For the detection of deep-sea natural gas hydrates, it is very important to accurately detect the ¹³CO₂/¹²CO₂ isotope ratio of dissolved gas in seawater. In this paper, a ¹³CO₂/¹²CO₂ isotope ratio sensor is investigated, which uses a tunable diode laser absorption spectroscopy (TDLAS) technique at 4.3 μm. The proposed sensor consists of a mid-infrared interband cascade laser (ICL) operating in continuous wave mode, a long optical path multi-pass gas cell (MPGC) of 24 m, and a mid-infrared mercury cadmium telluride (MCT) detector. Aiming at the problem of the strong absorption intensity of the two absorption lines of ¹³CO₂ and ¹²CO₂ being affected by temperature, a high-precision temperature control system for the MPGC was fabricated. Five different concentrations of CO₂ gas were configured to calibrate the sensor, and the response linearity could reach 0.9992 for ¹²CO₂ and 0.9996 for ¹³CO₂. The data show that the carbon isotope measurement precision was assessed to be 0.0139‰ when the integration time was 92 s and the optical path length was 24 m. The sensor is combined with a gas–liquid separator to detect the ¹³CO₂/¹²CO₂ isotope ratio of CO₂ gas extracted from water. Results validate the reported sensor system’s potential application in deep-sea natural gas hydrate exploration.

Enhancing the production capacity of natural gas hydrates (NGHs) is critical for its commercial development. Complex structure wells may efficiently increase drainage areas while enhancing exploitation efficiency. Based on the field data of China’s first offshore NGH test production, the numerical method was used to analyze the production performance of different complex structure well types by continuous depressurization production for 360 days under the preconditions of fixed effective completion length of 300 m and a pressure difference of 6 MPa. Results indicated that the complex structure well types deployed at the three-phase layer demonstrated superior production performance within 240 days of production; the DLW2 and HW2 well types stood out, with an average gas production rate Q_g reaching 43,333 m³/d and a specific production index J of 24.1. After 360 days of production, benefiting from multi-layer combined production, the Cluster vertical well deployed at the multi-layer had the best production performance, with an average Q_g of 34,444 m³/d and a J-index of 19.1. The research results provided insights into the complex structure well-type selection strategy for NGH depressurization in this sea area.

The concentration of dissolved carbon dioxide in seawater affects the formation of seabed biogenic minerals, and controls the development and evolution of marine organisms, which is one of the most important indices in the marine environment and geology survey. The concentration of free carbon dioxide in bottom seawater has important academic value for the exploration of seabed minerals, marine environment monitoring, the study of the carbon cycle in seabed sediments and even the global carbon cycle. Titration is used to determine the index, whose range is between 4 mg/L and 400 mg/L. However, the concentration of free carbon dioxide in bottom seawater in some sea areas is lower than this range, thus titration may not be suitable. A method for the determination of free carbon dioxide in artificial seawater, whose name is D.B.S/Henry’s Law, is described in this paper. D.B.S/Henry’s Law indirectly determines the content of free carbon dioxide in seawater instead of directly titrating with reagents. It is simple, fast and accurate, and it is based on the law of conservation of mass/Henry’s law and CO₂ dissolution mechanisms. This discovery mainly provides a new scientific perspective for the determination of trace free carbon dioxide concentration in seawater. In addition, Henry’s law constants of carbon dioxide in artificial seawater at 3–20 degrees Celsius are determined with D.B.S/Henry’s Law. These data will be used as an important reference for the determination of free carbon dioxide in seawater in the future. D.B.S/Henry’s Law may be used as a supplement of the methods specified in national and industrial standards, which are applied to the detection of carbon dioxide Henry’s constant and free carbon dioxide content in various temperature and salinity seawater. D.B.S/Henry’s Law provides a new perspective for CO₂ determination and has extensive practical application value.

Stepwise depressurization is an important depressurization strategy in the development of natural gas hydrates. This work numerically analyzes the effects of different depressurization gradients and constant pressure durations on gas and water production during stepwise depressurization extraction with a vertical well in the Shenhu Sea area hydrate reservoir of the South China Sea. The results indicate that stepwise depressurization can reduce water production and raise the gas-to-water ratio in the early stages of production while ensuring cumulative gas output. When the vertical well is deployed at the model’s center with a completion length of 70 m and a constant pressure duration of 10 days, a depressurization gradient of 0.5 MPa, stepwise depressurization by 6 MPa, and continuous production for one year is achieved. Compared with direct depressurization, its cumulative gas production is 2.966 × 10⁶ ST m³, which only decreases by 2.94%. However, it maintains a higher gas-to-water ratio in the early stages of production. Considering factors such as engineering operability, cumulative gas output, and gas-to-water ratio, it is recommended to use a small pressure gradient and a medium constant pressure stabilization time for stepwise depressurization Stepwise depressurization can maintain a high gas-to-water ratio while ensuring gas production and reducing water production can alleviate sand production problems and improve economic efficiency. The understanding gained from this work has reference value for the development of similar hydrate reservoirs worldwide.

A methane (CH₄) sensor based on off-axis integrated cavity output spectroscopy (OA-ICOS) was developed, equipped with two measurement schemes: direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). The sensor used an optical resonant cavity composed of two high reflection mirrors (reflectivity > 99%). With a cavity length of 7 cm, an effective optical path length of 10.8 m and a cavity volume of 8.9 mL were achieved. A distributed feedback laser was used to precisely target the CH₄ absorption line near 1.6537 µm. Compared with the original system, the cavity mode noise of the CH₄ sensor was further reduced by adding white noise perturbations. The white noise perturbations were generated by the broadband random noise from the signal generator. The special customized narrowband RF noise source was not required. The system complexity and cost could be reduced. In DAS mode, the signal-to-noise ratio (SNR) of the OA-ICOS was 16.2 and the minimum detection limit (MDL) was 2.2 ppm at 117 s. In WMS mode, the SNR of the OA-ICOS was 113.9 and the MDL was 1.2 ppm at 106 s. Compared with the results obtained from the WMS mode and DAS mode, the SNR and MDL was improved 7.0 times and 1.8 times, respectively. The proposed sensor system not only enabled high-accuracy trace gas measurement, but also demonstrated strong potential for applications due to its compact design and low cost.

In situ electric heating is an important method used to increase production capacity during the extraction of natural gas hydrates. This work numerically evaluated the sensitivity of different heating parameters on gas production behavior with a vertical well depressurization in the Shenhu Sea area hydrate reservoir, the production pressure difference of 4 MPa, and continuous depressurization for 1080 days. The results showed that the in situ electric heating method can effectively enhance production capability by promoting hydrate dissociation and eliminating secondary hydrates. Compared with scenarios without heating, implementing whole wellbore heating (100 W/m) increases cumulative gas production (V_g) by 118.56%. When intermittent heating is applied to the local wellbore (15 m) located in the three-phase layer (with an interval of 30 days) and stops heating in advance at 480 days, there is no significant difference in V_g compared to the whole wellbore heating case, and the cumulative heat input is only 4.76%. We recommend considering intermittent heating of the local wellbore and stopping heating in advance during vertical well depressurization as this approach significantly reduces heating energy consumption while simultaneously improving production capacity.