Pore structure of gas shale which possesses lots of nanopores is complex. NMR measurement can not only obtain accurate sample parameters but also provide an approach to quantitatively study pore structure. This paper Integrates NMR, mercury injection, nitrogen adsorption and X-ray diffraction to analyze the relationships of pore structure, surface to volume ratio and mineral components.
Both nitrogen adsorption and mercury-injection have some limitations in characterizating of pore structure of gas shale. In this paper we integrate pore size distribution curve of nitrogen adsorption and mercury-injection. Comparison and analyses of NMR T2 spectra and the integrated pore size distribution curve calculate NMR pore size distribution. Capillary pressure curve is constructed based on NMR T2 spectra for quantitatively studying pore structure of gas shale. The diagrams of relationship between characteristic parameters of pore structure, surface to volume ratio and mineral components such as clay minerals, pyrite and TOC are established.
NMR study has been implemented on 15 samples from Longmaxi formation. The results show the porosity range from 2.24% to 5.76%. The NMR pore size distribution curves is more precise than the curves that calculated only according to nitrogen adsorption or mercury-injection. Calculating separation coefficient, skewness, structure coefficient, median pressure and other parameters from constructed capillary pressure curves indicate high threshold pressure, poor connectivity, and complex pore size distribution. Porosity has a positive correlation with clay mineral content, TOC, pyrite. Most of surface to volume ratio and pore volume are provided by micropores and mesopores.
Using integration pore size distribution curve of nitrogen adsorption and mercury-injection to calculate parameter of NMR pore size distribution is a more exact approach than only using nitrogen adsorption or mercury-injection. Capillary pressure curve constructed by NMR T2 spectra can provide information of micropores and mesopores which mercury-injection cannot correctly characterize.