This paper presents a timely and detailed study of significant injection-induced seismicity recently observed in the Sichuan Basin, China, where shale-gas hydraulic fracturing has been initiated and the aggressive production of shale gas is planned for the coming years. Multiple lines of evidence, including an epidemic-type aftershock sequence model, relocated hypocenters, the mechanisms of 13 large events (M W > 3.5), and numerically calculated Coulomb failure stress results, convincingly suggest that a series of earthquakes with moment magnitudes up to M W 4.7 has been induced by “short-term” (several months at a single well pad) injections for hydraulic fracturing at depths of 2.3 to 3 km. This, in turn, supports the hypothesis that they represent examples of injection-induced fault reactivation. The geologic reasons why earthquake magnitudes associated with hydraulic fracturing operations are so high in this area are discussed. Because hydraulic fracturing operations are on the rise in the Sichuan Basin, it would be beneficial for the geoscience, gas operator, regulator, and academic communities to work collectively to elucidate the local factors governing the high level of injection-induced seismicity, with the ultimate goal of ensuring that shale gas fracking can be carried out effectively and safely.

To gain a better understanding of the effect of heat (e.g., magma intrusion, geothermal fluids and enhanced coal-bed methane recovery process) on coal reservoir properties, the pore structure and compressibility of coal matrix for low rank coal (0.69% Ro, m) with elevated temperatures were investigated by using multiple methods, including thermogravimetry-mass spectrometry (TG-MS), scanning electron microscope (SEM), N2 adsorption/desorption at 77 K and mercury intrusion porosimetry (MIP). The results from TG-MS showed that moisture and partial volatiles were removed from the coal matrix, and pore structure almost remained unchanged during the low heat treatment (25∼200°C). The micropores and transition pores consisted of more than 80% of the total pore volume based on the MIP. The pore structure was slightly changed following the temperature increase to 400°C, and the bound moisture and partial organics in the coal were released and decomposed by the increased heat, respectively. When temperature reached 400°C, organic matter decomposition of the coal released a large amount of hydrocarbon and micromolecule gases. The meso- and macropore in the coal were significantly developed, occupying ∼35% of the total pore volume. Although there was no large change in generated gas composition after 600°C, the pore volume and structures, including pore size distribution, pore volume and pore connectivity, were significantly changed based on the MIP. The pore structure acquired from MIP exhibited a deviation when the mercury intruded pressure reached 10 MPa. A fractal model was introduced to correct the MIP data and acquire the pore compressibility of the coal matrix. The results showed that the pore compressibility decreased with increasing pressure and temperature. Thus, this study provides significant implications of the pore structure evolution of underground coals that encounter heating.

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