“…X-ray CT images in this study are unable to map some microfractures that can be observed with SEM and thin sections (Fig. 7C,D) 6 .Figure 7Photomicrographs showing the pore systems of the sandstones. ( A ) Intergranular pores (red arrow) and intragranular pores (blue arrow).…”
Section: Resultsmentioning
confidence: 79%
“…Due to the tight nature of the matrix, the well-developed natural fracture systems play a significant role in natural gas transport from the reservoir to the wellbore 2 . Fractures are important in tight formations because they can constitute major paths for fluid flow and increase drainage surface area in oil and gas systems 6 . The successful exploration and efficient development of natural gas in Bashijiqike sandstones depend on the detailed characterization of pore network and fracture systems 2 , 5 , 7 .…”
Tight gas sandstone samples are imaged at high resolution industrial X-ray computed tomography (ICT) systems to provide a three-dimensional quantitative characterization of the fracture geometries. Fracture networks are quantitatively analyzed using a combination of 2-D slice analysis and 3-D visualization and counting. The core samples are firstly scanned to produce grayscale slices, and the corresponding fracture area, length, aperture and fracture porosity as well as fracture density were measured. Then the 2-D slices were stacked to create a complete 3-D image using volume-rendering software. The open fractures (vug) are colored cyan whereas the calcite-filled fractures (high density objects) are colored magenta. The surface area and volume of both open fractures and high density fractures are calculated by 3-D counting. Then the fracture porosity and fracture aperture are estimated by 3-D counting. The fracture porosity and aperture from ICT analysis performed at atmospheric pressure are higher than those calculated from image logs at reservoir conditions. At last, the fracture connectivity is determined through comparison of fracture parameters with permeability. Distribution of fracture density and fracture aperture determines the permeability and producibility of tight gas sandstones. ICT has the advantage of performing three dimensional fracture imaging in a non-destructive way.
“…X-ray CT images in this study are unable to map some microfractures that can be observed with SEM and thin sections (Fig. 7C,D) 6 .Figure 7Photomicrographs showing the pore systems of the sandstones. ( A ) Intergranular pores (red arrow) and intragranular pores (blue arrow).…”
Section: Resultsmentioning
confidence: 79%
“…Due to the tight nature of the matrix, the well-developed natural fracture systems play a significant role in natural gas transport from the reservoir to the wellbore 2 . Fractures are important in tight formations because they can constitute major paths for fluid flow and increase drainage surface area in oil and gas systems 6 . The successful exploration and efficient development of natural gas in Bashijiqike sandstones depend on the detailed characterization of pore network and fracture systems 2 , 5 , 7 .…”
Tight gas sandstone samples are imaged at high resolution industrial X-ray computed tomography (ICT) systems to provide a three-dimensional quantitative characterization of the fracture geometries. Fracture networks are quantitatively analyzed using a combination of 2-D slice analysis and 3-D visualization and counting. The core samples are firstly scanned to produce grayscale slices, and the corresponding fracture area, length, aperture and fracture porosity as well as fracture density were measured. Then the 2-D slices were stacked to create a complete 3-D image using volume-rendering software. The open fractures (vug) are colored cyan whereas the calcite-filled fractures (high density objects) are colored magenta. The surface area and volume of both open fractures and high density fractures are calculated by 3-D counting. Then the fracture porosity and fracture aperture are estimated by 3-D counting. The fracture porosity and aperture from ICT analysis performed at atmospheric pressure are higher than those calculated from image logs at reservoir conditions. At last, the fracture connectivity is determined through comparison of fracture parameters with permeability. Distribution of fracture density and fracture aperture determines the permeability and producibility of tight gas sandstones. ICT has the advantage of performing three dimensional fracture imaging in a non-destructive way.
“…Moreover, the lamina-induced fractures also correspond to the km-scale regional bedding fractures, which are induced by the formation beddings. These fractures have been the focus of conventional and unconventional oil and gas reservoir characterization including shale, tight oil [14][15][16][17][18][19][20][21][22][23], and Carboniferous rocks [24]. The fractures have also played an important role in methane gas emissions from coal seams [25][26][27].…”
Multi-scale bedding fractures, i.e., km-scale regional bedding fractures and cm-scale lamina-induced fractures, have been the focus of unconventional oil and gas exploration and play an important role in resource exploration and drilling practice for tight oil and gas. It is challenging to conduct numerical simulations of bedding fractures due to the strong heterogeneity without a proper mechanical criterion to predict failure behaviors. This research modified the Tien–Kuo (T–K) criterion by using four critical parameters (i.e., the maximum principal stress (σ1), minimum principal stress (σ3), lamina angle (θ), and lamina friction coefficient (μlamina)). The modified criterion was compared to other bedding failure criteria to make a rational finite element simulation constrained by the four variables. This work conducted triaxial compression tests of 18 column samples with different lamina angles to verify the modified rock failure criterion, which contributes to the simulation work on the multi-scale bedding fractures in the statics module of the ANSYS workbench. The cm-scale laminated rock samples and the km-scale Yanchang Formation in the Ordos Basin were included in the multi-scale geo-models. The simulated results indicate that stress is prone to concentrate on lamina when the lamina angle is in an effective range. The low-angle lamina always induces fractures in an open state with bigger failure apertures, while the medium-angle lamina tends to induce fractures in a shear sliding trend. In addition, the regional bedding fractures of the Yanchang Formation in the Himalayan tectonic period tend to propagate under the conditions of lower maximum principal stress, higher minimum principal stress, and larger stratigraphic dip.
“…X-ray computed tomography (X-ray CT) is a nondestructive imaging and analyzing technique [23][24][25][26][27][28][29]. It provides the feasibility in characterizing a large variety of materials in geoscience, including cleats/fractures, minerals inclusion, and pores [21,[30][31][32][33]. us, the volume variation of microstructures in coal with the specimen size can be quantitatively obtained by the X-ray CT in conjunction with the reconstruction algorithm and threshold segmentation technique.…”
Acoustic emission (AE) in coal is anisotropic. In this paper, we investigate the microstructure-related scale effect on the anisotropic AE feature in coal at unconfined loading process. A series of coal specimens were processed with diameters of 25 mm, 38 mm, 50 mm, and 75 mm (height to diameter ratio of 2) and anisotropic angles of 0°, 15°, 30°, 45°, 60°, and 90°. The cumulative AE counts and energy dissipation increase with the specimen size, while the energy dissipation per AE count behaves in the opposite way. This may result from the increasing amount of both preexisting discontinuities and cracks (volume/number) needed for specimen failure and the lower energy dissipation AE counts generated by them. The effect of microstructures on the anisotropies of AE weakens with the increasing specimen size. The TRFD and its anisotropy reduce as the specimen size increases, and the reduction of fractal dimension is most pronounced at the anisotropic angle of 45°. The correlation between TRFD and cumulative AE energy in the specimens with different sizes are separately consistent with the negative exponential equation proposed by Xie and Pariseau. With the specimen size gain, the reduction of the TRFD weakens with the increasing amount of cumulative absolute AE energy.
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