Underbalanced Perforation Characteristics as Affected by Differential Pressure

Abstract

Summary Underbalanced, tubing-conveyed completions are increasing because of the apparent advantages of cleaner perforations, reduced completion times, and stimulation treatments. Radial-flow Berea sandstone core specimens are used to determine the perforation characteristics resulting from time-dependent pressure differentials between core pressure and wellbore pressure during the completion process. The primary perforation characteristic studied [radial flow ratio (RFR)] is defined as the ratio of the perforated flow rate to the flow rate of the unperforated core. The perforation flow tests included pressure differentials from 500 psi [3450 kPa] overbalanced to 1,000 psi [6900 kPa] underbalanced, with immediate or delayed surging. The RFR was affected most by the 500-psi and 1,000-psi [3450- and 6900-kPa] underbalance. The surged RFR's were from 50 to 58% greater than the no-surge RFR's. Perforation hole volumes were increased as much as 55% by the surge, indicating some elimination of the compacted, damaged-zone material. Underbalanced surge conditions (1,000 psi [6900 kPa]) increased hole volume to four times that resulting from 500 psi [3450 kPa] overbalanced, no-surge conditions. Underbalanced surging, either at the instant of perforation or subsequently, appears to improve the flow characteristics of shaped-charge perforations by simultaneously reducing the thickness of the low-permeability crushed zone and increasing the perforation surface area.