Background and History of Oilfield Explosive Systems
- The Bullet Gun
- Development of Jet Perforating
- Future of well Perforating
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- The earliest methods of perforating cased hole completions involved the use of strictly mechanical means:
- A mechanical knife tool run on drill string was tripped into contact with the casing upon an upward pull on the drill pipe, opening a slit in the casing. It was slow and ineffective in penetration entirely through the cement layer.
- A second method used a star wheel similar to the rowel on a spur to puncture
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- Patented in 1926, the bullet gun, almost exclusively supplanted by the jet perforating gun, was the early method employed to perforate cased hole completions using energetic materials. The first bullet gun perforated well was completed in 1932.
- The gun system consists of:
- A gun body or carrier containing a series of individual “snub-nosed” guns.
- A series of short, (less than 2-inch-long), threaded gun barrels sealed to the gun body by an O-ring seal.
- A series of electrically actuated igniter charges, one per gun barrel.
- A series of propellant charges, one for each gun housed in combustion chambers in the gun body. Standard military propellants were most frequently used.
- A series of hardened steel bullets.
- A pressure seal preventing wellbore fluids from leaking into the gun barrels.
- Rapid burning of the propellant raised combustion chamber pressures to values in excess of 250,000 psi in the absence of chamber free volume, (much higher than conventional gun breach pressures).
- Typical muzzle velocities were in the 3,000 fps range.
- Barrel wear due to gas erosion and expansion severely degraded performance requiring frequent barrel changes to maintain adequate round velocities.
- The bullet gun perforating system was found wanting due to the need for constant monitoring of gun barrel dimensions accompanied by high maintenance costs.
- The energetic materials used in the propellant systems was temperature limited.
- Jet perforating possessed far greater performance potential.
- High temperature jet perforating was possible with the advent of aerospace type high temperature stable explosives with low failure diameters.
- High compressive strength formations result in very serious reduction in bullet penetration.
- Very little work has been done in bullet perforating for many years.
- No successful bullet gun was developed to perforate casing below production tubing.
- On the other hand:
- bullets do produce very consistent casing entrant hole diameters with practically no taper.
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- During the late 1930s and early 1940s, Henry Hans Mohaupt, a Swiss engineer, used the lined cavity principle to develop a series of military projectiles, including rifle grenades, mortar rounds and 100mm artillery projectiles. Test firing were conducted in at Thun, at Mohaupt’s laboratory, and the French Artillery Proving Ground.
- Mohaupt’s later work at the U. S. Army Aberdeen Proving Ground lead to the development of the 2.36 in. shoulder-launched rocket propelled anti-tank weapon. Immediate classification of the program excluded Mohaupt from further participation in the program.
- In 1945, Ramsey Armstrong started the Well Explosives Company, Inc of 3909 Hemphill Street in Fort Worth , (later Welex), to develop improved well perforating systems.
- At the close of the war, Henry Mohaupt and Robert McLemore joined Well Explosives Company. They considered that the lined cavity device would be more effective than the bullet gun in completing oil and gas wells.
A joint venture of Welex and BJ, Jet Research Center, was established on a remote, (at that time), farm property on Bowen Road in southwest Arlington, Texas to design, load, assemble and pack out the perforating devices
- The 8 July 1946 edition of Oil Weekly reported on the results of the first wells perforated by shaped charge jets. The wells were low production, open hole, stripper wells, whose production had slowed to a trickle. McLemore’s comment was, “Although the work performed to date with the formation penetrator has been entirely of the nature of improving secondary recovery by establishing channels in congested sand conditions, it is believed that there are many other applications which will prove to be of equal value, though possible not of widespread interest.”
- In 1948, two cased wells were jet perforated in Mississippi, recorded as the first cased hole completions jet perforated.
- Early perforators were also manufactured by the E. I. DuPont Company of Wilmington Delaware and the Atlas Corp. of Houston, Texas.
- Most early perforating charges employed wrought E.T.P. copper liners, cast pentolite or pressed RDX explosive fill and machined metal or die cast bakelite cases.
- Applications were for open-hole as well as cased hole completions.
- Typical performance from thirty gram charges was less than 4 inches penetration in steel.
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- Since the 1940s the jet perforating process evolved quickly, as it became apparent the jet perforating gun could operate over a wide range of gun/casing diameters, while the bullet gun ceased working at all in gun sizes less than 3 inches.
- By 1957, small caliber jet perforating guns were introduced that could be run through production tubing, opening the field to remedial re-perforating operations in existing well completions.
- Since early perforators were small caliber versions of military shaped charge designs, they were beset with the problem of slug plugging of the perforation tunnel, reducing the tunnel area for well inflow in the most critical portion of the tunnel. The technology of powered metal liner blends solved the slug plugging problem.
- The use of pentolite or other explosives using a TNT matrix were replaced by cyclic nitramines as the bottom hole temperatures encountered increased. TNT melts at approximately the boiling point of water, limiting operating temperature.
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- The length of the perforating gun string that can be run into the well on a single trip is limited and once the guns have fired, pressure equalization between the reservoir and the wellbore has been achieved limiting further cleanup of the perforation tunnels with additional runs.
- Tubing conveyed perforating guns first described in 1975 by Vann and Owens conveys all of the guns into the well on a single trip below a packer permitting control of wellbore pressure and thus providing for underbalance on all of the perforations enabling good perforation tunnel cleanup
- Perforation gun strings may also be conveyed on coiled tubing as well as jointed tubing strings.
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Along with the development of shaped charge perforators have been the development of:
- Tubing and casing cutters for pipe recovery
- Junk charges for downhole debris breakup
- Drill collar cutters
- Vent charges and tubing punchers
- Wellhead severing charges
- Specialty charges for offshore oil and gas production facility removal and demobilization
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There is no current replacement for the cost effectiveness of chemical explosives-propelled directed energy systems for performing useful work in a down-hole environment.
Perforating systems developments may include:
- More refined charge designs for improved efficiency
- Safer, more thermally stable explosives of increased output
- Perforator conveying systems of increased collapse pressure
- “Designer” perforators for specific formation lithology
- Lower cost RF safe initiators
- Safety systems to prevent inadvertent initiation of gun strings on the surface
- Improved shot detection systems for safer gun retrieval
- Refinement of well inflow computer models including transient pressure modeling