Communication Between Perforations: Solution Based on Good Practices and Numerical Simulations

Abstract

After almost 200 wells drilled in Cerro Fortunoso and more than 500 in the Barrancas and Ugarteche oilfields, located in Mendoza, Argentina, during the past 20 years, ongoing efforts are dedicated by operator and service companies to solve frequent communication between perforations (CBP) problems. Drilling parameters, well cleaning systems, cement slurries designs and acid stimulation treatments are some of the main aspects that have been reviewed and modified in order to solve this problem. However, random CBP keeps occurring during well completion jobs even when good cement bond logs (CBL) are achieved. The CBP events are typically observed on perforated zones that are separated 10 to 50 m between them, and usually occur either when performing swabbing, while pumping pre-flushes or during the acid treatments. These wells are subjected to high stresses generated by Andina tectonic. Therefore, wells are drilled with deviated trajectories usually up to 40° inclination angle (Cerro Fortunoso). In order to understand the CBP mechanism several studies were conducted. Image logs to obtain geomechanical properties as well as pumping pressure records and falloff profiles during communication events were analyzed. Interactions between the formation and completion fluids, as well as mechanical stresses numerical simulations of the open-hole and cemented-cased near wellbore conditions were conducted. The experimental tests performed with rock samples and completion fluids showed no significant interactions that could cause channeling due to formation shrinkage effects. On the other hand, the numerical simulations studies confirmed the data obtained from image logs and geo-mechanical studies, where high shear stresses generate wellbore oversize calipers and breakouts. The simulations also show that high stress levels are transmitted to the cement and the casing, reaching, in some cases their colapse limit. During the last year, CBP problems were diminished by introducing slight changes in the well design, mud density window and by improving casing centralization. Based on the numerical simulation results, the next step consists in using enhanced cement slurries designs with improved mechanical properties, in order to allow near wellbore stress redistribution. All these changes will avoid formation breakouts and cement rupture that could create fractures along the well trajectory, and therefore, will reduce the possibility for fluid communication during well completion jobs. Introduction CBP strongly affects stimulation treatments efficiency, generates difficulties on formation evaluation and may cause severe production losses due to water communication, as well as an unsuccessful secondary recovery, due to inappropriate water flooding. Several oilfields located in Mendoza state, such as Cerro Fortunoso, Barrancas and Ugarteche, have shown CBP problems since the mid 80´s. These events were particularly observed during acid stimulation treatments. This paper summarizes the results of an extensive study conducted between operator and service company in order to understand the CBP mechanism of these wells and determine the possible solutions to this problem. CBP problems in Mendoza oilfields After almost 200 wells drilled in Cerro Fortunoso and more than 500 in the Barrancas and Ugarteche oilfields, Mendoza, during the past 20 years, ongoing efforts are dedicated by operator and service companies to solve frequent communication between perforations problems. In the past two years a program was carried out were different aspect associated to well drilling and completion were analyzed in order to determine the causes and possible solutions to these problems. During the first 10 months of year 2004, fifty acid stimulation treatments were done, 12 of these presented CBP events. The communications occurred randomly, some of these during acid displacement, others before the acid had reached the formation and others even during previous swabbing operations.