52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: Shale gas reservoirs comprise of various minerals like clay and organic matters. After hydraulic fracturing, large number of water together with CH4 remains in reservoirs. In this case, fluid adsorption is one of obvious fluid-shale interactions and affects geomechanical properties of shale which are important for shale gas production. In this paper, experimental observations of shale property change due to fluid adsorption were illustrated and discussed. It was suggested that those experimental observations result from the internal strain between different minerals and microcracks initiation inside shale. Based on the poroelastic theory and rock damage theory, models of Young's modulus and permeability were established to consider effects of those physical processes. Those models were verified by over ten groups of experimental results. Meanwhile, an experimental method was proposed to identify the impact of gas adsorption on permeability. Finally, the impact of fluid adsorption on recovery rate of shale gas reservoirs was analysed through numerical simulations. The results illustrate that the fluid adsorption strongly affect the recovery rate, and indicate that those factors need to be seriously considered for shale gas production design.
The production challenges of shale gas reservoirs include quick decline and difficult refracturing. One of main factors causing those challenges is the strong changes of reservoir properties during long-term production. Besides pore pressure, fluid adsorption also affect changes of reservoir properties.
Two kinds of fluid-shale interaction are widely paid attention by experts. One is the gas adsorption. In shale gas reservoirs, methane is stored in both free-phase and adsorbed-phase forms (Jenkins and Boyer, 2008). Due to the existence of organic matter and the large surface area of nanopores, the content of adsorbed-phase methane can be up to 20%-80% (Curtis,2002; Wu et al., 2014). Methane is able to adsorb in organic matters which could induce a swelling strain of shale. The volumetric swelling strain of shale due to CH4 adsorption is around 0.1% at 10MPa (Chen et al., 2015). The pore size in shale usually ranges from 5 to 1000nm (Nelson,2009; Xiong et al., 2012), and is much less than that of other rocks, such as sandstone(Nelson,2009), so the low adsorption-induced swelling strain could also affect intrinsic permeability(Wang et al., 2015). The other fluid-shale interaction is water spontaneous imbibition.
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