Evaluation of Established Cleanup Models in Dynamic Underbalanced Perforating

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 159413, ’Evaluation of Established Perforation-Cleanup Models in Dynamic Underbalanced Perforating,’ by Dennis Haggerty, G.G. Craddock, and Clinton C. Quattlebaum, SPE, Halliburton, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 8-10 October. The paper has not been peer reviewed. Dynamic underbalanced (DUB) perforating is a process that creates a negative pressure differential, or underbalance, causing fluid to move toward the wellbore even in an initial overbalanced static condition. A DUB condition can be controlled by understanding and carefully managing the temporal pressure transients by use of multiple methods within the wellbore during and after gun-system detonation. Recently, a series of instrumented perforation experiments demonstrated that existing cleanup models do not accurately predict perforation cleanup when perforating in a DUB condition. Introduction Underbalanced perforating methods have been applied successfully since the 1950s, shooting both wireline and tubing-conveyed perforating guns. As shaped-charge jet-perforator systems became more advanced, using powdered metal liners, performance steadily improved. The art of minimizing the compacted and damaged area surrounding the perforation tunnel, commonly known as the crushed zone (Fig. 1), began shortly afterward. Much of the initial work involved shooting charges into prepared Berea sandstone cores while documenting the effect of a differential pressure toward the wellbore upon perforation efficiencies. As the benefits of an underbalanced pressure differential were observed, extensive testing established criteria for flow volumes and differential pressures required to remove or minimize the crushed zone created during the perforating event. Specifically documented was the role of trapped atmospheric pressure inside a perforating gun surrounding the shaped charge and components, known as free gun volume (FGV), which enabled the formation pressure to act as a differential and expel charge and crushed formation into the gun. As perforating research and field observations continued, a series of widely used and accepted formulas was established to document the magnitude of differential pressure required to ensure cleaned perforation tunnels. This paper reviews the effectiveness of each of these models, originally developed for a static underbalanced condition before perforating, to predict cleaning and removal of the crushed zone in a series of tests with a dynamic pressure differential. The test series uses an advanced perforation-flow laboratory to detonate an 11.1-g deep-penetrating shaped charge. Each charge will perforate a 24-in.-long, 7-in.-diameter Berea sandstone core with 3,950-psi applied pore pressure; 9,950-psi simulated overburden stress; and 4,900-psi wellbore pressure, creating a static 950-psi overbalance. Although the initial static condition will be overbalanced, a DUB condition will be established by wellbore and pore fluids filling the FGV upon detonation.