Effect of profile-control oil-displacement agent on increasing oil recovery and its mechanism

In recent years, chemical flooding has become significant in oilfield development. As its scope of application gets wider, choosing the proper chemical agent becomes an issue for field engineers. Different chemical agents have different oil displacement mechanisms, so engineers should choose the appropriate one for an oilfield application with different reservoir characteristics. The most important aspect of chemical profile-control oil-displacement technology is the selection of suitable agents for a reservoir. Petroleum engineers have considered the viscosity of a profile-control oil-displacement agent as the main technical index, and they developed salt-resistant polymers but the oilfield application results were insufficient. In this work, according to the actual need of the oilfield, the relationship between viscosity and mobility of the profile-control oil-displacement agent was studied. The results showed that the viscosity is the representation of friction, which is caused by the motion of the inner liquid layer, and it is related to the polymer molecular aggregation. Moreover, the principle of chemical profile-control oil-displacement technology is to improve the injection pressure and increase the absorption pressure difference (difference between injection pressure and absorption pressure of oil layer) of middle- and low-permeability oil layer and larger pore throat, thus realizing the goal of expanding swept volume. The mechanism of viscosity measurement and that of chemical profile-control oil-displacement are different, and there is no relationship between viscosity and oil recovery. Thus, the method of evaluating oil increasing effect by viscosity and mobility ratio (K/mu) is not logical. It was also important to achieve a good compatibility between the polymer molecular aggregation dimension and pore size so that the profile-control oil-displacement agent can be injected into a deeper area in the oil reservoir. This relationship can be evaluated by the agent transport capacity. Lastly, practice showed that irrespective of the viscosity, the profile-control oil-displacement agent could improve oil recovery if the agent is injected into the deep area in high-permeability layers.