Abstract: Method of Prediction of the Hydrogen Sulfide Content in Gas Based on Its Solubility in Water (Case Study of the Kochmes Field, Timan-Pechora Basin)

Lidia A. Faingersh, Marina A. Kalita

More than a hundred oil and gas fields have been discovered in the Timan-Pechora basin of Russia. Gas accumulations of the basin are typically characterized by a high hydrogen-sulfide content; the basin is among the largest world oil and gas provinces, rich in hydrogen sulfide. We proposed a new method for prediction of the hydrogen-sulfide content and tested this method on the Kochmes field located in the eastern part of the basin.

The Timan-Pechora basin is filled by predominantly carbonate rocks ranging in age from Ordovician through Early Permian and by Upper Permian-Mesozoic clastics. The sedimentary section contains evaporites at two stratigraphic levels, in the Silurian and Lower Permian. Carbonate rocks between these two evaporite layers are likely to contain gas accumulations rich in hydrogen sulfide. A gas pool with H₂S concentration of 3.6 percent was discovered in Silurian carbonates of the Kochmes field at a depth of 5,620 m.

The proposed method of prediction of the H₂S content is based on a hypothesis that phase equilibrium exists between free gas in the pool and dissolved gas in water underlying the pool. The constant of phase equilibrium is a ratio of mole fractions of a gas component in the gas phase and in water solution. Because of the high H₂S solubility and polarity, solutions of this gas in water under pressures exceeding 20 MPa do not obey Henry's law; therefore, complex physics chemical equations should be used to determine the constant of phase equilibrium for the hydrogen sulfide/hydrocarbon gas mixture.

Application of the method to the Kochmes field showed a close fit between predicted and measured values of the H₂S content in gas. The accuracy of calculations appeared to be within 10 percent, which is satisfactory and supports our principal hypothesis about the existence of phase equilibrium in the gas pool-bottom water system.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994