Most reservoirs in Zhongyuan oilfield of SINOPEC are characteristic of high temperature, high salinity and high heterogeneity. After long time water-flooded, the heterogeneity is getting worse and worse. To control conformance, a new gel was developed by using a new associating polymer (AP) with a special molecular structure and characteristics of high temperature and salinity resistance. Cross-linker and some other additives were screened to meet different gelation time and gel strength. Effects of temperature and salinity on gel stability were evaluated, and results show that the gel can still keep its 75~85% original strength under conditions of 95 °C and a salinity of 150,000mg/l after 6 months. Core flooding experiment results show that the gel has a much higher resistance to water than to oil and it is a good DPR (different permeability ratio) agent that benefits to improve gel treatment effectiveness. Twentysix wells were operated by using the new gel in Zhongyuan Oilfield in 2003, and average injected gel volume is 1100m3 per well. These applications have increased 5,274 tons of oil and the average benefit/cost ratio has achieved above 2.5 until now. Three well selection criteria and the injection process are summarized from the operations of the 26 wells treated. And the way how to improve the effectiveness of the gel in pilot tests is discussed.
When conventional polymer such as polyacrylamide is applied in conformance improvement of oil reservoirs with high temperature and high salinity, its molecular structure in solution is curly shaped under formation conditions, which results in a sudden drop in viscosity of the polymer solution [1–2], even with a ultrahigh molecular weight of 2200x104. Therefore, the application of such polymers in severely heterogeneous reservoirs of high temperature and high salinity is limited.
Associating Polymer (AP) is a new type of watersoluble polymer in which special hydrophobic molecular groups are added into the polymer chain. In solution, reversible physical cross-linking with strength will occur by the force of hydrophobic association among the polymer molecules, so that large spatial network structures are formed in the solution, with the polymer molecules appearing in comb type and star type (refer to Figure 1). This polymer solution can have a high viscosity under the condition of low molecular weight and low concentration, not necessarily relying on increasing single molecular chain size in the polymer to gain a high viscosity. This structure is reversible. It can be destructed by shear force and restored when shear force disappears, so the polymer solution has a better shearing dilution property. Under some conditions, the structure can be formed and strengthened more easily while the temperature and salinity increase, assuring a stable viscosity of the polymer solution under high temperature and high salinity.
According to the molecular structure of the AP, the problem that limit the application of conventional polymers under the conditions of high temperature (>80 °C) and high salinity(>100,000mg/l) can be resolved. In addition, the shearing restoration property of the AP can resolve the problem that the partially hydrolyzed polyacrylamide does not have resistance to shearing.
