A Multi-level Shale Gas Seepage Model Considering Stress/Desorption/Slippage Effect

During the extraction of shale gas, there are complex coupling behaviors of seepage and stress fields, and shale gas flow pattern are also different in the different scales of pore and fissure. so the study about the distribution of various physics fields and multi-level Seepage are becoming the important issues to be solved in the geothermal reservoir engineering. In this paper, theoretical analysis and numerical simulation are used for the study of multi physics coupling in geothermal reservoir. Partial differential equations of the shale gas movement of the whole process of are established. The model is applied to the numerical simulation of shale gas. The fracture distribution rule for shale gas production capacity is preliminary discussed. The following conclusions are obtained:(1)The flow rule of shale gas in porous media is discussed, the slippage effect of shale gas in the pores is analyzed, based on different Knudsen number, correct linear equation permeability is established. According to P-M permeability model, the thermal expansion is treated as adsorption swelling effect, the dynamic change of permeability and porosity of matrix and natural fractures are deduced, the multi-level coupling model of shale gas field is established.(2)Single well exploitation of shale gas is studied, research shows that pressure obeys the rule of distribution of concentric circles near the wellbore, in the early stage, gas pressure, fracture permeability decline very quickly. The closer the distance is to the wellhead, the more obviously permeability changes.(3)In well drilling, the influence of the boundary effect on the gas reservoir pressure and permeability is not big, but the two wells are close to each other, the flow field of one well will affect another well flow field, thereby affecting the gas reservoir pressure, the change of permeability, particularly the point between the two Wells. Along with the mining, gas velocity drops rapidly, falls to the lowest point in five years, shale gas reservoir is closed after 15 years.(4)When there are hydraulic fractures in shale gas reservoir, shale gas pressure drop very quickly in the early stage, at the time of gas of 1 year, gas pressure drops to 6.7×105Pa. Around the wellbore, the closer it gets to the fractures, the lower the pressure is.(5)We use Monte-carlo method to generate random fractures. The influence of nature and hydraulic fractures on the shale gas production is studied. Research shows that: natural fractures, the more the nature fractures are, the better the connectivity is. The connected natural fractures increase more than 1 time, when the natural fracture grows 1 time. When the hydraulic fractures interval is large, the number of connected natural fractures is directly proportional to the number of hydraulic fractures. Secondary fractures are helpful to improve the communication of the natural fractures, when the secondary fractures perpendicular to the main hydraulic fracture, the effect of secondary fracturing is better.