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

The movable fluid porosity can be used as a sensitivity factor to reflect the seepage capacity of tight reservoirs. However, the stress sensitivity and compressibility of samples differ and it is worth investigating whether the movable fluid porosity reflects the relative magnitude of the overburden permeability of the sample. In this study, the static pore information on three tight sandstones was characterized based on low-temperature nitrogen adsorption (LTNA), mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR) experiments. The stress sensitivity, compressibility, and movable fluid porosity of the samples at different pressures were discussed based on NMR, centrifugation, and permeability tests. The results show that the pore information on the nanopores of the samples obtained from LTNA, MIP, and NMR was consistent. Moreover, the NMR results reflected more submicron and micron pores in the samples compared to the MIP results that were affected by pore connectivity. The permeability was more sensitive to pressure than the porosity, and a small change in porosity was sufficient to cause a rapid decrease in permeability. The stress sensitivity and compressibility of nanopores were smaller than those of submicron and micron pores. The relative magnitude of stress sensitivity of micron pores and submicron pores varied from sample to sample. The variable compressibility coefficients of the samples decreased with increasing pressure (<12 MPa). The increase in the compressibility coefficients of nanopores and submicron pores for the samples at 12-15 MPa may result from the increase in the compressible space due to the transformation of the micron pores. The initial movable fluid porosity of the samples was positively correlated with the air permeability. The movable fluid porosity under pressure conditions (1-15 MPa) cannot reflect the relative magnitude of the overburden permeability, resulting from the difference in stress sensitivity between the porosity and permeability.