利用高分辨率工业CT成像系统(ICT)对致密砂岩裂缝进行三维定量评价具有不破坏样品的优点。利用二维切片、三维可视化软件对裂缝网格进行定量评价,首先扫描岩心样品产生灰度图像,同时测量相对应的裂缝面积、长度、张开度、孔隙度和密度;利用体积渲染软件对二维切片进行叠加产生完整的三维图像,开启的裂缝被标注蓝绿色、方解石充填的裂缝被标注品红色;利用三维计数方法计算开启的裂缝和开裂缝的比表面积、体积、裂缝孔隙度和张开度。通过ICT技术在大气压力下计算裂缝孔隙度和张开度比成像测井技术在地层真实压力下计算的数值要大,最后通过计算裂缝渗透率判断裂缝之间的连通性。裂缝密度的分布、裂缝的张开度决定了致密砂岩储层渗透率和产量。

Fractures are the main fluid-flow pathways in tight-oil sandstones, and they have significant influence on tight-oil distribution, exploration, and development. Cores and image logs are commonly unavailable because of their high costs. Therefore, employing conventional logs for fracture detection is imperative for tight-oil sandstones. Fractures, such as intragranular fractures, grain-edge fractures, and transgranular fractures, are abundant in the tight sandstones. Fractures improve storage and permeability and impact distribution of natural gas. 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. ICT has the advantage of performing three-dimensional fracture imaging in a nondestructive way. Fracture networks were quantitatively analyzed using a combination of 2-D slice analysis and 3-D visualization and counting. The core samples were 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) were colored cyan whereas the calcite-filled fractures (high density objects) were colored magenta. The surface area and volume of both open fractures and high density fractures were calculated by 3-D counting. Then, the fracture porosity and fracture aperture were estimated by 3-D counting. The fracture porosity and aperture from ICT analysis performed at atmospheric pressure were higher than those calculated from image logs at reservoir conditions. At last, the fracture connectivity was determined through the comparison of fracture parameters with permeability. Distribution of fracture density and fracture aperture determined the permeability and producibility of tight gas sandstones. Altogether, combined X-ray tomography, image processing help visualize and quantify the complexity and heterogeneity of naturally fractured geological samples in views of applications to integrated reservoir petrophysical and geomechanical characterization.