LAUSR

Abstract Tubing leakage is one of the main reasons to cause annular pressure in HPHT gas wells. This kind of annular pressure is called “sustained” because it is long-lasting and can not be eliminated by releasing. Sustained annular pressure is adverse to well barrier reliability, gas well management and repair. Even fire hazard and tubing fracture may happen. Therefore, it is necessary to study related characteristics of sustained annular pressure and fluid distribution, thus reducing potential risk. However, nearly all available research concentrates on cement integrity failure. So this paper develops a model of sustained annular pressure caused by multiple tubing leakage points. The wellbore temperature and pressure are calculated first as the gas leakage power, which considers the impact of pressure and temperature on gas properties. And then, tubing leakage point is classified as gas-leak and gas-liquid-leak. Fluid Leakage rate is calculated. Finally an equation is proposed to calculate the annular fluid distribution and annular pressure. To solve this model, the coupling relationship between leakage rate, sustained annular pressure and fluid distribution is analyzed. And then the well space and leakage time are divided into short segment. Two typical situations are selected as example. One is multiple gas leakage points. The other is combination of gas-leak and gas-liquid-leak points. By the proposed model, characteristics are analyzed, including annular pressure, pressure balance position, annular gas volume and liquid level. Also, sensitivity of factors is evaluated, including leakage point position, size, production rate and bottom pressure. The results indicate that rising process of sustained annular pressure can be divided into three zones(a, b, c). Annular pressure should be released to zone c when acoustic log is applied to locate leakage points. Method based on U-principle is not suitable to locate multiple tubing leakage points. Both pressure and liquid level should be monitored to evaluate tubing integrity. For multiple gas-leak points, position and size of leakage point have impact on maximum annular pressure and final liquid level. When gas-liquid-leak point exists, sustained annular pressure and liquid level continue changing, so annular fluid distribution and its change law can help to judge the existence of gas-liquid-leak points. Increase of production rate and decrease of bottom pressure can be considered as potential measures to reduce the rising rate and maximum sustained annular pressure under both two situations. This paper can help to diagnose tubing integrity by the characteristics of annular pressure and fluid distribution. Acoustic log is recommended to locate tubing leakage point. Potential measures are discussed to reduce maximum annular pressure. The proposed model can also be applied to calculate maximum annular gas volume and gas leakage rate when parameters of tubing leakage points are known, thus helping to assess potential risk.