Shaped-charge perforating

In a cemented casing (bottomhole designs), the reservoir is separated from the well by steel and cement. The connection is made by perforating: tunnels (channels) through the pipe and the cement sheath into the rock. All the inflow goes through them, and acid, fracturing fluid and gravel are pumped through them. Perforating lets us choose the intervals: we open the oil-bearing part of the reservoir and leave alone the water-bearing part, or a weak part that tends to produce sand. Later we can perforate new intervals.

There are several methods. Bullet perforating, in which powder charges fire steel bullets, was the first, and in strong rocks it makes shallow tunnels. Abrasive-jet perforating cuts holes with a jet of liquid carrying sand through nozzles, and drilling perforators drill holes without any impact load on the casing and the cement. But the great majority of wells are opened with shaped charges (shaped charges): this is fast, comparatively cheap and makes deep tunnels. In Russian-language practice, perforating, together with other work with explosives in a well, is called “perforating and blasting operations”.

A shaped perforating charge is a charge of explosive in a case, with a thin metal liner lining a conical cavity on its front side. The charges of a gun are set off almost at the same time by a detonating cord (a cord filled with explosive that carries the detonation) that runs along the whole gun.

The detonation runs through the explosive at about 8–9 km/s and squeezes the liner towards the axis. The metal does not melt. Under a pressure of tens of gigapascals it flows like a liquid. The inner layer of the liner is squeezed forward as a thin shaped-charge jet, while the outer, more massive layer moves slowly and forms the slug. A smaller part of the metal goes into the jet, usually not more than a third. The head of the jet flies at a speed of about 6–8 km/s, and the tail is several times slower. Because of this difference the jet stretches on its way and then breaks into particles. From the detonation to the stop of the jet in the rock, only fractions of a millisecond pass.

Liners are either solid, usually of copper, or pressed from metal powders. A solid liner leaves a massive slug that can plug the tunnel, while the slug of a powder liner crumbles into small particles. The jet needs a short distance to form. This distance is given by the air gap inside the case and by the wall of the gun body. But water between the gun and the casing slows the jet down: the larger the clearance (the gap between the gun and the casing), the shorter the tunnel, especially with deep-penetrating charges.

The pressure at the front of the jet is much larger than the strength of steel and rock. So, as a first approximation, strength is ignored, and the jet and the target are treated as ideal incompressible fluids (fluids that cannot be compressed and have no strength). Imagine that we ride along with the bottom of the growing tunnel. The metal of the jet then runs onto the bottom at a speed $v - u$, and the material of the target runs onto it at a speed $u$. The stagnation pressures on the two sides must be equal (the stagnation pressure is the pressure that a flow reaches when it is brought to rest).

How deep will the jet pierce? Each element of it is “used up” at the bottom of the tunnel during the time while the metal runs onto it at a speed $v - u$. During this time the bottom moves forward at a speed $u$. The ratio $u/(v - u) = \sqrt{\rho_j/\rho_t}$ follows from the previous formula, and for a continuous jet of length $L$ it gives the density law (the derivation is in the advanced section):

The same mass of explosive can give a narrow, long tunnel or a wide, short hole. The shape and the material of the liner decide which. So charges are divided into two classes.

For an ordinary well with natural inflow, the length of the tunnel matters: it must pass through the zone damaged by the drilling fluid and the cement (tunnels and the damaged zone). For a gravel pack and for hydraulic fracturing, the hole matters more: liquid with a high concentration of gravel or proppant goes through it at high speed. If the hole is smaller than about six diameters of the largest grain, the grains form an arch and block it, and pumping stops. For 20/40 gravel (a sieve size: the largest grains are 0.84 mm) this is about 5 mm, and for 12/20 proppant (up to 1.68 mm) about 10 mm. Big-hole charges give holes with a margin. A gravel pack inside the casing also needs a high shot density, because the whole flow goes through the tunnels filled with gravel (losses in tunnels filled with gravel).

For hydraulic fracturing, the tunnels must point in the direction of the future fracture, perpendicular to the smallest horizontal stress (orientation of the fracture). So we use a phasing of 60° or less, so that at least one tunnel is close to the needed direction, or oriented perforating. The interval for one fracture is made short, a few metres, so that the fracture starts at the chosen place. When pumping must be shared between several intervals, the opposite is done: few holes are made, and the pressure drop across them evens out the rate (limited entry).

Special low-power charges pierce only the tubing, without damaging the casing behind it. They are used to connect the tubing with the annulus if there is no sliding sleeve (a circulation valve) or if it does not open (circulation valves).

The charges are assembled in a perforating gun, a device that holds them at the needed depth and in the needed direction and protects them until the shot.

Carrier gun (hollow carrier)

The charges sit inside a sealed steel tube at atmospheric pressure. The carrier takes the pressure and the temperature of the well, protects the charges from the liquid, and after the shot takes most of the debris out of the well. The wall of the carrier is thinned opposite the charges, with scallops (recesses), so that the jet spends less energy and the burrs stay in the recess, and the carrier can be pulled out freely. After the shot the carrier swells a little, and this is allowed for when we check that the gun will pass through the casing or tubing. Carriers can be reusable (the carrier is reloaded) or single-use.

Expendable gun (exposed)

Each charge is in its own sealed capsule, and the charges are fixed on a strip, a wire or a rod. With no carrier, a larger charge fits in the same outside size, and the tunnels are longer. But the charges and the detonating cord are exposed to the pressure, the temperature and the liquid, the remains of the capsules and the strip stay in the well, and the blast hits the casing directly.

