Extensive pore-structure studies reveal that the fabric of shale systems are comprised primarily of micro- nano meter matrix pores and complex natural fractures. Measured permeability of shale samples containing natural fractures varies significantly with different cores in the same shale gas reservoir. The significant variation is mainly caused by the stochastic distribution of natural factures. These natural fractures’ effects on permeability under reservoir conditions significantly influence oil and gas production. This work proposes a predictive permeability model that considers the gas transport mechanisms both in the natural factures and matrix pores. In this work, permeability variation is accomplished by considering that the shale core consists of matrix pores and natural fractures. A simple conceptual flow model with shale gas apparent permeability is developed that couples the cubic flow effects of natural fractures and micro- and nano- scale seepage effects of matrix micro- and Nano- pores based on the principle of equivalent flow resistance and the Tandem -Parallel rules of circuit theory. The matrix apparent permeability is derived based on the dusty gas model (DGM) by considering the contribution of Knudsen diffusion and gas slippage. The fracture apparent permeability is established based on a parallel-plate model by considering the influence of the fracture density. Then the comprehensive apparent permeability of shale samples is achieved by coupling the matrix apparent permeability and the natural fractures apparent permeability. The matching results with experiment permeability of the Cambrian Niutitang formation from Sichuan Basin in South China show that the naturally fractured shale permeability is strongly affected by the natural fractures’ characteristic parameters. This research proves that shale measured permeability is the result of seepage interaction between the natural fractures and matrix space. The new permeability model, which overcomes the defect of conventional models that only considered the contribution of the fracture system or the matrix system, can accurately estimate the total contributions of matrix pores and natural fractures. In turn, this will enable engineers to complete the shale gas simulation studies more readily. In the future, this new methodology can be used to develop the next generation reservoir simulators for the shale reservoirs.
展开▼
