Ammunition and explosives – Blasting – Detonation wave modifying
Reexamination Certificate
1999-04-22
2001-05-01
Nelson, Peter A. (Department: 3641)
Ammunition and explosives
Blasting
Detonation wave modifying
C102S307000, C102S313000, C102S476000
Reexamination Certificate
active
06223656
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the production of hydrocarbons from a borehole. More particularly, the invention relates to a method and apparatus for perforating and fracturing a formation surrounding a borehole.
2. Description of Related Art
Techniques for perforating and fracturing a formation surrounding a borehole are known in the art. The most common technique for perforating and fracturing a formation to stimulate production includes the steps of: 1) penetrating a production zone with a projectile, such as a shaped charge; and 2) hydraulically pressurizing the borehole to expand or propagate the fractures initiated by the shaped charge.
Modern shaped charges are widely used for both military and commercial applications. Although the main operation is remarkably similar in both applications, there are at least two significant differences in the devices actually employed. One difference is cost. Military applications generally demand much higher performance and, in particular, high reproducibility. This, in turn, requires the liner portion of the shaped charge to be forged and precision machined.
In the commercial use of the shaped charge in oil or gas well stimulation, the jet from the shaped charge is employed to create a flow path from the reservoir to the wellbore. In this application, a large number of perforators is inserted into the wellbore in what is called a gun. Although there are three basic types of guns, perhaps the most common is the casing gun, which can be run into the well on a wireline or conveyed by tubing. The charges are contained in a steel tube, protected from impact and from the well fluids, and are arranged so that they face radially outward from the vertical axis of the carrier. In these devices, the liners are pressed using powder metal technology and are relatively less expensive than those used in typical military uses, e.g., missile warheads.
Another factor that distinguishes commercial shaped charges from those used in weapons is standoff, i.e., the distance from the liner base to the target (usually measured in charge diameters). The penetrating effectiveness of a shaped charge jet is markedly enhanced by standoff. The reason is that shaped charge jets normally are formed with a high axial velocity gradient, the tip moving at speeds of 6-10 km/s. The standoff distance allows the jet to stretch or elongate before encountering the target and, to first order, the depth of penetration is directly proportional to the length of the penetrator. There is an optimum standoff. If the distance to the target is too great, the penetration can be much less than if there were no standoff. This occurs because the jet can only stretch a given amount before breaking; once broken the particles are easily deflected by small perturbations and no longer produce a coherent, unidirectional penetrator. With optimal standoff, typically 6-8 charge diameters (CD), the penetration can be enhanced by 50% or more, relative to that achieved with zero standoff. Commercial perforators, however, are rarely able to operate at more than 1 CD because they must fit inside the casing gun which, in turn, must fit inside the casing.
Techniques to increase the efficiency of hydrocarbon production in the borehole utilizing guns and pressurizing sections of the borehole have been described in the recent past. For example, Petitjean, U.S. Pat. No. 5,355,802, describes a method and apparatus for firing a shaped charge through a gas zone of propellant combustion gases. A propellant is ignited downhole, releasing gas into the borehole to pressurize a portion of the borehole. The firing of the shaped charges is delayed until the pressure level is significantly above the breakdown pressure of the formation, but still below that of the casing. Although such a technique can improve the penetration effects of shaped charge perforators, a need still exists to continually improve penetration of a reservoir surrounding a borehole.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for perforating and fracturing a formation surrounding a borehole and propagating that fracture to increase the efficiency of hydrocarbon production in the borehole. The invention is advantageous because it benefits from the energy of shaped charges to perforate and initiate fractures in the formation. In addition, it provides better propagation of the fractures. The greater efficiency is achieved by pressurizing an interior section of a sealable container of a downhole tool, such as a casing gun, using a light gas, i.e., a gaseous substance which has a density less than air at the same conditions of temperature and pressure. The light gas is usually supplied (pressurized) and sealed at the earth's surface; although gas-generating materials which release the light gas within the gun prior to the firing of the shaped charges can also be employed. The travel of the shaped charge jet in the light gas atmosphere results in a longer, more narrow and stable jet—thus greater penetration.
According to one aspect of the invention, a casing gun, containing shaped charges surrounded by the pressurized light gas within the gun, is positioned in a production zone of a borehole. The shaped charges are fired and their liners collapsed within the light gas atmosphere. The resulting shaped charge jet perforates the casing gunwall, penetrating through the wellbore fluids, through the well casing wall, into the reservoir rock and concomitantly the escaping light gas from within the gun increases the pressure level in the production zone. The pressure level in the production zone can be increased to significantly above the breakdown pressure of the formation. To maximize the efficiency of the technique in a cased hole, the pressure level within the gun can approach the maximum that can be applied to the wall of the gun and/or well casing; however, penetration of a shaped charge jet has been shown by experiment to be enhanced by at least 40% (vs. air) by imploding a liner in the light gas atmosphere at pressures in the range from about 1,500 psia to about 5,000 psia.
The fired shaped charges, creating the perforation tunnel through the wall of the casing gun and well casing, help to initiate fractures at particular locations in the borehole. Thus, the shaped charges are designed to accomplish a dual purpose. First, the shaped charges perforate the well casing. Second, after passing through the well casing they continue their penetration into the formation sometimes initiating a fracture. Such penetrations travel deeper than the procedures of previously known techniques. Increased efficiency is achieved at the initial penetration by increasing the jet length by squeezing on its periphery, which also produces a highly stabilized shaped charge jet. This is enabled by the firing of the shaped charges through the pressurized gas zone of substantially lower density within the gun instead of the higher density of conventional surrounding gases, such as air. The less dense, light gas zone permits effective collapse of the liner of the shaped charge.
The invention provides superior results to those obtained by the prior art because unlike the prior art, the pressure within the tool can be maximized at the time the shaped charges are fired, thus providing increased jet length and stability, and the shaped charge liners and jet can function within a light gas atmosphere to improve jet penetration. Unlike techniques that release gas from a casing gun into the well casing outside the casing gun (via gas propellant materials) as described by Petitjean, the present invention allows the shaped charge liner to collapse against a less dense gas, thus initiating the formation of the shaped charge jet within the casing gun to create greater jet length for extended penetration. Upon firing of the shaped charges, the method of the invention provides increased perforation of the well casing and initiation of the fracture in a single step.
REFERENCES:
patent: 2759418 (1956-
Nelson Peter A.
The Regents of the University of California
Thompson Alan H.
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