Perforating for Zero Skin: A Study of Productivity Improvement in Ecuador

Abstract

Abstract The most significant components in the skin equation that do not depend on nature are invasion damage and perforating-induced damage. The oil industry has approached the invasion issue by using stimulation treatments intended to regain the permeability lost through formation damage, or by deep-penetration perforating aimed at bypassing the invasion damage. Perforating is known to create a low-permeability crushed zone in the formation around the perforating tunnels that also constitutes a flow restriction, which can impair the well's productivity or injectivity. A perforating design that guarantees bypassing the invasion and at the same time eliminates the perforating tunnel damage is crucial in obtaining a zero-skin completion and maximum productivity or injectivity. This paper introduces an objective methodology to design perforating jobs for zero-skin and maximum productivity. The method is based on theoretical development and experimental studies carried out on cores from three fields in Ecuador's Oriente basin. Simulations using experimental data allowed for modeling the performance of the perforating charges and for determining accurate values for the penetration depth in the actual reservoir rock. The depth of invasion for each of the reservoirs of interest was obtained from modern logs in the case of new wells, or from a statistical study based on pressure transient tests in the case of old completions. By coupling the actual penetration depth of the charges with the radius of invasion, objectively designing perforating jobs to bypass the invasion damage became a reality. Solving the second part of the problem involved finding a way to produce clean perforating tunnels. Recent research work has shown that fluctuations in wellbore pressure during the first milliseconds after guns fire actually govern the cleanup of the perforating tunnels and not the initial static underbalance, as originally thought. A perforating job that bypasses invasion, combined with a new design that guarantees a dynamic underbalance at all times, has produced outstanding results in over 40 perforating jobs in Ecuador. Historical field data for conventional underbalance perforating in comparable wells show average skin values of +13 vs. -0.5 using the proposed methodology, with typical production level gains of 300 to 400 BOPD.