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.
Formation fluids are displaced by drilling mud filtrate as a result of pressure overbalance during drilling. This process changes the petrophysical properties of the near-wellbore zone and creates an invasion zone that has a complex radial profile characterized by the partially decreased porosity, permeability, and altered saturations. Further, at the completion stage the well is cased, cemented, and then perforated to re-establish connection between wellbore and reservoir. During perforation, a shaped charge produces a jet of dense material traveling at very high velocity which penetrates casing, cement, and formation. The resulting tunnel is a rugose tapered cylinder roughly characterized by its diameter and total depth of penetration.One of the main goals of perforated completion is to ensure fluid flow from the productive reservoir interval to the wellbore. Equally important is the ability of the jet to penetrate beyond the zone of formation damage caused by drilling, connecting the wellbore to the virgin reservoir and alleviating the effect of formation damage on production. The ability to predict the invasion depth and the depth of penetration of downhole perforators is therefore critical for pre-job completion modeling.This work presents the results of numerical modeling predictions of both drilling mud filtrate invasion during drilling and jet penetration in rock during perforation. The invasion model is further applied to the well data interpretation, and a good agreement with log resistivity profile is shown. In addition, we review and discuss various empirical methods currently used in the industry to predict penetration depth. Despite a variety of available methods and published experimental data, penetration depth results are often inconsistent with each other and are of questionable use in predicting actual downhole penetration.We highlight the importance of combining accurate invasion and penetration models for the successful pre-job completion planning. The results should be used further with the well-inflow model to maximize well productivity and minimize the effect of formation damage.
With the steadily increasing demand for oil and gas, operators have been forced to explore deeper and hotter areas to find the most prolific reservoirs. This paper hightlights the design and qualification steps performed during development of a 7.0-in. O.D. 16 shots per foot (SPF) 30,000 psi perforating gun system. Finite-element modeling was used to optimize the gun design for 30,000 psi external pressure, and 400° F. As part of the qualification requirements, ballistic survival testing was conducted using low-debris gravel pack shaped charges.Comprehensive knowledge of the post-perforated condition of the gun body is required, as its failure may lead to an expensive fishing operation and possible damage of the oil well casing. Survival testing was conducted under two different conditions, no load on the gun body and the second contion with a tensile load equivalent to the weight of 1,500 ft of perforating guns.The methodology of finite-element modeling is discussed along with experimental data obtained by collapse testing of perforating hollow carriers (HC) of different diameters and shot densities. Data from computer simulations and physical testing in the lab are presented to provide better insight into the behavior of perforating guns under external pressure. The data from the survival tests showed virtually no change in the swell of the HC when subjected to a high tensile load as compared to the no-load condtion. This testing method provides insight into the behavior of perforating systems under different loading parameters.The results described in this paper highlight the need to apply rigorous methods for design and qualification of ultra-high pressure gun systems when exploring new oilfield frontiers characterized by extremely harsh environmental conditions.
Historically, the performance of shaped charge jet perforators has been evaluated using concrete targets under ambient conditions, thereby failing to clearly characterize their performance in downhole conditions. In recent years, testing using sandstone targets (API RP-19B Section 2) and various numerical models have been adopted as a better means of quantifying shaped charge performance. However, it is important to note that such testing and modeling methods do not still account for the complex shaped charge physics in downhole conditions, which includes pressures (wellbore to pore to confining), rock properties (bedding plane orientations, porosity, permeability, UCS, etc.), cement and casing material characteristics, etc. In fact, the results from standard tests or numerical models have been proven to be misleading while designing and optimizing shaped charges for reservoir conditions. As a result, the advent of perforation flow laboratories and advances in shaped charge research and engineering has been the key motivation for this work. In this work, we have employed a scientifically engineered approach to determine and optimize shaped charge performance. A comprehensive test program in conjunction with computational modeling was utilized to design, test and improve shaped charge performance under reservoir-specific conditions. For this purpose, methods relating to API RP-19B Section 4 testing and advanced hydrodynamic solvers were integrated into a scientific workflow. The results from this study demonstrate the following: First, pore pressure does influence shaped charge performance. Second, the best way to characterize the performance of a shaped charge is to conduct a laboratory test under true reservoir conditions. Third, designing a shaped charge around performance at true downhole conditions has enabled significant performance and productivity improvements. Utilizing advanced numerical modeling and extensive testing, we have achieved significant increase in shaped charge performance (penetration and flow area) in some of the most challenging reservoir conditions. The new class of reservoir-driven shaped charges is aimed towards increasing production or injection by deeper formation connectivity with the wellbore as well as ensuring perforation contribution efficiency.
The performance of shaped charge jet perforators in complex downhole environments remains the status quo for successful cased-hole completions. Traditionally, these perforators have been evaluated using concrete targets under ambient conditions. The designs based on these evaluations were therefore optimized for conditions that don't reflect true downhole environments. Recent testing specifications in sandstone targets (API RP-19B Section 2) have also failed to consider the effects of complex well conditions and rock properties. In addition to the perforator itself, the overall completion design must be considered to properly optimize the perforating event. This is typically done using empirical correlations and first-order modeling software. While beneficial, these methods can misrepresent the complex interactions among the perforator, the reservoir, and the wellbore in the unique set of conditions. Ultimately, this can all lead to the deployment of a perforator optimized with conditions (not tailored to the reservoir) and utilizing generic practices, thereby leading to adverse implications on the overall well completion. The use of perforation flow laboratories and advances in shaped charge research and engineering have been employed in this study to understand and optimize shaped charge performance at downhole conditions. The perforators that result from a comprehensive test program in conjunction with multi-physics computational modeling are tailored for improved shaped charge performance under reservoir-specific conditions. These bespoke perforators undergo further testing as part of a more comprehensive optimization program to design the overall application on a well-specific basis. A similar scientific approach is also applied for the overall application optimization by integrating laboratory testing, CT imaging, computational flow dynamics, and dynamic event modeling to eventually upscale the results to the field applications. The results of these processes demonstrate the significant performance improvement in the shaped charge tailored for true reservoir conditions. Field applications examined in this study demonstrate specific uses of optimized perforators, as well as example workflows of how an integrated testing and modeling process can insure the perforators are applied properly. Comparisons demonstrate productivity improvements through these engineered processes. The new class of reservoir-driven shaped charges is aimed towards increasing production or injection by deeper formation connectivity with the wellbore as well as ensuring perforation contribution efficiency. The productivity gains in these case studies demonstrate the potential improvement in well completions through the use of engineered processes both for initial product design and specific application optimization in perforated completions.
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