The Research Of Gas Supplying And Exhausting Mechanism In Volcanic Rock Reservoir

By the end of 2007, the XuShen gas field and ChangLing gas field in the volcanic rock of SongLiao Basin have explored its total demonstrated reserves for 3197 x 108m3, which is the largest in volcanic gas reservoirs been found in the world. Before the experiment, large number of papers have been read about the structure of volcanic reservoir, The experimental technology included the NMR analysis, the constant rate mercury injection technology, centrifugalization experiment and conventional physical simulation experimental technology of water flooding and gas flooding, the theory methods contained Petrophysics, permeation fluid mechanics and gas reservoir engineering theory and so on. The purpose of this paper is find out the gas supplying and exhausting mechanism at pressure difference, imbibition, diffusion and compaction in volcanic rock reservoir using of different kinds of volcanic rocks from XuShen gas field. After analyzing the experimental results the following understanding can be obtained:First, the cracks are the superiority passageway, gas moved faster in cracks than in the matrix under the same pressure grads; The greater pressure gradient or permeability, greater gas flow rate; the less of the difference between the matrixes and the cracks, the greater of the ability of the matrix in the system; In the course of the exploration of the volcanic gas reservoir the perforation should be at the layer of cracks and the production pressure should be amplified. Meanwhile, the problem of the difference of the permeability between matrixes and cracks should be noticed, the less difference of the permeability between matrixes and cracks and the greater of the production pressure are the suitable production system.Second, The imbibition efficiency decrease with the increasing of flooding rate in dry cores, but increase with the increasing of flooding rate until 0.04 ml/min, and then decrease with the increasing of flooding rate in cores with bound water saturation, the efficiency of imbibition decrease with the increasing of initial water saturation, The imbibition is obvious in the high water stage; In the same core, the maximum imbibition efficiency is found in dry condition, the efficiency of dynamic imbibition lower, but this can tread as the upper limit of imbibition in actual production process, the least efficient happened in bound water condition.Third, The natural gas diffusion course in volcanic reservoir was mainly influenced by diffusion media, rock water saturation, temperature and porosity of the rock. The descending order of the diffusion coefficient in five different types of volcanic rocks was:rhyolite, tuff, crystal tuff, volcanic breccias, fusion sintering breccias; the diffusion coefficient volcanic rocks sharp decline with the water saturation increases, the diffusion coefficient in completely dry rock is 600 times greater than that in totally saturated rock samples; the diffusion coefficient has little to do with the temperature when it is over 100℃, the diffusion coefficient is 10-5cm2/s at the underground gas reservoir; and less than 1% of gas can be get with diffusion in production.Fourth, The compaction is strong at the beginning of production, the rock deformation is huge and the gas compacted out is large, but there is only 2% of gas getting from compaction totally.Fifth, in the whole course of gas exhausting and supplying, the pressure difference is absolutely the main influence factor; compaction can force out some gas, but it also can reduce the rock permeability by compressing thethroat, increasing the resistance to gas flowing; Diffusion exists anytime and anywhere, but it can be ignored in production; in the case of water, imbibition can produce a certain amount of gas, accounting for 10%, it is depending on the condition of water situation of the gas reservoir.