Perforacion Direcciona

lDIRECTIONAL DRILLING TECHNOLOGY
1. Overview of Directional Drilling Technology
While the use of directional (or horizontal) drilling technology has increased dramatically
since the mid-1980's, the technology itself dates back to 1891, when the first patent was granted for
equipment to place a horizontal hole from a vertical well. In 1929, the first truly horizontal wells
were drilled atTexon, Texas and many horizontal wells were drilled in the USSR and China during
the 1950's and 1960's, with limited success. Weakening of oil prices, coupled with the need to
reduce finding costs and the development of new downhole devices, resurrected horizontal drilling
technology in the late 1970's and early 1980's.

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A directionally drilled well
is defined as a well bore that
intersects a potentially productive
f o r ma t i o n a n d d o e s n o t
intentionally exit the formation for
the remaining footage drilled.
Generally, this means that the well
is spudded like a conventional
vertical well, and at a
predetermined "kick-off" point
(KOP), the well is deviated from
the vertical so that thewell bore
enters the formation roughly
parallel to the bedding plane. In
addition to directionally drilled
wells from the surface, some mine
operators drill directional wells
from within the mine working for
degasification and geological
control. Currently, there are six
different techniques available for
drilling horizontal holes, as shown
in Figure 1 and outlined in Table 1.

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Ultra-Short Radius
Zero

TARGET

ZONE

In-Mine Horizontal Well

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Figure 1. Classification of Directional Wells

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Table 1

Classification of Directional Wells

Type
Radius

Radius
(Feet)

Achievable Lateral
Length (Feet)

Method

Zero

0

10

Telescopic probe with hydraulic jet

Ultra-Short

0.5-5.0

200

Coiled tubing withhydraulic jet

Short

35-45

1,500

Curved drilling guide with flexible drill pipe;
entire string rotated from surface

Medium

300-500

1,500

Steerable mud motor used with compressive
drill pipe; conventional drilling technology can
also be used

Long

1,800-2,800

1,500 +

In-Mine

N/A

5,000

Conventional directional drilling equipment
used; very long curve lengthof 2,800 to
4,400 feet needed to be drilled before
achieving horizontal
Uses underground drilling rigs with steerable
motors and position systems to achieve long,
in-seam boreholes

Directional drilling for coalbed degasification is an outgrowth of the techniques developed
for degasification through the use of in-mine horizontal holes and surface vertical, stimulated wells
("conventional"CBM wells) using modified oilfield technology. In-mine horizontal holes have the
advantage of relatively low drilling costs and the ability to intersect the coalbed cleat or fracture
system at right angles to the dominant fracture direction. However, in-mine drilling requires
underground access to the coal and facilities that often interfere with the mining cycle (Diamond and
Oyler, 1986).Additionally, the requirement of access to the coal can limit the value of the
horizontal degasification holes, because of the limited time and/or distance they can be drilled ahead
of mining. Generally, horizontal wells only produce for a three to six-month period before being
mined through.
The various difficulties associated with degasifying coal seams via in-mine drainage
horizontal wellsled the industry to the use of hydraulically fractured vertical wells drilled from the
surface to degasify the coal seam in advance of mining. At first, many mining companies expressed
concern that the hydraulic fracturing process would damage the integrity of the mine roof, thus
creating hazardous mining conditions. These fears were largely put to rest after the publication of
a U.S. Bureau...