Branching structure is often used as a supporting structure of the grid shell due to its geometrical and force-transferring features, and the rationality of its shape is very important. The “physical” and “numerical” hanging models can be used for the joint form-finding of the branching structure and free-form grid shell. However, slack elements may exist in the equilibrium model which corresponds to the inefficient members in the form-found branching structure. To solve this problem, a form-finding method of branching structure based on dynamic relaxation is proposed in this study. The proposed method clusters the elements of the branching model and equalizes the axial forces of the elements in the same cluster, in other words, there are no slack elements in the equilibrium branching model. This method overcomes the defect that the equilibrium branching model may have slack elements and needs many manual adjustments during the procedure of determining the rational shape of a branching structure, and effectively prevents the inefficient members existing in the form-found structure. Numerical examples are provided to demonstrate the characteristics of the proposed method and its effectiveness is verified as well.
The exploitation of challenging oceanic gas hydrate reservoirs with low permeability and permeable boundary layers faces the challenges of methane leakage and low production. Considering this aspect, a novel five-spot injection–production system combined with hydraulic fracturing was proposed. In particular, the potential of this development mode, including hydrate dissociation, gas production, and gas capture, was evaluated in comparison with a three-spot injection–production system. The results showed that increasing the fracture conductivity cannot prevent CH4 leakage in the three-spot, and the leakage accounted for 5.6% of the total gas production, even at the maximum fracture conductivity of 40 D·cm. Additionally, the leakage amount increased as the well spacing increased, and the leakage accounted for 36.7% of the total gas production when the well spacing was 140 m. However, the proposed development mode completely addressed CH4 leakage and significantly increased gas production. The average gas production rate reached 142 m3/d per unit length of the horizontal section, which was expected to reach the commercial threshold. The variance analysis indicated that optimal plans for the challenging hydrates in the Shenhu area were well spacing of 100–120 m and fracture conductivity greater than 20 D·cm.
Fine-grained natural gas hydrate (NGH) reservoirs are widely distributed across the world and bear more than 90% of global NGH. However, it is difficult to exploit this kind of NGH reservoir economically and environmentally using conventional methods. Water-jet cutting is an efficient and environmentally friendly technique for mining such hydrate reservoirs, as the production process does not depend on mass and heat transfer within the formations. In this work, a series of physical experiments were conducted to clarify the erosion performance of marine hydrate-bearing sediment (HBS) impacted by water jets. The results show that the accumulation of sediment particles and hydrate particles at the bottom of erosion hole severely inhibits the vertical erosion of HBS by water jet. For a particular jet flow rate, the jet distance has an optimal value, which is between 4 mm and 28 mm. Moreover, the upwelling flow containing solid particles has a significant impact on the erosion of the hole top. In reservoirs with a low hydrate saturation (20–40%) and reservoirs with a high hydrate saturation (60–80%), the erosion holes exhibit a gourd shape and a bamboo shape, respectively. In addition, the volume erosion efficiency and the depth erosion efficiency are more sensitive to the variation in jet flow rate than jet distance and hydrate saturation. This study can provide theoretical and technical support for the application of water-jet cutting in the exploitation of marine HBS.
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