| Natural gas hydrate is a kind of gas hydrate, ice alike crystalline compound composed of water and natural gas molecules with cage structure. Because of its appearance and could ignite with fire, so it is also called "flammable ice","solid gas" or "gas ice". The molecules of natural gas including hydrocarbon (CH4、C2H6、C3H8、C4H10and homologous series) and non-hydrocarbon gases (CO2、N2、H2S etc.), occupied the cavities in water cages. Since methane is the main component of natural gas, the natural gas hydrate with more than99%of methane molecules is commonly referred to as methane hydrate. Gas hydrate is a kind of metastable material, can only be stable under the condition of high pressure and low temperature, mainly exist in bottom sea continental shelf sediments, arctic permafrost, and widely distributed in the slope zone of mainland and islands, uplift of active and passive continental margin, arctic continental shelf and some deep water environment of ocean and inland lake, and even be found in outer space.Since the natural gas hydrates were found in permafrost and seafloor in1960s and1980s respectively, its significance in resource, environment and global warming has evoked the attention of governments, petroleum giants and academic institutions. The study on gas hydrates including flow assurance, drilling operation safety, geological hazard, carbon cycle and climate change, etc., has become a hotspot in earth science and energy research. The level of natural gas research is a reflection of the comprehensive science and technology and potential of sustainable development of a nation to some extent.Studies indicated that the future research of gas hydrate have transformed from the basic research and development plan to applied research such as comprehensive gas hydrate exploitation. The United States has successfully implemented hydrate research program, and owns offshore and Alaska permafrost that have abundant hydrate distribution, is one of the leading countries in gas hydrate research, and have planned to exploit gas hydrate in north slope of Alaska permafrost before2015, and in marine sediments before2025. In addition, Japan, Canada, Germany, Britain, Russia, Norway, South Korea, India, China and other countries have also carried out a large amount of hydrate research. Japan has also planned to commercially exploit gas hydrate before2018. Nigeria, Spain, Chile, Colombia, Peru and New Zealand also showed a strong interest in hydrate research.Preliminary explorations show that the hydrate resource is abundant in China, and the hydrate research has important strategic significance in meeting the energy demand of the country. The gas hydrate research program has been listed in the major project of large scale oil-gas filed and coal bed in the "national medium and long-term (2006-2020) science and technology development plan" in China. In accordance with the macroeconomic policy, the gas hydrate research related scientific activities including investigation, technical system, experimental simulation and environmental and climate issues have increased significantly. Natural gas hydrate has become an important research focus of energy science, and deeply involved in the climate, environment, high technology and global sustainable development.Gas hydrate exploration is the main source of data and support for basic research, such as physical property of hydrate reservoir, geological accumulation and application research including resource evaluation and production technique of gas hydrate. The prospecting methods applied to the hydrate formation are categorized to geologic, geophysical, geochemical and coring drilling. Among them, the coring drilling is the most direct means of exploration, the samples of in-situ hydrates can be directly obtained from hydrate bearing formation for analysis, whereas the geophysical well logging is the method that most close to the in-situ state of hydrate formation when coring is difficult. Therefore, these two methods play an important role in gas hydrates exploration, and neither could be done without drilling.Usually marine hydrate formation is in poor degree of consolidation, if it is drilled by underbalanced drilling methods, is not conducive to borehole stability. Hydrate decomposes under reduced pressure, which also significant reducing the strength of formation, leads to severe borehole wall instability. Therefore, it is preferable to maintain borehole pressure greater than the formation pore pressure while less than fracture pressure during the process of hydrate drilling operation. Under the condition of overbalance, the drilling fluid (i.e. the water based drilling fluid) displaces the original pore fluid (gas and water) around the borehole wall and invades the formation by the pressure differential. The practice of drilling operation indicates that the invasion of drilling fluid would alter the rock properties at the borehole wall, including rock strength, pore pressure etc. Different to invasion of common oil and gas reservoir, the invasion of drilling fluid in gas hydrate formation accompanied by hydrate decomposition caused by drilling tools friction and the temperature of drilling fluid. As a result, the instable of borehole wall accelerated. The resistivity and wave speed are mostly influenced by gas hydrate dissociation amongst all the geophysical logging parameters. The marine hydrate bearing sediments are poorly consolidated, so the acoustic speed is highly affected by compact coefficient, therefore the result of resistivity is more stable and reliable. However, the circulation and invasion of drilling fluid have great influence on resistivity logging. For instance, the high concentration salt and polymer water based drilling fluid systems were applied in order to inhibit the reformation in the borehole and protect the marine environment. The invasion of highly mineralization filtrate would severely alter the characteristics of reservoir and accuracy of resistivity logging. The salt, acting as thermodynamic inhibitor, causes the curve of phase equilibrium moving towards left, results in further gas hydrate dissociation, and has greater effects on evaluation of logging, stability of formation around borehole wall and safety within the borehole. Therefore, the study on the dynamic properties and influence on the formation of invasion of gas hydrate bearing formation by water based drilling fluid coupled by gas hydrate dissociation is meaningful and valuable on safety management, accurate identification and evaluation of geophysical logging, protection of gas hydrate reservoir, assessment of gas hydrate resource and environment, and implementation of well observatory system in IODP.Based on the analysis above, it is important to understand the properties of invasion of gas hydrate formation by drilling fluid, and carry out study by experimental simulation and numerical modeling. The main contents and path of research including:(1) Analysis of the typical characteristic of marine and permafrost gas hydrate bearing formations, and preparation of artificial gas hydrate sample.The research on gas hydrate bearing formation of major ocean and permafrost shows that the gas hydrate distributed in sandstone pore is the most typical hydrate reservoir, and is the most promising energy resources. In the paper, the physical properties of actual gas hydrate bearing formation were firstly summed up. Then the main mineral component, rock particle size, pore distribution and connectivity and seepage characteristics of disseminated type of hydrate formation were analyzed. The artificial core preparation device was independently developed for the preparation of rock sample, in order to simulate the real gas hydrate reservoir.The mercury intrusion instrument and permeability measuring apparatus were applied for the analysis of pore structure and testing of permeability of artificial rock sample. After analyzing the physical property of the artificial stone sample, the optimization of formulation and manufacturing technique of artificial core sample were formed.