Abstract:The South China Sea has been confirmed to be rich in oil and gas resources as well as natural gas hydrate resources. However， hydrate-bearing formations are often encountered during the drilling of oil and gas wells. Cementing is a critical step in oil and gas development. In deepwater drilling， the heat released during cement hydration can potentially induce hydrate decomposition， compromising formation stability and even affecting cementing quality. This study utilized numerical simulation methods， focusing on the hydrate-bearing formation at the SH7 site in the Shenhu area of the South China Sea GMGS-1 project. A numerical model for cementing was established to analyze the issues caused by cement slurry invasion into hydrate-bearing formations and the impact of cementing process parameters. The study found that an increase in cement hydration heat release rate significantly advanced the onset of gas and water influx， as well as increased its volume. The cementing pressure differential had a minor impact on the influx phenomenon， but it suppressed the influx when exceeding a certain threshold. Prolonging the pressure maintenance period significantly delayed the initiation of influx and reduced its volume. Therefore， it is recommended in practical engineering to use low-heat cement， extend the pressure maintenance period， and avoid excessively high cementing pressure differentials in the early stages to minimize hydrate decomposition and mitigate the occurrence of influx. This research provides a theoretical foundation for the cementing of hydrate-bearing formations， which is of great significance for enhancing the safety and efficiency of cementing operations.

Abstract:Natural gas hydrate is a type of solid clean energy with huge reserve， which is therefore considered as a substitute for traditional fossil fuels and have attracted much attention around the world. Due to its occurrence in low-temperature and high-pressure marine and permafrost environments， the key to achieving commercial exploitation is to find out economical and efficient exploitation methods. Based on the current research status of laboratory research， numerical simulation， and on-site experiments， the exploitation effects of methods such as depressurization， heat injection， chemical inhibitor injection， CO2 displacement and combination method were analyzed， and the advantages and limitations of each method were discussed. The existing exploitation methods are mainly limited by low reservoir permeability and poor thermal conductivity， and have not been able to achieve long-term continuous gas production. To address the above issues， the in-situ resistance heating method for reservoirs is proposed to improve thermal utilization efficiency， and it is believed that hydraulic fracturing and permeability enhancement technology is an effective measure to increase reservoir permeability and assist in efficient gas production through production methods such as depressurization； Regarding the potential instability of reservoirs caused by hydrate exploitation， it is believed that the use of CO2 replacement method can strengthen the reservoir， and the use of supercritical CO2 injection technology can improve the CO2 replacement rate.

Abstract:Natural gas hydrates is a new type of energy， characterized by huge reserves， cleanliness and high efficiency. China is rich in hydrate resources， and the safe and efficient exploitation of hydrates can help realize the transformation of energy structure and the strategic goal of “double carbon”. Hydrate reservoirs are generally characterized by non-rock formation， weak cementation and strong temperature and pressure sensitivity. The application of water jet technology to hydrate drilling process can effectively solve the engineering problems of well borehole collapse and formation destabilization caused by conventional methods. This paper summarizes the existing water jet drilling hydrate technology and water jet drilling tools， and analyzes the influencing factors and breaking mechanism of hydrate-containing sediment breaking process under the impact of water jet. Finally， the research direction for the application of water jet technology in the hydrate drilling process is prospected. This work can provide support for the research of natural gas hydrate development technology in China.

Abstract:The shape memory polymer sand control system has been successfully applied in the second phase of natural gas hydrate production in Japan. However， the effects of the shape memory on the pore-permeability characteristics and sand control performance of polymers is rarely researched. Therefore， in this paper， a shape memory polymer sand control material taking the polyurethane as the matrix is developed. The polymer’s shape memory characteristics as well as the changes in pore-permeability and mechanical strength before and after the shape memory process are investigated and the sand control performance is also evaluated. The results revealed that the developed porous polyurethane shows excellent shape memory characteristics and pore-permeability properties： the shape fixed rate exceeded 98%， the shape recovery rate reached 100%， the shape memory temperature was 59.8°C， the permeability remained at approximately 10D before and after the shape memory process， and the compressive strength was maintained at around 1MPa. The mercury injection tests indicated that the shape memory process is a compression-rebound process for the polyurethane material with large pores， which will lead to some changes in the internal structure of the material， resulting in a slight decrease in compressive strength and a small increase in permeability after complete shape recovery. The sand control performance tests indicated that a small amount of sand production occurred only in the initial test， and the presence of sand particles caused some damage to the permeability of the polyurethane material， however， the permeability could still be maintained at around 10 D for the fully recovered polyurethane. According to the comprehensive performance analysis of the above materials， the developed shape memory polyurethane material can be used in conjunction with a mechanical sand control screen pipe， which could meet the requirements of sand control in hydrate production wells.

Abstract:To address the issue of wellbore instability resulted from hydrate decomposition during offshore drilling for natural gas hydrate， this paper developed a dual-purpose additive which possess both hydrate decomposition inhibition and filtration reduction properties. It was synthesized using 2-acrylamido-2-methylpropane sulfonic acid， dimethyldiallyl ammonium chloride and dimethoxy methylvinylsilane-modified cellulose as the starting materials （named as “CAD”）. The molecular structure was characterized through infrared spectroscopy. Thermal analysis revealed that the product decomposition initiated at around 290°C， demonstrating commendable thermal stability. The hydrate decomposition evaluation experiments indicated that the presence of 1% of the dual-function additive extended the total decomposition time of hydrates by approximately 1-fold and reduced the hydrate decomposition rate by 19.8%， which highlights the outstanding performance of hydrate decomposition inhibition. Filtration in freshwater-based slurry and 5 wt% NaCl saline-based slurry measured to be 6.8 and 8 mL respectively， signifying effective filtration reduction capabilities for hydrate reservoirs. The filtration for the freshwater-based slurry amounted to 6.5 mL at low temperatures， indicating the dual-function additive has favorable performance of low-temperature rheology. All these research offers substantial support for the advancement of high-performance drilling fluids designed for natural gas hydrates.

