Configuring Dynamic Underbalance to Achieve Perforation Tunnel Cleanup in a Gas Well at High Static Overbalance

Abstract

Abstract The strategy of improving gas production by removing perforation debris from perforation tunnels with dynamic underbalance (DUB), even in an overbalanced (OB) wellbore setting, was advanced by conducting a series of perforation tests. Based on API RP 19B (2014) Section 4, perforation tests were simulated matching well configurations and conditions in a high-pressure gas field to help improve production. Simulation models that included field DUB were used to adjust the laboratory test configuration such that a similar DUB response could be achieved and the degree of perforation tunnel cleanup quantified. These models of the subject formation indicated that to achieve at least a 90% cleanout of flow-obstructing material in the perforation tunnels, a post-perforating instantaneous wellbore pressure drop slightly greater than 5,000 psi would be necessary. Laboratory components were adjusted to mimic the downhole tubing conveyed perforating (TCP) string and vent chambers to achieve this magnitude of instantaneous pressure drop, or DUB. Gas-flow measurements immediately followed to determine productivity. Laboratory results indicated that the higher the DUB, the higher the degree of open perforation tunnel and, consequentially, productivity. Computed tomography (CT) images of the target cores after perforating showed a direct correlation between DUB and the extent of an open or clear perforation tunnel. An additional method of gas-flow measurement that accounts for inertial flow effects (Barree and Conway 2004) further validated the observation that as the DUB increased, the absolute open-flow potential (AOFP) increased. Viewed another way, both mechanical and inertial-based skin were reduced by the effective cleanup with higher DUB. The DUB occurrence timeframe is approximately one second in the API RP 19B (2014) Section 4 tests and approximately two seconds in the surge models, which considers the entire perforating string. These tests and early field indicators show that despite perforating at a high static OB, optimizing DUB effects with added volume space in the gun string improves perforation cleanup, which results in productive perforations.