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.
Highlights
In recent years, unconventional fossil resources such as shale gas, coal-bedded methane, tight gas, and tight oil have derived remarkable success in North America and China [1,2,3]
Pores were identified through scanning electron microscopy (SEM) observation and classified into three types: residual interparticle pores, intergranular pores, and dissolution pores (Figure 2)
The pore-size distribution (PSD) from nuclear magnetic resonance (NMR) was calculated from the T2 spectrum with reference to the literature based on the theory discussed in Section 3.2.5 [40]
Summary
Unconventional fossil resources such as shale gas, coal-bedded methane, tight gas, and tight oil have derived remarkable success in North America and China [1,2,3]. Many researchers have tried to enhance oil recovery from these tight-rock reservoirs [5, 6]. These investigations have included quantitative characterization of pore structure using various techniques [7,8,9], connectivity analysis of tiny pores [10,11,12,13,14], and predictions of migration of movable fluids and oil/gas
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