Patent Grant
4598776
This application is related in subject matter to: U.S. patent application Ser. No. 743,580, entitled "Method and Apparatus for Initiating Subterranean Well Perforating Gun Firing from Bottom to Top"; U.S. patent application Ser. No. 743,429, entitled "Perforating Gun for Initiation of Shooting from Bottom to Top"; and U.S. patent application Ser. No. 743,578, entitled "Boosterless Perforating Gun and Method of Assembly". 1. Field of the Invention The invention relates to a method and apparatus for effecting the reliable firing of a perforating gun for a subterranean well, and particularly a multisection gun with each section having a plurality of shaped charges mounted for substantially concurrent firing to produce perforating zones on the order of 50 to 2,000 feet in length. 2. History of the Prior Art The utilization of a plurality of vertically and angularly spaced shaped charges to effect the perforation of a subterranean well represents the modern approach to achieving perforation of the well casing and the adjoining production formation. Such shaped charges are generally substantially concurrently fired by the ignition of a primer cord which is positioned adjacent the ignition ends of each of the shaped charge containers in the gun section. It is now not uncommon for the zone to be perforated to extend from 50 to 1,000 feet in length, thus necessitating the fabrication of the perforating gun as a plurality of axially stacked, substantially identical gun sections. The transmission of the firing energy from the uppermost section to the lowermost section is commonly accomplished by the mounting of booster charges on each of the ends of a length of primer cord which is utilized only within an individual gun section. The booster charges are required to insure that sufficient detonation energy is transmitted from the bottom end of one primer cord to the top end of the next primer cord to insure the successive detonation of all primer cords. There have been many instances of unsuccessful transfer of detonation energy from one gun section to the next, necessitating the very costly and time-consuming procedure of removing the apparatus from the well in order to reconnect the uppermost primer cord where detonation failed, to a suitable booster charge and, in turn, position a new firing mechanism in proximity to the booster charge. The exact cause of failures to transfer detonation energy from one gun section to the next is not readily pinpointed. Of course, a defective length of primer cord or a defective booster would interrupt the detonation of the particular primer cord. Booster charges are conventionally secured to the ends of the primer cord by a crimping operation, and this operation may very well damage the explosive core of the primer cord. The primer cord may have picked up water or may have become desensitized by grease. Any of these adverse elements can cause the detonation velocity of the primer cord to drop off to a low order, insufficient to effect the firing of the shaped charge containers and, in fact, causing the interruption of the detonation wave down the primer cord. Thus, in a 2,000-foot multisection perforating gun, it is not uncommon for several hundred feet of gun to be successively fired, only to have the detonation wave interrupted for one or more of the above stated reasons and thus leaving a very large number of shaped charges which have not been fired. The only cure is to expend the necessary rig time to pull out all of the gun sections and then reinsert in the well a length of gun equivalent to the extent of the production zone which was not perforated by the initial firing of the perforating gun. Obviously, any procedure or apparatus which would provide greater assurance that all of the successive lengths of primer cord will be detonated, thus hopefully insuring the detonation of most, if not all, of the shaped charges, can be of tremendous value to the oil well completion industry. The invention is particularly applicable to multisection perforating guns to be utilized in a subterranean well when a substantial length of production zone is to be perforated in a single operation. A multisection perforating gun of conventional configuration is assembled in conventional fashion through the axial stacking of a plurality of substantially identical perforating gun sections. Each gun section comprises a plurality of vertically and angularly spaced, shaped charge containers supported by suitable carrier. An individual length of primer cord is provided for each gun section, which cord passes closely adjacent to the ignition end of the shaped charge containers disposed in the particular gun section. A conventional booster charge is secured to both the top and bottom ends of the length of primer cord in the gun section and these booster charges are utilized to insure the transmission of detonating energy from the uppermost gun sections to each of the successively lower gun sections or vice versa. In accordance with this invention, a backup or duplicate system for transmission of detonating energy from one gun section to the next lower