Wells – Processes – Perforating – weakening – bending or separating pipe at an...
Reexamination Certificate
2000-07-21
2003-02-18
Bagnell, David (Department: 3672)
Wells
Processes
Perforating, weakening, bending or separating pipe at an...
C166S055000, C102S312000
Reexamination Certificate
active
06520258
ABSTRACT:
BACKGROUND
The invention relates to explosive devices, such as perforating guns and other types of explosive devices used in wellbore applications.
To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and one or more guns fired to create openings in casing and to extend perforations into the surrounding formation.
A perforating gun typically includes a gun carrier on which multiple shaped charges are mounted. One type of shaped charge is the capsule shaped charge, which is sealed by a capsule to protect explosive material from corrosive fluids and elevated temperatures and pressures in the wellbore. Other types of shaped charges include non-capsule charges that are carried in sealed containers or hollow carriers.
Referring to
FIG. 1
, a generally conical shaped charge
10
includes an outer case
12
that acts as a containment vessel designed to hold the detonation force of the detonating explosion long enough for a perforating jet to form. Common materials for the outer case
12
include steel or some other metal. With a capsule charge, the outer case
12
may be part of the capsule housing, and a cap (not shown) is attached to the front of the case
12
to keep the explosive
16
and generally conical liner
20
sealed from the wellbore environment. A non-capsule charge may be arranged as illustrated in
FIG. 1
, with the liner
20
exposed.
The main explosive charge
16
is contained inside the outer case
12
and is sandwiched between the inner wall of the outer case
12
and the outer surface of the liner
20
. A primer column
14
is a sensitive area that provides the detonating link between a detonating cord
15
(attached to the rear of the shaped charge) and the main explosive charge
16
. A detonation wave traveling through the detonating cord
15
initiates the primer column
14
when the detonation wave passes by, which in turn initiates detonation of the main explosive charge
16
to create a detonation wave that sweeps through the shaped charge
10
. The liner
20
collapses under the detonation force of the main explosive charge
16
. Material from the collapsed liner
20
forms a perforating jet that shoots through the front of the shaped charge
10
, as indicated by the arrow
22
.
The diameter and depth of a perforation tunnel created in a well formation is determined by the speed and geometry of the perforating jet as it enters the formation. The symmetry and stability of the perforating jet, which are important to promote a long straight perforation tunnel, may be adversely affected by shock waves generated by detonation of neighboring charges. As a perforating jet enters the surrounding wellbore liquid, the jet creates a cavity inside the liquid. The shock waves from the charge itself and from surrounding charges can collapse the cavity so that the liquid can interfere with the jet.
To reduce charge-to-charge interference, some predetermined separation is needed between shaped charges in a perforating gun. In conventional systems, perforator performance decreases with increasing shot density (above some critical value of shot density) and with increasing gun-to-casing clearance (the amount of water or other liquid the perforating jet has to traverse). The performance decrease is typically greater for perforating systems with capsule charges because of the direct coupling of the exploding charge case to the wellbore fluid. The cause of the performance degradation may be due to the interaction between explosive induced shock in the wellbore fluid and either the perforating jet or the perforator itself during formation of the jet.
Another issue associated with perforating and other types of explosive systems is the potential for damage to downhole equipment. For example, the perforating gun itself, the casing, and other components may be damaged by the shock induced by an explosion.
Another type of interference is “pre-shock” interference, in which the detonation wave traveling through a detonating cord (e.g., the detonating cord
15
in
FIG. 1
) interferes with the performance of the shaped charge. The strand of detonating cord
15
may be attached to a plurality of shaped charges that are mounted on the gun carrier. For a single-directional perforating gun, such as a 0°-phased perforating gun, the strand of detonating cord
15
extends generally along a straight line. The shaped charges may also be mounted in a phased arrangement, such as a spiral arrangement or some other phasing pattern. With shaped charges arranged in a spiral arrangement, the detonating cord extends in a generally helical fashion. In some other phased arrangements, such as a ±45° twisted arrangement, the detonating cord
15
may be weaved in a fairly tortuous path across the rear surfaces of the charges. In all these arrangements, the detonating cord
15
traverses across substantial parts of the rear surfaces of the outer case
12
of the shaped charges
10
.
As illustrated in
FIG. 1
, the detonating cord
15
makes contact with, or is in near proximity to, a substantial portion of the rear surface of the shaped charge
10
. A detonating wave travels through the detonating cord
15
at high speed, typically about 6-8 km/s (kilometers per second). The detonation wave transfers energy to the primer column
14
to detonate the shaped charge
10
. However, the detonation wave also transfers a high pressure shock, referred to as pre-shock, to the portion of the outer case
12
in contact with or in close proximity to the detonating cord. The pre-shock may also be transferred from the detonating cord to the outer case
12
through a liquid (such as water in the wellbore). Since the outer case
12
is typically made of a metal such as steel, which is a material having high shock transmissibility, the shock transferred to the explosive
16
may be significant.
Thus, an instance in time before the initiation energy of the detonating cord
15
reaches the primer column
14
, a pre-shock may have been applied through the outer case
12
, which is communicated into the explosive
16
. The propagation of the pre-shock wave through the outer case
12
and the explosive
16
may interfere with the initiation front from the primer column
14
into the explosive
16
. This may cause an asymmetry in the resultant collapse of the shaped charge liner
20
. Possible adverse effects of such pre-shock interference may include one or more of the following: the perforating jet may have a crooked (rather than a straight) tip, and the cross-section of the jet may be elliptical rather than generally circular. Such adverse effects may reduce the penetration depth of a perforating jet produced by the shaped charge.
In some more severe situations, particularly with insensitive explosives having relatively slow detonation speeds, a mis-fire may occur due to the pre-shock wave reaching the explosive
16
through the outer case
12
before the main initiation front through the primer column
14
. In this case the pre-shock wave densifies the explosive
16
before the main initiation front reaches the explosive
16
, which may cause the mis-fire.
Some conventional methods of reducing unwanted pre-shock may include the following. A separation gap may be provided between the detonating cord and the outer case. Another solution is to provide a longer primer column
14
. The thickness of the outer case
12
may also be increased to increase the length of the path that the pre-shock wave has to traverse before encountering the explosive
16
of the shaped charge. Another solution involves reducing the amount of explosive in the detonating cord to reduce the pre-shock level. Another technique is to use a detonating cord with conventional plastic jackets of standard thicknesses instead of metal jackets. Although such solutions reduce the effects of shock to some degree, they
Behrmann Lawrence A.
Fayard Alfredo
Grove Brenden M.
Markel Daniel C.
Yang Wenbo
Bagnell David
Dougherty Jennifer R.
Griffin Jeffrey E.
Jeffery Brigitte
Schlumberger Technology Corp.
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