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Chemical and physical structure properties of coal macromolecules are the main microscopic factors affecting coal wettability. Numerous studies on coal structures have found that lignite, as a coal of low metamorphic degree, shows more-complicated macromolecular structures than other types of coal, and the macromolecular structure of lignite can better promote coal wettability than other low metamorphic coals with similar physical pore structures. To investigate the underlying reasons, lignite samples from the main coal producing areas of China were analyzed via X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, and physical methods were used to construct two-dimensional and three-dimensional models of lignite macroscopic molecular structures to study its effect on coal wettability. The results indicated that aromatic layer spacing of lignite was considerably large and showed a low degree of aromatization with irregular arrangement and less directional degree. The macroscopic molecular structure of lignite was composed of 3–4 effectively stacked aromatic layers and alicyclic layers of hexagonal or pentagonal structures. The alicyclic ring structure was well-developed in the atom radial direction and had a high content of active ingredient, which was prone to attractive interaction with other molecules. The molecular formula of the lignite sample was determined to be C180H145O31N5S and the adsorption and spreading of wetting agents were found to be mainly dependent on the hydrophilic ability of coal surface. Furthermore, the microscopic pore stacking stereochemical structure of lignite indicated the existence of hydrophilic groups in surface functional groups around molecular nucleus, which resulted in the formation of infiltration points in the coal crystallite nucleus structure. Meanwhile, the absence of delocalized electron in the atom radial direction of amorphous lignite atoms could induce a relatively poor ability of attracting negative charge, and thus lessening the repulsive interaction to the electronegative ions in aqueous solutions, which promoted the wetting effect to some extent.

The normal-to-shear fracture compliance ratio is commonly used as a fluid indicator. In the seismic frequency range, the fluid indicator lies between the values for isolated fluid-filled fractures and dry fractures, and it is not easy to discriminate the fluid content. Assuming that the fracture surfaces are smooth, we use [Formula: see text], with [Formula: see text] and [Formula: see text] representing the normal fracture weakness of the saturated and dry rock, to indicate fluid types, and to define a fluid influencing factor. The fluid influencing factor is sensitive to the fluid properties, the aspect ratio of the fractures, and the frequency. Conventionally, the amplitude versus offset and azimuth (AVOA) inversion is formulated in terms of the contrasts of the fracture weaknesses across the interface, assuming that the fractures are vertical with the same symmetry axis. We consider fractures with arbitrary azimuths, and develop a method to estimate fracture parameters from wide-azimuth seismic data. The proposed AVOA inversion algorithm is tested on real 3D prestack seismic data from the Tarim Basin, China, and the inverted fracture density show good agreement with well log data, except that there are some discrepancies for one of the fractured reservoir sections. The discrepancies can be ascribed to neglect of the dip angle for the tilted fractures and the conjugate fracture sets, and to the validity of the linear-slip model. The fractured reservoirs are expected to be liquid saturated, under the assumption of smooth fractures. Overall, the inverted fracture density and fluid influencing factor can be potentially used for better well planning in fractured reservoirs and quantitatively estimating the fluid effects.

The aim of this paper was to develop a model that can characterize the actual micropore structures in coal and gain an in-depth insight into water’s seepage rules in coal pores under different pressure gradients from a microscopic perspective. To achieve this goal, long-flame coals were first scanned by an X-ray 3D microscope; then, through a representative elementary volume (REV) analysis, the optimal side length was determined to be 60 μm; subsequently, by using Avizo software, the coal’s micropore structures were acquired. Considering that the porosity varies in the same coal sample, this study selected four regions in the sample for an in-depth analysis. Moreover, numerical simulations on water’s seepage behaviors in coal under 30 different pressure gradients were performed. The results show that (1) the variation of the simulated seepage velocity and pressure gradient accorded with Forchheimer’s high-velocity nonlinear seepage rules; (2) the permeability did not necessarily increase with the increase of the effective porosity; (3) in the same model, under different pressure gradients, the average seepage pressure decreased gradually, while the average seepage velocity and average mass flow varied greatly with the increase of the seepage length; and (4) under the same pressure gradient, the increase of the average mass flow from the inlet to the outlet became more significant under a higher inlet pressure.

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