Investigation of the Velocities of Coals of Diverse Rank under Water- or Gas-Saturated Conditions for Application in Coalbed Methane Recovery

Abstract

Coalbed methane recovery enhanced by hydraulic or nonaqueous fracturing methods has been studied for decades, and it is of significance to evaluate fracturing results and scope for field applications. Monitoring variation in velocity is one way to explain fracturing effects. However, the existence of residual water or gas within cracks or pores may affect velocity measurements, and the correlation between velocity and inherent coal attributes (such as density and porosity) has not been studied comprehensively. In this paper, coal of different ranks (lignite, bituminite, and anthracite) was prepared under water and gas saturation to approximately simulate the residual water and gas in cracks under field applications. Correlations between the velocity and coal attributes were studied. For both water- and gas-saturated cores, the diverse velocity distributions were highly correlated to rank and saturation media. The longitudinal ultrasonic pulse velocity (UPVp) and transverse ultrasonic pulse velocity (UPVs) of different cores were distributed differently. For coal saturated with water or gas, the UPVp values of lignite, bituminite, and anthracite had positive linear correlations with the corresponding UPVs values. The discrete velocity ratio data were fit as negative linear correlations with UPVs, and different coals had different declining degrees, the difference of which might be attributed to the characteristics of structural cracks and the inherent properties of the coal, such as grain size and pore shape, which result in decreasing coal integrity and strength. Moreover, the difference in acoustic resistance between coal and fluids might have an inverse impact on the acoustic energy, and a larger difference might cause a large amount of energy to dissipate and finally cause the velocity to decrease. Under water and gas saturation conditions, the UPVp showed a positive linear correlation with density and a negative linear correlation with porosity. Finally, a potential field application was designed on the relations between the velocity and the elastic parameters to estimate fracturing effects by monitoring the petrophysical parameters of coal lithologies.

Novel Analytical Model of Shale Spontaneous Imbibition Considering the Hydration Effect

“…On the basis of the imbibition length from eq , to obtain the imbibition weight, it is assumed that the pore volume in shale is composed of multiple capillaries [capillary­( i ), where i = 1, 2, 3, ..., n ], shown as Figure . In the multiple capillary model, we assume that each capillary has the same average pore diameter (λ a ) and tortuosity (τ), thus having the same imbibition length [ L s ( i ) and L t ( i ), where i = 1, 2, 3, ..., n ] and sectional area of each capillary [ A p ( i ), where i = 1, 2, 3, ..., n ], shown as Additionally, on the basis of the rock physics theory, it gives two traditional assumptions that the total sectional area of the pore ( A p ) is equal to the sum of A p (1), A p (2), A p (3), ..., and A p ( n ) and the porosity of the sectional area is approximately equal to the porosity of the volume, meaning that ϕ = V p / V = A p / A . Then, the volume of imbibition liquid at any given time [ V i ( t )] can be calculated, expressed as Finally, when the volume and density of the liquid in the shale sample are combined, we can obtain the imbibition amount, shown as where w ( t ) is the imbibition amount (%), W ( t ) is the weight of the shale sample at t (g), and W o is the weight of the original shale sample (g).…”

“…Additionally, on the basis of the rock physics theory, it gives two traditional assumptions that the total sectional area of the pore (A p ) is equal to the sum of A p (1), A p (2), A p (3), ..., and A p (n) and the porosity of the sectional area is approximately equal to the porosity of the volume, 52 meaning that ϕ = V p /V = A p /A. Then, the volume of imbibition liquid at any given time [V i (t)] can be calculated, expressed as…”

Failure characteristics of thick hard roof stratum under hydraulic pre-splitting and its application in a coal mine, Dongsheng mining area

“…The initial pressure step and periodic pressure step were reduced accordingly. Hydraulic penetration fracturing technology has also been applied to improve the low permeability of tight coal seams for the productive extraction of coalbed methane (Jiang et al, 2018;Xu et al, 2019Xu et al, , 2017 Hydraulic pre-splitting technology has been discussed in detail for control and prevention for geodynamic hazards and enhanced permeability of tight coal seams. However, the influence of hydraulic fractured stratum on the development height of water-conducting fracture has not been fully understood.…”