This paper presents the results of successful applications of polymer gels to control undesirable water production in mature fissured reservoirs in northern Italy.

The presence of natural fractures creates hydraulic communication through an interval to the high-pressure aquifer. This allows the water to percolate up to the production interval causing a negative effect upon the oil production. This situation represents one of the most complex problems to diagnose and solve.

The shutoff of water produced through fissures and fracture networks involves injection of gel solutions of polymer, maturely crosslinking inside the reservoir. The ability to inject these polymers and their subsequent long-term stability under reservoir conditions are requirements for a successful treatment.

Two case studies are provided where a systematic methodology was employed to correctly diagnose and solve this type of problem.

The first case discussed is where a bottom zone was finally shut off. Diagnosis through production logs permitted us to understand the situation and treat the problem accordingly with the use of flowing polymer gels. Wellbore isolation from the upper oil-producing level was achieved with an inflatable cement retainer set in a blank casing section. The treatment and setting of the plug were performed through coiled tubing.

The second case treatment was performed under challenging technical and operational conditions due to high injection pressures (9000 psi) and elevated temperatures (165°C). The applied methodology included the use of diagnostic plots based on the historical behavior of the water/oil ratio as a function of time, cement bond logs, a pressure and temperature survey during actual production to determine the origin of the water influx, and the simulation of the bottomhole temperature (BHT) profile. Gel characteristics are also provided along with details of laboratory testing and procedures.

Both cases illustrate the use of the dual-injection technique, which involves simultaneous injection of permanent gel into the water zone and protective fluids into the oil zone, followed by a post-flush stage to eliminate gel from potentially productive fissures.

Pre-and post-treatment production data,analysis,and evaluation of results are included in both cases along with production and gel stability data with time.

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