The Flow Mechanism And Numerical Simulation Research Of Shale Gas Based On The Dual Media Model

The occurrence, the percolation mechanism and the mining method of shale gas reservoir are different from conventional gas reservoirs, this problem has not fully understood by people. First, the permeability of the shale gas is extremely low, generally less than 1 micro darcy, the size of pore is micro/nano orders of magnitude. In the micro-nano pore, the effect which is caused by collisions between the gas molecules and pore wall slip(Knudsen) is dominant, and the Darcy’s Law is no longer applied. Second, the content of the adsorption state of shale gas is between 20%~85% of total gas. Therefore, the adsorption state of shale gas in shale gas seepage effects on gas reservoir production must be considered. Again, the reservoir is composed of matrix system and natural fracture system, the natural fracture system is development and the matrix system is more complex. Now the study of shale gas reservoirs percolation mechanism on considering low permeability and dual medium also is less. Therefore, completing the flow mechanism and numerical simulation research of shale gas based on the dual media model has important theoretical and practical significant.Based on analysis of the reservoir’s capability, migration and production mechanisms of shale gas and for better charactering the adsorption-desorption mechanism of shale gas on shale reservoirs, isothermal langmuir adsorption model is introduced into mass conservation equation. Knudsen number is used to describe the flow regime of shale gas in reservoirs, the molecular diffusion and slippage effect are considered. According to the desorption, diffusion and slipping effect, the mathematical model of shale gas for dual porosity media is established, and deduced the vertical well and horizontal well’s production capacity formula of shale gas. Using finite difference method for the numerical solution of the mathematical model, the seepage mechanism of shale gas is deeply discussed and at the same time, the sensitivity analysis of shale gas production capability is carried out. The main results are as follow.(1) It’s important to decide the flow regime when considering the flow mechanism of shale gas. The slippage effect must be included in shale gas flow model of low permeability. Beskok-Karniadakis permeability correction factor considers the collision of gas and the collision between gas and wall. It can be used to represent gas flow in detail and fully. When the gas flow belongs to slip flow regime, Beskok-Karniadakis permeability correction return to Klinkenberg correction.(2) Under the vertical well and horizontal well production conditions, Lungmuir volume constant has a big impact on the productivity, the bigger the Lungmuir volume constant, the more the shale gas reservoir adsorption in the volume per unit volume; shale reservoirs thickness also have a big influence on the productivity and little on the pressure. This is because under the vertical well production condition,as the condition of the reservoir all perforated, the greater the thickness, the greater the elastic energy, and under the horizontal well production condition, the thicker the reservoir, the more the shale gas reserves; Besides in the horizontal well production condition, the length of horizontal well can greatly increase production capacity, because the longer the horizontal well length means the well contact with shale reservoir area increases, the corresponding seepage area greatly increased; Other sensitive parameters influence on shale reservoir capacity is relatively small;(3) By comparing the horizontal well and vertical well production capacity curve, horizontal well has a big advantage in exploitation of shale gas, so in the same geological and production conditions, the priority should be given to choose horizontal well drilling.