Experimental study on the seepage characteristics of bituminous coal under the conditions of liquid nitrogen fracturing

Abstract

In coalbed methane (CBM) exploitation, liquid nitrogen (LN2) fracturing has attracted much attention as a promising and effective means to enhance coal seam permeability. Considering the bituminous coal in the Xutuan coal mine of Anhui Province, China, the permeability characteristics of coal samples under different pressure and stress conditions, and a single LN2 treatment and cyclic fracturing conditions were studied. Additionally, an evaluation of the macroscopic cracking behavior of coal was carried out. LN2 treatment can be very effective in promoting the generation of macroscopic fissures in the coal, and the degree of damage increases with the number of fracturing cycles. The results of a single LN2 treatment showed that under a constant confining pressure, the permeability increased exponentially with the increase in gas pressure. The cyclic LN2 treatment showed that permeability first decreased and then increased with increasing gas pressure, which is consistent with the quadratic function distribution. The permeability of the coal sample increases exponentially with the number of cycles, but the growth rate gradually slows down. For the same absolute fracturing time, cyclic fracturing had a better fracturing effect than a single cycle, and the ratio of the permeability increment is 126.3%‐213.8%. The research results provide some guidance for the development of the LN2 freeze fracturing technology.

Advances in Liquid Nitrogen Fracturing for Unconventional Oil and Gas Development: A Review

“…The ultralow temperature of LN 2 induces temperature gradients and strong thermal stresses , in coal and rock masses and causes deformation between rock and mineral particles, freezing of pore fluids, and frost heave. Severe thermal shock causes physical changes to the walls of fractures, prevents the complete closure of hydraulic and thermally induced fractures, and thermally induces microcracks that are orthogonal to hydraulic cracks. , Because of the large differences between the thermal properties of the matrix and those of the internal mineral particles, freezing and thawing result in heterogeneous deformation and promote the initiation of new cracks. , The width, density, and connectivity of cracks in the coal increase considerably on both macro and micro levels. − The initial pores and cracks in the matrix are destroyed, and a main fracture network is created. Figures and show the microscopic features of coal fractures.…”

“…125,126 Because of the large differences between the thermal properties of the matrix and those of the internal mineral particles, freezing and thawing result in heterogeneous deformation and promote the initiation of new cracks. 127,128 The width, density, and connectivity of cracks in the coal increase considerably on both macro and micro levels. 129−131 The initial pores and cracks in the matrix are destroyed, and a main fracture network is created.…”

Experimental Study on Evolution of Fracture Network and Permeability Characteristics of Bituminous Coal Under Repeated Mining Effect

“…Due to coal anisotropy, a coal sampleÕs sensitivity to axial pressure and confining pressure is different. Effective stress is difficult for describing accurately the effect of stress variations in different directions of heterogeneous coal on its permeability (Chen et al, 2019). Generally, when a coal body is in the elastic stage, the confining pressure will control directly the opening of seepage channel (Zou et al, 2015).…”

Variation law of roof stress and permeability enhancement effect of repeated hydraulic fracturing in low‐permeability coal seam

“…As the injection pressure increases, the penetration rate is exponentially increasing. 27 In summary, a lot of research has been reported on the law of hydraulic fracturing crack propagation and permeability. However, there are few studies on the large-scale stress transfer after hydraulic fracturing in low-permeability coal seams and the investigation of gas drainage effects.…”