Seismic modeling of acid-gas injection in a deep saline reservoir

Charles P. Ursenbach, Donald C. Lawton

A number of projects involve the injection of acid gas into non-commercial reservoirs as a means of disposal. Acid gas removed from natural gas is composed of carbon dioxide, hydrogen sulfide, methane, and other residual gaseous hydrocarbons. It may also contain water, but this is often removed prior to acid-gas injection.

A case is studied of non-aqueous acid gas injection into a saline dolostone reservoir. Seismic monitoring of an injection plume requires knowledge of acid gas acoustic properties, and for a non-aqueous acid gasubstitution, if the assumptionassumption that fluids do not interact with the rock matrix must be considered carefully in view of the acidity of the injected fluid, particularly for carbonate rocks. Studies have not yet clarified the rate of reaction of acid gas with carbonate rocks. However, a mineral such as dolomite will react more slowly than calcite, and it is therefore reasonable to assume that substantial reaction will occur on a longer time scale than the early stages of injection. This would justify using the standard Gassmann's equation to monitor initial injection progress.

The feasibility of monitoring is judged by the sensitivity of traveltimes and reflection coefficients to fluid substitution. Using acid-gas properties from the Peng-Robinson equation of state and fluid substitution effects from Gassmann's equations, the traveltime difference is seen to be on the order of a quarter millisecond for each 10 m thickness of the acid-gas plume for average dolostone properties, and up to a half millisecond for softer dolostones. Minor changes in reflection coefficient are also observed, but full analysis with well logs would be required for a useful assessment of this effect.