Fracturing is an important stimulation technique for naturally-fractured and low-permeability reservoirs. Based on the characteristics of fluid percolating in porous medium containing artificial fracture, natural fracture and matrix, we establish tri-medium percolation model of fracturing naturallyfractured reservoirs. In this paper, the macro-fracture and micro-fracture in the same grid are named as composite natural fracture. Taken mass exchange between macro-fracture and matrix into account, the multi-medium percolation model with macro-fracture is set up.
The simulator of post-frac performance of fractured reservoirs is developed. Using the data of Tahe Oil Field in the northwestern China, we studied the effects of natural fracture and artificial fracture on post-frac performance and concluded that both of them have important meanings for fracturing optimum design of naturally-fractured reservoirs. The simulation results show that both the damage of natural fracture and the decrease of fracture conductivity with time greatly affected the post-frac productivity. Considering the damage of natural fracture, post-frac productivity may decrease correspondingly with the increase of the treatment scale. A reasonable treatment scale, therefore, should be selected. The productivity of fracturing well with macro-fracture is higher than that of fracturing well without macro-fracture and the time that water breaks through the well with macro-fracture may be earlier. Also the example of productivity history matching was given.
Tahe Oil Field lies in the north of Tarim basin. The main pay formation belongs to Ordovician formations. Natural fractures and caves are widely developed in these formations. The main storage space includes tectonic fractures, cavern pores and kluftkarren. These caves and fractures are the main storages while the tectonic fractures are both the main storages and flowing passages for reservoir fluids.
Studying the percolation model with macro-fractures and predicting the productivity of fracturing wells are the basis of fracturing optimization design. During the numerical simulation, we always divide the heterogeneous formation into many grids and presume that one of the grids is homogeneous. In the traditional double-medium model, matrix and microfractures are continuously distributed in the whole percolation micro-unit. In fact, they are not continuous in the present of macro-fractures. Considering the artificial fractures formed by fracturing, the percolation mediums that we studied include: matrix, micro-fractures, macro-fractures and artificial fractures.
In this paper, to facilitate the actual application and to reduce the difficulty of solutions, the macro-fractures are presumed as a series of continuous planks. These macro-fractures are spliced into micro-fractures, then the reservoir simulation will be much more convenient and these anomalistic and short macrofractures in the grids can be treated better. The percolation characteristics can be embodied in the micro-fracture systems. In this paper, the medium bearing macro-fractures and microfractures is called composite natural fracture medium.
Basic Assumptions
The basic assumptions for establishing the percolation model are as follows. 2.Matrix blocks are the main storage space, and both natural fractures and artificial fractures are the flow channels. They are made up of discontinuous units separated by the fracture system.
