Evolution of coal petrophysical properties under microwave irradiation stimulation for different water saturation conditions

Coalbed methane (CBM) reservoirs generally have very low permeability and requires stimulation to make gas extraction economical. Hydraulic fracturing has been widely applied to enhance CBM productivity, but this technology has a number of key limitations, including fractures only propagate along existing joints, large amount of water usage, and potential environmental impact. Microwave irradiation technology can likely overcome the above limitations. In this study, the effect of microwave irradiation on the petrophysical properties of an unconstrained bituminous coal was comprehensively investigated through a suite of integrated diagnostic techniques including Nuclear Magnetic Resonance and X-ray Computed Tomography. A series of experiments were conducted both on centrifuged samples and on samples with different water contents ranging from 1 to 15%. The mineral removal and moisture evaporation due to the microwave selective heating lead to the enlargement, opening, and interconnection of coal pores. The NMR-determined porosity increases linearly with the microwave power while grows exponentially with respect to water contents. When the water content is above 6%, the porosity increases by around 98~211%. The fracture volume and coal permeability increase while the P-wave velocity decreases with increasing water contents. Microwave irradiation is effective in enlarging mesopores and macropores and in enhancing the pore connectivity. The significant enhancement of coal permeability and pore fracture structure indicates that the microwave irradiation is effective in improving gas productivity thus has the potential to become a new CBM reservoir simulation technology

Sensitivity analysis on the microwave heating of coal: a coupled electromagnetic and heat transfer model

Microwave irradiation has gained widespread popularity in coal processing. However, the effects of microwave parameters and coal properties on the heating behavior remain much less explored. In this study, a coupled electromagnetic and heat transfer model was developed to investigate the microwave-coal interactions. Results show that microwave heating is sensitive to frequency: an optimal frequency of 2.45 GHz was found whereby the maximum temperature and energy efficiency can be achieved. In addition, the increase in the microwave power contributes to the thermal heterogeneity and energy consumption, suggesting that low powers are suitable for uniform heating and energy saving, while high powers apply to differential and rapid heating. As the loss factor increases, the temperature and energy efficiency increase while the thermal heterogeneity decreases. Increasing the loss factor could not only achieve rapid and uniform heating but also save energy. Furthermore, the optimal coal size for enhancing microwave energy absorption could be obtained when the coal diameter is 50 mm, with heights ranging from 60 mm to 100 mm. Outcomes of this study can be used to identify the electric and thermal fields of coal and thereby help to optimize the microwave applicators

Kinetic Analysis of Thermal Decomposition Process of Emulsion Explosive Matrix in the Presence of Sulfide Ores

This study aims to characterize the whole reaction process of (i) emulsion explosive matrix and sulfide ores, and (ii) ammonium nitrate and pyrite by the thermodynamics analysis method. A series of experiments were carried out at atmospheric pressure from 25 °C to 350 °C at four heating rates (3, 5, 10, and 15 K/min) and the Coats–Redfern method was applied to calculate the apparent activation energy of samples at different heating rates. The results show that the thermogravimetric (TG) curve of sulfide ores and emulsion explosive matrix can be divided into four stages: the water evaporation stage, the dynamic balance stage, the thermal decomposition stage, and the extinguishment stage. However, the thermal decomposition process of ammonium nitrate and pyrite can be divided into the dynamic balance stage, the thermal decomposition stage, and the burnout stage. The ignition temperature (T0) and maximum peak temperature (Tm) of the samples increased with the heating rate, but the shape of the TG/DTG (Derivative Thermogravimetric) curve was not affected. The results show that the reaction process of sulfide ores and emulsion explosive matrix is similar to the reaction process of pyrite and ammonium nitrate. The thermal stability of emulsion explosive matrix decreases when sulfide ores are added. By contrast, when pyrite is added, the thermal stability of the ammonium nitrate decreases more significantly.Safety and Security Scienc

Experimental study on the petrophysical variation of different rank coals with microwave treatment

To maximize coalbed methane recovery, the reservoir is often stimulated because of its low permeability. An exploratory study on improving coal porosity and permeability by microwave treatment was proposed. Pore size distribution of four unconstrained coals (lignite, subbituminous, bituminous, and anthracite), before and after microwave treatment, were evaluated using the nuclear magnetic resonance. With continuous exposure to microwaves, the pore size distribution of coals (especially the lignite coal) extends, and the pore volume and connectivity increase. In addition, coal porosity and permeability evaluated based on the Schlumberger Doll Research model increase by 33–72% and 73–181%, respectively. The mechanism was revealed by combining P-wave tests, thermal imaging and X-ray computed tomography scanner. The moisture and minerals bounded in pores are selectively heated and then detached, and, as a result, the pore structure is opened. Continued exposure to microwaves rapidly converts the mobilized moisture into super-heated steam. Under the steam pressure, pores and fractures generally expand. Borrowing from microwave-assist oil recovery, we presented a conceptual design of a CBM extraction borehole with microwave irradiation