An optimized calculation method of critical erosion flow rates of UGS injection/production wells

Critical erosion flow rate is the key factor restricting the injection/production capacity of an injection/production well. At present, it is commonly calculated according to API RP 14E standard and its calculation result tends to be conservative. So far, however, there is no definite laboratory experiment or field data that can prove that critical erosion flow rate can be increased on the basis of API RP 14E. To deal this end, the concept of critical erosion flow rate was proposed based on corrosion rate for the first time in this paper. Then, a laboratory equivalent simulation experiment under real injection and production conditions was carried out by comprehensively taking into account the factors influencing string erosion (including temperature, pressure, gas component, water content, solid particle content and string material) while introducing the wall shear stress. Accordingly, the critical erosion coefficient (C) under experimental working conditions was calculated. Finally, a C value chart for three kinds of strings that are commonly used on field (N80, SM80S and S13Cr) was established. And the following research results are obtained. First, solid particle content, water content, CO2 differential pressure and wall shear stress are the main erosion controlling factors. Second, solid particle content is the most significant factor that affects the erosion of N80, SM80S and S13Cr strings, and erosion of N80 and SM80S strings is more sensitive to wall shear stress and water content. Third, as for S13Cr string, the C value can be 100 when the solid particle content is lower than 250 mg/L, 180 when the fluid contains liquid but no solid particles, and 275 when the fluid is gas phase. Fourth, as for N80 and SM80S strings, the C value can be in the range of 100–180 based on different water content and wall shear stress when the fluid contains liquid but no solid particles, and 275 when the fluid is in gas phase. Fifth, in view that the fluid produced from the injection/production wells of Hutubi gas storage has a water content of 0.0010‰ without solid particles, S13Cr is adopted as string material and the C value is set at 180. It is shown in the laboratory erosion experiments that no erosion trace occurs on the string samples under injection and production conditions and the erosion rate is extremely low without point erosion. In conclusion, the C value chart established in this paper is reliable and can provide the guidance for the scientific and reasonable determination of critical erosion flow rate.