BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a pictorial drawing illustrating a perforating gun system.
FIG. 2 is a cross-sectional view of a shaped charge that is utilized in the perforating gun system of FIG. 1.
FIG. 3 is a pictorial drawing which illustrates a jet that has been produced by a shaped charge as illustrated in FIG. 2 where the jet is in the process of penetrating the target material.
FIG. 4 is a pictorial drawing which illustrates a jet whose penetration velocity is below the acoustic velocity of the formation material and from which the shock wave has become detached.
FIG. 5 is a pictorial drawing which illustrates the effects of the detached shock wave of FIG. 4 encountering a preexisting fracture or other discontinuity in the formation material.
FIG. 6 is pictorial drawing which illustrates a jet whose penetration velocity is below the acoustic velocity of the formation encountering a region in the formation which is in tension, the tension having been created due to the shock wave pulling away from the jet/target interface.
FIG. 7 is a pictorial drawing which illustrates the effects of the situation in FIG. 6.
FIG. 8 is a pictorial drawing of a jet whose penetration velocity exceeds the acoustic velocity of the formation material and to which the shock wave remains attached.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
It will be appreciated that the present invention may take many forms and embodiments. In the following description, some embodiments of the invention are described and numerous details are set forth to provide an understanding of the present invention. Those skilled in the art will appreciate, however, that the present invention practiced without those details and that numerous variations from and modifications of the described embodiments may be possible. The following description is thus intended to illustrate and not limit the present invention.
Referring first to FIG. 1, a perforating system 100 in accordance with the present invention comprises at least one perforating gun section 101, with two such gun sections 101 being illustrated in FIG. 1. Each of the perforating gun sections 101 comprises a plurality of radially-oriented shaped charges 102, which may, for example, be placed at points along a spiral that extends around the longitudinal axis of perforating gun 101. The shaped charges 102 are detonated using firing head 103 to form perforations in the formation 106. The perforating system may comprise additional joints of tubular members 104, and the number of tubular members 104 that are used in the perforating system will be determined by the depth to which the perforating gun sections 101 are to be lowered in wellbore 105.
With reference now to FIG. 2, each of the shaped charges 20 used in the perforating gun system 100 of FIG. 1 includes a metal liner 21, a metal case 22 and a main body of high explosive material 23 disposed between the metal liner 21 and the metal case 22. The apex 24 of each shaped charge 20 is adapted to receive a detonation signal from firing head 103 to detonate the shaped charge 20.
With reference to FIGS. 2 and 3, when the shaped charge 20 is detonated, it produces a jet 30. Jet 30 first penetrates the casing and then enters the formation material 106, which is the target material. As noted above the jet-target interface 31 will advance at a penetration velocity U according to the formula:
u/(v−u)=sqrt(ρj|ρt)
where V is the velocity of the jet, ρj is the jet density and ρt is the target density.
With reference to FIG. 4, it is well known that a shock wave can precede the advancing jet-target interface 31. It is believed that any discontinuities (fractures, laminations, etc.) within the formation 106 can affect the passing shock wave 32. Specifically if the discontinuity exhibits an impedance mismatch (acoustic impedance=density*acoustic velocity), this would cause some portion of the incoming shock to be reflected, and some to be transmitted. A compressive shock will always be transmitted beyond the discontinuity, but the reflected shock wave could be either compressive or tensile, depending on whether impedance increases or decreases at the interface. In either case, the reflected wave traveling back may disturb the incoming jet 30.
With reference to FIG. 4, there is illustrated a situation where shock wave 32 which has become detached from the jet-target interface 31, where the jet-target interface 31 advances at a penetration velocity that is less than the acoustic velocity of formation 106. In FIG. 5, the detached shock wave 32 of FIG. 4 has encountered a fracture 34, which results in a portion 35 of the shock wave 32 being reflected back toward the jet-target interface 31. This reflected portion 35 of shock wave 32 adversely affects the penetration velocity of the jet-target interface 31.
In addition to shock reflection (which is a consequence of impedance mismatch), it is believed that the compressive shock wave 32 traveling ahead of the jet-target interface 31 may be followed by a region in tension, since the velocity of the detached shock wave is greater than the penetration velocity of the jet-target interface 31. With reference to FIG. 6, such a region in tension 36 is illustrated. If the formation material 106 has a low tensile strength, the region 36 may serve to open existing factures or may create and open new fractures, where such opened existing fractures or new fractures are designated 38 in FIG. 7. This situation produces the same impedance mismatch previously discussed, with the result being that the reflected portion 35 of shock wave 32 adversely affecting the penetration velocity. Since the situation illustrated in FIG. 7 gap-target), total penetration effectiveness (depth) of the perforating jet is reduced.
Referring to FIG. 8, a charge in accordance with the present invention, is provided for a perforating gun, where the charge, when detonated, produces a jet 30 with a penetration velocity that will always exceed the acoustic velocity of the formation 106. In such a situation, shock wave 32 remains attached (or nearly attached) to the jet-target interface 31, and the perforation is expected to extend deep into the formation 106. Using a charge having the characteristic illustrated in FIG. 8 is especially useful when the formation 106 is a fractured material, such as coal, or has a low tensile strength.
Patent Application
20070295235
