Seepage Characteristics And Numerical Simulation Of Shale Gas Reservoirs

Shale gas is a kind of unconventional natural gas,stored in the reservoir mainly in adsorption state and free state.Shale gas transports in shale reservoir by diffusion,slippage,Darcy flow and non Darcy flow.Doing research on shale gas accumulation and migration mechanism can help us exploring the inherent law of gas flow in different migration model,which can create theoretical basis to improve the gas production rate and ultimate recovery.Based on accumulation and migration mechanism of shale gas,a mathematical model of unstable seepage in dual-porosity shale gas reservoir is developed while considering desorption effect,diffusion,slip flow effect.According to the mathematical model,multistage fracturing horizontal well pressure response curve is drawn,the influence of fracture spacing,fracture half length,desorption time,skin factor on pseudo pressure of the bottle hole is analysed by the curve.ECLIPSE software for numerical simulation is used for single well simulation research of TJ1HF Longmaxi shale gas well in South Sichuan and influence evaluation of fracture parameters and reservoir parameters which have significant impact on cumulative gas production.The main research results are as follows:1.Geological features,reservoir characteristics,reservoir depth,reservoir thickness,reservoir temperature and pressure of Longmaxi shale reservoir in south Sichuan basin and major American shale gas reservoir were compared and analysed,the main characteristics of Longmaxi shale reservoir in south Sichuan basin are deep burial depth of reservoir,thickness,a high content of clay,high temperature and high pressure and thin single layer of shale.2.The adsorption of shale gas is controlled by potential energy between gas molecules and solid molecular.According to adsorption potential theory,the adsorption potential and adsorption quantity curve can be established,characteristic curve is unique for the same adsorption system.The adsorption data of any pressure and temperature can be predicted if the characteristic curve has been established by a set of data under a certain temperature.3.Migration model is classified according to the K_nudsen number,the apparent permeability is represented to show the influence of diffusion,slippage effect,the ratio of apparent permeability and absolute permeability is used for quantitative analysis.4.According to the characteristics of shale gas in adsorption-desorption,diffusion,slippage and seepage during accumulation,migration and production,a mathematical model of unstable seepage in dual-porosity sealed shale gas reservoir is developed while considering diffusion and desorption effect.By solving the model several typical curves of bottom-hole pressure are obtained.The curves are divided into seven stages,that is to say the flow period is divided into seven periods.The influence factors of different stages are studied respectively.5.ECLIPSE software for numerical simulation is used for single well simulation research of TJ1HF Longmaxi shale gas well in South Sichuan.On the basis of history matching,the sensitive parameters that influence the cumulative gas production of 30 years were analyzed.Meanwhile,orthogonal design table of L₂₇（3⁶）is used to analyse the sensitive parameter such as horizontal section length,fracture length,adsorption gas content,Langmuir pressure,diffusion coefficient,interporosity flow coefficient.