Deep water shallow natural gas hydrate (NGH) is a kind of clean energy and has entered the commercial exploitation stage. However, it produces a lot of seabed sediment in the process of large-scale mining, which not only easily causes undersea natural hazards, but also leads to pipeline equipment blockage and high energy consumption in the mining process. A downhole solid–liquid separator can effectively separate natural gas hydrate from sand and backfill sand in situ, which can effectively solve this problem. In this paper, the safety of a downhole solid–liquid separator desander under torsion conditions is determined by a test method. A numerical simulation method was used to simulate the tension and pressure of the downhole solid–liquid separator, and a modal simulation analysis and erosion analysis of the downhole solid–liquid separator were carried out. The experiments showed that the downhole solid–liquid separator could withstand 30 KN/m of torque, and a numerical simulation analysis showed that it could withstand 30 MPa of pressure and 50 KN of tension. The results show that the maximum stress is 116.56 MPa, and the maximum allowable stress is 235 MPa. The modal analysis showed that the downhole solid–liquid separator produces resonance at a frequency of about 93 Hz, resulting in large deformation, which should be avoided as far as possible. Through the erosion analysis, the life of the downhole solid–liquid separator was determined to be about 2.3 years. Numerical simulation and experimental results show that the designed downhole solid–liquid separator for natural gas hydrate can ensure safety.

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Aiming at the problems of pipeline blockage and equipment wear caused by large sand production in shallow gas hydrate mining on the seabed, based on the solid-state fluidization mining method, the idea of in situ separation of natural gas hydrate is proposed, and the downhole design is based on the physical parameters of hydrate mixed slurry. For the in situ helical separator, the CFD-Fluent commercial software was used to establish an analysis model and optimize the response surface of the model. The effects of the three-stage variable-pitch helix and blades on the performance of the separation device were investigated. The simulation results and response surface optimization were conducted through experiments to verify the accuracy. The results show that the third-stage pitch has the greatest impact on the separation efficiency and pressure drop, while the first-stage pitch has the least impact. The pressure drop and separation efficiency are fully considered. After the response surface optimization, the optimal three-stage pitch is the first-stage pitch x1 = 72.227 mm, the pitch of the second stage x2 = 105 mm, and the pitch of the third stage x3 = 124.817 mm. The separation efficiency of the optimal structure is verified by experiments. Compared with the previously used fixed-pitch downhole cyclone separator, the three-stage variable pitch cyclone, the separator improves the separation efficiency from 88.29% to 97.16% while keeping the pressure drop unchanged.

Oily sludge is recognized as hazardous waste. To reduce the potential danger and harmful factors of oily sludge, it is very important to analyze its environmental risk. In this paper, the characterization of oily sludge from Shengli Oilfield in China was tested experimentally, including the composition content, particle size, microscopic morphology, heavy metal content, organic composition, inorganic composition, and thermogravimetric analysis, which were used to analyze environmental risks. The results show that the oil content of oily sludge is as high as 10.3%, which will cause serious pollution. It is calculated that China can recover 772.5 million liters of oil and reduce 553.9 million kg of carbon emissions compared with incineration in one year, if the oily sludge can be managed effectively. The content of heavy metals such as Ba, Zn, Cr, As, Ni, Se, Be, and Hg in oily sludge exceeds the standard. It will restrain the self-healing ability of soil, pollute groundwater, and endanger animals and plants. The organic matter of oily sludge is concentrated in C11 to C29. It contains a large amount of benzene series and polycyclic benzene hydrocarbons, which can lead to cancer in the human body. Inorganic substances in oily sludge are mixed with some additives, which can not only reduce the toxicity of heavy metals, but also be used as building materials. The median particle size D50 of oily sludge is 0.91 μm, and it spreads all over the narrow pores. Generally, it needs to be treated under high temperature conditions, which will cause secondary pollution to the environment. The research content of this paper provides a theoretical reference for the management of oily sludge.