Measurement and modeling of adsorptive–poromechanical properties of bituminous coal cores exposed to CO2: Adsorption, swelling strains, swelling stresses and impact on fracture permeability

Abstract

International audienceThe reservoir response of unmineable coal seams to primary and enhanced natural gas recovery is strongly affected by gas sorption and the swelling properties of the coal reservoir rock. In-depth understanding of the process of gas sorption/desorption in the coal matrix, induced deformation and measurement of relevant physical parameters are critical for predictive reservoir management. Models used in industry practice are based on swelling strains measured in “free” swelling coal or on empirical correlations between strain and adsorption, and predict permeability changes based on changes of porosity or stress calculated assuming an analogy with thermoelasticity. However, not only coal seams are subjected to in-situ stresses and geometrical boundary conditions but also sorption and strain are strongly coupled. Representative experiments and a truly coupled model for coal seams are needed in challenging applications. We present a set of triaxial testing measurements on 38 mm diameter fractured sub-bituminous/bituminous coal cores exposed to CO2. Testing includes the measurement of fluid uptake, adsorption-induced strains and stresses, and the impact on simultaneously measured permeability. Noteworthy, we measured increases in effective stress of up to 29 MPa when injecting CO2 at 5 MPa and preventing the coal core to swell. The results are analyzed with a poromechanical model in which coal matrix microporosity and adsorption-induced phenomena are embedded into a fractured reservoir rock with transverse isotropic properties. The adsorptive–mechanical coupling in the coal matrix is integrated through an adsorption stress function and fractured coal permeability is estimated as a function of Terzaghi's effective stresses (parallel and perpendicular to the bedding plane). The experimental results and model predictions help identify the characteristic response of coal microporosity and cleat macroporosity on the poromechanical response of coal cores, and suggest that order of magnitude changes of reservoir permeability observed in the field are linked to sorption-induced change on Terzaghi's effective horizontal stress under laterally constrained displacement condition. Together, the modeling and experimental characterization offer unprecedented insights into the mechanics of coal

Adsorptive-mechanical properties of reconstituted granular coal: Experimental characterization and poromechanical modeling

“…The macromechanical parameters K and ϕ c0 are known from Table 1 In order to simulate isochoric experiments, we first find the macroscopic volumetric strain that matches the experimentally measured initial axial stress σ 0 . A blind prediction of uptake amount n T and total stress σ with coal matrix parameters provided by Espinoza et al (2014) does not yield a good match with experimental results. The model overestimates uptake amount by 30% and adsorption-induced swelling stress by 60%.…”

“…In the microporosity, fluid is adsorbed and generates adsorption-induced swelling strains or stresses. A detailed formulation of the poromechanical model for the microporous coal matrix and extension to double porosity are available elsewhere (Brochard et al, 2012;Nikoosokhan et al, 2012;Espinoza et al, 2013Espinoza et al, , 2014. Here we summarize the basis of the theoretical formulation and present the final constitutive equations.…”

“…(13) contains Table 2 Poromechanical parameters for various coal matrices. (*) (Espinoza et al, 2014), (**) average from (Espinoza et al, 2013). The macroscale model parameters are obtained from the stressstrain oedometric data in Fig.…”

Strength Reduction of Coal Pillar after CO2 Sequestration in Abandoned Coal Mines

“…Fractured coal possesses dual porosity system: (a) the cleat macroporosity system, and (b) the microporosity of coal matrix. Espinoza et al [62,63] proposed a double porosity poromechanical model and considered the strains as a function of stresses, fracture pore pressure, and the pressure-dependent adsorption stress developed by the coal matrix.…”

Hydromechanical modelling of shaft sealing for CO2 storage

“…However injection of carbon dioxide into coal formations triggers complex coupled phenomena between the fluids and the coal formation. In particular CO 2 adsorption onto the coal particles induces swelling and important permeability modifications of the porous medium (see for instance Gray, 1987;Seidle and Huitt, 1995;Connell, 2009;Chen et al, 2010, Espinoza et al, 2014. As a consequence, the injection rate of carbon dioxide into the formation can decrease drastically and jeopardize the viability of the operations.…”