INTRODUCTION:
Shaped charges represent a special case of hypervelocity penetration in rocks and soils. They are commonly used to perforate well casings in the petroleum industry and are also in less frequent use in the mining industry. In this paper, we examine the penetration depth as a function of rock properties and stress environment.
Figure 1 illustrates a typical shaped charge and the sequence of events as it detonates. After being initiated by a detonating cord at its base, the detonation wave sweeps forward along the liner and pushes it inward. Where the liner material collides along the symmetry axis, extremely high pressures are produced resulting in a jet of liner material as shown in the sequence of sketches.
The resulting jet is usually not molten but consists of solid metal with density near that under ambient conditions. The jet acts much like a fluid because its failure strength is much less than the stresses caused by the detonation wave. The difference in velocity between the tip (typically 7 km/s) and tail (1 k m/s) of the jet means that its length increases and its diameter decreases with time and travel distance. While military shaped charges typically use solid metal liners, oil-well perforators are made with pressed metal powders. This results in a lower-density jet composed of loose particles with virtually no strength at all, and avoids possible plugging of the perforation with metal.
The principle measure of charge performance is penetration depth. Although they were initially intended simply to create holes in well casing, numerical models such as those by Locke (1981), and Tariq (1987), demonstrated that increasing the penetration into the rock itself significantly improves the production rate of a well. For this reason, emphasis has shifted to optimizing penetration depth, even though shock damage to the rock, and the presence of charge debris, combine to reduce flow rates into a perforation below expected values.
