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.
Effective coalbed methane (CBM) drainage is a topical research issue around the world. CBM also occupies an increasing proportion of the resource structure in China, and its efficient use is likely to ease the pressure on other diminishing fossil fuel reserves [
The UPV testing method in a laboratory setting uses a pair of transducers (one transducer as a signal emitter and another transducer as a receiver) to quantify ultrasonic pulse transmission through a sample using an oscilloscope. There are two main parameters of interest: longitudinal ultrasonic pulse velocity (UPVp) and transverse ultrasonic pulse velocity (UPVs), as shown in Figure
Schematic diagram of the UPV device used in this study.
Vilhelm et al. [
UPV testing has also been used to evaluate coal properties and various influencing factors such as confining stress, temperature, moisture content, and porosity. The ultrasonic wave velocity of coal under lower confining stress increases with rank because of fracture closure [
The velocity monitoring method has been applied to fracturing estimation; however, residual water or gas occupies pores and cracks after fracturing. The impact of residual water or gas on the accurate evaluation of velocity and the correlations between inherent attributes (porosity, density, and rank) have not been studied comprehensively. In this paper, three different ranks of coals, including 24 lignites, 26 bituminites, and 26 anthracites, are tested (UPVp and UPVs) under both water and gas (air) saturation. The correlations among velocity, density, and porosity are described, and the statistical results can provide reference parameters for further field monitoring in coal physics in terms of CBM reservoir fracturing processes.
Lignite, bituminite, and anthracite samples were collected from the Shengli Coal Mine, Inner Mongolia; the Datong Coal Mine, Shanxi; and the Yangzhuang Coal Mine, Huaibei; respectively. The large coal blocks were wrapped with preservative film and transferred to the State Key Laboratory of Coal Resources and Safe Mining in Xuzhou, Jiangsu, before being cored to produce 5 cm diameter cylinders with 10 cm height (Figures
Images of 76 coals with different ranks. (a) Overall view of the 76 coal cores, (b) lateral display of lignite cores, and (c) sketch image of UPV test method.
Detail properties of the coals with different ranks.
Type | Density (g/cm3) | Porosity (%) | Proximate analysis (%) | Maceral analysis (%) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Lignite | 1.28-1.55 | 1.07-3.17 | 11.37 | 14.63 | 53.41 | 20.59 | 0.32 | 79.5 | 15.5 | 3.6 | 1.4 |
Bituminite | 1.23-1.42 | 0.3-4.0 | 8.83 | 3.30 | 29.64 | 58.23 | 1.13 | 58.7 | 32.6 | 3.4 | 5.3 |
Anthracite | 1.29-1.61 | 0.3- 2.9 | 2.10 | 7.73 | 6.48 | 83.69 | 3.05 | 86.4 | 10.5 | 1.5 | 1.6 |
The UPV test equipment used to record the UPVp and UPVs values of the cores under water and gas saturation (the gas during the experiment and referred to in the paper is air) was an HS-YS4A Sonic wave parameter tester (Tianhong Electronics, Xiangtan, China). This apparatus operates with a high signal-to-noise ratio and has low temperature excursions and high repeatability with low failure rate. The instrument has two emitter options, 160 V or 1000 V, and the amplifier can be regulated with multistage attenuation processes.
Gas saturation was carried out in an autoclave with a pressure of 1.5 MPa to ensure that air occupied the internal pores or cracks in the cores, and water saturation was undertaken using a vacuum pump (ZN-BSJ, Suzhou Niumag Analytical Instrument Corporation, Suzhou, China) by depressurizing the cores to approximately -1.0 MPa, thus allowing water ingress into the pore spaces of the core.
The ambient temperature during the experiments was 25°C, and to eliminate the high-frequency effect on fluid-saturated samples when measuring velocity, the 160 V emitter was chosen with a frequency of 50 kHz. The experimental procedure was carried out in the following steps:
All the cores were placed in a vacuum drying oven at 60°C for 72 h to remove the original gas and water from the cores, and their masses were tested using an electronic balance and were recorded as The cores were placed in the autoclave at a pressure of 1.5 MPa for 72 h to achieve gas saturation, the velocity parameters were tested using the velocity apparatus by connecting the core surface and the transducers, and the values were recorded as UPVp-gas and UPVs-gas The cores were then immersed in the water saturator, and all the air was exhausted to ensure that water occupied the pores, at a pressure of -0.95 MPa for 72 h. The mass of water-saturated cores was recorded as
Figure
Box plots of different coal rank samples at (a) water saturation and (b) gas saturation condition.
UPVp and UPVs velocity ranges and mean values for the three types of coal are listed in Table
UPVp and UPVs velocity ranges and the mean values of three ranks of coals.
Water saturation condition (km/s) | Gas saturation condition (km/s) | |||||||
---|---|---|---|---|---|---|---|---|
UPVp | UPVpmean | UPVs | UPVsmean | UPVp | UPVpmean | UPVs | UPVsmean | |
Lignite | [1.18, 2.17] | 1.86 | [1.13, 1.87] | 1.57 | [0.61, 1.67] | 1.22 | [0.44, 1.23] | 0.90 |
Bituminite | [1.61, 2.17] | 1.95 | [1.04, 1.89] | 1.59 | [0.71, 1.85] | 1.34 | [0.64, 1.72] | 1.07 |
Anthracite | [1.63, 2.25] | 1.93 | [1.26, 1.87] | 1.58 | [1.12, 1.94] | 1.61 | [0.74, 1.69] | 1.21 |
Given the different velocity distributions of the three ranks of coal, it is necessary to analyze correlations between the UPVp and UPVs under the two different conditions. The scatter of velocity data and correlations of fit are shown in Figure
Scatterplot of velocity data and goodness-of-fit correlations between UPVs and UPVp of lignite, bituminite, and anthracite cores under water and gas saturation.
