Abstract
Shale gas is an important hydrocarbon resource in a global context. It has had a significant impact on energy resources in the US, but the worldwide development of this methane resource requires further research to increase the understanding of the relationship of shale structural characteristics to methane storage capacity. In this study a range of gas adsorption, microscopic, mercury injection capillary pressure porosimetry and pycnometry techniques were used to characterize the full range of porosity in a series of shales of different thermal maturity. Supercritical methane adsorption methods for shale under conditions which simulate geological conditions (up to 473 K and 15 MPa) were developed. These methods were used to measure the methane adsorption isotherms of Posidonia shales where the kerogen maturity ranged from immature, through oil window, to gas window. Subcritical methane and carbon dioxide adsorption studies were used for determining pore structure characteristics of the shales. Mercury injection capillary pressure porosimetry was used to characterize the meso and macro porosity of shales. The sum of the CO2 sorption pore volume at 195 K and mercury injection capillary pressure pore volumes (1093–5.6 nm) were equal to the corresponding total pore volume (< 1093 nm) thereby giving an equation accounting for virtually all the available shale porosity. These measurements allowed quantification of all the available porosity in shales and were used for estimating the contributions of methane stored as ‘free’ compressed gas and as adsorbed gas to overall methane storage capacity of shales. Both the mineral and kerogen components of shale were studied by comparing shale and the corresponding isolated kerogens so that the relative contributions of these components could be assessed. The results show that the methane adsorption characteristics were much higher for the kerogens and represented 35–60% of the total adsorption capacity for the shales used in this study, which had total organic contents in range 5.8–10.9 wt%. Microscopy studies revealed that the pore systems in clay-rich, organic-rich and microfossil-rich parts of shale are very different, and also the importance of the inter-granular organic-mineral interface.
Highlights
The development of shale gas as a methane resource started in the US and it has become an increasingly important component of natural gas supply in the past twenty years [1]
It was necessary to provide information on supercritical methane adsorption characteristics under simulated geological conditions in order to gain an insight into the variation of sorption capacity with temperature and pressure
The results show that the Mercury injection capillary pressure (MICP) range (< 1093 nm) studied covers the upper macropore range in this suite of Posidonia shale samples
Summary
The development of shale gas as a methane resource started in the US and it has become an increasingly important component of natural gas supply in the past twenty years [1]. There are large potential shale gas reserves in the rest of the world, which could be exploited in the future. The economics of shale gas reservoirs depends on the Gas-in-Place (GIP) and the methane extraction rate that can be achieved [2]. This depends on the properties of the shale and the fracturing of the shale matrix induced by the hydraulic fracking and horizontal drilling processes. Methane is stored as either compressed gas or physisorbed gas in porous structures in organic-rich shales and the quantities of these components are very difficult to predict. The economic potential of the resources needs to be established unequivocally, in order to justify extraction of shale gas
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