For many decades
the only way we could extract natural gas was to drill a well straight down
into the ground. However, in many instances, this is not possible, not
economically feasible, or simply not efficient. Technological advances now
allow us to efficiently deviate from 'straight line' drilling, and steer the
drilling equipment to reach a point that is not directly underneath the point
of entry. While what is known as 'slant drilling', where the well is drilled at
an angle instead of directly vertical, has been around for years, new
technology is allowing for the drilling of tightly curved well holes, and even
wells that can take a 90 degree turn underground.
Directional
drilling is the process of drilling a curved well, in order to reach a target
that is not directly beneath the drill site. This is useful in many
circumstances where the area above the targeted deposit is inaccessible. For
example, to reach reservoirs that exist under shallow lakes, protected areas,
railroads, or any other area on which the rig cannot be set up, directional
drilling is employed. It is also useful for long, thin reservoirs. These types
of reservoirs are not efficiently mined with a vertical completion. However,
horizontal entry into the reservoir allows it to be drained more efficiently.
Directional drilling is especially useful for offshore locations. The cost of
offshore drilling rigs can make it uneconomical to drill a single well. With
directional drilling, the offshore rig can gain access to deposits that are not
directly beneath the rig, meaning that 20 or more wells can be drilled from a
single rig, making it much more cost effective to drill offshore.
Horizontal
Drilling
The difference
between traditional directional or slant drilling and modern day horizontal
drilling, is that with directional drilling it can take up to 2,000 feet for
the well to bend from drilling at a vertical to drilling horizontally. Modern
horizontal drilling, however, can make a 90 degree turn in only a few feet! The
concept of horizontal drilling is not new. In fact, the first patent for
horizontal drilling was issued in 1891 to Robert E. Lee, for drilling a
horizontal drainhole for a vertical well. The advances in technology and the
increasing focus on accessing less accessible reservoirs to meet rising demand
have allowed for a proliferation of horizontal drilling.
Horizontal
drilling technologies have been heralded by many as the greatest advances since
the conception of the rotary drilling bit. Horizontal drilling now accounts for
5 to 8 percent of active onshore wells in the U.S., and seems to be increasing
every year. The ability of horizontal drilling to reach and extract petroleum
from formations that are not accessible with vertical drilling has made it an
invaluable technology. Horizontal drilling allows for an increase in the
recoverable petroleum in a given formation, and even increases the production
in fields previously thought of as marginal or mature. Horizontal drilling also
allows for more economical drilling, and less impact on environmentally
sensitive areas. In fact, in some areas in which drilling is not allowed for
environmental reasons, it is possible to drill horizontal wells to the targeted
deposit without harming the environment above. In addition, with this
technology, fewer wells are needed to produce the same amount of hydrocarbons.
A number of
advances were crucial to the progression of horizontal drilling.
Measurement-while-drilling technology (or 'borehole telemetry') has allowed
engineers and geologists to gain up-to-the-minute subsurface information, even
while the well is being drilled. This avoids some of the complications of
normal logging practices, and greatly increases the drilling engineer's
knowledge of the well characteristics. Steerable downhole motor assemblies have
also allowed for advances in horizontal drilling. While conventional drilling
occasionally employs the use of downhole motors just above the drill bit to
penetrate hard formations, steerable drilling motors allow the actual path of
the well to be controlled while drilling.
There are three
main types of horizontal wells; short-radius, medium-radius, and long-radius.
Short-radius wells typically have a curvature radius of 20 to 45 feet, being
the 'sharpest turning' of the three types. These wells, which can be easily dug
outwards from a previously drilled vertical well, are ideal for increasing the
recovery of natural gas or oil from an already developed well. They can also be
used to drill non-conventional formations, including coalbed methane and tight
sand reservoirs.
Medium-radius
wells typically have a curvature radius of 300 to 700 feet, with the horizontal
portion of the well measuring up to 3,500 feet. These wells are useful when the
drilling target is a long distance away from the drillsite, or where reservoirs
are spaced apart underground. Multiple completions may be used to gain access
to numerous deposits at the same time.
Long-radius
wells typically have a curvature radius of 1,000 to 4,500 feet, and can extend
a great distance horizontally. These wells are typically used to reach deposits
offshore, where it is economical to drill outwards from a single platform to
reach reservoirs inaccessible with vertical drilling.
To give an idea
of the effectiveness of horizontal drilling, the U.S. Department of Energy
indicates that using horizontal drilling can lead to an increase in reserves in
place by 2% of the original oil in place. The production ratio for horizontal
wells versus vertical wells is 3.2 to 1, while the cost ratio of horizontal
versus vertical wells is only 2 to 1. In carbonate formations, where 90 percent
of horizontal drilling is done, productivity of horizontal wells is almost 400
percent higher than vertical wells, while they cost only 80 percent more.
Horizontal
drilling is an important innovation that will likely find countless new
applications as the technology is developed. With increasing demand for natural
gas, innovations like these will be invaluable to securing and bringing to
surface these much needed hydrocarbons.
Reference:
“Directional and
Horizontal Drilling”.
George Gilbert Mattew
Course: KL4220 Subsea Pipeline
Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D./ Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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