Development And Experimental Research Of Artificial Hydrate-bearing Cores

Natural gas hydrates are widely distributed in the ocean sediments, permafrost regions and lake sediments. The large amounts of gas hydrate reserves make them significant to solving the growing tension of traditional fossil fuel crisis, if they are successful exploited. Natural gas hydrates have long been seen as the effective alternative energy of the oil, natural gas and other conventional energy sources. However, there are still many key technical problems need to be solved during the process of commercial exploitation, while low level of exploitation technology and big security risk in mining process are two of the most important issues. The studies on physical parameters response of hydrate-bearing sediments during hydrates decomposition and drilling fluid invasion, mechanical properties change with the hydrate saturation during exploitation, microscopic distribution of pore hydrates and dynamics of hydrate decomposition process will have important theoretical and practical significance to hydrates safe and efficient exploitation.However, in recent years, due to the high costs and technical difficulties of obtaining in-situ hydrate-bearing sediments, domestic and foreign researchers have to replace the natural sediments with artificial cores. Artificial core technology in conventional oil and gas field is relatively mature than that in unconventional oil and gas field. And the latter has central problems like big differences with in-situ sediments in physical parameters, small dimensions, poor pore homogeneity, which all will easily influence the accuracy and guidance of the subsequent experimental results.To solve the above problems, this paper conducts studies on artificial cores production device and production process, gas production from hydrate-bearing sediments simulations, and analyzes the mechanism of physical properties change characteristic from the microscopic view of hydrate decomposition. This paper is divided into seven chapters, the main contents are as follows:Chapter 1:The properties of gas hydrates and the impacts of gas hydrates exploitation on global energy, climate and environment are introduced. The main technical problems faced by gas recovery are put forward. And the research purpose and main contents are presented.Chapter 2:The development of gas hydrates exploration and the main drilling coring technologies are reviewed. The technical difficulties and high costs of obtaining in-situ hydrate-bearing sediments are analyzed and understood.Chapter 3:Density, permeability, porosity and mechanical strengths of in-situ hydrate-bearing sediment skeleton were simulated by artificial cores. The quantitative impacts of main factors on physical parameters of artificial cores and the mechanism were determined. On the basis, a set of artificial core simulation technology which could product artificial cores close to the in-situ hydrate-bearing sediments was summarized. At the same time,Chapter 4:Firstly the physical parameters of in-situ hydrate bearing sediments in the Gulf of Mexico were simulated by artificial cores. Then the Artificial cores were used in the gas recovery and drilling fluids invasion experiments, to acquire the change law of temperature, pressure and resistivity of hydrate-bearing sediments when hydrates are during thermal decomposition and sediments are invaded by drilling fluids. These can provide technical guidance for drilling fluid and logging technology of hydrate-bearing sediments.Chapter 5:Studies of the mechanical properties of hydrate-bearing sediments during gas recovery process are conducted. The shear strength, peak strength and residual strength of hydrate-bearing sediments were tested, and numerical relationships between cohesion, angle of internal friction and hydrate saturation were analyzed. Besides, the impact mechanisms of hydrate saturation on cohesion and interaction angle was analyzed by comparing with other researchers’ conclusions.Chapter 6:The impacts of pore hydrate decomposition on permeability and mechanical stability of hydrate-bearing sediments are studied from the perspective of pores. Based on the observation of pore water transport, and the changes of sand particles bond strength during hydrate decomposition, the impact mechanism of hydrate distribution patterns and saturation changes on pore permeability and mechanical strength are analyzed and concluded.Chapter 7:The main conclusions and innovation points of this paper are summarized, and the shortcomings and future research direction are present.Through theoretical and experimental researches on these issues, main understanding and conclusions are as follows:(1) The particle size distribution, density, permeability, porosity and mechanical strength of optimal artificial core formula are very close to the physical properties of hydrate-bearing sediments in Alaska North Slope.(2) Drilling fluid invasion often accompanied by drilling in hydrate-bearing sediments. Drilling fluid temperature, pressure difference between liquid column and pore pressure, salinity have major impacts on the stability of hydrate in near wellbore pores, which will lead to the decomposition of hydrates and change the pore properties, electrical properties and mechanical properties of sediments thereby, the accuracy of logging and borehole stability will be affected.(3) During drilling fluid invasion into hydrate bearing sediments under the pressure and temperature difference, the temperature and pressure increase while the resistivity firstly increases and then decreases, the permeability is significantly impacted by the water transport mutation. Besides, the transfer rate of pressure in pore space is faster than heat, which will easily led to the in-situ gas and water forming hydrate when the pressure increases while the temperature keeps the same. Gas hydrate will decompose when temperature reach the equilibrium point, the decomposed water and gas transfer into the deep pores and reform hydrates, which will led to the increase of resistivity. The temperature of drilling fluid is the major reason of hydrate decomposition, while pressure difference is helpful to increase pore water pressure, and maintain the hydrate stability. High salinity drilling fluid is not only conducive to form high pressure difference, but also inhibit methane from reforming hydrate near the wellbore wall or in the drilling fluid.(4) The reduction of hydrate saturation reduces the adhesion between particles. In the destruction process, hydrates also play a role and affect the friction properties of hydrate-bearing sediments. With the decrease of saturation, shear strength and cohesion of hydrate-bearing sediments decrease rapidly, while the relations between shear strength, cohesion and saturation show approximately linear relationship. However, when impacted by saturation reduction, the internal friction angle maintain unchanged, then decreases rapidly. Otherwise, the brittle of sediments enhances, softening phenomenon is more evident when hydrate saturation decreases.(5) Different distribution patterns and saturation of pore hydrate have quite different influence on mechanical strength of sediments during hydrate decomposition. With the change of saturation, hydrate decomposition behavior gradually transfer from pore spaces, adjacent surface of particles to particle surfaces, the contribution to sediments mechanical strength also increases. Hydrate decomposition reduces the bond strength between particles, resulting in the migration of particles and bentonite, the clog of pore channels, so that the decomposed water cannot be readily discharged. Compared to large diameter particles, hydrate decomposition has more apparent impacts on the mechanical strength of hydrate-bearing cores made by the small diameter particles.