Numerical Simulation Of Shale Gas Occurrence And Seepage Mechanism

Shale gas reservoir is different from conventional gas reservoir. It has complicated occurrence form and seepage mechanism that exist in porous medium with absorbed states and free states including desorption, darcy flow, slippage and diffusion. Developing numerical simulation study on shale gas reservoir occurrence mechanism, transport mechanism and shale gas reservoir production dynamic aimed to explore seepage law and production performance character on fluid in porous medium and create conditions for shale gas reservoir efficient development.This paper research shale gas reservoir occurrence mechanism and fluid transport mechanism in porous medium using isothermal adsorption experiment and variable internal pressure stress sensitivity experiment and combining with shale reservoir characteristics and on the base of that creates shale gas reservoir gas and water horizontal well numerical model. In the model, absorption and desorption in matrix system and diffusion and slippage characteristics in matrix and fracture system consider the influence of diffusion, slippage and stress sensitivity on seepage. Using IMPES method and programming by Matlab, it gets fracture phase pressure, saturation distribution, matrix system pressure and absorption gas distribution and studies shale gas reservoir horizontal well production dynamic change rules. This paper obtains main results as follows:(1) Using volume method and material balance method principle, develops shale isothermal adsorption experiment and results meet Langmuir isothermal adsorption law. According to results and isothermal adsorption thermal theory, predicts shale absorption gas quantity under different temperature and pressure. On the base of shale matrix occurrence model, gets the relation of shale free gas with absorption phase density, gas relative molecular quality, reservoir depth, temperature and pressure. At the same time, analyzes the influence of shale gas occurrence to absorption phase density, gas relative molecular quality, reservoir depth, temperature and pressure.(2) According to shale gas reservoir characteristics, occurrence and transport mechanism and combined with variable internal pressure stress sensitivity experiment, creates apparent permeability model considering slippage, diffusion and stress sensitivity and analyzes the influence of apparent permeability on pressure, aperture and stress sensitivity.(3) The higher the stress sensitive coefficient, the stronger the shale stress sensitive, the lower apparent permeability; the smaller bore diameter, the weaker that stress sensitive coefficient impact on apparent permeability, when bore diameter less than 5nm, the affection of stress sensitive coefficient on apparent permeability can be ignored. Therefore, it could be neglect the affection of stress sensitive coefficient on gas transport in shale gas matrix system.(4) According to production data in field, it verifies numerical model reliability and analyzes the influence of adsorption gas percent content, natural fracture permeability, Langmuir pressure constant, horizontal well length, number of fracture section and fracture distance on shale gas production dynamic change rules.(5) The higher stress sensitive coefficient, the accumulative gas production and adsorbed gas desorption quantity is reduced by numerical simulation, it can’t be ignored in the process of shale gas reservoir development.(6) Simulation under different occurrence forms of shale gas reservoir in exploitation of dynamic change, it is concluded that the accumulative gas production increases with the depth of the reservoir, the cumulative desorption rate rise increased before they are reduced; The higher the adsorption gas, the lower gas production rate, the greater the amount of adsorbed gas desorption.