Riserless mud-recovery (RMR) drilling technology was widely applied in recent years. Compared with traditional deepwater drilling, RMR drilling can improve drilling efficiency, reduce risks, and minimize environmental effects. This paper focuses primarily on the stability of a mud-return circulation system in an RMR system. First, various factors that affect the stability of a mud-return circulation system are analyzed. An analytical model for the skid-and-mud-return line is established. Second, relevant data are derived from theoretical calculations and experiments. ABAQUS software is used to analyze the effects of each factor on the stability of the mud-return circulation system. The influencing patterns of each factor on the stability of the mud-return circulation system are summarized. Furthermore, the stability of the system under different operating conditions is analyzed based on the coupling of multiple factors. The support vector regression with derivative significance weight analysis (SVR-DWSA) algorithm is employed to perform a weight analysis of the effect on the system’s stability. Finally, based on the research findings on the stability of the mud-return circulation system, relevant conclusions and recommendations are drawn. The results of this study provide valuable references for the application of RMR technology.

Deepwater natural gas hydrate (NGH) exploration faces environmental and economic challenges due to the conventional seabed discharge of drilling mud. To address this, we developed an improved riserless mud recovery (RMR) system using hybrid risers to create a stable, decoupled circulation pathway. This innovative design fulfils three key operational needs: (1) closed-loop drilling fluid circulation with dual-gradient control, (2) completion fluid and test production returns, and (3) mechanical decoupling between the floating platform and subsea riser string. Numerical analysis shows that the system maintains operational stability under wave loading while effectively isolating seabed equipment from platform motion. Simulation results indicate that optimal performance requires a top tension ratio exceeding 1.5 and a submersion depth of 150–250 m, achieving a 70–80% reduction in drilling fluid consumption with full recovery capabilities. The hybrid riser-based RMR system provides a practical solution for sustainable NGH development, offering superior water depth adaptability for both drilling and completion operations in deepwater environments.

Riserless Mud Recovery (RMR) technology, as an emerging and efficient drilling method, is advantageous to reduce the shallow flow hazards and the number of casings. The wave current effect is one of the reasons limiting the application of RMR technology in deep and ultra-deep water, and fewer quantitative and qualitative analyses of the effect of the current are made on the stability of conductors. This paper investigates the influence of the overturning moment generated by the continuous subsea internal wave flow and the soil resistance to the conductor. The numerical simulation software ABAQUS is used to study the effects of sea state recurrence period, seabed soil properties, conductor material, driving depth in the mud, and conductor wellhead height on the stability of the conductor, and the influence weights of the factors affecting the stability of the conductor are analyzed using the weight analysis algorithm of extreme learning machine-mean impact value (ELM-MIV). Finally, the qualitative and quantitative analyses affecting the stability of the conductor are carried out, which provide reference values for the application of the RMR technology.

The degree and amount of sand production in hydrate reservoirs is related to the selection of stable production processes, but there is currently a lack of quantitative sand production prediction research using real logging data and formation samples from hydrate reservoirs. To reveal the dynamic change characteristics of in-situ reservoirs during hydrate decomposition, and explore quantitative prediction methods for guiding production practice, it is conducted a series of numerical simulations and quantitative prediction experiments. The numerical simulations are carried out using different sand-out prediction methods by using hydrate logging data during drilling, while quantitative prediction experiments of water production and sand-out are carried out based on in-situ reservoir samples. Our experiments indicate that hydrate mining is facing a serious risk of sand-out. The particle transport in the reservoir changes from “large-channel seepage” to “umbrella seepage” and then to “uniform fine flow” as the replacement flow rate decreases. A quantitative prediction model for water and sand production is also established. As a result, our study can provide support for the advancement of technology for long-term stable production and sand control of hydrates, laying the groundwork for developing a stable production plan for natural gas hydrates in offshore areas and determining the optimal depressurisation method.

Natural gas hydrate is widely distributed, shallow-buried, clean, and pollution-free and has enormous reserves, it is regarded as the alternative clean energy source in the oil and gas field with the most potential. Pressure coring is the only way to drill for gas hydrate core on the surface under in situ conditions, which is of great value for analyzing its occurrence conditions and reserves comprehensively. Based on this, a new wireline pressure coring system (WPCS) with a ball valve seal was designed and developed in this paper; it was applied in the deep sea for the first time in the South China Sea hydrate survey voyage of the Guangzhou Marine Geological Survey (GMGS). A total of 15 runs of deep-sea gas hydrate drilling and coring applications were carried out, and they tested well. The experimental water depth was 1700–1800 m, and the coring depth below the seafloor was about 100–150 m. The formation consisted of sandy hydrate and argillaceous hydrate. The results showed the following. (1) The success rate of ball valve turn-over could reach almost 100% in the argillaceous hydrate reservoir, although there are some isolated cases of pressure relief. Meanwhile, drilling in the sandy hydrate reservoir, the success rate was only 54.55%. (2) When drilling in the argillaceous hydrate reservoir, the core recovery rate could reach 80%, while in the sandy hydrate reservoir, it was almost 0%. In practice, the sandy formation with gas hydrate is stiff to drill compared to the performance in argillaceous formations. After our analysis, it was believed that the ball valve and core tube could be easily plugged by sand debris during the sampling of sandy hydrate formation. Moreover, the sandy core is easily plugged into the core liner because of the high friction of sand grains in clearance. (3) The pressure-holding effect of the core drilling tool was related to the formation of hydrate, the sealing form of the ball valve, and the environmental pressure. Sandy hydrate formations often caused the ball valve to jam, while the muddy hydrate formation did not. The research results of this paper have reference value for the further optimization of the WPCS structure, the optimization of drilling parameters, and the design parameters of the ball valve structure, which could be better used for the pressure coring of gas hydrate and subsequent research work in the future.

With the steady progress of the global energy transition and the pursuit of “dual carbon” goals, Offshore Carbon Sequestration (OCS) has emerged as a pivotal strategic pathway within Carbon Capture and Storage (CCS) initiatives aimed at mitigating climate warming. Nevertheless, the drilling of OCS injection wells faces severe challenges, including narrow geological pressure windows, high risks of shallow geohazards, stringent environmental protection standards, and prohibitive construction costs. Riserless Mud Recovery (RMR) technology, as a novel and eco-friendly deepwater drilling technique, provides innovative technical support for OCS by establishing a closed-loop seafloor circulation system that achieves dual-gradient pressure control and “near-zero discharge” of drilling fluids. This paper systematically reviews the development history and technical principles of RMR. By integrating the specific requirements of OCS injection well drilling—such as wellbore integrity, environmental protection, and shallow hazard mitigation—the study provides an in-depth analysis of the application potential of RMR in drilling CO₂ injection wells within shallow formations. Furthermore, it demonstrates the engineering feasibility of RMR across technical, environmental, and economic dimensions. Building on this analysis, the paper discusses current technical challenges regarding key equipment research and development, adaptability to complex operating conditions, enhancement of intelligent control systems, and the establishment of technical standards. It also outlines the prospects for the integrated development of RMR with emerging fields, including hydrate-based carbon sequestration, intelligent drilling and completion, and carbon sequestration in far-reaching deep-sea areas. The research indicates that RMR technology can effectively resolve the dual constraints of cost control and environmental protection in OCS drilling. With breakthroughs in critical hardware, such as high-displacement subsea lift pumps, and the deepening of cross-disciplinary integration, RMR is poised to become an essential technical pillar in the field of offshore carbon sequestration.