole section can be
drilled as a straight tangent section without additional directional work.
Likewise, in troublesome zones, such as underpressured sands or reactive
shales that can increase the risk of stuck pipe, it may be desirable to avoid
directional work that requires sliding (drilling without rotating the drillstring).
Thus, the design of a build section may need to include a short tangent
through the troublesome section that will allow it to be rotary-drilled as
rapidly as possible.
When the various constraints are considered, a feasible and optimized
directional trajectory plan should result. An optimized wellpath often cannot
be described by the simplest geometry that can be conceived to connect a
series of targets. Even with a simple build-and-hold profile, additional
wellpath optimization is possible. For example, drilling experience in an area
should allow for the definition of the typical walk rate (the tendency of the
BHA to turn slightly in the azimuthal direction) for certain BHAs in that area.
With walk rates defined, the well can and should be "led," or initially directed
away from the target in the direction opposite to the anticipated walk. If the
well is properly led, steering will not be required in this interval, since the
natural walk tendency will gradually bring the well into the target. If the
wellpath had been designed as a straight line from one target to the next,
frequent steering would be required throughout the interval to counteract
the natural walk tendency.
Modeling the Wellpath
In addition to refining the trajectory plan to account for drilling tendencies
such as walk, trajectory planning in development projects must also account
for the location of existing wells and the requirement that the planned well
safely bypasses all existing wellbores. This aspect of planning, known as
"collision avoidance," must account for the uncertainty associated with the
ability to survey the well.
Wellbore trajectory calculation methods use data points called survey
stations, each of which consists of inclination, azimuth, and measured depth.
Directional measurements are normally provided by MWD sensors, and
measured depth is provided by traveling block sensors or by pipe tally. Many
survey models are available (Bourgoyne et al., 1991; Craig and Randall,
1976), and each is based on different assumptions on the shape of the
wellbore between survey stations. Except for the tangent method, most
models provide virtually identical results. The most commonly used survey
calculation method is the minimum curvature method (Figure 2-3).