The team will field test the newly developed ‘quad porosity model’ in the next few months at an exploratory shale gas site.
‘The challenge for the team at the outset was to understand shale gas reservoirs in order to develop a predictive tool for better forecasting and economics,’ said project lead Deepak Devegowda of the Mewbourne School of Petroleum and Geological Engineering at Oklahoma. ‘Shale gas reservoirs are complex systems, unlike conventional reservoirs.’
The key factor influencing the storage and transport of fluids in shale gas reservoirs is the inorganic and organic pore spaces in the rock.
Standard equations used to describe gas transport cannot be applied to the small pores in the organic material where a significant portion of the gas is stored. The research team has shown permeability enhancement effects of up to two orders of magnitude in very small pores and this, in part, explains how gas is produced from these extremely tight formations.
Additional complexity arises due to adsorption of gas in the organics in a high-density layer adjacent to the pore walls. While current numerical reservoir simulators are sophisticated in terms of their algorithms and computational efficiency, they are restricted to modelling viscous flow.
Using scanning electron microscopy, the team was able to adapt these models and demonstrate that gas shales can be characterised by four porosity systems — hence the ‘quad porosity model’.
‘Developing a realistic simulator is an exciting challenge,’ said co-investigator Faruk Civan of the Mewbourne School. ‘Our work focuses on understanding and testing the theoretical description of the mechanisms of gas storage and fluid (gas/liquid) transfer in such an intricate system of inorganic and organic pores, and natural and induced fractures. OU is pioneering permeability measurement, which incorporates all flow regimes. We can also determine properties of shale rock.’
The three-year, $1.5m (£1m) project was funded by the Research for Partnership to Secure Energy for America and a consortium of nine oil- and gas-producing companies.