Self-Diverting Acid for Effective Carbonate Stimulation Offshore Brazil

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 165089, ’Self-Diverting Acid for Effective Carbonate Stimulation Offshore Brazil: A Successful History,’ by A.T. Jardim Neto, C.A.M. Silva, R.S. Torres, R.L. Farias, F.G.M. Prata, and L.A.M. Souza, Baker Hughes, and A.Z.I. Pereira, A. Calderon, and E.F. Sandes, Petrobras, prepared for the 2013 SPE European Formation Damage Conference and Exhibition, Noordwijk, the Netherlands, 5-7 June. The paper has not been peer reviewed. This work describes the positive results experienced when a self-diverting acid system based on a viscoelastic-surfactant (VES) technology was introduced for carbonate-reservoir stimulation offshore Brazil. The self-diverting (SD) VES (SD-VES) promotes viscosity development when the acid comes in contact with the carbonate formation. Since the SD-VES was introduced in this environment in 2009, more than 40 wells have been treated with the system. Introduction Matrix acidizing is frequently used to stimulate carbonate reservoirs offshore Brazil. In these treatments, a proper diversion technique is required to direct the treatment fluid to lower-permeability or more-damaged zones and ensure the treatment of the entire production interval. The chemicals developed and used as diverting agents include polymer gels, foams, oil-soluble resins, and rock salt, among others. Offshore Brazil, polymer-based systems have been applied successfully. Such systems are easier to handle than particulates, and diversion is achieved through the formation of natural resistance to viscous flow. Generally, the treatment is divided into three phases: a regular 15-wt% hydrochloric acid (HCl) is pumped first, followed by a gelled 15-wt% HCl, and finally by a pill of insitu- crosslinked gelled 3-wt% HCl. In most cases, several stages are pumped, repeating the three phases in order to cover the entire interval. Despite the success experienced with polymer systems, concerns about the potential damage to the formation provided an impetus for the successful introduction of a viscoelastic-acid system offshore Brazil.