Page 202 - Introduction to Petroleum Engineering

PERFORATING 189
Some hydrocarbon formations consist of poorly consolidated sand. This sand
can damage tubing or casing and can plug up surface equipment. To prevent produc-
tion of sand, a variety of screened liners are used, sometimes combined with a
“gravel pack” in the annular region between the liner and the formation. The
“gravel” consists of sand that is about six times larger than the mean size of sand in
the formation.
Many coalbed methane wells are cased, cemented, and then perforated. Some
are completed with open holes. Some open holes are expanded by very high rate gas
production that blows coal fragments and powder out of the well and forms a
large cavity.
For oil and gas wells in shale formations, wells are usually drilled horizontally to
maximize contact between the well and the very‐low‐permeability formation. Some
operators case and cement the horizontals, and some case them without cementing.

10.3 PERFORATING

If a well is cased or lined and then cemented, the connection between the inside of
the casing and the surrounding formation is established by shooting holes through
the casing and cement and into the formation. In the 1930s and 1940s, perforations
were shot with large caliber bullets. But developments in high explosives during
World War II led to shaped charges for perforating. A few operators have used water
jets with abrasives to perforate casing.
A shaped charge for perforation consists of a cup‐shaped device with high
explosive between an outer metallic cup and an inner tungsten layer. The inner tung-
sten layer becomes a high‐speed metal jet when the explosive detonates. The metal
jet punches a hole through the casing and the cement and into the formation. This
tunnel in the formation is surrounded by a zone of crushed rock with the remains of
the metal jet at the end of the tunnel.
To prepare for perforating, the shaped‐charge cups are mounted on a steel strip or
tube in a spiral pattern with a detonator cord running from the top to the bottom of the
strip or tube. The detonator cord is wrapped to touch a primer on each cup. The strip
or tube is then mounted in a steel pipe, or hollow carrier, to complete the “gun.” At the
top of a gun, a blasting cap is connected to the detonation cord. The blasting cap can
be activated by an electric signal through wireline to the surface or by a pressure
switch. The length of perforation guns varies from 1 ft to about 20 ft, but guns can be
linked to form longer perforating assemblies. The diameter of guns ranges from about
2 to about 7 in. Larger diameter guns can carry larger shaped charges.
Perforating guns can be lowered to the target formation on wireline or tubing.
Gravitational force is sufficient to pull the guns on wireline for vertical and modestly
deviated wells. For horizontal wells, tractors are often used to pull the guns on wire-
line. When using wireline, the gun assembly will normally include a casing‐collar
locator. The collar‐depth data from the locator on the gun is correlated to the collar
depths observed during cement‐bond logging, which are correlated with the gamma
log from the cement‐bond log to the open‐hole logs. With the gun at the correct
location, the wireline operator fires the shaped charges with an electric signal.