Using Associated Polymer Gels to Control Conformance for High Temperature and High Salinity Reservoirs
Authors: Zhao H. , Zhao P. , Bai B. , Xiao L. , Liu L. 
Published: March 2010

Abstract
Most reservoirs in the Zhongyuan oilfield are characterized by high temperature, high salinity, and serious heterogeneity. Extensive waterflooding in the reservoir has magnified the effect of these heterogeniuties. To improve the poor conformance, a new kind of gel was developed using a water-soluble hydrophobically associating polymer (AP). Effect of the concentration of AP, crosslinker, and additives on gelation time and gel strength were studied. Thermo-stability of the gel was evaluated and results show that the gel can still retain 75 &#x98; 85% of its original strength under conditions of 95 °C and salinity of 150,000 mg/l after six months. Core flooding results have illustrated that the plugging efficiency of the gel is more than 90% for selected applications. Twenty-six wells were treated using the new AP gel in the Zhongyuan oilfield in 2003. The response of treated wells and connected producers is reviewed. The average injected gel volume was 1,100 m3 each well. These treatments have resulted in approximately 6,000 m3 (5,300 tons) of incremental oil. Selection criteria for the optimum type of wells to be treated and injection procedures are summarized from the 26 applications.

Introduction
Gel treatment has been proved to be a cost effective method to control conformance for mature water/CO2 floods reservoirs (1–4).Traditionally, partially hydrolyzed polyacrymide (HPAM) is used to synthesize these crosslinked polymer gels. However, the traditional polymer molecular structure of these polymers tends to degrade under high salinity or hardness conditions in the injected brine, resulting in a sharp drop in viscosity of the polymer solution(5–6). Also, high molecular weight polymer tends to be irreversibly degraded by high shear rates, such as those encountered in pumps in the near wellbore region. Therefore, the application of traditional polymers of this type in high temperature and high salinity reservoirs is limited.
Associating polymer (AP) is a new type of water-soluble polymer in which a small number of special hydrophobic molecular groups are directly attached to the polymer chain(5–10). In aqueous solution, the hydrophobic groups of these polymers can associate to minimize their exposure to the external fluid, resulting in a star type of large spatial network structures, as shown in Figure 1. The association results in an increase of the hydrodynamic size of polymer that increases solution viscosity. This polymer solution can have a high viscosity at a relatively low molecular weight and low concentration. This associated structure is reversible. The fluids tend to be thixotropici in nature (viscosity is reduced by increased shear rates).
Most reservoirs in the Zhongyuan oilfield have high salinity and temperature and serious heterogeneity(11). The harsh reservoir conditions limit the application of traditional polymer HPAM. Therefore, AP associated polymer gel was synthesized and evaluated to control conformance in the Zhongyuan oilfield.

Experiment Results
Synthesis of the AP Gel

AP Concentration Effect
Synthesized Mazai formation water was used to prepare the polymer solution and the gel was formed at 95 °C. After three days' aging, the gel viscosity was measured at a shear rate of 12 rpm.