Effect of paraffin precipitation on core flow in porous media capacity during fracturing

Abstract

Low-permeability sandstone reservoirs have become an important exploration and reserve increase target in China, but most low-permeability sandstone reservoirs with high wax content and high freezing point will reduce their production after fracturing injection, especially in winter. Therefore, it is necessary to analyze the field fracturing construction measures and reservoir characteristics. Generally, the fracturing fluid injected by fracturing operation is at room temperature, even lower than 5 °C in winter, and the viscosity and freezing point of crude oil in this block are high. Therefore, the research focuses on the properties of crude oil and the influence of temperature reduction on oil-bearing cores. In order to find out the mechanism of reservoir cold damage, this paper first observed the pore throat of rock sample slice after displacement experiment, and found that there were black paraffin particles in some pore throats; then the distribution of core pore size after displacement was analyzed by T2 spectrum of NMR, and it was found that the proportion of small pores increased gradually with the decrease of temperature. Finally, the displacement experiment is used to simulate the change of permeability in the process of formation cooling and heating after field fracturing. The permeability of crude oil is tested by changing the ambient temperature of displacement experiment. It is found that the permeability before cooling is larger than that after heating recovery, and the process is irreversible. Therefore, the cause of cold damage is that the injection temperature of fracturing fluid is far lower than the formation temperature and the precipitation of paraffin during the cooling process of crude oil. For reservoirs with high wax content and high freezing point, paraffin precipitation caused by cold damage has a great influence on formation permeability, and cold damage can be prevented by changing injection fracturing fluid temperature or adding paraffin inhibitor.