Ammunition and explosives – Blasting – Detonation wave modifying
Reexamination Certificate
2002-01-17
2003-12-30
Nelson, Peter A. (Department: 3641)
Ammunition and explosives
Blasting
Detonation wave modifying
C102S476000
Reexamination Certificate
active
06668726
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to liners for shaped charges and more particularly to liners for shaped charges of the type used in perforating gun systems.
BACKGROUND OF THE INVENTION
Development of an oil or gas well often involves fixing a tubular steel casing in cement in an underground well borehole. Holes, or perforations, are subsequently created in the steel well casing and surrounding cement in order to gain access to the surrounding formation, e.g., an oil or gas bearing stratum. Such holes are generally created through a process known also as perforation using a perforating gun.
Perforation is a process of piercing the well casing, the surrounding cement, and rocks in the surrounding formation to provide fluid communication between the oil or gas deposit and the interior of the well. Explosive charges are typically used to pierce the well casing. Perforation involves introducing a firing device, commonly termed a perforating gun, into the well, positioning the perforating gun at a desired depth, and detonating the gun. The process of locating a perforation gun in position is sometimes referred to as “delivering” the gun. After detonation, the gun may be retracted or released and dropped to the bottom of the well. If discarded, the size of the gun is limited by the depth of the bottom hole available.
Several perforating methods have been used to deliver and detonate a perforating gun. For example, the “wireline” process involves attaching a perforating gun, or string of guns, to a long, flexible steel cable paid out from a truck-mounted drum at the surface. An electrical conductor, paid out with the cable and connected to the gun, or the gun in the string nearest to the surface, carries an electrical signal to energize the perforating gun detonator. Alternatively, tube conveyed perforating (TCP) employs straight production tubing to carry or convey the perforating gun to a desired position in the well. The gun may be activated by a drop bar. The gun may also be activated by hydraulic means. For example, fluid in the tubing may be pressurized sufficiently to activate a hammer and firing pin of a percussion detonator on the gun.
An explosive charge is typically contained in a charge assembly that may include a casing and a metal liner. The casing may typically have a recess having an inner wall structure and an opening. The explosive charge is packed against the inner wall of the charge casing. The metal liner may line the explosive charge opposite the casting. As such the explosive charge is contained between the liner and the casing. The shape of the explosive charge is defined by the space between the inner wall of the casing and the metal liner and is thus referred to as a “shaped charge”. This space often has a concave, typically generally conical, shape. The term “shaped charge” may also refer to a charge assembly. The shape of such a charge may be varied, depending on the pattern of perforations desired, the size and number of perforations desired, and the depth of the perforations in the surrounding mineral bearing stratum.
A perforating gun may typically include an elongate member in the nature of a hollow tube having openings, or seats, into which the shaped changes are mounted. Several charge assemblies are generally arranged along the length of the elongate member. Typically, a detonation cord runs along the perforation gun between, and is connected to, the charges. Typically, the shaped changes are mounted such that the wide part of the conical shape faces radially outwardly, i.e., away from the gun, and toward the wall of the well bore, generally having a central axis aimed in a plane transverse to the length of the elongate member. Different explosive charges may face radially outwardly in different angular directions in the plane or in spaced, parallel planes to produce a helical perforation pattern.
When detonated, each shaped charge produces a compressive shock wave. This may collapse the liner and propel the central portion of the liner at a very high speed, possibly of the order of about 10,000 m/sec, thereby forming an explosive central jet. This jet pierces the well casing and the surrounding cement, and penetrates some distance into the oil bearing formation. The differently facing charges explode radially outwardly in different angular directions into the oil-bearing formation. The result is a perforated wall, like a colander, that facilitates entry of oil or gas into the well.
The outer, slower moving portion of the liner may have a tendency to form a slug, sometimes called a “carrot” due to its shape. The slug can cause numerous problems. The slug may embed itself in the exit hole of the perforating gun and cause mechanical difficulties in removing the perforating gun from the well borehole. The slug, when embedded in the perforation pierced by the explosive jet, may tend to impede the outflow of oil or gas, thus reducing the performance of the well. Sometimes, the slug may be carried by the gas or oil flow to the surface and cause malfunction of surface devices. The slug may also fall from the perforation gun down the well borehole into other downhole devices, possibly causing these devices to malfunction.
These problems caused by slugs have long been recognized. Efforts continue to be made to minimize or eliminate the formation of slugs. For example, U.S. Pat. No. 5,098,487, issued to Brauer et al. on Mar. 24, 1992 (“Brauer”), gives an account of various solutions directed at minimizing slug formation.
The majority (perhaps up to 90%) of liners used in the field are constructed of compacted metal powders. Metal powder liners tend to pulverize upon striking the well casing, and thus do not tend to cause undue formation of slugs. However, this type of liner may tend to have other disadvantages. They tend to be used in a green (i.e., unsintered) state, and as such may tend also to be relatively fragile. Care must be taken in their handling and assembly. Sintering, such as the process disclosed in U.S. Pat. No. 6,012,392, issued to Norman et al. on Jan. 11, 2000 (“Norman”) may reduce this fragility, but is sometimes considered an unnecessary manufacturing step, particularly when it is often desirable for the device to fragment upon detonation.
Compacted metal powder liners in the green state also tend to be porous. There may be water at a depth in the well at which the shaped charge is to perforate holes in the well casing. Water may leak more easily through a porous liner and dampen the explosive charge lined by such a liner. This may cause detonation difficulties.
Often, liners made of compacted metal powders tend to be formed individually. Compared with liners formed in batches, individually formed liners may tend to have increased cost, and their product quality may tend to be less consistent and reliable. Additionally, because liners made of unsintered metal powders often pulverize upon striking the well casing, they may tend to be less effective for perforating large holes.
“Large holes” in this context may be holes of diameter up to about 1 inch. “Large holes” are often required for wells of heavy oil. Heavy oil, having higher viscosity, may tend to flow more easily from the surrounding oil bearing formation through these large holes and into the well. To obtain good production performance from such a heavy oil well, deep penetration with a depth of up to 30 inches may often be desired as well. Solid liners made of relatively heavy material may tend to be more effective for producing holes satisfying those requirements. This type of liner typically accounts for most, if not all, of the remaining 10% of liners in use. However, this type of liner has the tendency of forming relatively large slugs.
Zinc and zinc alloys have been used as a material for the outer casings of shaped charges. A casing made of zinc or a zinc alloy may tend to disintegrate without forming significant debris upon explosion of the explosive charge contained inside such a casing. The long held belief has been that some other material is required f
Innicor Subsurface Technologies Inc.
Nelson Peter A.
LandOfFree
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