Study On The On-board Detection System And Analysis Methodology For Natural Gas Hydrate Core Samples

Natural gas hydrate(NGH)is a clean and efficient new energy source with huge global reserves which is regarded as one of the most valuable unconventional energy sources in the 21st century.China's hydrate reserves in South China Sea amount to 80 billion tons of oil equivalent,which is the sum of China's reserves of oil and natural gas,and has a broad prospect of development.After the preliminary survey of gas hydrate exploration in the sea area,China has now entered the critical period of detailed drilling exploration and trial production in the prospective mining area.Accurate acquisition of the characteristics of hydrate reservoirs in the trial mining area is the first prerequisite for high-precision exploration and exploitation.NGH and its reservoir characteristics are extremely sensitive to the environmental conditions.The in-situ detection and basic physical property analysis of the NGH core are the main difficulties in current research.Therefore,in view of the key technical problems of in-situ detection in the NGH pressure core samples,this paper develops an ultrasonic inversion method,a visualization method of microscopic seepage and an in-situ mechanical test method for NGH core under high pressure(?30 MPa).An integrated on-board detection system has been formed including ultrasonic detection,X-ray CT scan and triaxial mechanical test.Moreover,the system has been used in South China Sea to complete the field analysis and evaluation of NGH reservoirs for many times,the flow and method of NGH core analysis are put forward,and the basic physical and chemical properties of NGH core in South China Sea are obtained.Firstly,an in-situ detection device and analysis technique for hydrate saturation measurement in NGH cores during pressure retaining transfer are developed.An integrative system is developed including a rotated ultrasonic detection system and a core constrained device for NGH cores at high pressure(?30 MPa).Through frequency screening,the optimal frequency for NGH cores in the target drilling area in South China Sea is obtained,which is 100 kHz.The diffracted problem in ultrasonic detection is solved.It is indicated that the hydrate formation will lead to the displacement of the peak frequency of the transmitted wave to low frequency under the water-saturated condition,while under the gas-saturated condition,the transmitted wave only accompanied with the change of the peak intensity.The first period of the transmitted wave is extracted by the spectral ratio method,and the attenuation coefficient of the wave is determined by combining with the fast Fourier transform,it is found that the attenuation coefficient increases with the increase of hydrate saturation.The inversion model of hydrate saturation is established for the ultrasonic data analysis of on-site pressure core samples.Secondly,a helical scan X-ray CT system on-board is developed by combining 3D visualization equipment and methods for obtaining the hydrate forms of occurrence and microscopic seepage characteristics in natural cores.The multi-layered structure of the core holding device is designed to solve the problem of ray attenuation and rotation eccentricity.The in-situ scan and 3D structure reconstruction of NGH core samples in South China Sea have been finished,the spatial distribution of each component and the form of hydrate occurrence in the natural cores have been visualized.The pore network model of natural cores is established based on CT images.and the seepage characteristic parameters related to hydrate skeleton structure in cores are extracted.The relative permeability and capillary force of gas-water two-phase seepage flow in natural cores with different burial depths and saturation are compared and analyzed.Thirdly,in order to obtain the in-situ mechanical parameters of the natural core to build the strength criteria,a set of NGH core sample transfer and triaxial test system is developed.The sample transfer device can realize the segregation of the sampled core through the separation node in the in-situ state of the sample,and the mechanical property test of the natural core sample can be realized through the transfer to the triaxial pressure chamber.A pressure chamber with a viewport is designed to provide visual analysis of core failure modes.The influence of burial depth and hydrate saturation on strength and deformation characteristics are analyzed.It is found that the core buried at 170 m presents more obvious strain hardening phenomenon than the one at 120 m.These results provide an important technical means for the construction of hydrate true core strength criterion.The author goes to Liwan and Qiongdongnan Basin in South China Sea by CNOOC 708 geological exploration ship,and carries out field sea test research using the on-board detection system.Multi-depth experiments are performed to obtain the physical and chemical properties including sediment particle size and chloride ion concentration of pressure core samples at the whole station.The test results indicate that the particle size distribution and chloride ion concentration abnormally change at the buried depth range of 120?170 m.Combined with the logging data while drilling,it can be determined that the depth range mentioned above is the main area of hydrate reservoirs.On the basis of the above work,the author participates in the formulation of the CNOOC enterprise standard for basic physical and chemical properties test and analysis of NGH core samples.Two sets of analysis procedures for non-pressure and pressure core samples are summarized and sorted out through field tests of many sea trials,which provides basic data support for gas hydrate trial production in South China Sea.