Correlations between UPVp and UPVs for lignite, bituminite, and anthracite under water or gas saturation.
Condition | Equation | ||
---|---|---|---|
Lignite | Water saturation | 0.681 | |
Gas saturation | 0.853 | ||
Bituminite | Water saturation | 0.659 | |
Gas saturation | 0.775 | ||
Anthracite | Water saturation | 0.801 | |
Gas saturation | 0.809 |
Dependent on Figure
Because the different cores have various UPVp and UPVs values, a parameter
Correlations between Vr-water and UPVs-water and between Vr-gas and UPVs-gas for (a, b) lignite, (c, d) bituminite, and (e, f) anthracite.
Linear fit equations of
Condition | Equation | ||
---|---|---|---|
Lignite | Water saturation | 0.459 | |
Gas saturation | 0.515 | ||
Bituminite | Water saturation | 0.837 | |
Gas saturation | 0.456 | ||
Anthracite | Water saturation | 0.399 | |
Gas saturation | 0.684 |
For one particular coal with certain porosity, elastic waves show different responses to different fluids occupying the cracks; for example, the UPVp has higher sensitivity with a small amount of gas compared to the UPVs, and the UPVp value might decrease by a larger degree than the UPVs value [
Moreover, under water saturation conditions, the
Figure
Scatterplot and distribution of velocity data and the corresponding trends between UPVp and density for (a) lignite, (b) bituminite, and (c) anthracite at water/gas-saturated conditions.
Correlations between UPVp and porosity under water and gas saturation.
Condition | Equation | ||
---|---|---|---|
Lignite | Water saturation | 0.406 | |
Gas saturation | \ | \ | |
Bituminite | Water saturation | 0.689 | |
Gas saturation | 0.430 | ||
Anthracite | Water saturation | 0.759 | |
Gas saturation | 0.472 |
It should be noticed that there were several outliers of UPVp scatter for lignite and bituminite due to their high dispersion, and these outliers were labeled using the dotted cycles. The fitting difference might be explained from three aspects: (a) the presence of complex components, such as grains with different sizes, mineral distribution, or incomplete evolution of the plant material in the coal, leads to greater anisotropy in the lignite coal matrix, resulting in random distributions of density and wave velocity [
The presence of fissures always has a significant impact on pulse wave transmission, and wave velocity increases parallel to the bedding planes, but decreases in the perpendicular direction [
The porosity calculated in equation (
Scatterplot and linear correlations between UPVp and porosity under water and gas saturation for (a) lignite, (b) bituminite, and (c) anthracite (d) showing a sketch of wave transmission within the coal matrix.
As shown in Figure
Linear fit equations between UPVp and porosity at water and gas saturation.
Condition | Equation | ||
---|---|---|---|
Lignite | Water saturation | 0.433 | |
Gas saturation | 0.564 | ||
Bituminite | Water saturation | 0.533 | |
Gas saturation | 0.526 | ||
Anthracite | Water saturation | 0.706 | |
Gas saturation | 0.769 |
Based on the above results and the reported results from Kahraman [
Potential application of velocity measurements by hydraulic fracturing or CO2 fracturing during enhanced CBM recovery.
A number of drill groups are advanced into a coal seam with each group containing a single fracturing fluid (water or LCO2) injection hole and eight monitoring holes (
Thus, it is possible to locate petrological changes caused by the mechanical effects of water or CO2 injection, rapidly identify the effective fracture zone, and deduce fracture orientations. Depending on the collected velocity data, it is possible to forecast changes in physical parameters when subjected to crustal stress. This potential application could save a significant amount of core drilling work and help to obtain useful information about coal at depth in real time.
Although correlations among velocity, density, and porosity of water and gas-saturated coal cores have been identified, there is still a need for further study. For example, velocities recorded in different cores with various fluid media and fluid contents should be further investigated and velocity anisotropy should be quantified in future research. Additionally, correlations between velocities of different coal types under fluid saturation and the relevant elastic properties should be tested.
Based on this study, the following major conclusions can be drawn:
The velocities recorded in the three different coal ranks displayed various distributions and discrete degrees, likely related to rank and saturation media. By comparing the range of the UPVp and UPVs box plots, it is evident that the UPVp is more accurate or reliable for characterizing the existence of micro- or macrocracks with the assistance of adsorbed water The UPVp values of lignite, bituminite, and anthracite coals showed positive linear correlations with their corresponding UPVs values, under water and gas saturation. The UPVp values correlated positively with the density of water/gas-saturated cores and correlated negatively with the porosity of both prepared cores The ratio of UPVp/UPVs had a negative linear correlation with the UPVs values for lignite, bituminite, and anthracite, and the different decline trends might be attributed to coal rank. The coals with lower rank had large amounts of fissures. The larger difference of acoustic resistance between coal and gas dissipated a large amount of energy by means of wave reflection or refraction
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was financially supported by the National Key Technologies Research & Development program (2018YFC0808403), the National Natural Science Foundation of China (51774278), the Natural Science Foundation of Jiangsu Province (BK20170001), and the Jiangsu Province Fifth 333 High-Level Talents Training Project (BRA2018032).
The total data in the paper are listed as the following table: parameter data of lignite cores under water/gas-saturated conditions, parameter data of bituminite cores under water/gas-saturated conditions, and parameter data of anthracite cores under water/gas-saturated conditions.