The low rank coalbed methane (CBM) has great potential for exploration and development in China, but its exploitation level is low at present stage. The pores are the storage space of CBM, so recognizing its structural characteristics has very important practical significance for the development of CBM. The samples of No. 4 and upper No. 4 coalbed in Dafosi were selected to carry out the analysis of mercury injection test, nitrogen adsorption test and scanning electron microscopy to study the different lithotypes of the pore structure, pore throat distribution and fracture character of low rank coal reservoir. The results showed that micropore of low rank coal in Dafosi relatively developed and the pore volume of vitrain was equivalent to durain. The pore throat of durain was larger than vitrain, the connectivity was better and the fissures were more developed. The percolation capacity and reservoir performance of upper No. 4 coal was better than No. 4 coal. Generally, the potential of exploration and development of upper No. 4 coal in the study area was better than that of No. 4, and the developed area of durain was more beneficial for the development of CBM.
The CBM’s exploration and development degree of low rank coal area is low in China, only achieving preliminary development in the mining of Xinjiang Fukang, Shaanxi Binchang etc. [
As a storage space for CBM, the pore directly affects the adsorption, accumulation and migration of CBM, and finally affects its output [
In this paper, coal samples were collected from low rank coal in Dafosi mine field. The coal samples of No. 4 vitrain, No. 4 durain and upper No. 4 (the upper coalbed of No. 4) durain in Dafosi were carried out to mercury pressure test, liquid nitrogen test and electron microscope scanning. Then the pore structure, pore type and fracture characteristics of different lithotypes in low rank coal reservoirs were analyzed, which provided the basis for the development of CBM in later stage.
The experimental samples were collected from No. 4 and upper No. 4 coal seam of Dafosi field in Binchang mining. The coal lithotype included vitrain, clarain, durain and fusain, and the vitrain and clarain were easily recognized by the naked eye. Samples of vitrain and clarain were obtained by hand picking and stripping (
After the coal samples were processed, conventional industrial analysis and helium porosity were carried out. Moreover, Auto Pore9505 mercury porosimetry, ASAP2020 specific surface tester and EVO-MA15 scanning electron microscope were used for the mercury injection test, liquid nitrogen test and scanning electron microscope observation.
The serial number of each sample and the results of industrial analysis were shown in
Samples | Mad/% | Aad/% | Vdaf/% | FCad/% |
---|---|---|---|---|
No. 4 vitrain | 6.06 | 9.40 | 36.57 | 53.62 |
No. 4 durain | 4.32 | 14.04 | 26.73 | 59.82 |
Upper No. 4 durain | 5.25 | 13.02 | 26.84 | 59.79 |
The method of helium porosity was used for the determination of porosity (φ). The principle is as follows. With a known volume of standard body, the gas makes an isothermal expansion at an atmospheric pressure of the coal sample at the initial set pressure. When the gas diffuses into the pores of the coal sample, the effective pore volume and particle volume of the measured coal sample can be calculated, and the porosity of coal sample can be calculated according to gas equation. The experimental results showed that the porosity of the No. 4 coal vitrain was 4.9%, greater than 3.9% of the durain, which might be the reason that the density (ρ) of the durain (1.48 g/cm3) was greater than that of vitrain (1.19 g/cm3). When Jin X.L. et al. [
Mercury intrusion test was an indirect experimental method for pore volume, aperture and its distribution [
The pore throat size was mainly characterized by mean and median radius of throat. The mercury injection of 3 samples showed that, for No. 4 coal, the pore throat of the durain was larger than that of the vitrain, which was beneficial to the migration of gas molecules. The average pore throat size of No. 4 coal was as same as upper No. 4 coal. The mean radius of pore throat showed the central value of the normal distribution of pore throat radius. The median throat radius was the highest, and the radius of throat radius decreased at both sides. In addition, the pore throat median radius of durain was larger than that of the vitrain in No. 4 coal by experiment. The results showed that the pore throat of the durain was larger than that of the vitrain in general. Similarly, the test results showed that the gap throat of upper No. 4 durain was larger than of No. 4 coal.