(2) The simulation research on invasion of gas hydrate bearing formation by drilling fluid.The optimized artificial core samples were selected for simulating the skeleton of gas hydrate bearing sediments. With the application of "comprehensive natural gas hydrate seepage exploitation device" which was self-designed, the research were carried out related to the invasion of simulating gas hydrate bearing formation by drilling fluid, aimed at understanding the change of physical property of formation during the process of drilling fluid invasion. That is obtaining the basic rules of gas hydrate decompose, the evolution of pressure and temperature of formation, and the change of electrical properties during the process as a reference to the result of numerical simulation in the subsequent chapters.(3) Numerical simulation of invasion of gas hydrate bearing formation by drilling fluid.Since the simulation experiment was limited by the function of equipment and time expense, implementing the issues related to invasion of hydrate formation by drilling fluid with numerical simulation method is a good supplement for experimental research, and the efficiency of research would also enhanced. The numerical simulation took the gas hydrate bearing formation in Shenhu area, northern South China sea as background, utilizing numerical simulation exploitation software TOUGH+HYDRATE developed by Lawrence Berkeley national laboratory to analyze the dynamic process of drilling fluid invasion of marine gas hydrate and its influence on the formation at certain conditions, and evaluated the factors that involved during the process.The paper consists of six parts. The main contents are as follows:In the first chapter, firstly the natural gas hydrates were briefly introduced, and the course and state of art of gas hydrate research home and abroad were overviewed. The global distribution, assessment of reserves, as well as gas hydrates in nature, possible environmental problems brought about by human exploration and development were also introduced. The main gas hydrate exploration programs and progresses were summed up. The main methods of gas hydrate exploration were introduced, and the difficulties and problems faced with during drilling operation were analyzed. The problems related to invasion of gas hydrate bearing formations by drilling fluid were introduced and the aim and meaning of the research were also illustrated with the main content and technical route.In the second chapter, the physical properties of actual gas hydrate bearing formation were summed up and analyzed, for the purpose of understanding the basic characteristics of the object of research. The distribution patterns of in situ gas hydrate reservoir were summarized and classified from the macro, meso and micro dimension. The basic physical properties of gas hydrate bearing formations, such as electrical, acoustic, thermal, and porosity, gas hydrate saturation, permeability and mechanical properties were summarized as well. Finally, the three approaches, experimental simulation, actual core analysis and logging interpretation for obtaining the physical properties of actual gas hydrate were compared.In the third chapter, the technique of artificial core sample preparation for simulating actual gas hydrate bearing formation was studied. First chose the appropriate technique of artificial core sample from the conventional methods available. Then the basic physical parameters of the simulating formation were determined before the design of the orthogonal experiment scheme. By comparing the permeability and porosity of the core sample prepared with the actual gas hydrate bearing formation, and evaluating and analyzing the result of experiment, formulation of core and technique, a set of artificial core sample formulation and technique suitable for simulating gas hydrate were obtained. Cores with vary porosity and permeability could be prepared for related experimental tests. This work makes up for the shortage of natural hydrate core sample, therefore it’s important for the ongoing of the simulating experiment and facilitates the future work in the laboratory.In the fourth chapter, the basic rules and behaviors of invasion of gas hydrate bearing formation by drilling fluid were studied by experimental simulation. At first, the process and principle of the invasion were analyzed qualitatively. Then the comprehensive simulating device for seepage and exploitation were introduced. The device was designed for the study of the invasion of gas hydrate formation by drilling fluid, with other functions as well. By utilizing the device and optimized artificial core samples, the experiment of gas hydrate forming and dissociation were conducted, which built the basis for the simulating experiment of gas hydrate invasion. The brine was applied to simulating the marine drilling fluid and the experiment of drilling fluid invasion were carried out. The problems were studied by investigating the evolution of pressure, temperature and resistivity during the process of invasion. The results indicated that the invasion of drilling fluid would disturb the in-situ gas hydrate bearing formation,In the fifth chapter, based on the result of experimental modeling, the problems of drilling fluid were further studied by numerical simulation. Firstly, the numerical simulation exploitation software TOUGH+HYDRATE was introduced and the setup of theoretical model was analyzed. Then the gas hydrate drilling program (GMGS-1) at Shenhu area, northern South China sea were taken as a background, the dynamic process of invasion of marine gas hydrate bearing formation by drilling fluid and its influence on the formation. The factors affecting the invasion of drilling fluid including density, temperature, and salinity of the drilling fluid, absolute permeability and gas hydrate saturation were analyzed as well.In the last chapter, the main conclusions, understandings and innovations were demonstrated, and the shortcomings and suggestions were also proposed. |
PDF Full Text Request