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Fire In The Hole. The perforating engineer places the shot where it is supposed to be. Checking the depth one last time, he or she activates the various safety lockout mechanisms and turns up the powerstat until the gun fires.
Blasting Cap Detonated. Millions of electrons travel at near the speed of light down the wireline conductor and into the detonator. The "electric match" nichrome bridgewire rapidly heats until the ignition mix, in which it is embedded, bursts into flame. This detonates the primer or initiator charge (often Lead Azide), which in turn detonates the higher power base charge (RDX or other high explosive material).
Det Cord Initiated. Energy is transferred from the blasting cap detonator to the detonating cord to which it is tightly coupled. As the det cord fires, the shock wave accelerates past the speed of sound within an inch or so. The shock wave is now traveling at 20,000-25,000 feet per second as it winds its way past charge after charge. It takes less than 500 microseconds to initiate 40 charges in a ten foot gun.
Charge Detonated. As the det cord explosive shock wave reaches each shaped charge at T=0, it detonates the fine grained pure high explosive primer at the base of the charge. The explosive wave front advances to the main explosive body of the charge, attaining full speed and pressure just before it reaches the apex of the liner. The front is moving at about 25,000 feet per second and develops pressures between four and five million psi.
It is now T+6 microseconds since charge initiation.
Liner Collapse. When the explosive wave front reaches the liner, it collapses toward the axis of the cone. Further impetus is provided by the superheated explosion gases that are confined by the still intact, but rapidly disintegrating charge case. The collapse speed of the liner accelerates rapidly, usually reaching a maximum of around 8,000 feet per second, then declining.
It is now T+10 microseconds.
As the circumferential liner particles collapse in a rapidly decreasing circle into a tumultuous collision centered on the axis of the cone, collision pressures as high as 15 million psi are generated. When the collapsing particles reach the cone axis, radial components of momentum are cancelled and are now focused on propelling the particles forward along the axis of the cone. At the collision point, the metal quickly divides into two highly-focused forward moving axial streams; about 40% of the mass of the liner goes into the high velocity jet, while about 60% goes into the lower velocity slug. All components are now rapidly accelerating at various speeds away from the point of detonation.
It is now T+15 microseconds.
The jet is now moving at over 21,000 feet per second, while the slug is trailing at a a little over 1,600 feet per second, but the jet and slug components combine to form a continuous stream of metal particles. With increasing distance from the liner apex, the liner diameter increases, thus limiting the space between the charge case and the liner for explosive material. Thus, the base or skirt of the liner receives less explosive impulse than does the apex. This produces a velocity gradient that accounts for the elongation of the jet as it approaches the casing. The stretching of the jet combined with increasing mass, maximizes the penetration capability of the jet. The shock wave now reaches the base (skirt or largest diameter) of the liner.
It is now T+20 microseconds.
Almost 35% of the total explosive energy has been transferred to the jet. Total liner collapse is about to occur.
It is now T+50 microseconds.
Target Penetration. The jet tip is now well formed and racing toward the gun housing (or port plug) with millions of pounds of penetrating pressure; this metal flows plastically aside as though it were a liquid. The wellbore fluid offers even less resistance, forming a high pressure bubble that quickly collapses. The casing metal is shoved aside as the jet races through the steel and on to the cement sheath. With more than 90% of the original jet energy remaining, at a velocity close to 20, 000 feet per second, the jet easily penetrates the cement sheath and slams into the producing formation, pulverizing the rock matrix. The jet continues to crush the rock, and displaces both solid rock and pore fluids radially from its path. The penetration velocity is now around 12,000 feet per second (for Berea sandstone). The jet makes a bigger hole than the jet diameter (a 3 mm jet can easily make a 13 mm hole) because the tremendous inertia imparted to the displaced rock particles continues to expand the diameter of the hole. Material is compacted, not burned, in an action similar to driving a nail into a block of wood (casing weighs the same before and after perforation). Even though the temperature of the newly formed jet may be as high as 1000°C, there is no time for heat transfer, thus no fusing, glazing, or scorching of the perforation walls. The displaced very finely crushed rock matrix is actually jammed into the pore throats of the adjacent rock. This, in conjunction with the already compressed perimeter matrix, forms the "altered zone" of reduced permeability, typically about an inch thick.
As the slower after portions of the jet continue to penetrate the formation, progressively more of the rock matrix at the end of the tunnel is forced aside. This continue until the energy level of the jet, marked by ever slower velocity, is no longer able to force material out of its path. The actual penetrating process is complete.
It is now T+150 microseconds.
The tail end particles of the jet remain in motion, but cannot penetrate the formation. These particles are simply impacted onto the formation face, and depending on charge design, may form a "carrot". The slug usually breaks up before reaching the perforation.
It is now T+2,500 microseconds.
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