Gas Migration from a Tight-/Shale-Gas Reservoir to an Overlying Aquifer Through Long Fractures, Conductive Faults and Abandoned Older Wells
Abstract
Natural gas from shale reservoirs has become an increasingly important energy resource in recent years. However, the environmental challenges posed by hydraulic fracturing (a necessary stimulation method in tight- and shale-gas reservoirs) remain poorly characterized. There exist theoretical risks of leakage of contaminants from such reservoirs through hydraulically-induced fractures into groundwater resources, but no rigorous model-based analysis has been performed to assess the magnitude of these risks. The mechanisms and quantities of fluids that may realistically be transmitted through induced fractures and faults between geological strata are unknown. Possible exacerbating factors in shale gas well completion or stimulation design are likewise unknown. Quantification of these factors is necessary to quantify possible environmental risks and to aid the industry in the continuing development of sustainable hydraulic fracturing practices. We used the TOUGH+RealGasH2O code to model the two-phase flow of water and gas through long conductive features (such as fractures, conductive faults and abandoned older wells) connecting shale gas systems to shallower aquifers. The complex 3D domains in this study involve Voronoi grids describing challenging geometries that include vertical wells (in the aquifers and abandoned older gas wells), the hydraulically fractured system along long horizontal wells, and thin vertically extensive features intersecting multiple geologic strata. We investigate various configurations of the fractured system, we determine the upper limit if the possible contaminant transport solutions stemming from "worst-case scenarios", and we conduct a thorough sensitivity analysis to determine the dominant mechanisms, conditions and parameters. These include the conductivity of vertically extensive faults and fractures, the relative pressure differential of the underlying shale layer and the aquifer, the permeabilities of the productive intervals, the vertical distances between layers, etc..
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H11C1202M
- Keywords:
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- 1805 HYDROLOGY / Computational hydrology;
- 1832 HYDROLOGY / Groundwater transport