Quantitative Characterization of Pore Connectivity and Movable Fluid Distribution of Tight Sandstones: A Case Study of the Upper Triassic Chang 7 Member, Yanchang Formation in Ordos Basin, China

Abstract

The pore connectivity and distribution of moveable fluids, which determines fluid movability and recoverable reserves, are critical for enhancing oil/gas recovery in tight sandstone reservoirs. In this paper, multiple techniques including high-pressure mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and microcomputer tomography scanning (micro-CT) were used for the quantitative characterization of pore structure, pore connectivity, and movable fluid distribution. Firstly, sample porosity and permeability were obtained. Pore morphology and the 3D distribution of the pore structures were analyzed using SEM and micro-CT, respectively. The pore-size distribution (PSD) from NMR was generally broader than that from MIP because this technique simply characterized the connected pore volume, whereas NMR showed the total pore volume. Therefore, an attempt was made to calculate pore connectivity percentages of pores with different radii (<50 nm, 50 nm–0.1 μm, and 0.1 μm–1 μm) using the difference between the PSD obtained from MIP and NMR. It was found that small pores (r<0.05 μm) contributed 5.02%–18.00% to connectivity, which is less than large pores (r>0.05 μm) with contribution of 36.60%–92.00%, although small pores had greater pore volumes. In addition, a new parameter, effective movable fluid saturation, was proposed based on the initial movable fluid saturation from NMR and the pore connectivity percentage from MIP and NMR. The results demonstrated that the initial movable fluid saturation decreased by 14.16% on average when disconnected pores were excluded. It was concluded that the effective movable fluid saturation has a higher accuracy in evaluating the recovery of tight sandstone reservoirs.

Characterization of Movable Fluid Distribution in Tight Oil Reservoirs by NMR and Centrifugation

“…NNR and Centrifugation. NMR techniques have been are already widely applied to study the distribution of fluids in tight sandstone reservoirs because of their efficiency, nondestructiveness, and accuracy [23,24]. The main principle is that fluids present in porous media generate detectable signals when subjected to an external magnetic field, and the intensity of the signal is proportional to the number of hydrogen nuclei in the sample [24].…”

Applicability of Nuclear Magnetic Resonance Experiment in Analyzing Pore and Fluid Distribution Characteristics of Tight Sandstone: A Case Study in the Julu Area, Bohai Bay Basin, China

“…Two T 2 cutoff values have been used to divide the T 2 spectrum into fully movable, partially movable, and fully immovable parts. , The movable fluid is mainly distributed in large pores and pores controlled by large pore throats, and the bound fluid is present in small pores and large pores controlled by small pore throats . Centrifugal force controls the theoretical minimum pore radius corresponding to the moving fluid. , However, the T 2 signal changes completely in the short relaxation range after the centrifugation of some samples. − This phenomenon suggests that the short relaxation signal of the T 2 spectra of these samples may reflect macropore information to some extent. It might be possible to analyze the applicability of T 2 in terms of the lower limit of the centrifugal radius.…”