Calculation of leakage rate in casing annular of high pressure gas well containing acidic media

The primary cause for failure of gas well integrity is pipe string leakage, which leads to anomalous annular pressure in gas wells and hence threatens safe production of gas wells. The leakage rate of annulus is the core parameter to judge whether the gas well integrity fails. At present, some companies abroad have instrument which can measure the annular leakage rate at field; but there is no reliable domestic method to determine the annular leakage rate. This paper presents two ways to calculate the annular leakage rate: one is the safety valve method, which makes reference to the leakage rate of downhole safety valve; the other is differential method, which establishes the theoretical model for gas to be leaking in the annulus and determines the boundary conditions by the leakage model through small holes on gas lines. Due to the effect of field pressure relief data, the downhole safety valve method is not applicable on site, so the differential method may be used to calculate the annular leakage rate. The differential method was used for example verification in DN2 gas field of Tarim Oilfield, and the result agreed well with the real situation, showing that this method for annular leakage rate calculation is reliable and is of some reference significance to the field.