Reservoir simulation of free and adsorbed gas production from shale

•Triple porosity model for adsorbed and free gas production from shale.•Pseudo mixed gas approach to identify contribution of adsorbed and free gas production.•For deep shales gas production is mainly from compressed free gas.•Adsorbed gas production is relatively more difficult from deep shales.

Shale gas has become a key natural gas resource and has been a great success in the USA and a few other countries. The gas production behaviour from shale is different to that from conventional gas reservoirs or coal seam reservoirs, primarily because of the different gas storage and flow behaviours in shale. Gas is stored in shale mainly as adsorbed gas in the pores of organic matter and clay minerals and as free gas compressed in other pores. Thus to understand how free and adsorbed gas contribute to the total gas production is a key question for shale gas reservoir engineering. One method to answer this question is by using reservoir simulation; however, this will require a reservoir simulator representing the shale gas storage and flow behaviours. In this work, a triple porosity model was applied in the reservoir simulator SIMED II, which was designed for dual porosity coal seam reservoirs, to account for both gas storage mechanisms of adsorption and free gas. A unique aspect of this new development is that the adsorbed gas, matrix free gas and fracture free gas are identified as different gas types but having the same gas properties. Thus the flow and production of gases with different storage mechanisms are directly identified in the simulation and output. The developed simulator was validated through history matching production data of a vertical well from the Barnett Shale. Then a series of parameter sensitivity analyses was carried out to investigate the impact of reservoir properties on adsorbed and free gas production. The results show that the contribution of adsorbed gas on the total gas production is generally low and is dependent on many factors such as the reservoir permeability, porosity and adsorption behaviour, and hydraulic fracturing effect.