Hongxing Zhou

A Mathematical Model of Coupled Gas Flow and Coal Deformation with Gas Diffusion and Klinkenberg Effects

The influence of gas diffusion behavior on gas flow and permeability evolution in coal seams is evaluated in this paper. Coalbed methane (CBM) reservoirs differ from conventional porous media and fractured gas reservoirs due to certain unique features, which lead to two distinct gas pressures: one in fractures and the other in the coal matrix. The latter pressure, also known as the sorption pressure, will be used in calculating sorption-based volume changes. The effective stress laws for single-porosity media is not suitable for CBM reservoirs, and the effective stress laws for multi-porosity media need to be applied. The realization of the above two points is based on the study of the two-phase state of gas migration (involving Fickian diffusion and Darcy flow) in a coal seam. Then, a general porosity and permeability model based on the P-M model is proposed to fit this phenomenon. Moreover, the Klinkenberg effect has been taken into account and set as a reference object. Finally, a coupled gas flow and coal deformation model is proposed and solved by using a finite element method. The numerical results indicate that the effects of gas diffusion behavior and Klinkenberg behavior can have a critical influence on the gas pressure, residual gas content, and permeability evolution during the entire methane degasification period, and the impacts of the two effects are of the same order of magnitude. Without considering the gas diffusion effect, the gas pressure and residual gas content will be underestimated, and the permeability will be overestimated.

Characteristics of gas disaster in the Huaibei coalfield and its control and development technologies

The Huaibei coalfield is in the East China Economic Area, which is rich in coal and gas resources. However, hundreds of coal and gas outburst accidents have occurred because of the complex geological structures of the coalfield. Based on theoretical analysis and field statistics, the characteristics of regional geological structures and the coal measure strata evolution in the Huaibei coalfield were researched, and gas resource distribution and gas parameters were statistically analyzed to determine the dominant controlling factors of gas occurrence and gas dynamic disaster. The results indicated that the Huaibei coalfield has undergone complex tectonic evolution, causing obvious differences in gas storage in different blocks of different mining areas, which exhibits a pattern of high amounts of gas in the south and east, and low amounts of gas in the north and west. The coal seam and gas occurrence have a bipolar distribution in the coalfield caused by multiple tectonic movements, and they are deeply buried. Horizontal tectonic stress plays a dominant role in gas outburst, and the thermal evolution and trap effects of magma intrusion increase the possibility and extent of gas outburst. Considering coal seam and gas occurrence characteristics in the coalfield, we propose a new technology for deep coal reservoir reconstruction which combined present underground regional gas control methods and surface well extraction methods. The technology has three effects: developing gas resources, improving coal mining safety level and reducing greenhouse gas emissions, which has been practiced to be effective in coal mines in the Huaibei coalfield.

Fissure evolution and evaluation of pressure-relief gas drainage in the exploitation of super-remote protected seams

Abstract Based on nonlinearity contact theory and the geological structure of the Xieqiao Coal Mine in the newly developed Huainan coal field, rock movements, mining fissures and deformation of overlying strata were simulated by using the interface unit of FLAC3D to evaluate the pressure-relief gas drainage in the exploitation of super-remote protected seams. The simulation indicates that the height of the water flowing fractured zone is 54 m in the overlying strata above the protective layer. The maximum relative swelling deformation of the C13 coal seam is 0.232%, while the mining height is 3.0 m and the distance from the B8 roof to the C13 floor is 129 m, which provides good agreement with a similar experiment and in situ results. The feasibility of exploitation of a super-remote protective coal seam and the performance of the pressure-relief gas drainage in a super-remote protected layer are evaluated by comparisons with practice projects. It demonstrates that the relieved gas in the super-remote protected layers could be better drained and it is feasible to exploit the B8 coal seam before the C13 super-remote protected coal seam. The method is applicable for the study of rock movements, mining fissures and deformation of the overburden, using the interface unit to analyze the contact problems in coal mines.

Study of Drainage and Percolation of Nitrogen–Water Flooding in Tight Coal by NMR Imaging

Abstract N2 flooding in pores and water percolation in fissure is analyzed in-depth to investigate the permeability reduction caused by coalbed methane displacement in low-permeability coal seam in China. Using low-field nuclear magnetic resonance imaging (NMRI), the transverse relaxation time spectrum (T2 spectrum) was carried out for tight intact and fractured coal. The T2 spectra were obtained for saturated samples containing adsorption pores (AP), percolation pores (PP), and migration pores (MP), as well as one-, two-, and three-dimensional spatial distributions of water content. The spatial distribution of the water in the displacement process of tight coal was finely characterized by low-field NMRI. Our results show that the water displacement by N2 in AP, PP, and MP requires a high to low pressure gradient and they play different functions of percolation. And the study revealed that there is no obvious gas–liquid interface during N2 flooding in tight coal. The results show that PP play an important role in water transfer and pressure transmission during the N2 flooding process, which assists in forming connected channels from nonpenetrative cracks. PP themselves can also form percolation channels and transmit pressure.

On Acoustic Emission and Post-peak Energy Evolution in Beishan Granite Under Cyclic Loading

Cyclic tests (loading–unloading) provide an effective way for a better understanding of damage and deformation characteristics in rocks. In general, elastic and plastic strain components can be distinguished during cyclic experiments (Elliott and Brown 1986). In addition, the influence of accumulated damage on rock strength can be studied by this method (Martin and Chandler 1994). Detectable acoustic emission (AE) signals are emitted when the specimen is loaded, and AE monitoring techniques have been used to provide a detailed view of fracture nucleation and growth (Lockner et al. 1991; Thompson et al. 2005, 2006; Cai et al. 2001, 2007; Cai and Kaiser 2005). Since the AE events are closely related to the damage or fracturing of the rock, rock degradation or damage can thus be measured (Falmagne et al. 1998). Furthermore, energy is dissipated during the whole failure process and can be quantified (Xie et al. 2009, 2011; Wasantha et al. 2014). In this study, cyclic tests were carried out on Beishan granite chosen as potential host rock for a high-level radioactive waste (HLW) repository in China (Wang 2010). AE recording system was utilized to monitor the fracturing process during the test. In addition, based on stress–strain curves, the energy evolution in the post-peak stage of Beishan granite was studied.