The sorting coefficient can directly reflect the concentration or uniform degree of pore throat distribution, and is a measure of the standard deviation of pore throat size. The throat sorting coefficient of vitrain and durain of No. 4 coal sample was more than 3.0. The poor sorting quality also showed that the coal reservoir permeability of the sample was bad. Sorting coefficient of durain was smaller than that of vitrain, which indicated durain’s sorting was slightly better. Through the pore throats were distributed in different pore sizes and mesopore number was far greater than vitrain, pore throat was still concentrated in micropores and small pore segments (
Samples | r/μm | r50/μm | Sp | Sk | Kg | Pd/MPa | P50/MPa | WE/% |
---|---|---|---|---|---|---|---|---|
No. 4 vitrain | 3.18 | 0.01 | 4.98 | 1.42 | − | 0.03 | 93.03 | 63.88 |
No. 4 durain | 4.83 | 0.02 | 3.39 | −1.37 | − | 0.02 | 44.42 | 36.56 |
Upper No. 4 durain | 4.90 | 0.05 | − | −1.03 | 4.66 | 0.02 | 14.34 | 42.45 |
“−” indicated that data was not tested.
and the throat intruded by mercury was in a larger throat state. The distribution of the durain’s pore throat was negatively skewed, and the throat was in a small throat state. Besides, the pore throat of durain of upper No. 4 was larger than that of No. 4. The kurtosis of pore throat reflected the ratio of two tail stenter in the distribution of amplitude. This experiment measured only the kurtosis of upper No. 4 coal. The larger numerical value indicated that the pore throat distribution of different sizes was relatively dispersed in the coal reservoir of upper No. 4 coal.
The seepage capacity of pore throat can be described by the mean and median radius of the pore throat, in addition to displacement pressure and median pressure. The displacement pressure Pd refered to the capillary pressure corresponding to the largest connected pore throat in the pore system. The smaller the driving pressure was, the bigger the pore throat was connected, and the better the connectivity and the seepage capacity of the pore throat would be. The experimental results showed that the displacement pressure of durain was smaller than vitrain in No. 4 coal, and its seepage ability was relatively better. It was conducive to the migration of gas molecules consistent with throat radius test results; the durain’s displacement pressure of upper No. 4 coal was bigger than No. 4 coal, which showed upper No. 4 coal had better seepage ability. Mean pressure was the corresponding capillary pressure when mercury saturation was 50%. It was a measure of the trend of capillary pressure distribution. The greater the median pressure was, the more compact the sample was and the poorer the ability to store gas would be. The mean pressure of durain of No. 4 coal was smaller than that of vitrain, which was more conducive to the storage of CBM. So the storage capacity of upper No. 4 coal was better than that of No. 4.
By the analysis of curve shape of
durain was much larger than the vitrinite (60.20%), which was close to 100%. It showed that the connectivity of the pore throat of durain was excellent and almost all pores were occupied by mercury; the mercury injection saturation of vitrain was small and the pore volume not occupied by mercury was larger, so the whole pore connectivity of vitrain was poor. Mercury injection curves of three kinds of coal samples showed hysteresis in different degrees. The efficiency of mercury ejection withdrawal of vitrain (63.88%) was higher than durain (No. 4 durain was 36.56% and upper No. 4 durain was 42.45%). The reason was that the pressed mercury could exit effectively because of thick pore throat, in spite the overall coal pore connectivity of vitrain was poor. It was consistent with the results shown by the deflection of the hole throat. Compared with durain of No. 4 coal, the pore connectivity of upper No. 4 durain was better. The mercury injection curve of No. 4 durain had longer straight section and smaller slope. It showed that the throat distribution of upper No. 4 durain was smaller than that of No. 4, which was consist with the results of pore throat skewness and kurtosis analysis.
