Explosive shaped charges have been the primary means of creating a flow passage from a hydrocarbon bearing reservoir into a wellbore for almost six decades, and overall well productivity is directly related to penetration depth. While penetration depth performance has improved dramatically over that time frame, current models of penetration are inaccurate. The largest source of error is a direct result of the reliance on unstressed concrete penetration as a measure of downhole shaped charge performance into real rock. Following a brief review and discussion of several industry models, the results of an ongoing experimental program will be discussed. This program has produced several hundred data points spanning multiple shaped charges, rock types and strengths, with liquid and gas pore fluid, and over a range of overburden stresses and pore pressures. This extensive data set has confirmed well-known aspects of the underlying physics, such as the reduction in penetration with increasing effective stress and/or rock strength. Previously unknown aspects have been revealed such as the “ballistic” pore pressure coefficient which is analogous to the quasi-static Biot coefficient. A new correlation is proposed which combines rock strength and stress into a single term. Gas pore fluid in dry sandstone generally results in reduced penetration depth compared to brine saturated sandstone, but the reduction appears to be dependent on rock strength as well. Shaped charge performance into gas saturated rocks was also observed to be less sensitive to effective stress. Rock composition also effects penetration depth, as equivalent strength sandstone and limestone exhibit significantly different penetration depths. A discussion is included regarding the impact of decades of reliance on surface concrete penetration on shaped charge development. A likely shift is predicted within the industry as developers begin to make charges optimized for real rock performance. Surface concrete performance testing is best suited for system-level interference and entrance hole evaluation as originally intended.
Explosive shaped charges have been used to perforate oil and gas wells since the 1940's. Based on military technology fielded in World War II, these small explosive devices create metallic “jets” traveling at several km/sec, perforating steel casing, cement, and formation rock. Typically, multiple charges are configured in a perforating “gun”, and initiated in rapid succession by high explosive detonating cord, which has been initiated by a detonator. The primary objective of perforating a cased wellbore is to establish efficient flow communication with the reservoir. The key perforating parameters which influence reservoir deliverability are well known, and the relative influence of each has been quantified by a number of researchers [1, 2, 3, 4, 5, 6]. These key parameters include shot density, phasing, depth of penetration (DoP), tunnel diameter, and the nature of any permeability-impaired (“crushed”) zone which remains surrounding the perforation tunnels. Shot density and phasing are fixed system parameters, and therefore their values at downhole conditions are known. However, downhole values of perforation tunnel depth, diameter, and crushed zone characteristics cannot be known with certainty. These quantities must be estimated with predictive models.
