Correlating Surface and Downhole Perforation Entry-Hole Measurements Lead to Development of Improved Perforating Systems

Abstract Studies have shown that achieving a consistent perforation hole size in casing (i.e., entry hole) and zero-phase perforation gun orientation led to improved treatment distribution among multiple perforation clusters in plug-and-perf limited entry treatments. In addition to reducing variation in the perforation entry hole by establishing uniformity in gun clearance and angle of incidence of the perforation jet at the wall of the casing, oriented perforating has been shown to minimize the tendency of proppant to separate from the fracturing fluid while travelling across the perforated intervals (inertial effect) and mitigate non-uniform entry-hole erosion due to gravity-induced proppant stratification. The primary goal of this study was to determine the controllable perforating-gun elements and accessories that effect the accuracy of gun orientation and entry-hole dimensions. Surface tests were conducted at manufacturing facilities for determining the characteristics of the entry holes in pipe produced by shaped explosive charges using various system configurations and the robustness of various gun orientation devices. Promising perforating systems were then used in wellbores to create calibration entry holes (downhole tests) that were measured for equivalent diameter and orientation accuracy using high-resolution acoustic imaging prior to conducting treatments. This process enabled components of the perforating system influencing entry-hole size and gun orientation to be evaluated and modified, as necessary. Elements of the perforating system and downhole environment which influenced entry-hole size and consistency included casing type, cement sheath characteristics, perforating gun clearance and orientation, perforating charge type and density, packing arrangement of multiple charges, charge tube and charge carrier design, gun detonation system, hydrostatic pressure, and locking devices. Achieving tight control of these elements significantly reduced variation in entry-hole size. Deviations from surface and downhole testing results were commonly attributed to using perforating system elements in the field that differed from those used by the manufacturer in surface testing. Factors affecting gun orientation accuracy and consistency included weight bar type, gun string length, weight, and stiffness, the presence of modified standoff bands, progressive gun deformation during firing, wellbore tortuosity, and self-orienting devices. Several orientation systems were found that achieved orientation within the target 20-degree window. To assess the value of this workflow process, the paper includes information on the results of diagnostic tests for evaluating the accuracy of the ultrasonic measuring device, the derivation process used for determining coefficients for a two-component perforation erosion model and the use of the derived erosion-rate coefficients for computing the mass of proppant that enters each perforation and perforation cluster during a frac treatment.