Dynamic Underbalanced Perforating System Increases Productivity and Reduces Costin East Kalimantan Gas Field: A Case Study

摘要

New gas wells at Tunu field in East Kalimantan are normallyperforated using the extreme underbalanced (EUB) technique,in which a large static pressure differential between thewellbore and formation, or initial underbalance, is set beforethe gun is fired. This requires an operation to unload wellboreliquid, a wireline operation to set an anchoring tool, andseveral slickline operations to run and retrieve the gun string.The required degree of underbalance depends primarily onrock properties such as permeability, porosity, and strength.A new approach produces a well dynamic underbalance(the transient underbalance just after creating the perforationcavity). By basing the job design on the properties of thereservoir, wellbore, and gun string, the technique consistentlyminimizes perforation damage and thus maximizesproductivity. This technique is based on recent single-shotperforating experiments that show that it is the maximumdynamic underbalance—not the initial underbalance—thatgoverns perforation cleanup. The wellbore pressure was foundto vary considerably during the first half-second after thecharges were detonated. This variation in wellbore pressurecan be manipulated to give a large dynamic underbalance.Perforating with the new technique and electric wirelinewas performed on 10 wells in the field. Software thatconsidered wellbore geometry, fluid, gun-string selection, andreservoir properties was used to design a specific gun-stringconfiguration and to predict the productivity increasecontributed by the underbalance. The jobs were performedsmoothly with significant cost savings compared toconventional EUB operations and yielded an increasedproductivity. Details of the perforating jobs with the newtechnique will be described, including a cost reduction ofapproximately 40% and a significant production increase.Testing revealed an average skin value for four wells of -0.29, versus the average skin of 4.73 from 35 conventional EUBperforating jobs executed during the multi-year interval from2000 to 2004.