Characterization of Full Pore and Stress Compression Response of Reservoirs With Different Coal Ranks

Abstract

The accurate characterization of coal pore structure is significant for coalbed methane (CBM) development. The splicing of practical pore ranges of multiple test methods can reflect pore structure characteristics. The pore\fracture compressibility is the main parameter affecting the porosity and permeability of coal reservoirs. The difference in compressibility of different coal rank reservoirs and pore\fracture structures with changing stress have not been systematically found. The pore structure characteristics of different rank coal samples were characterized using the optimal pore ranges of high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen adsorption (LT-N2A), low-pressure carbon dioxide adsorption (LP-CDA), and nuclear magnetic resonance (NMR) based on six groups of different rank coal samples. The compressibility of coal matrix and pore\fracture were studied using HPMI data and NMR T2 spectrum under effective stress. The results show that the more accurate full pore characterization results can be obtained by selecting the optimal pore range measured by HPMI, LT-N2A, and LP-CDA and comparing it with the NMR pore results. The matrix compressibility of different rank coal samples shows that low-rank coal > high-rank coal > medium-rank coal. When the effective stress is less than 6 MPa, the microfractures are compressed rapidly, and the compressibility decreases slowly when the effective stress is more than 6 MPa. Thus, the compressibility of the adsorption pore is weak. Nevertheless, the adsorption pore has the most significant compression space because of the largest proportion in different pore structures. The variation trend of matrix compressibility and pore\fracture compressibility is consistent with the increase of coal rank. The compressibility decreases with the rise of reservoir heterogeneity and mechanical strength. The development of pore volume promotes compressibility. The research results have guiding significance for the exploration and development of CBM in different coal rank reservoirs.

Analysis of Stress Sensitivity and Movable Fluid Characteristics of Tight Sandstone Based on NMR and Permeability Experiments

“…In general, the variable compressibility coefficient decreases as the pressure increases. 40,44 The results show that the compressibility coefficients of the samples were different at different pressure ranges and the compressibility coefficients of the pores of different sizes showed different trends with increasing pressure (Figure 8d−f). At pressures less than 12 MPa, the total pore compressibility coefficients of samples NS1, NS2, and NS3 all decreased with increasing pressure, indicating a gradual decrease in compressible space.…”

A comprehensive insight into the effects of acidification on varied-sized pores in different rank coals

“…However, neither LPGA-CO 2 , LTGA-N 2 or MIP can independently elucidate the complicate pore structure characteristics, which was limited by the test principle. Previous studies have demonstrated that the above three test methods have different optimal performance intervals: LPGA-CO 2 can accurately characterize micropores, LTGA-N 2 is suitable for characterizing mesopores, and MIP is more accurate for characterizing macropores (Wang et al, 2019;Wei et al, 2019;Lu et al, 2021;Wei et al, 2021). Referring to previous studies, the pore size distribution data of micropores (below 2 nm) from LPGA-CO 2 , mesopores (2-50 nm) from LTGA-N 2 , and macropores (above 50 nm) from MIP were directly combined and respectively connected at 2 nm and 50 nm to obtain the full-sized pore size distribution dataset.…”

“…From previous relevant researches, we can see that various advanced techniques have been employed to analyze the changes of coal pore structure under acidification. Nevertheless, due to the limitation of the testing principle, these studies can only detect pore structure characteristics within a specific range of pore size (Zhang et al, 2017;Liu X. et al, 2019;Wei et al, 2019;Qin et al, 2020;Lu et al, 2021;Mou et al, 2021). For example, neither LTGA-N 2 nor MIP can accurately characterize pores with diameters below 2 nm, which is attributed to the extremely slow nitrogen diffusion rate under low pressure and the limitation of the maximum mercury intrusion pressure (Mou et al, 2021).…”

Construction and Programming of Bivariate Model for Unsaturated Soil Seepage Calculation

“…(Jennings and Burland, 1961;Fredrond and Laharzo, 1997;Xie and Zhiyan, 2006;Han et al, 2016;Fan et al, 2019;Jiang et al, 2021). Many researchers (Wang et al, 2013;Chen, 2014;Zhou et al, 2018;Shao et al, 2019;Xie et al, 2019;Huang et al, 2021;Kang et al, 2021;Lu et al, 2021;Sun et al, 2021; have studied unsaturated seepage from different aspects. (Yu et al, 2008;Han et al, 2018 studied the stability of unsaturated soil slope under unsteady seepage condition.…”