Through-tubing gun

A gun of small diameter, from about 35 to 90 mm depending on the tubing bore. It passes through the tubing string and shoots into the casing below its end. It lets us open the reservoir when the tubing string is already run and the wellhead is installed, including underbalanced. The price is small charges, short tunnels and a large clearance to the far wall.

Casing gun

A carrier gun of the largest diameter that passes in the casing, usually from 86 to 127 mm (3⅜–5 in), and larger in wide casings. It is run before the tubing string is installed, on a cable or on pipe (Perforating operations: gun conveyance and procedures). It has large charges, any phasing, and a shot density of up to several tens of holes per metre.

A gun in the casing is seldom exactly in the centre. A small gun in a wide casing ends up at one of the walls (the low side in a deviated well). The clearance to the near wall is almost zero, and to the far wall there is more than 10 cm of water. So such guns usually shoot with 0° phasing and are turned, by a magnetic or eccentric device that presses the gun against the wall, so that the charges face the wall. A large gun leaves a small clearance all round, and any phasing suits it.

Explosives slowly decompose when heated, so the allowed temperature depends on how long the charges will stay in the well: hours when run on a cable, a day or more when run on pipe. For hot wells, heat-resistant explosives are used, and for each charge the manufacturer states the allowed combination of temperature and time.

The shot density $n_s$ is the number of holes per metre, from one or two up to several tens. English-language data sheets give shots per foot (spf; 1 shot per foot ≈ 3.28 shots per metre). The phasing is the angle between the directions of neighbouring charges: 0° means all in one direction, 180° in two opposite directions, and 120°, 90°, 60° or 45° in a spiral.

The shot density and the phasing decide the perforation skin (calculation by the method of components), the flow speed in each tunnel (and so the sand production and the inertial losses, which are the extra pressure losses at high flow speed), and the strength of the casing. Dense perforating with a phasing of 45–90° comes closest to an open hole. A phasing of 0° makes the flow go around the casing and gives a larger skin, but for a gun pressed to the wall it gives the longest tunnels. Low vertical permeability and thin layers call for a higher shot density: a thin layer between two shots otherwise hardly works.

Each hole weakens the casing, and the burrs inside get in the way of running packers (seals for the well) and tools. At ordinary shot densities and hole diameters in a well-cemented casing, the strength falls only moderately. But very dense perforating with large holes, thin-walled pipes and poor cement can lead to cracks in the casing. The allowed combinations are set by the gun manufacturer and by calculation.

Manufacturers compare charges by the standard method API RP 19B (this role was earlier played by the document API RP 43). It includes:

• firing a section of a gun into a casing pipe cast in concrete in a barrel, at surface conditions: the diameter of the holes in the pipe, the total length of the tunnels and the height of the burrs are measured, and this is the “data-sheet” tunnel length (penetration)
• a single shot into a sandstone sample squeezed by pressure as in the reservoir, followed by pumping fluid through the tunnel to estimate its productivity
• firing after the charges have been held at a high temperature
• measuring the debris that the gun leaves in the well, and the swelling of the carrier

A concrete target is convenient for comparing charges with each other, but it does not predict the tunnel in the reservoir. The tunnel length in the reservoir becomes smaller when:

• the casing is stronger and thicker, because more of the jet goes into the steel
• the rock is stronger: the uniaxial compressive strength of different reservoirs differs by tens of times (rock strength)
• the effective stress is larger (the difference between the overburden pressure, which is the weight of the rock above, and the pore pressure, which is the fluid pressure in the pores; it grows as the reservoir pressure falls during production)
• the clearance between the gun and the casing is larger
• the charges have lost part of their power from a long stay at high temperature

So the tunnel length for the inflow calculation is taken not from the catalogue but from a forecast for the particular reservoir. The forecast comes from tests on rock samples under stress and from manufacturers' programs that convert the data sheet for the rock strength, the stresses, the casing and the clearance. The length in the data sheet and in the forecast is usually counted from the inside wall of the casing, but the skin calculation needs the length of the tunnel in the rock beyond the wall of the wellbore, $l_p$. So the casing wall and the cement sheath are subtracted from it.

The jet pushes the rock aside; it does not remove it. Around the tunnel a ring of crushed and compacted grains is left, about a centimetre thick, with a permeability several times lower than that of the reservoir. This is the crushed zone. The tunnel is left with debris of rock, particles of the liner and of the charge case, and, with a solid liner, the slug. How much this lowers the productivity is calculated in the lesson skin of the crushed zone: the crushed zone can easily give a larger skin than all the geometry of the perforations.

If at the moment of the shot the pressure in the well is higher than the reservoir pressure (overbalanced perforating), the well fluid, together with the solid particles of the drilling fluid, at once rushes into the fresh tunnels and plugs them together with the debris. Later, during production, the flow in each tunnel is small, and the plugged tunnels never clean up. Only some of them work.

In underbalanced perforating, the pressure in the well is lower than the reservoir pressure. In the first fractions of a second after the shot, the reservoir gives the tunnel a short, strong pulse of inflow. The pressure difference across the loose, crushed rock and the force of the flow tear off the debris and part of the crushed zone and carry them into the well. It is this instant pulse that does the work, not the production afterwards. The lower the permeability of the reservoir, the larger the pressure difference needed for cleaning. A similar pulse can be created without a permanent underbalance, from empty spaces in the gun that fill with liquid at once after the shot and for a moment lower the pressure at the tunnels (dynamic underbalance).

The underbalance cannot be as large as we like. In weak rocks it destroys the walls of the tunnels and causes sand production (Sand production: types, prediction and cost of the problem), and too large a pressure difference is also dangerous for the casing. How the underbalance is chosen, and how to perforate underbalanced when the gun is run on a cable or on pipe, is the subject of Perforating operations: gun conveyance and procedures.