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A common problem during the lifetime of oil and gas producing wells is the surge of the water-cut. High water-cut leads to poor economics due to oil productivity losse and increased water handling and operational cost. A widely used chemical water shut-off method relies on gelling systems consisting of a water soluble polymer and a crosslinking agent. This paper reports an experimental study of the shut-off water flow in fractured carbonate system using a polymer gel consisting of a copolymer of acrylamide-sodium acrylate crosslinked with chromium acetate. The experimental study consists of two parts:the rheological characterization of the gel in presence of high salinity formation water (total dissolved solid of 172,000 ppm) andcore flood experiments using a length-wise artificially fractured limestone core (fracture width of 1 mm). From the rheological experiments, it was found that the gel system made with high saline formation water is stable at high temperatures (at 85 °C) for 4 days. More crosslinking of polymer chains occurred in presence of formation water. The core flow experiments revealed that gel is strongly blocking agent reducing the fracture conductivity by four orders of magnitude. Introduction A well known problem during the lifetime of oil and gas wells is the production of unwanted water. Excessive water production increases production operation costs due to artificial lifting, water handling and separation and other problems such as increased corrosion rates, higher tendency to make emulsion and scale formation. Moreover, environmental concerns exist about the disposal of large amounts of the produced water on- and offshore. These problems ultimately lead to a premature end of the economic life of the well. For this reason extensive efforts have been done for many decades to find ways to keep the water in the reservoir as much as possible, along with other methods to manage produced water. Several authors [Arnold et al. 2004; Sparlin and Hagen 1984; Seright et al. 2001] have reviewed the main sources of produced water in oil and gas wells and the main methods of the existing solutions to reduce or completely shut-off water production. The use of polymer gels emerges as one of the most effective ways to reduce or completely block water production in many reservoir related water production problems. Polymer gels can be used either in injection or and in production wells [Moradi-Araghi 2005]. When used in injection wells, gels divert the injected water toward less permeable layers and help improve the reservoir sweep efficiency. This experimental work is concerned with ways of shutting-off water production through fractures connecting wells to water sources in a carbonate reservoir. A polymer gel consists typically of a water soluble polymer and a crosslinking agent. The low viscosity solution containing the polymer and the crosslinker, often called gelant [Seright 1996], is converted into the rubber-like gel structure through a cross-linking reaction in which polymer chains are linked together to make a three dimensional network. Depending on whether the crosslinking agent is an organic compound or an inorganic metal ion, covalent or ionic bonds are made between the polymer chains. Gels consisting either of copolymer of acrylamide-t-butyl acrylate and polyethyleneimine [Eoff et al. 2006; Al-Muntasheri et al. 2007] or of partially hydrolyzed polyacryalmide and chromium acetate [Seright 1999; Willhite and Pancake 2004] are well known examples of covalently and ionicaly crosslinked gels. Two main applications of polymer gels have been reported in the literature. Several researchers studied performance polymer gel system in the near well bore area to decrease water permeability much more than oil permeability, the so-called disproportionate permeability reduction effect [Seright 2006; Stavland et al. 2006]. The other type of gelling system application is to seal a high permeable system [Sydansk et al. 2004], which is responsible to increase abruptly unwanted produced water. This method which is typically applied far from the well bore vicinity is aimed to decrease water permeability as much as possible in the reservoir condition.

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Polymer gel treatment is an economic and effective method to reduce excessive water production in hydrocarbon reservoirs. However, there exist unsuccessful applications of polymer gel due to a mismatch of theoretical and experimental results in field conditions. In this study, a gel treatment experiment was implemented using a novel test method which includes a unique two-dimensional coreflooding setup and a new procedure of simultaneous oil and water injection. To form the gel in situ, a Cr(III)-acetate-hydrolyze polyacrylamide (HPAM) gelant was used. The results showed that polymer gel could be successfully applied to water shut-off (WSO) treatments in low-permeable porous media with radial flow. The residual resistance factors for oil and water were 78.50 and 1.96, respectively. Polymer gel also showed disproportionate permeability reduction (DPR) behaviour in coreflooding experiments. The flow resistance to water was 40 times greater than that to oil. In gel treatment, high gel selectivity (DPR scale) of 0.83 was measured by simultaneous oil and water flow to the core, and the determined oil cut was much greater than the water cut, whereas a lower well production rate led to a higher water cut. In addition, a water blockage problem was examined in simultaneous injection at constant pressure. No observation of water in the outlet was reported, though water saturation through the sandpack was increased to 84 %. Finally, this paper suggests a new experimental test to increase the chances of successful WSO treatment under laboratory circumstances close to the field conditions.