EVALUATION OF THE SPREAD OF ACID-GAS PLUMES INJECTED IN DEEP SALINE AQUIFERS IN WESTERN CANADA AS AN ANALOGUE FOR CO_2 INJECTION INTO CONTINENTAL SEDIMENTARY BASINS

摘要

For the past 15 years, oil and gas producers in the Alberta Basin in western Canada have increasingly turned to disposal of acid gas (H_2S and CO_2, with minor traces of hydrocarbons) by injection into deep geological formations. By the end of 2003, more than 4.5 Mt of acid gas, ranging in composition from 2% H_2S and 95% CO_2 to 83% H_2S and 14% CO_2, have been injected at 39 locations. The acid gas is injected into deep saline aquifers at 24 sites, and into depleted oil or gas reservoirs at the remainder, at rates that range from 1x10~3 to 500x10~3 m~3/d. At in situ conditions, the flow of the injected acid gas is driven and controlled by the hydrodynamic force due to injection and by differences in density and viscosity between the injected gas and the formation water. In a sedimentary basin extensively drilled for hydrocarbon exploration and production like the Alberta Basin there are wells that penetrate the disposal aquifer in the vicinity of the injection well that can be reached by a migrating plume of acid gas. Thus, the acid-gas disposal operations in western Canada represent an analogue for CO_2 storage in geological media in mature continental sedimentary basins like those in North America where oil has been discovered and produced since the 19th century and where many wells can be encountered by a plume of injected CO_2.rnTo assess the potential for upward leakage along improperly completed and/or abandoned wells, it is necessary to estimate the evolution and spread of the plume of injected acid gas and identify the existing wells within the plume's reach. A semi-analytical solution has been developed and applied to the radial spread of a plume of injected acid gas around an injection well. The flow of the injected acid gas is driven by the injection hydrodynamic force and by the density difference between the acid gas and formation water (buoyancy), and is controlled by the mobility ratio between the two fluids. The analytical derivation explicitly shows that the plume evolution and speed of spread depend on two classes of parameters: aquifer characteristics, such as porosity, permeability and thickness; and injection and fluid characteristics, such as injection rate, and fluid density and mobility. The effect of buoyancy is small and the plume evolution is controlled by the mobility ratio between the injected gas and the formation water, with gravitational forces playing a minor role in segregating the two fluids, when acid gas of high mobility contrast is injected at a high injection rate (strong hydrodynamic force) into a thin aquifer with low permeability and porosity. Buoyancy is important and even may dominate the transport problem in the case of low injection rates of light acid gas into thick aquifers with high porosity and permeability. An immediate and simple application of this semi-analytical solution is the explicit prediction of the maximum extent of an injection plume, which depends on the mobility contrast between the two fluids, on injection rate, on the length of time since the start of injection, and on aquifer thickness and porosity. Application to the case of acid-gas injection operations in western Canada shows that the respective acid-gas plumes have probably spread at distances that, depending on the particular case, range from approximately 150 m to less than 2100 m from the injection well. This method allows for the easy identification of existing wells that may be reached by a plume of injected CO_2 or acid gas.