Accurate Natural Gas Viscosity Correlations for High and Ultra High Pressure and Temperature Gas Systems

Accurate modeling of natural gas viscosity under high and ultra-high pressure temperature reservoir conditions is crucial for efficient reservoir simulation, flow assurance and production optimization. Conventional viscosity correlations often fail in such ultra-high conditions due to nonlinear gas behavior. Its behaviour under ultra-high reservoir conditions is still uncertain despite being thoroughly investigated at low to moderate temperatures and pressures. As exploration moves deeper into ultra-high pressure high temperature (UHPHT) reservoirs, where accurate viscosity predictions are essential for accurate simulation, reserve estimation and field planning, this limitation becomes more significant. Most existing correlations are calibrated on dataset below 10,000 psia and 300 °F, raising concerns when applied to ultra-high reservoir conditions. This study developed two new empirical correlations capable of accurately predicting natural gas viscosity in ultra-high reservoir environments, based on extensive 460 laboratory measurements with temperature and pressure range of 212–392 °F and 5,000–20,000 psi, respectively. Model validation confirmed the superior predictive performance of the proposed empirical correlations PLM and POLM compared with existing correlations, yielding average absolute relative errors of 1.25% and 1.38% and high coefficients of determination (R²) of 0.98 and 0.97, respectively. These results indicate the reliability and accuracy of the developed correlations for prediction under the investigated conditions. These correlations provide reliable tools for gas viscosity prediction in high and ultra- high pressure and temperature gas systems.