Accurate Gas-Viscosity Correlation for Use at HP/HT Conditions Ensures Better Reserves Estimation

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 124734, ’More-Accurate Gas-Viscosity Correlation for Use at HP/HT Conditions Ensures Better Reserves Estimation,’ by Ehsan Davani, SPE, Kegang Ling, Catalin Teodoriu, SPE, William D. McCain Jr., SPE, and Gioia Falcone, SPE, Texas A&M University, prepared for the 2009 SPE Annual Technical Conference and Exhibition, New Orleans, 4-7 October. High-pressure/high-temperature (HP/HT) gas reservoirs have pressures greater than 10,000 psia and temperatures higher than 300°F. Modeling the performance of these reservoirs requires understanding gas behavior at elevated pressure and temperature. Gas viscosity is used to model the gas mobility in the reservoir and can have a significant effect on reserves estimation during field-development planning. Accurate measurements of gas viscosity at HP/HT conditions are extremely difficult. Public-domain databases of hydrocarbon-gas viscosity were reviewed for validity of gas-viscosity correlations and their applicability range. Introduction The growing demand for natural gas is driving the search for new deeper sources of gas, many of which encounter HP/HT conditions. Among gas properties, viscosity is seldom measured in the laboratory and, typically, is estimated by use of correlations. At HP/HT conditions, reservoir fluids will be very lean gases, typically methane with some level of impurity, and therefore the gas properties may be different from those of gases at lower pressures and temperatures. A review of large databases of published viscosity data for pure methane and mixed hydrocarbons revealed limitations in terms of experimental conditions, data quantity, and in some cases accuracy. The full-length paper details many of these limitations. A review of available gas-viscosity correlations also was performed, which showed that these correlations were developed from experimental data taken at low-to-moderate pressures and temperatures and that their applicability at HP/HT conditions could be limited. Available Correlations National Institute of Standards and Technology (NIST). NIST developed computer software to predict thermodynamic and transport properties of hydrocarbon fluids. The software program uses the principle of “extended corresponding states” and was developed from pure-component and mixture data. The maximum pressure and temperature that can be used in the program are 44,100 psia and 1,340°F, respectively. The NIST gas-viscosity values closely match most of the published data, and the predictions generally are reliable for HP/HT conditions in the absence of real HP/HT gas-viscosity measurements. Lee, Gonzalez, and Eakin (LGE) Correlation. The LGE correlation is based on measured data of pure-component gases and eight natural gases with specific gravities less than 0.77. The correlation can be used to estimate gas viscosity, provided that the molecular weight and density at the relevant conditions are known.