Petrophysical characterization of shale reservoir based on nuclear magnetic resonance (NMR) experiment: A case study of Lower Cambrian Qiongzhusi Formation in eastern Yunnan Province, South China

In order to characterize the petrophysical properties of shale using NMR technique, eight shale samples from the Lower Cambrian Qiongzhusi Formation in the eastern Yunnan province were measured by porosity and permeability tests, field emission scanning electron microscopy (FE-SEM) and NMR experiment. Pore types were obtained from the shape and distribution of transverse relaxation time (T2) spectrum. Residual porosity and movable porosity could be well estimated based on T2 spectrum area fraction. On the basis of Coates model, we proposed a regional Coates model to calculate the NMR permeability of shale. A method for determining T2cutoff of shale samples was also expounded. Moreover, the specific surface area distributions and pore size distributions could be obtained based on the mathematical equation of T2. Results show that T2 spectrums of shale samples at water-saturated condition can be divided into unimodal and bimodal T2 spectrums. Continuous bimodal T2 spectrums reflect the samples with good connectivity between small pores and large pores, whereas discontinuous bimodal T2 spectrums reflect that the connectivity between small pores and large pores is poor. Shale samples with higher bound water content have a greater T2cutoff. The NMR permeability is close to gas log permeability, which proves the applicability of regional Coates model. In eight shale samples, transition pores account for the largest proportion, followed by mesopores, indicating that transition pores and mesopores are the major sites for the accumulation of shale gas.