Permeability Response Characteristics of Primary Undeformed Coal and Tectonically Deformed Coal under Loading–Unloading Conditions in Huainan Coalfield, China

Abstract

Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore–fissure structure analysis, mechanical property test, and permeability experiments under different stress loading–unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore–fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading–unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading–unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

Microwave-Mediated Stabilization and Carbonization of Polyacrylonitrile/Super-P Composite: Carbon Anodes with High Nitrogen Content for Lithium Ion Batteries

“…The larger capacities in the sloping region were due to the higher N atomic content and increased surface area (Figures 4b,S6). [29][30][31]60,61 More pyridinic-N and graphitic-N in M1000W resulted in the exposure of more active sites for Li ions and enhanced electronic conductivity. 29−34 Therefore, facile redox reactions could be achieved, particularly at high current densities.…”

“…The capacity increase in the plateau region is influenced by the larger crystallite size, which improves electronic and ionic transport. 61,62 The M1000W demonstrated an initial Coulombic efficiency (ICE) of 48.6%, whereas the T1000°C displayed an ICE of 52.4%. The low ICEs were due to the formation of an SEI layer on the carbon surface, which occurred at ∼1.0 V as a result of electrolyte reduction and unwanted side reactions (Figures S7a,b).…”

“…A higher fraction of N dopants and larger crystallite size in the direction of L c could enhance both Li ion and electron transport. 61,62 The diffusion properties of T1000°C and M1000W during charging and discharging were checked by using CV studies, as depicted in Figure S8a,b. T1000°C and M1000W exhibit similar oxidation/reduction potential peaks.…”

Stress Relief and Permeability Enhancement with Hydraulic Fracturing in Overlying Key Strata of Deep and Soft Coal Seams

“…At present, gas control technologies and processes are mainly consisted of hydraulic technology, gas-phase fracturing, deep-hole pre-splitting blasting, etc. − The hydraulic process mainly forms macroscopic cracks through high-pressure hydraulic fracturing, which creates a flow channel for gas migration to improve the permeability of coal seams. − However, the cracks formed by the hydraulic measures have a short maintenance time, the fracture channels are quickly closed, and under the action of water, coal slime is generated to block the cracks, which accelerates the decline of permeability of soft coal seams, and the local permeability enhancement effect is slowed down or even stopped. The deep-hole pre-splitting blasting (explosive) produces dynamic pressure shock, stress wave superposition, and explosive gas drives the crack growth of coal seam, which results in the local fracture field and improves the gas drainage efficiency of coal seam. , Gas-phase fracturing mainly uses the impact load generated by the instantaneous release of liquid CO 2 to act on the coal body to form a crushing zone, realizes the pressure relief and permeability enhancement of the coal seam, improves the gas drainage efficiency, and achieves the purpose of permeability enhancement. , Gas-phase fracturing and deep-hole pre-splitting blasting only use local coal.…”

“…9 − 11 The hydraulic process mainly forms macroscopic cracks through high-pressure hydraulic fracturing, which creates a flow channel for gas migration to improve the permeability of coal seams. 12 − 15 However, the cracks formed by the hydraulic measures have a short maintenance time, the fracture channels are quickly closed, and under the action of water, coal slime is generated to block the cracks, which accelerates the decline of permeability of soft coal seams, and the local permeability enhancement effect is slowed down or even stopped. The deep-hole pre-splitting blasting (explosive) produces dynamic pressure shock, stress wave superposition, and explosive gas drives the crack growth of coal seam, which results in the local fracture field and improves the gas drainage efficiency of coal seam.…”