Experimental Study Of Micro-structure And NMR Features Of Volcanic Gas Reservoir

In recent years, a large number of volcanic oil and gas reservoirs have been successively discovered in Songliao Basin, Bohai Bay, Huanghua, Jungger and Kelamayi Basin. Within the above reservoirs, the gas reserve exceeds 3000 billion cubic meter. But the mature development strage about volcanic gas reservoir has not been shaped until now due to the human's bias to this kind of reservoir and its complicated features. Nowadays, nuclear magnetic resonance (NMR) logging is one of the most effective technologies to research and evaluate reservoir. But unfortunately, there has been few research results on volcanic gas reservoir using NMR. It can not only provides useful information on formation evaluation but also supports decision making in development strage.Firstly, the micro-pore-throat features is studied using the constant rate mercury injection technology. The experimental results show that the radius of pore throat is very small and more than eighty percent radius are lower than 0.5μm. The capacity of permeability of volcanic reservoir is controlled by the pore throat radius. There was a better pertinence between the permeability and the average pore throat radius. The research also indicates that the main pore radius is 150μm-200μm. The pore radius of different lithlogies gives the following subsequence (from large to little): granite porphyry, volcanic breccias, rhyolite and rhyolite chondrule. Compared to low permeability sandstone gas reservoir, the throat radius is smaller but the pore radius and pore-to throat ratio is larger. Meanwhile, the correlation between porosity and the throat or pore volume per volume of volcanic reservoir is better than that between permeability and the throat or pore volume per volume.Secondly, the NMR experiment shows that there was clearly difference amongy the T2 spectrums of different lithlogies. The error between the NMR porosity and the conventional porosity was closely with the lithlogy. The NMR porosity of Jilin rhyolite, tuff and Daqing rhyolite, lava brecciaas well as Xinjiang andesitic basalt close to their conventional porosity. The feature of micro-structure of reservoir and their elements have serious effect on this error. Lastly, this paper gave a simple method using to correct NMR porosity by mathematical fitting.The most suitable centrifugal force using to demarcate the movable fluid T2cutoff is 2.76MPa which was determined by different centrifugal forces experiment. The low limit of throat radius is 0.05μm which can provide strong theory foundation in making minimum parameters of reservoir. The average value of the T2cutoff of Daqing, Jilin and Xinjiang volcanic reservoir is 49.31ms. The sequence of the value of the T2cutoff among these three regions (from large to small) is Daqing, Jilin and Xinjiang. The feature of the micro-structure and their elements were the two most important factors which affected the value of the T2cutoff.The movable fluid saturation of the volcanic gas reservoir increases as the increation of the permeability. The comparison among the NMR results, gas injection ones and that of gas reservoir practical production datas reveals that 2.76MPa is the most suitable centrifugal force and can been used to calculate reservoir movable fluid saturation.