Experiment Investigation of NMR Porosity for Gas-bearing Shale

Abstract

The pore structure of gas-bearing shale is very complex. NMR is a key means to detect pore structure of shale both in laboratory and in downhole. Porosity measurement is fundamental for NMR application. In the paper we collected eighteen shale samples in Sichuan Basin to investigate NMR porosity. T2 measurement under three conditions, dry, water-saturated, and desaturated, was made. Otherwise the gas porosity with helium and the water porosity with the weight method were obtained. The results show that NMR signal in the dry shale sample is induced by the bound water. The water porosity is greater than the gas porosity because of clay mineral absorbing water. T2 distribution of shale is narrow and T2 is also short. Water-saturated NMR porosity is greater than the water porosity, but the difference between NMR porosity of the water-saturated and the dry sample is well correlated with the water porosity. With comparing of the saturated and the desaturated, the NMR porosity changes little. Less bound water is unable to move or evaporate to produce NMR signal in the dry shale sample.