Productivity of a cased-and-perforated well depends on completion parameters (charge, shot density and phasing), formation properties (lithology, anisotropy, permeability, fluid properties and saturations), and environmental conditions (pressure and temperature). A shaped charge produces a jet of dense, typically solid material traveling at very high velocity which penetrates casing, cement, and formation. This penetration process creates a tunnel in the rock connecting the reservoir and the wellbore. Stress waves generated during penetration damage the rock around the tunnel, creating a complex zone of mechanically deformed rock material. Explosive and metallic debris may mix with damaged rock material. Depending on the specific conditions (mechanical rock properties, permeability, fluid viscosities, interfacial tensions, pressures and fluid storage volumes), reverse surge flow following the jet penetration partially or completely clears the tunnel of the charge and rock debris. The resulting tunnel is a rugose, tapered cylinder roughly characterized by its diameter and total depth of penetration. The degree of cleanup and total depth of penetration are two of the most critical parameters influencing flow performance of a perforated well.
This work focuses on predicting penetration depth and providing a summary of recent progress and advances in this area. A review of industry penetration prediction methods is presented, including relationships based on surface perforation tests in concrete targets, as well as correlations of penetration depth with properties that could be measured by downhole logging tools. Despite a variety of available methods and published experimental data, penetration depth results are often inconsistent with each other and of questionable use in predicting actual downhole penetration. Moreover, most studies have concentrated on the influence of a single penetration variable. There have been no systematic studies of the interaction of parameters on perforation penetration.
This paper emphasizes the critical need for such a study, given the difficulty of downhole measurements of penetration depth to finally achieve reliable penetration predictions, and suggests future directions and conditions necessary for such a study.
Introduction
Prior to the adoption of perforation technology in the oilfield industry, wells were completed "open hole" or "shot hole" (barefoot), sometimes employing liners. The perforated-casing completion was an important and necessary development as wells got deeper, and reservoir conditions became more complex. Early gun perforators were "bullet" devices, using actual projectiles (usually steel bullets) to penetrate the well casing. Prior to the introduction of lined shaped-charge perforators, almost all research emphasis was placed on increasing the depth of bullet penetration. Shaped-charge perforators were adapted for oilfield industry from the military in the 1950s (Pugh et al. 1952; Eichelberger and Pugh 1952; Turechek and Lindsay 1953; Eichelberger 1956) and have displaced the old bullet perforators (nearly to extinction) since then. This paper is focused on depth of penetration (DOP) of downhole shaped-charge perforators and summarizes recent progress and advances in understanding and predicting this important parameter.
Productivity of cased-and-perforated wells is influenced by many different factors, including completion (charge characteristics, shot density and phasing), formation properties (lithology, anisotropy, permeability, fluid properties and saturations), and environmental conditions (pressure and temperature). The main goal of natural-perforation completion is to optimize production flow by re-establishing connectivity between wellbore and reservoir, and maximizing the effective wellbore diameter. Drilling and completion operations result in various types of formation damage, including drilling damage (mechanical damage, mud filtrate invasion, and fines migration), perforation damage (crushed zone of reduced permeability around perforating tunnels), partial penetration, well deviation, etc.