Comparisons of Pore Structure for Unconventional Tight Gas, Coalbed Methane and Shale Gas Reservoirs
Authors: Yu-jiang Shi , Yang Hua , Zhiqiang Mao , Liang Xiao , Hai-tao Zhang , Xiao-ming Yang , Tai-ping Zhao , Tain-ping Liu 
Published: October 2013

Extended abstract
Tight sands gas, coalbed methane and shale gas are three kinds of typical unconventional natural gas. With the decrease of conventional oil and gas reserves, unconventional reservoirs, especially unconventional gas-bearing reservoirs have become the most important supplement and alternative resources. At present, studies of the tight sands gas, coalbed methane and shale gas have become the hotspot of energy research all over the world. As the unconventional gas-bearing reservoirs, they all characterize as ultra-low porosity and permeability, the pore sizes are nanoscale. For tight gas sands, the porosity is main lower than 10.0%, and the permeability is always lower than 0.5 mD, the pore size ranges from 300 to 900 nm, the proportion of nanoscale pore is nearly 85.0%, and the critical pore radius of the lower limit is 20 to 60nm (Zou et al., 2011). For coalbed methane reservoir, the porosity is always lower than 6.0%, and the permeability is lower than 0.002mD, the pore size is lower than 130nm (Yan et al., 2008). In the shale gas formation, the porosity is lower than 10.0%, and permeability is even lower than 0.002mD, the pore size ranges from 80 to 200nm (Erik et al. 2013). The information of pore structure is very important in unconventional gas-bearing reservoirs evaluation. The coalbed methane and shale gas are authigenic reservoirs, and the gas contents are dominant by the adsorbed gas (Sun et al., 2011). Hence, the developmental degree of micropore is of great importance in effective unconventional coalbed and shale gas reservoirs evaluation. Zhao et al. (2012) pointed out that the more development of micropore volumen, the higher content of gas in coalbed methane reservoir. However, in tight gas sands, the micropore content is inversely proportional to the gas production. In this paper, to understand the pore structure of the effective unconventional gas bearing reservoirs, we review the microstructure of these three kinds of unconventional gas bearing formation, and compare the NMR T2 spectra at laboratory and field conditions. The results illustrate that in tight gas sands, the lab NMR T2 spectra in the Xujiahe tight gas sands of Sichuan basin are dominant as unimodal distribution; the T2 relaxation time of the main peak is lower than 100.0 ms. The Bound fluid volume (BFV) ranges from 35.0% to 60.0%, and the average value is 44.3%, and the NMR T2 distributions of field condition are also main unimodal, the T2 distribution are wide in formations with high gas production, and on the contrary, for formations with low gas production, the T2 distributions are narrow and unimodal, and the largest T2 transverse relaxation time is relative lower. In coalbed methane and shale gas reservoirs, the lab NMR T2 spectra are also main unimodal, and the largest T2 transverse relaxation times are far less than that of the tight gas sands. In field condition, the T2 spectra in coalbed methane formations are narrow, and the largest T2 transverse relaxation times are lower than those of the adjacent strata (Adrian et al., 2010). At present, we cannot collect the field NMR logs, hence, the NMR T2 distribution for shale gas reservoir in field condition cannot be compared. In the meanwhile, comparisons of lab NMR spectra for core samples drilled from coalbed methane and shale gas reservoirs illustrate that there are the rapid relaxation components, and a insulated peak with ultra-short relaxation time exist in the NMR T2 spectra under fully water saturated, and there are T2 spectra in dry core samples. For core samples drilled from coalbed methane reservoirs, the relaxation time of insulated peak is lower than 2.0 ms, and for core samples drilled from shale gas reservoirs, the relaxation time of insulated peak is lower than 1.0 ms. With the lab NMR experimental measurements of braise, Guo et al. (2007) and Zhao et al. (2011) pointed that the signals on NMR spectra with rapid relaxation time are contributed by the blind pore. Odusina et al. (2011) and Sulucarnain et al. (2011) also obtained the same conclusion that the signals on NMR spectra with rapid relaxation time for core samples drilled from shale gas reservoirs are contributed by the disconnected blind pore.