Study On Reservoir Wettability Under The Action Of Viscoelastic Surfactants

Surfactant is a significant chemical agent which can be used for EOR. It can enhance oil recovery by the mechanism of reducing oil-water interfacial tension, changing wettability of rock surface and so on.Viscoelastic surfactant(VES) with dual function of oil displacement and flow control has the potencial of enhacing oil recovery just like polymer/surfactant compound system.Currently, research of wettability changed by surfactants is mainly focused on static wettability.The understanding of dynamic wettability change caused by surfactant flow in pores medium and the displacement mechanism is not clear, especially the effect when surfactant has the character of viscoelastic is still of blank. This thesis aimed at static and dynamic wettability under the action of VES and the main contents completed were as follows:Firstly, we tested and calculated surface tension and viscoelastic parameters of two kinds of viscoelastic surfactant system. Water wet and oil wet solid surface were simulated on the basis of the experiment above. With the aid of contact angle tester, we probed the wettability change of solid surface under the effect of different concentrations of VES and effect factors such as time, temperature, water quality, pH value to the change of surface wettability were considered. In addition, Amott/USBM method was used to explore the changes of core wettability index before and after the action of VES. Core displacement device was used to study the change of core wettability and efficiency of oil displacement caused by VES. Finally, micro displacement device was utilized to compare the effect of four kinds of displacement systems on different kinds of residual oil on oil-wet surface, mechanism of wettability change caused by VES was analyzed from the micro-level.The experimental results show that the viscoelasticity of VES may lead to the fact that contact angle changes with time while tested. After the action of VES under 73?, water wet quartz piece changes from the low concentration of monolayer adsorption to a high concentration of double layer adsorption. However, the mechanism of wettability changing of oil wet quartz is to depend on the electrostatic repulsion to improve wettability which differs from water wet quartz piece. Contact angle of oil wet quartz affected by VES is negatively related to the duration and temperature. When VES flows in the core, the degree of core wettability index influence:aging time>VES concentration>injection rate>pore throat size. Relative permeability curve has obvious changes when flooded by viscoelastic surfactant, polymer or polymer/surfactant compound system after water flooding compared with pure water flooding. The curves appear an "isotonic point" under certain conditions. Because of the ability of wettability change and recoverable reticular structure, the oil displacement efficiency of VES is slightly higher than polymer/surfactant system. Combing with micro-experiment, VES have greater extent and displacement efficiency than glycerol and conventional surfactants.The effect on oil-wet blind side is remarkable and displacement depth is greater than glycerol and polymer.This adsorption/stripping mode can make the residual oil which was hard to be moved by water flooding movable. So that, oil wet surface changed to water wet, oil displacement efficiency was improved as a result.This paper devoted to study the mechanism of reservoir wettability alteration caused by VES, which has a significant guiding to enhance oil recovery of low permeability reservoirs.