2.1. Materials

Xanthan polymer (Mw~5 × 10⁶ Da) was provided by AVA Drilling Fluids and Services Co., (Roma, Italy). For each experiment, polymer solution was prepared at constant temperature by slow addition of known amounts of gum in a reactor containing DW, IW or RW. Acrylamide monomer (AAm), from Sigma-Aldrich Chemie Gmbh (Munich, Germany), was purified by double recrystallization from methanol, vacuum dried at 40 °C, and then stored in a desiccator over silica gel. Initiator, Ammonium persulfate (APS), acetone, and methanol were all supplied by Panreac Monplet and Esteban, S.A. (Barcelona, Spain) and used without additional purification.

Deionized water was employed in the grafting reaction and preparation of polymer-DW solutions. In most oilfields, injection water and reservoir formation brine (both contain salts) are essential water sources; their impact on rheology of polymer solutions is very significant for the flooding. The brine composition has a considerable influence on the aqueous phase viscosity. In accordance with the field geology, two brines have been used for polymer mixing: Liassic brine as injection water, Devonian one as reservoir water. Table 1 shows the chemical composition of these brines. The internal pressure in the well is of 131.1 bars; the well internal temperature is about 68 °C.

Chemical composition of brines used for rheology tests.