Research On The Physical Characteristics And Seepage Mechanism Of Shale Gas Reservoir

In the recent years, the energy demand is growing rapidly in the world, especiallyin the biggest developing country，China. The shortage of oil&gas in China reached600million barrels per day and4billion cubic meters per year respectively asrevealed by BP statistical review of world energy2013. However, the latest statisticsof EIA show that the global recoverable reserves of shale gas approximately reach203.19trillion cubic meters, and about31.57trillion cubic meters of them in Chinawhich ranks the first in the world. In the next several decades, shale gas resources arebecoming an important energy source to meet the rising energy demands.Shale gas is an unconventional gas reservoir which differs from conventional gasreservoir in the geological conditions, the reservoir conditions, the accumulationmodel, percolation mechanism and development method. Shale has the poor reservoirproperties (low porosity and permeability) which determine the flow ambient of gas.The internal pore of shale contains non–organic&organic pore, natural fractures andartificial fractures. The micro-fractures are the main gas flow channels, while matrixpores (between microns and nanometers) as reservoir spaces. Shale gas is mainlybased on the occurrence of free gas and adsorbed gas, while a small amount ofdissolved gas dissolved in the organic matters，bitumen and the pore water. Thecomplicated gas flow process which is caused by the special reservoir conditions ofshale includes the Darcy seepage, diffusion, and adsorption&desorption. Manyfactors affect the gas flow process, such as the stress sensitivity, matrix shrinkage andthe gas slippage effect.In this paper, according to the law of conservation of matter，a1-D macro-poregas flow model is established, which is based on the analysis of shale gas reservoirproperties and gas flow mechanisms. Then a simulator is developed with computerprogramming language. The numerical results indicate that the gas slippage anddiffusion result the pore pressure drop faster, while the stress sensitivity and gasdesorption slow down the pressure decrease; the slippage effect, gas diffusion and gasdesorption help to increase the cumulative gas production, while the negative effect ofstress sensitivity is very serious. So controlling the pressure drop is an importantmeans to achieve the effective development of shale gas reservoirs, and to slow downthe stress drop as much as possible to maximize gas recovery rate. By analyzing thesensitivity of different parameters, we get that the poorer the stress sensitivity of shale reservoirs, the smaller Langmuir’s volume, the bigger the gas diffusion coefficient,the better the permeability of shale, the larger the micro-pore diameter,the easier thegas flow in the shale,and the greater the shale gas cumulative production.