Gelation of a Water-Shutoff Gel at High Pressure and High Temperature: Rheological Investigation

Polymeric gels are used to reduce or shut off excess water in petroleum reservoirs. The strength of polymeric gels needs to be high enough to withstand the water flow. This study investigates the strength of an organically crosslinked polyacrylamide (PAM) with polyethylenimine (PEI) as crosslinker. Dynamic testing measurements at high temperatures up to 130 degrees C (266 degrees F) and high pressure (500 psi) were used. During the gelation process, the rate of crosslinking increases with increased temperature. However, after the crosslinking reactions were completed, G' was lower at higher temperatures than at lower temperatures. Polymer concentration has a stronger impact on G' in comparison with the crosslinker. Salinity of mixing water has a negative impact on G'. High G' was observed in distilled water (1044 Pa) in comparison with field water (725 Pa) at 130 degrees C (266 degrees F). Moreover, samples prepared at alkaline and acidic conditions produced stronger gels compared with samples prepared at neutral conditions. However, the gelation time is longer at a pH of 7. Ammonium chloride is found to be a very effective retarder in comparison with NaCl. The yield stress of the gel was correlated to the pressure gradient for the gel extrusion through fractures. It was found that gels with 7:0.3 and 5:0.3 wt% PAM/PEI gave pressure gradients of 155.5 kPa/m (55.2 psi/m) and 3572 kPa/m (518 psi/m), respectively. The decrease or increase in gel strength was explained through the impact of the different parameters on PAM hydrolysis.