The adsorption properties of methane (CH4) have a great influence on shale gas exploration and development. The surface chemistry characteristics of nanopores are key factors in adsorption phenomena. The clay pores in shale formations exhibit basal surface and edge surfaces (mainly as A and C chain and B chain surfaces in illite). Little research regarding CH4 adsorption on clay edge surfaces has been carried out despite their distinct surface chemistries. In this work, the adsorption of CH4 confined in nanoscale illite slit pores with basal and edge surfaces was investigated by grand canonical Monte Carlo and molecular dynamics simulations. The adsorbed phase density, adsorption capacity, adsorption energy, isosteric heat of adsorption, and adsorption sites were calculated and analyzed. The simulated adsorption capacity compares favorably with the available experimental data. The results show that the edge surfaces have van der Waals interactions that are weaker than those of the basal surfaces. The adsorption capacity follows the order basal surface > B chain surface > A and C chain surface. However, the differences of adsorption capacity between these surfaces are small; thus, edge surfaces cannot be ignored in shale formation. Additionally, we confirmed that the adsorbed phase has a thickness of approximately 0.9 nm. The pore size determines the interaction overlap strength on the gas molecules, and the threshold value of the pore size is about 2 nm. The preferential adsorption sites locate differently on edge and basal surfaces. These findings could provide deep insights into CH4 adsorption behavior in natural illite-bearing shales.

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Water always occurs in gas shales, especially during the treatment of shale gas hydraulic fracturing. In sharp contrast to the prevailing view that water film is ubiquitous in shale formations, we observed an unusual phenomenon that water bridge instead of water film dominates in some illite and kaolinite slit pores when we are investigating the coexisting pattern of water and methane inside shale nanopores using molecular dynamics simulations. The network orientation structure and hydrogen bond of water molecules are analyzed, and the results indicate that appearance of water bridge is attributed to the strong internal, self-generated electric field induced by surface charge contrast between different pore surfaces. Four factors can significantly influence this self-generated electric field strength: pore surface chemistry, mineral type, pore shape, and pore size. When the pore size is within several nanometers, a small charge difference could induce strong electric field and change the structural properties of water clusters. The water film or water bridge inside shale nanopores alters the hydraulic diameter of the pore and the fluid flow pattern. These findings may provide a better and microscopic insight of the water−gas flow behavior and the electric field inside clay nanopores.

Summary Recent molecular-dynamics (MD) simulation of methane flow through nanoscale kaolinite channels shows that the gas molecules accumulate near the kaolinite wall, which will reduce the flowpath of the gas through tight porous media. Considering this gas-accumulation effect, and on the basis of the corrected second-order slip boundary condition (BC) proposed by Zhang et al. (2010), a permeability-correlation model is proposed for nanoscale flow in highly compacted shale reservoirs. Full-derivation detail of this model is presented along with a comparison with several existing correlations. Results show that, with the increase of the Knudsen number (Kn), the molecular-accumulation effect has an obvious negative effect on the shale permeability, which should not be neglected in further investigation. The parametric investigation of the model proposed shows that the permeability is mostly decided by the pore-wall structure of shale matrix and only slightly influenced by the gas property.

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