Hydraulic fracturing characteristics and evaluation of fracturing effectiveness under different anisotropic angles and injection rates: An experimental investigation in absence of confining pressure

The transport properties of clay-rocks are controlled by their complex pore network and the reactive and negatively charged clay mineral surfaces. The Pore Size Distribution (PSD) and the geometrical features of the pore network constitute key parameters governing the migration of solvate elements through the clay formations. Upon the last few years, some works have contributed to improve the knowledge of some clay-rich rock pore networks by using innovative microscopic imaging techniques (FIB-nt, BIB, X-ray…). Even if these techniques allow imaging the mesostructure scale organization of clay-rocks and clay materials, these studies have however highlighted several limitations to fully represent the spatial distribution of pores in terms of PSD, geometry and connectivity. In the present work, we demonstrate that a multi-scale characterization of pore network of clay materials based on FIB-nt and TEM techniques is feasible and could be robustly achieved for providing some useful input data in order to assess a future modeling task. The studied material was a compacted clay materials composed of illite, a non-swelling mineral. Illite was chosen in order to represent an analog system mimicking the clay matrix of the clay-rocks. This compacted clay plug allows working on homogeneous system with a controlled porosity, facilitating the comparison between different methods. All the techniques used in the present work whether bulk or microscopic methods show that all results converge to a similar and entire PSD (figure 1). The main improvement of this work was to demonstrate how the usual resolution limitations and the data handling of the FIB-nt techniques could be improved in order to recognize a fully connected pore network.

Optimization of pore network characterization of compacted clay materials by TEM and FIB/SEM imaging

Sensitivity analysis on the effect of natural fractures and injected fluid on hydraulic fracture propagation in a fractured reservoir

Fe3O4@ZnO nanocomposites (NCs) were synthesized to improve the stability of the wormlike micelle (WLM) network structure of viscoelastic surfactant (VES) fracturing fluid and were characterized by Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Then, an NC-enhanced viscoelastic surfactant solution as a fracturing fluid (NC-VES) was prepared, and its properties, including settlement stability, interactions between NCs and WLMs, proppant-transporting performance and gel-breaking properties, were systematically studied. More importantly, the influences of the NC concentration, shear rate, temperature and pH level on the stability of NC-VES were systematically investigated. The experimental results show that the NC-VES with a suitable content of NCs (0.1 wt.%) shows superior stability at 95 °C or at a high shear rate. Meanwhile, the NC-VES has an acceptable wide pH stability range of 6–9. In addition, the NC-VES possesses good sand-carrying performance and gel-breaking properties, while the NCs can be easily separated and recycled by applying a magnetic field. The temperature-resistant, stable and environmentally friendly fracturing fluid opens an opportunity for the future hydraulic fracturing of unconventional reservoirs.

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Experimental Study on the Stability of a Novel Nanocomposite-Enhanced Viscoelastic Surfactant Solution as a Fracturing Fluid under Unconventional Reservoir Stimulation

3D natural fracture model of shale reservoir based on petrophysical characterization

Numerical simulation of hydraulic fracturing process in a naturally fractured reservoir based on a discrete fracture network model

Pores of shale exhibit multiscale characteristics, and pore characterization is challenging due to the complexity of pore systems. Currently, research is focused on nano-submicron pores, but the structure of micrometer-scaled pores is not well understood. In this research, an investigation of the three-dimensional pore network of the Chang 7 shale in the Ordos Basin of China was conducted, in order to provide an insight into the full characteristics of pore systems. Nano-CT and micro-CT scanning technology was used to comprehensively delineate the pore structure at different scales, for further understanding the gas storage mechanism in shale rocks. Results show that the radius of micro-scale pores ranges from 1 to 15 μm, with an average of 2.8 μm, and pores with radii of 1–5 μm occupy approximately 90% of all the pores. For the nano-scale pores, the size ranges from 86 to 2679 nm, with an average of 152 nm, yet it has a rather concentrated distribution within 300 nm. The nano-scale pores constitute most of the pore amount in the shale, whereas the micro-scale pores constitute most of the pore volumes. Moreover, the results show that more than 70% of nano-scale pores in the Chang 7 shale are isolated pores, indicating that pore bodies formed in the shale reservoir have poor connectivity. Positive linear relationships between pore sizes and the number of pore throats at the micro-scale and nano-scale were both obtained, suggesting that larger pores tend to have better connectivity than smaller pores.

Zhihong Zhang

Papers

Numerical simulation of hydraulic fracturing process in a naturally fractured reservoir based on a discrete fracture network model

Opportunities in Measuring Multiscale Pore Structure of the Continental Shale of the Yanchang Formation, Ordos Basin, China