Numerical Simulation for Shale Gas Flow in Complex Fracture System of Fractured Horizontal Well

Abstract

Complex fracture systems including natural fractures and hydraulic fractures exist in shale gas reservoir with fractured horizontal well development. The flow of shale gas is a multi-scale flow process from microscopic nanometer pores to macroscopic large fractures. Due to the complexity of seepage mechanism and fracture parameters, it is difficult to realize fine numerical simulation for fractured horizontal wells in shale gas reservoirs. Mechanisms of adsorption–desorption on the surface of shale pores, slippage and Knudsen diffusion in the nanometer pores, Darcy and non-Darcy seepage in the matrix block and fractures are considered comprehensively in this paper. Through fine description of the complex fracture systems after horizontal well fracturing in shale gas reservoir, the problems of conventional corner point grids which are inflexible, directional, difficult to geometrically discretize arbitrarily oriented fractures are overcome. Discrete fracture network model based on unstructured perpendicular bisection grids is built in the numerical simulation. The results indicate that the discrete fracture network model can accurately describe fracture parameters including length, azimuth and density, and that the influences of fracture parameters on development effect of fractured horizontal well can be finely simulated. Cumulative production rate of shale gas is positively related to fracture half-length, fracture segments and fracture conductivity. When total fracture length is constant, fracturing effect is better if single fracture half-length or penetration ratio is relatively large and fracturing segments are moderate. Research results provide theoretical support for optimal design of fractured horizontal well in shale gas reservoir.

Numerical Analysis for Dynamic Propagation and Intersection of Hydraulic Fractures and Pre-existing Natural Fractures Involving the Sensitivity Factors: Orientation, Spacing, Length, and Persistence

“…Gas production is affected by the length of the natural fractures, and relatively long fractures with high hydraulic fracturing conductivity are required to obtain high gas production. ,, Gas reservoirs with a high natural density and shorter primary fractures are superior . There is a discernible linear relationship between the cumulative gas production and fracture permeability .…”

“…Therefore, numerical methods and models that consider the effects of natural fractures have been developed, such as the finite element method (FEM), discrete element method (DEM), extended finite element method (XFEM), displacement discontinuity method (DDM), and cohesive zone method (CZM) . The discrete fracture network (DFN) was developed to effectively simulate natural fractures in tight fractured reservoirs, ,− which can flexibly control the two- and three-dimensional geometries of natural fractures. Therefore, this model was regarded as a potential technique for simulating natural fractures.…”

Transient Pressure Analysis of a Multiple Fractured Well in a Stress-Sensitive Coal Seam Gas Reservoir

“…Recently, Wei et al [10,11] and fractured wells with multiple radial artificial fractures in stress-sensitive CBM reservoirs. Recently, Wei et al [10,11] and Yuan et al [12] numerically investigated the flow mechanism for fractured horizontal wells in shale gas reservoirs, and there are many similar features between CBM reservoirs and shale gas reservoirs, such as the gas adsorption-desorption, gas diffusion, and stress sensitivity of the reservoir permeability, etc.…”

Gas Transport in Shale Nanopores with Miscible Zone

“…Surface diffusion of adsorbed gas cannot be neglected in smaller pores and increases significantly with the increase of the maximum adsorption capacity [11]. By defining the ratio of the average free path of gas to the characteristic dimension as the Knudsen number K n , the gas transport mechanism can be divided into viscous flow, slip flow, transition flow, and molecular free flow [12][13][14]. Therefore, when establishing the flow equation, the flow boundary cannot be regarded as a nonslip boundary, and it needs to be corrected by adding gas slip [15].…”