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Using structurally weak materials to construct blast barriers is a promising approach for blast mitigation because these materials have multiple potential energy-dissipation modes and barely produce secondary hazards. This research experimentally and numerically investigated the blast mitigation effects of different hollow cylindrical barriers using water or/and polyurethane (PU) foam. In view of the characteristics that momentum acquired by the barrier debris might increase the near-field blast loading, a sliding impulse witness device was designed to determine the loadings induced by the air shock waves and debris impact. Six explosive experiments were performed to study the influences of the barrier’s materials and structures in terms of pressure, impulse, structure response, etc. Numerical models corresponding to experiments were established, which can reproduce the dynamic response of the barriers. The experimental results show that among water or/and PU barriers, the protective performance of the PU_water barrier has advantages in comprehensive evaluation, whose mitigation efficiency for a scaled mass is 48% higher than that of the water barrier. The corresponding impulse is significantly lower because the foam is restrained by water, without premature disintegration to form a large debris impact. However, the pressure distribution of different barriers was not improved with the material arrangement, since that the main factor affecting it is the deflection of the blast wave through shadowing effect. The simulation results further revealed that although the dominant energy absorption mechanism and efficiency of the foam can be changed by material arrangement, the energy dissipated by the foam is negligible compared to the heat of explosion, resulting in a minimal effect on the overpressure distribution.

Deep shale gas reservoirs buried underground with depth being more than 3500 m are characterized by high in-situ stress, large horizontal stress difference, complex distribution of bedding and natural cracks, and strong rock plasticity. Thus, during hydraulic fracturing, these reservoirs often reveal difficult fracture extension, low fracture complexity, low stimulated reservoir volume (SRV), low conductivity and fast decline, which hinder greatly the economic and effective development of deep shale gas. In this paper, a specific and feasible technique of volume fracturing of deep shale gas horizontal wells is presented. In addition to planar perforation, multi-scale fracturing, full-scale fracture filling, and control over extension of high-angle natural fractures, some supporting techniques are proposed, including multi-stage alternate injection (of acid fluid, slick water and gel) and the mixed- and small-grained proppant to be injected with variable viscosity and displacement. These techniques help to increase the effective stimulated reservoir volume (ESRV) for deep gas production. Some of the techniques have been successfully used in the fracturing of deep shale gas horizontal wells in Yongchuan, Weiyuan and southern Jiaoshiba blocks in the Sichuan Basin. As a result, Wells YY1HF and WY1HF yielded initially 14.1 × 10 4 m 3 /d and 17.5 × 10 4 m 3 /d after fracturing. The volume fracturing of deep shale gas horizontal well is meaningful in achieving the productivity of 50 × 10 8 m 3 gas from the interval of 3500–4000 m in Phase II development of Fuling and also in commercial production of huge shale gas resources at a vertical depth of less than 6000 m.

Post-fracturing evaluation by using limited data is of great significance to continuous improvement of the fracturing programs. In this paper, a fracturing curve was divided into two stages (i.e., prepad fluid injection and main fracturing) so as to further understand the parameters of reservoirs and artificial fractures. The brittleness and plasticity of formations were qualitatively identified by use of the statistics of formation fracture frequency, and average pressure dropping range and rate during the prepad fluid injection. The composite brittleness index was quantitatively calculated by using the energy zones in the process of fracturing. It is shown from the large-scale true triaxial physical simulation results that the complexity of fractures is reflected by the pressure fluctuation frequency and amplitude in the main fracturing curve, and combined with the brittleness and plasticity of formations, the fracture morphology far away from the well can be diagnosed. Well P, a shale gas well in SE Chongqing, was taken as an example for post-fracturing evaluation. It is shown that the shale beds are of stronger heterogeneity along the extension directions of horizontal wells, and with GR 260 API as the dividing line between brittleness and plasticity in this area, complex fracture systems tend to form in brittleness-prone formations. In Well P, half of the fractures are single fractures, so it is necessary to carry out fine subsection and turnaround fracturing so as to improve development effects. This paper provides a theoretical basis for improving the fracturing well design and increasing the effective stimulated volume in this area.

In the Southeastern Sichuan Basin, the deep shale reservoirs (with vertical depth over 2800 m) are complicated and diverse in reservoir mineral compositions and pore structural characteristics, with the obvious rock plasticity and nonlinear fracturing features and the high absolute difference between maximum and minimum principal stresses, due to the effect of geological setting and diagenesis. Consequently, staged fracturing operations often suffer from high fracturing pressure and propagating pressure, small fracture width, low sand–fluid ratio and fracture conductivity and difficult formation of volume fractures, which seriously influence the post-fracturing shale gas productivity. In this paper, a new combined fracturing mode (pretreatment acid + gelled fluid + slickwater + gelled fluid) and its supporting technologies were developed after a series of analysis and studies on deep rocks in terms of mechanical property, earth stress characteristics, fracturing characteristics and fracture morphology characteristics. Field application shows that geologic breakthrough was realized in Longmaxi Fm of Lower Silurian in Well Dingye 2HF, with absolute open flow (AOF) of 10.5 × 10 4 m 3 /d after fracturing. And it was expected to reach commercial breakthrough in Qiongzhusi Fm of Lower Cambrian in Well Jinye 1HF, with AOF of 10.5 × 10 4 m 3 /d after fracturing. Finally, the following conclusions are reached. First, it is hard to form complex fractures in deep shale and the fracturing technologies applicable for it should be different from those used in mid–deep zones. Second, the established fracturing pressure model can provide an effective way for deep-zone fracturing pressure prediction. Third, reducing operation pressure is one of the key measures to ensure successful deep-zone fracturing. Fourth, besides good material basis, it is crucial to increase the complexity of induced fractures and generate high-conductivity fractures in order to guarantee successful fracturing in deep shale.