Search

The composition of gas released under vacuum by crushing from the gas shale of Longmaxi Formation in Upper Yangtze Plate, Southern China was systematically investigated in this study. The effect of residual gas release on pore structures was checked using low-pressure nitrogen adsorption techniques. The influence of particle size on the determination of pore structure characteristics was considered. Using the Frenkel-Halsey-Hill method from low-pressure nitrogen adsorption data, the fractal dimensions were identified at relative pressures of 0−0.5 and 0.5−1 as D1 and D2, respectively, and the evolution of fractal features related to gas release was also discussed. The results showed that a variety component of residual gas was released from all shale samples, containing hydrocarbon gas of CH4 (29.58% −92.53%), C2H6 (0.97% −2.89%), C3H8 (0.01% −0.65%), and also some non-hydrocarbon gas such as CO2 (3.54% − 67.09%) and N2 (1.88%−8.07%). The total yield of residual gas was in a range from 6.1 μL/g to 17.0 μL/g related to rock weight. The geochemical and mineralogical analysis suggested that the residual gas yield was positively correlated with quartz (R2=0.5480) content. The residual gas released shale sample has a higher surface area of 17.20−25.03 m2/g and the nitrogen adsorption capacity in a range of 27.32−40.86 ml/g that is relatively higher than the original samples (with 9.22−16.30 m2/g and 10.84−17.55 ml/g). Clearer hysteresis loop was observed for the original shale sample in nitrogen adsorption-desorption isotherms than residual gas released sample. Pore structure analysis showed that the proportions of micro-, meso- and macropores were changed as micropores decreased while meso- and macropores increased. The fractal dimensions D1 were in range from 2.5466 to 2.6117 and D2 from 2.6998 to 2.7119 for the residual gas released shale, which is smaller than the original shale. This factor may indicate that the pore in residual gas released shale was more homogeneous than the original shale. The results indicated that both residual gas and their pore space have few contributions to shale gas production and effective reservoir evaluation. The larger fragments samples of granular rather than powdery smaller than 60 mesh fraction of shale seem to be better for performing effective pore structure analysis to the Longmaxi shale.

Marine shales in southern China experiences multiple complex tectonic movements, which highly affects shale pore characteristics and pore evolution. The effects of tectonic deformation on nanopore structure and petrophysical properties of organic shales remain unclear. Three ductile deformed shale samples were collected and analyzed through an integrated experiment procedure for a comparative research against undeformed shale samples. Field emission-Scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption and mercury injection porosimetry (MIP) analyses were performed to characterize pore structure of the shales. The results reveal that the mineral compositions of these shale samples are controlled by sedimentary environment and source and exhibits no obvious relationship with tectonic deformation. With increasing deformation degree, the micropore proportion gradually decreases (from 17% to 6%), and the macropore proportion increases from 16% to more than 20%. Micropore volumes and proportion in the fold core samples are the smallest (4.2 × 10−3 cm3/g and 6.3%) and account for only one-third of the undeformed shale values, while the meso- and macropore values are larger-1.2 times those of the undeformed shale values. The evolution characteristics and deformation modes of organic matter (OM), interparticle (interP) and intraparticle (intraP) pores and microfractures under ductile deformation were examined. All pore types become interconnected during deformation and form microfractures ranging from a few nanometers to tens of microns, which is the main reason for the enhanced shale connectivity caused by deformation. Under the same porosity conditions, the deformed shale permeability is much higher than the undeformed shale permeability, and stronger deformation corresponds to higher permeability. The fold core permeability is generally higher than the limb permeability and exceeds 30 times. The different ductile deformation positions were divided into five levels, and the corresponding reservoir quality was evaluated. This study is of great significance to better understand the pore structure evolution process for shale exploration and development under tectonic deformation.