Novel Insights into the Movable Fluid Distribution in Tight Sandstones Using Nuclear Magnetic Resonance and Rate-Controlled Porosimetry

To characterize the movable fluid distribution in tight sandstones and determine its influencing factors, 20 tight sandstone core samples from the Chang-6 and Chang-8 Members of the Yanchang Formation in the Ordos Basin, NW China, were analyzed using nuclear magnetic resonance (NMR) and rate-controlled porosimetry (RCP). The conventional method of movable fluid analysis divides the whole NMR T-2 spectrum into two parts, namely unmovable and movable fluids, based on a single T-2cutoff value. However, this conventional method ignores pore size wherein fluid is partially movable. In this paper, the fluid in tight sandstones is divided into three parts, i.e., completely unmovable, partially movable and completely movable fluids. Based on the throat size distribution obtained from RCP, T-2 spectra were converted into corresponding pore-throat sizes with the nonlinear conversion method, and then the critical pore-throat sizes for fluid distribution were derived. The research results show that the average residual liquid saturation was 46.14% under a centrifugal pressure of 900 psi. On average, the completely unmovable fluid occupied approximately 18% of the total pore volume of tight sandstones, the partially movable fluid accounted for approximately 72% of the total pore volume, and the completely movable fluid accounted for only approximately 10% of the total pore volume. The pore throats filled with completely unmovable fluid contributed very little to the permeability. Compared with the Chang-8 Member, the tight sandstones of the Chang-6 Member had higher completely unmovable fluid saturation and larger critical pore-throat size of the completely unmovable fluid.