New mathematical formulations for accurate estimate of nitrogen leakage rate using distributed temperature sensing in Mechanical Integrity Tests

Current practices for estimating calculated nitrogen leakage rate (CNLR) have been a great challenge in Mechanical Integrity tests (MIT) for underground solution-mined storage caverns due to the insufficient wellbore temperature measurements. The use of a traditional temperature logging system for detecting gas leakage relies on the existence of steady-state temperature conditions which is difficult to validate with current measurement technologies. Accurate estimation of CNLR is essential for making correct decisions on the use of underground caverns. Recently, an advanced MIT system with a new measurement technique, namely distributed temperature sensing (DTS), was developed to overcome the limitations of the conventional measurement system. DTS produces continuous measurements of real-time temperature data throughout the wellbore which are used to estimate the gas leakage rate. In this paper, new mathematical formulations for the DTS system are developed to adapt the new measurement technique and allow incorporating real-time temperature measurements in estimating both volume- and mass-based CNLR. The new mathematical formulations are applicable for unsteady-state temperature conditions and account for the compressibility effect of nitrogen gas. To evaluate the efficiency of the new mathematical formulations, a computational fluid dynamics (CFD) model for a solution-mined wellbore was developed and validated. The CFD model was then used to predict the thermodynamics response of complex leakage scenarios with minimum detectable leak rates (MDLR) for nitrogen and brine. The results demonstrate the ability of the DTS method utilizing the new formulations to improve the accuracy in estimating MDLR compared with conventional temperature logging methods. The advantage of using a mass-based formulation versus volume-based formulation is also highlighted.