An Effective Matrix Diversion Technique for Carbonate Formations
Authors: Saxon A. , Chariag B. , Abdel Rahman M. Reda 
Published: December 2007

Abstract
The removal of formation skin damage from carbonate reservoirs is ordinarily accomplished by stimulation with hydrochloric acid. Uniform coverage of the zone being treated during matrix acidizing stimulations of carbonate reservoirs is an accepted requirement for a successful treatment. However, the task of achieving efficient diversion and uniform zonal coverage is often difficult due to permeability heterogeneity within the treated interval.
This paper describes a technique that has been used successfully to divert treating fluids in carbonate formations.

Introduction
Effective placement of the stimulation fluids evenly across the zones being treated has always been a prime concern during matrix acidizing of carbonate formations. This problem is compounded when treating long perforated intervals, zonal heterogeneities or open hole intervals (especially in horizontal completions).
The paper describes a technique that has been used with success in the U.A.E. to divert treating fluids in carbonate formations. Actual data from several wells, both vertical and horizontal, that have been treated in different fields is presented. The methodology of candidate well selection, treatment design and execution is described along with documented pre and post stimulation treatment production log assessments that verify the success of the technique.
The diversion technique utilises coiled tubing (CT) for fluid deployment and diversion of the treating fluid is accomplished by employing a temporarily activated cross-linked gelled acid system. The creation of the diverter gel structure is dependent on the pH of the fluid and dissipates upon spending of the acid.
The impact, in terms of improvements in injectivity or productivity, on the wells treated using this process over other types of diversion techniques utilised in the area, was significant and is documented.
Laboratory study data, from carbonate core flow tests, is also included to illustrate how the diversion process physically manifests itself and is used to substantiate the field successes that have been encountered using this matrix diversion technique.

Diversion Technique

Coiled Tubing
It is generally accepted, that where possible and economically viable, coiled tubing should be employed for the placement of matrix treating fluids in both cased and open hole situations. Bull heading matrix treatments in carbonate reservoirs tends to result in non-uniform treatments. This is especially true when long, open hole, sections are being treated.

Diverting Agent
The effectiveness of using particulate diverters in carbonate formations has been questioned and it is not possible, in most instances, to use them in conjunction with coiled tubing. Foam has been used with success in carbonate reservoirs for some time but requires, in most cases, that additional equipment is mobilised to location for on-site generation of the foam. Consequently, a need arose for an effective matrix diverting agent for use in carbonate reservoirs that could be placed through coiled tubing, where it was not anticipated to require nitrogen on location for post treatment well lifting operations. A derivative of a fluid originally designed for controlling fluid loss in acid fracturing treatments found application as a such a diverting agent.
The fluid in question was an acid based system that develops a cross-linked gel structure as a function of changes in pH. Essentially, the system cross-links at approximately pH 3 and forms a firm cross-linked structure. P. 473^