Abstract:Natural gas hydrate is an efficient and clean energy， which is widely distributed in the sedimentary strata in the South China Sea. China has successfully carried out two trial productions in 2017 and 2020 respectively. However， due to the special occurrence conditions of marine natural gas hydrate， there are still some problems with single well trial production， such as small production range and short time for high and stable production. In order to improve the exploitation range of hydrate， the two-dimensional hydraulic fracturing numerical model is studied based on cohesive units. Through simulation， the half length and width of cracks of the two models 100m×100m and 20m×20m are compared. It is concluded that at the injection pressure of 30MPa， the half length of the crack is 6m for both models， and the maximum width is 5.8mm and 5.5mm respectively. The more accurate experimental results can be obtained by constructing a larger model. Moreover， the variation law of fracture width with injection time is studied. With the continuous increase of injection pressure and injection volume， the initial fracture width increases rapidly， and then the fracture propagates “step by step” under the action of in-situ stress and injection fluid pressure. The research has been successfully applied in the hydraulic fracturing model analysis of unconventional energy reservoirs such as shale gas， coalbed methane， and provides some technical guidance for marine natural gas hydrate reservoir hydraulic fracturing .

Abstract:The exploration and exploitation of natural gas hydrate need to obtain the pressurized core from the formation for physicochemical property testing， reserve evaluation and mining technology research. However， in the process of drilling， gas hydrate is easily decomposed with the change of temperature and pressure， which brings great difficulties to the core drilling of hydrate. In order to solve the pressure maintaining reliability problem of gas hydrate pressure maintaining coring drill， the project team developed a new type of gear and rack closed ball valve for gas hydrate pressure maintaining coring drill. The ball valve was driven by lifting force of rope fishing. The structure and working principle of the pressure maintaining core drill are introduced in detail. The specific pressure of working seal of ball valve and lifting force required for fishing inner pipe are calculated. The indoor test and sea test of the pressure maintaining core drill are introduced. The research shows that the hydrate pressure retaining coring drill has the advantages of simple coring process， good reversing and sealing performance of ball valve， high success rate of coring， and all functions and performance indexes meet the design requirements.

Abstract:Obtaining in-situ natural gas hydrate reservoir cores through drilling and coring， and conducting pressure transfer and testing to obtain parameters of core physical， chemical， and mechanical properties， is one of the key technical methods for conducting marine natural gas hydrate exploration work. This paper summarizes the relevant data of natural gas hydrate core pressure retaining transfer system at home and abroad， and makes a comprehensive summary from the aspects of working principle， structural characteristics and test application， systematically reviews the research and development status of natural gas hydrate pressure retaining transfer and test system， and compares the typical core pressure retaining transfer and test system at home and abroad from the aspects of compatibility and key parameters. Some suggestions for the research and development of pressure retaining transfer system of natural gas hydrate in China are put forward

Abstract:The depressurization method is a common approach for the exploitation of natural gas hydrate reservoirs in marine areas. This method can induce complex multi-physical coupling responses in the near-wellbore reservoir， leading to pressure changes， temperature variations， hydrate decomposition， deterioration of reservoir mechanical properties， and formation subsidence. This study utilizes a fully-coupled hydro-thermo-mechanical numerical model to analyze the mechanical property deterioration and subsidence characteristics of marine natural gas hydrate reservoirs caused by depressurization in horizontal wellbores， characterizing the multi-field coupling response laws of the horizontal wellbore and surrounding reservoir， and identifying the influencing factors of reservoir mechanical property deterioration and subsidence. The simulation results indicate that the affected area of reservoir pressure and temperature changes is much larger than the decomposition front of hydrates， and the distribution of effective normal stress varies significantly in different directions. The deterioration area of cohesion induced by depressurization is highly correlated with the plastic zone and hydrate decomposition zone. The subsidence characteristics in the shallow and deep areas relative to the horizontal wellbore exhibites distinct features. The simulation results provide a reference for the stability analysis of marine natural gas hydrate reservoirs during depressurization exploitation through horizontal wellbores.

Abstract:The gas hydrate reserve is abundant in the Shenhu Sea area， which provides a large amount of energy reserve for our country， and the problem of the shortage of energy can be solved effectively by means of efficient exploitation. Inhibitor injection method is one of the main methods to extract gas hydrate. Methanol is an excellent inhibitor with good inhibition and low viscosity. In this paper， a three-dimension， three-phase and four-component numerical model for gas hydrate exploitation in the sea area was established according to the actual geological parameters. The dynamic characteristics of hydrate exploitation in hydrothermal methanol solution were studied by means of numerical simulation using the horizontal well layout method with production wells on both sides and the injection well in the middle. The extraction effects of single depressurization method and hot water injection method were compared with that of the methanol inhibitor injection and the results show that the later can increase the reservoir temperature and promote the decomposition of hydrate， which improves the shortcomings of the former two methods. It has higher initial gas production rate， higher initial gas-water ratio and higher hydrate decomposition effect， which is a competitive exploitation method.