Applications of a Polymer Gel for Establishment of Zonal Isolations and Water Shut-Off in Carbonate Formation
Authors: Perez D. , Fragachan F.E. , Ramirez A. , Ferraud J.P. 
Published: December 2007

Abstract
This paper presents the results of successful applications of polymer gels to control water production in Mexico. Three case studies are provided where a systematic methodology was employed to correctly diagnose near-wellbore water channeling behind casing. The methodology discusses the use of diagnostic plots based on the historical behavior of the water-oil ratio as a function of time. Including, correlation with information from original cement bond logs, oxygen activated logs during actual production to effectively determine the origin of the water, and saturation logs to determine the actual levels independently of the salinity of the water been produced.
In addition, the paper presents successful applications of polymer gels to re-establish zone isolations in the three case studies mentioned above. It discusses gel placement and presents the procedure followed in each case, evaluation of a water injectivity test followed by a temperature log prior to gel placement to determine height propagation of the water and anticipate potential zone damage of adjacent producing intervals during gel placement. One case is discussed where a new interval was completed perforating through the gel, with excellent results. The other case, presents a zone abandonment with gel were positive pressure tested with 500 and 1,000 psi well head pressure at 2,500 meters. In all cases advantages of gel treatments over common cement squeeze are discussed.
Finally, results of the treatments performed are discussed including the analysis of pressures recorded during gel placement. The oil and water production prior and after treatment are presented.

Introduction
One of the main problems encountered in old wells and in wells that were originally cemented under low reservoir pressure, consist in granting hydraulic isolations between the different intervals to allow proper production of the zones of interest. The lack of isolation has caused, among other things, undesired movements of fluids behind casing generating confusion of the actual levels of the oil-water contact and originating premature abandonment of oil reserves. This paper presents a methodology followed in the north of Mexico to correct channeling of water behind pipe using chromium crosslinked polymer gels. It presents the advantages of using gels over cement, including flexibility for their pumping without a workover rig, higher control in setting time, easy-to-clean, no milling time, and superiority regarding operation cost without risking treatment effectiveness.
Included in the methodology is the candidate selection using diagnostic plots which allows to identify, among others, near wellbore flow which correlates with cement bond logs indicating poor cement.
Finally, three field case studies corresponding to the north of Mexico, Poza Rica, are discussed. Analysis of the information available is discussed in each case, including saturation logs, production logs, density logs, and water flow, based on the activation of oxygen, to monitor the movement of water through a channel. Corrections to these flow of water is presented including a detail overview of the execution and of the results showing the effectiveness of the treatments.

Near Wellbore Flow
The problems associated with water production, and its control, represent a great challenge to the reservoir and workover engineers. The key of the problem lies in defining the origin of the water and determining whether the water production of a given interval is necessary to the associated oil production. Therefore it is required to define two kinds of water production: bad and good. P. 415^