Inaccurate Gas Viscosity at HP/HT Conditions and its Effect on Unconventional Gas Reserves Estimation

Viscosity is one of the gas properties that is always uncertain at high-pressure and high-temperature (HPHT) conditions because no reliable correlation has been developed for HPHT and accurate measurement of gas viscosities at HPHT is difficult. However, the industry currently does not pay sufficient attention to the need for accurate gas viscosity values in HPHT gas fields' development. We show that small errors in gas viscosity affect the inflow performance relationship (IPR) curves and eventually change the reserves estimate for HPHT conditions in ways that can drastically influence production forecasting. A sensitivity analysis for a synthetic case at these conditions shows that a 1% error in gas viscosity measurement results in a 1% error in gas flowrate, which in a large scale of gas production may severely underestimate or overestimate recovery from an HPHT field. This linearity between gas viscosity and gas flowrate continues to ±10% errors. An evaluation of gas viscosity correlations commonly used in the industry shows that none of them is reliable at HPHT conditions, mainly for two reasons: measured data are not available to confirm their consistency and they do not account for impurities in the gas. Clearly, the industry needs accurate gas viscosity measurements and needs a new procedure to develop correlations based upon accurate HPHT gas viscosity data