Novel Acid-Diversion Technique for Matrix Acidizing in Production and Injection Wells: Case Histories of Alger
Authors: Kritsanaphak Kritsana , Vasquez Julio Estuardo 
Published: May 2016

Abstract
Successful matrix acidizing requires an even distribution of the acid treatment to remove near-wellbore formation damage throughout the perforated interval. In the absence of diversion, acid treatments tend to predominantly enter the highest-permeability zone, thus bypassing the lower-permeability, or most damaged, layers. In some cases, the acid might also break into a nearby water-bearing zone, which can result in a significant increase in water production after the treatment. This paper presents the field implementation of an associative polymer (AP) system that has been used to effectively divert matrix acid-stimulation treatments in producer and injector wells in an Algerian oil field. Additionally, this AP system can potentially reduce formation water production or redistribute water in injectors.
The AP system is placed as a low-viscosity solution (&lt; 2 cp) in alternating stages with the acid throughout the entire treatment. This AP system immediately adsorbs to the surface of the rock, reducing formation permeability to water with little or no effect on the permeability to hydrocarbons. The subsequent acid stages are diverted to other zones because the aqueous-based acid treatment will find a restriction in the zones where the AP chemical has been adsorbed. It does not rely on viscosity or solids to provide diversion (unlike the plugging mechanism of typical particulate diverters).
Case histories are presented of producer and injector wells where this novel acid-diversion technique positively changed oil production, including a case where water production was decreased from 75 to 60%. The acid-stimulation treatments included alternating stages of acid (9:1 hydrochloric:hydrofluoric[HCl:HF]) and the AP system. More than 1,100 jobs have been performed worldwide with the AP system in sandstone and carbonate formations up to 350°F. This technique has been implemented in matrix and frac acid treatments.

Introduction
One of the largest Algerian oil fields has productive reservoirs in the Triassic Argilo-GreseuxInferieur (TAGI) sandstones. The oil wells are typically drilled to a depth of 3250 to 3450 m, and multiple zones are perforated for production from the reservoir depth of 3000 to 3300 m. These wells have been producing for sometime, with some water cuts ranking from 0 to 97%,requiring workover operations to recomplete wells or installation of a gas-lift system for continuing production. In most cases, production decline was observed after completing workover operations, which was a result of formation damage caused by the workover operations.
Matrix acidizing is a preferred method to remediate formation damage and restore production. Without diversion, the acid tends to predominantly enter the highest-permeability layers and bypass the most damaged layers. In some cases, the acid can also enter into the water-production zone or break into a nearby water-bearing zone, which can result in increased water production after the treatment. In attempts to perform matrix acidizing in multiple zones with existing water production, placement technique and diversion are crucial for successful acid stimulation without increasing the water production. Various placement techniques have been used, and the most reliable method is the use of mechanical isolation devices, such as using a straddle packer for selective zone injection (Reza et al. 2006). However, this technique is not always practical, cost-effective, or feasible. Without mechanical isolation, some type of diverting agent must be used.