Prediction analysis of downhole tubing leakage location for offshore gas production wells

Industry experience has shown that downhole tubing leakage is the most challenging issue resulting in sustained casing pressure in natural gas production wells. This article presents a novel tool based on pressure difference and probability distribution for predicting the tubing leakage location for an offshore gas production well with sustained casing pressure. Combined with the formation information, well structure, annular fluid level and dynamic production parameters of a gas well, the model of pressure difference between tubing and production casing annulus is established. However, these pressure-based predictions suffer from various sources of uncertainties, such as the variations in reservoir conditions, and measurement errors. Bayesian inference is introduced to handle these uncertainties in leakage location forecasting effectively. Dynamic confidence intervals under two kinds calculation modes are applied to calculate the leakage depth within a certain range. The probabilistic distribution of leakage location is statistically predicted under different wellhead pressure difference using Monte Carlo simulation. A case study on a specific offshore gas well with the field data is presented to illustrate the feasibility of the proposed method and to demonstrate that the downhole tubing leakage location prediction contributes to the safety and integrity management of offshore production wells in an economical and convenient way.