Two-scale analysis of a tight gas sandstone

Tight gas sandstones are low porosity media, with a very small permeability (i.e., below 1 mD). Their porosity is below 10%, and it is mainly composed of fine noncemented microcracks, which are present between neighboring quartz grains. While empirical models of permeability are available, their predictions, which do not compare well with macroscopic measurements, are not reliable to assess gas well productivity. The purpose of this work is to compare the permeability measured on centimetric plugs to predictions based on pore structure data. Two macroscopic measurements are performed, namely dry gas permeability and mercury intrusion porosimetry (MIP), together with a series of local measurements including focused ion beam and scanning electron microscopy (FIB-SEM), x-ray computed microtomography (CMT), and standard two-dimensional (2D) SEM. Numerical modeling is performed by combining analyses on two scales, namely the microcrack network scale (given by 2D SEM) and the individual 3D microcrack scale (given by either FIB-SEM or CMT). The network permeability is calculated by means of techniques developed for fracture networks. This permeability is proportional to the microcrack transmissivity, which is determined by solving the Stokes equation in the microcracks measured by FIB-SEM or CMT. Good correlation with experimental permeability values is only found when using transmissivity from 3D CMT data.

Figure 1

The tight sandstone sample T-2390-82. (a) Definition of the four zones on the upper surface where 2D SEM imaging is performed. (b) Samples for macroscopic gas permeability measurements. Dry gas permeability $K_{\mathrm{gas}}$ is measured along the vertical $z$ axis prior to transversal overcoring along the radial $r$ axis (parallel to the biggest cracks in zone 1) to a diameter of 20 mm The permeability of the smaller 20mm diameter sample is also along the $r$ axis.

Figure 2

Indirect PSD results for T-2390-82 sandstone. (a) Nitrogen adsorption as a function of the relative pressure $P/P_o$. Data are for adsorption (black solid line, +), desorption (red broken line, $\circ$). (b) Mercury intrusion porosimetry (MIP) results (plotted after blank cell and conformance corrections [15]). Data are for intrusion (red solid line, $\circ$), cumulated intrusion (black broken line, $+$).

Figure 3

Two dimensional SEM measurements of T-2390-82 sandstone. (a) SEM1 is located in the homogeneous zone 4, and observed with the SE detector (microcracks appear light grey); the corresponding trace map is given in (b). (c) SEM2 is from the most porous and cracked zone 1, and it is observed with the BSE detector (microcracks appear black); it is interpreted as a trace map in (d).

Figure 4

2D SEM of T-2390-82 sandstone: close-up views of single micro cracks between two quartz grains, highlighting their highly variable width, from (a) less than 50 nm and up to (b) several microns. (a) and (b) are taken from zones 4 and 1, respectively.

Figure 5

Three different micro-CT samples of T-2390-82 sandstone: (upper left) 20 mm diameter and (lower left) its 3D nonconnected pore volume rendering; (upper middle) 5 mm wide parallelepipedic sample and (lower middle) its 3D nonconnected pore volume rendering; (upper right) 0.9 mm section rod and (lower right) its 3D connected pore volume rendering.

Figure 6

The initial FIB-SEM pore volume sample for T-2390-82 sandstone in (a) with $674\times 562\times 300$ parallelipipedic voxels of size $48.8\times 35.71\times 50{\mathrm{nm}}^3$. In (b), the percolating component is retained along the $y$ axis with cubic voxels of size 48.8 nm.

Figure 7

The relative pore volume (in percent) obtained by different direct or indirect methods, for T-2390-82 sandstone. Data are for XRT 0.6 $\mu \mathrm{m}$ (o, black solid line), XRT 4.37 $\mu \mathrm{m}$ ($\times$, black broken line), XRT 13.1 $\mu \mathrm{m}$ ($\times$, black solid line), N2 adsorption (+, red solid line), MIP (1) ($\times$, red broken line), 2D SEM medium density zone ($⊲$, blue solid line), 2D SEM high density zone ($⊳$, blue broken line), FIB-SEM $48.8\times 35.7\times 50$ nm ($\square$, red broken line).

Figure 8

Subsamples of the FIB-SEM pore volume sample (displayed in Fig. 6) taken from T-2390-82 sandstone. Zone 1: sech1 (a) and sech11w (b). Zone 2: sech2 (c).

Figure 9

Subsamples of the FIB-SEM pore volume sample (displayed in Fig. 6) taken from T-2390-82 sandstone: (a) zone 3 with remnants of zone 1 and its mirror image; (b) subsample of the wing and its mirror image along the $y$ axis.

Figure 10

3D rendering of the pore volume of the CMT rod-shaped sample taken from T-2390-82 sandstone (see Fig. 5 right). (a) The percolating component of the full sample is completed by its mirror image along the $y$ axis. (b) A permeable central portion of (a). (c) A less permeable central portion of (a).