gun section is provided. A length of flame-conveying element or fusible cord, preferably a flexible tubing element comprising a plastic tube having an explosive powder adhered to its interior bore, is inserted in each gun section in parallel relationship to the primer cord. In accordance with one embodiment, the upper end of the flame-transmitting tubing is adjacent the upper end of the primer cord and its upper end is ignitable by the lower booster charge in the upwardly adjacent gun section. The lower end of the flame-transmitting tubing is secured to any convenient medial portion of the length of primer cord disposed in the respective gun section. Intermediate the flame-transmitting tubing and the primer cord, a secondary booster charge is connected. The energy of this booster charge is sufficient to convert the relatively low-deflagration rate of flame-transmission produced by the flame-transmitting tubing to a detonating wave of sufficient intensity to detonate the adjacent medial portion of the primer cord. Such booster charges are commonly referred to as DDT charges, such as those sold under the trademark "PRIMADETS" by the Ensign Bickford Co. of Simsbury, Conn. In accordance with another embodiment of the invention, a length of detonation-transmitting tubing is inserted in each perforating gun section at the factory and passes through an axially parallel aperture in the coupling nipple secured to the upper end of the perforating gun section. The upper end of the detonation-transmitting tubing is disposed in a circular configuration and lies against the upper end face of the connecting nipple. The lower end of the detonation-transmitting tubing passes through a similar radially displaced axial passage provided in the connecting sub and terminates in a charge, such as a DDT charge, immediately above the top end of the next gun section. When the nipple on the top end of one perforating gun section is secured to the connecting sub on the lower end of another perforating gun section, then the DDT charge is disposed immediately above a portion of the circle formed by the top end of the detonation-transmitting tubing and effects the firing of such tubing to transmit the firing energy of the next section. The primer cord is conventionally mounted in an axial passage and terminates at the top end of the gun section in a booster charge, and at the lower end of the gun section in another booster charge connected to the end of the primer cord. Thus, the detonation of the booster charges of the primer cord will effect the transmission of energy across each of the connections of the gun sections and the firing of the multiplicity of gun sections, or alternatively, the detonation of the DDT charge will effect the detonation of the adjacent booster charge. In addition to a detonation-transmitting element, there are also other devices sensitive to shock or percussion from preceding guns which could convert a low-order detonating condition back a high order. Since the deflagration rate of the detonation-transmitting tubing is substantially lower than the detonation rate of any conventional primer cord, if the transfer of detonating energy from the lowermost booster charge of an upper section to the uppermost booster charge of the next lower section is accomplished in conventional fashion, the primer cord will have been detonated throughout the entire lower section before the detonation transmitted by the tubing reaches the auxiliary booster charge. Thus, nothing happens, because proper detonation of the primer cord in the particular gun section has been achieved. On the other hand, if the transfer of detonating energy from the lowermost gun section is interrupted for any cause, the detonation transmitted by the flame-transmitting tubing will effect the detonation of the auxiliary booster charge and it in turn will effect the detonation of the medial portion of the primer length in the respective gun section in one modification and detonation of the primer cord booster charge in the other modification. Such primer cord can detonate both upwardly and downwardly from the point of attachment of the auxiliary booster charge and thereby effect the detonation of all of the shaped charges contained within the particular gun section, as well as the bottom booster charge secured to the bottom end of the particular primer cord for transmission of detonating energy to the next lower gun section. It is thereby assured that the transmission of detonating energy from each successive gun section to the next lower section will occur, even though the detonation of the primer cord conventionally accomplished by the auxiliary spaced, adjacent booster charges may, for some unknown reason, fail. Other advantages of the invention will be readily apparent to those skilled in the art from the following detailed description, taken in conjunction with the annexed sheets of drawings, on which is shown two embodiments of the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2925775 | McKee | Feb 1960 | |
| 3648785 | Walker | Mar 1972 | |
| 4011815 | Garcia | Mar 1977 | |
| 4491185 | McClure | Jan 1985 |