Pore volume (V): The experiment of liquid nitrogen adsorption (
Sample | V/mL・g−1 | Vmi/% | Vs/% | Vme/% | Vma/% | S/m2・g−1 | Vmi/% | Vs/% | Vme/% | Vma/% |
---|---|---|---|---|---|---|---|---|---|---|
No. 4 vitrain | 0.01894 | 50.11 | 36.69 | 13.20 | − | 13.1979 | 90.27 | 9.22 | 0.51 | − |
No. 4 durain | 0.01676 | 50.00 | 35.68 | 14.32 | − | 11.5236 | 90.74 | 8.61 | 0.65 | − |
Upper No. 4 durain | 0.02547 | 12.35 | 57.16 | 23.12 | 7.31 | 20.7443 | 51.32 | 47.49 | 1.02 | 0.17 |
coal seam depth and coal accumulation environment. The total pore volume of vitrain was equivalent to durain in No. 4 coal and upper No. 4 durain was bigger than that of No. 4, which indicated that upper No. 4 durain was more favorable to the storage of CBM, consistent with the mercury pressure test analysis.
Specific surface area (S): The distribution of specific surface area of the sample was consist with pore volume distribution. The micropore of No. 4 coal accounted for more than 90% of the surface area, which contributed largest for specific surface area. Small pore surface area accounted for less than 10%. For upper No. 4 durain, the contribution of micropores and pores to the specific surface area was about 50%. As for the total specific surface area, vitrain was equivalent to No. 4 durain and upper No. 4 durain was larger than that of No. 4, indicating that upper No. 4 durain had better adsorption capacity.
The adsorption and desorption curves of liquid nitrogen adsorption method can also be applied to the pore morphology analysis of coal. The loop formed by the separation of adsorption curve and desorption curve is called adsorption hysteresis loop. The shape and position of hysteresis loop can reflect the morphological characteristics of the pore [
In
The liquid nitrogen absorption/desorption curves of vitrain and durain of No. 4 were similar. As the relative pressure decreased, the desorption curve decreased slowly, and the hysteresis loop was formed due to the simultaneous existence of the open hole and the half closed cell. When the relative pressure dropped to 0.5 (the corresponding aperture was about 4 nm), the condensation of the “ink bottle” hole was about to evaporate. If the pressure decreased slightly, the liquid would be poured out, showing a sharp decline in the curve. When the
relative pressure was less than 0.5, there was only half closed hole in the corresponding pore section, and the desorption curve coincided with the adsorption curve basically. In addition, when the relative pressure was more than 0.5, the adsorption/desorption curves of upper No. 4 durain was similar to that of No. 4 durain. The difference was that the open hole and half closed hole existed at the same time when the pore size of upper No. 4 durain was less than 4nm. When the relative pressure was greater than 0.5, there was hysteresis.
The scanning electron microscope test results can reflect more than 0.1nm of the hole and fissure development, coal surface features, microstructure and mineral composition and other information [
In
The pore structure, type and distribution were the important parameters of coal reservoir. The difference of the pore system, coal formation environment, metamorphism and late tectonism were closely linked, therefore resulting in the coal reservoir had a strong heterogeneity. For the exploration and development of CBM, favorable reservoirs and unfavorable reservoirs were formed naturally. The study of the pores in the coals of different lithotypes was helpful to the understanding of the properties of the coal reservoirs and the evaluation of the reservoir properties, so as to guide the exploration, selection and development of CBM in the later period.
1) In the No. 4 coal seam of Dafosi, the micropore of vitrain developed better. The pores of durain were distributed in each pore section, and the pores were dominated by micropores. The pore volume and the specific surface area of vitrain was equipment to durain. Compared with vitrain, the durain had larger pore throat, better connectivity, more developed fracture and better percolation capacity and reservoir performance.
2) For different coal seams, the porosity, pore volume and specific surface area of upper No. 4 were more developed than that of No. 4. The pores of upper No. 4 were distributed in each pore section, and the pores were dominated by micropores. The pore throat was large; the fissure developed vertically in two directions, and the pore connectivity was good. The percolation capacity and reservoir performance of the upper No. 4 were better than that of No. 4.
3) For Dafosi field, the upper No. 4 was more favorable for the exploration and development of CBM. For the different of lithotype of low rank coal, the development of CBM may be more favorable for the areas where the durain is more developed.
Ma, D.M., Li, Q., He, Q. and Wang, C.T. (2017) Pore Characteristics of Vitrain and Durain in Low Rank Coal Area. Journal of Power and Energy Engineering, 5, 10-20. https://doi.org/10.4236/jpee.2017.511002