Water Control while Acidizing: Case Histories and Lessons Learned After More than 2,000 Well Interventions
Authors: Vasquez Julio Estuardo 
Published: June 2014

Abstract
Effective diversion of acidizing treatments is a key parameter to successfully removing near-wellbore (NWB) formation damage, especially when dealing with long intervals and/or highly heterogeneous formations with high permeability contrast. Additionally, acid stimulating wells already producing at high water cut can many times result in significant increase to the undesired water product rather than the hydrocarbon rates. This paper discusses the application of a novel solids-free acid diverter (SFAD) system that was originally developed for water control. However, because of the unique set of properties exhibited by the system, its application was extended to acid stimulation with two main objectives:effective acid diversion andwater control.
The SFAD system performs as a low-viscosity, solids-free diverter agent, opposed to the plugging mechanism of typical particulate or viscous diverters, helping guide acid away from high-permeability zones and into lower-permeability zones. The SFAD system is typically placed in alternating stages with the acid throughout the entire treatment.
This SFAD system is based on a hydrophobically modified water-soluble polymer that, once adsorbed to the surface of the rock, selectively reduces the effective permeability to any aqueous-based fluid (including acid) with little or no damage to oil or gas production, in both injection and production modes. The hydrophobic modification to the base polymer chain adds unique associative properties to the system that allows diversion of water-based treatments. The system changes the effective permeability to water of the zones where it is adsorbed; the subsequent acid stage is then diverted to other zones. Because this system does not affect hydrocarbon permeability, it does not require the use of breakers or a cleanup stage, eliminating possible negative impact to post-stimulation well productivity. In many instances, a reduction in water cut has been observed after the acid stimulation treatment, which is attributed mainly to the SFAD property to selectively reduced water permeability.
To date, more than 2,000 treatments have been performed with this SFAD system during matrix and non-matrix acid stimulation treatments. Although the primary objective of the SFAD system has been acid diversion, it has been also used for water control when acidizing high-watercut wells and/or acidizing nearby a water-producing zone. This paper discusses the SFAD system performance testing and a wide variety of case histories detailing the applications described in different types of reservoirs and wellbore completions. Onshore and offshore case histories are presented.

Introduction
A fundamental objective in the production and operation of hydrocarbon reservoirs is to produce the maximum percentage of the deposit while, at the same time, attempting to maintain initial investment and operating expenses at the lowest level possible to obtain this recovery. However, the produced hydrocarbons are often accompanied by large amounts of undesired water production that can impact hydrocarbon production and the overall profitability of the wellbore and reservoir. Additionally, when stimulation completion techniques, such as hydraulic fracturing or acid stimulation, are implemented, water production is often increased and exacerbated.