According to the data such as outcrops, cores, well logging with imaging, and thin sections, the natural fractures are developed in ultra-low permeability reservoirs. The fracture types are with tectonic shear fractures as the dominated which appear in groups and en echelon arrangement. Natural fractures are most developed in sandstone and siltstone, but rarely developed in mudstone.

The tectonic fractures show themself in four groups of the NE, NW, near EW and near SN orientations. Influenced by the heterogeneity of rock mechanics and the current stress field, fractures in NE-orientation are with wide opening and high permeability. Therefore the NE-orientation is the main flow direction of fluid in this area. On the basis of the traditional fracture description methods, a geological model of reservoirs considering the natural fractures is established with the help of modern fine reservoir description and fracture modeling techniques. The planar distribution laws of effective natural fractures that tally with production performance are determined by application of a numerical reservoir simulation inversion technique. A model considering natural fractures and non-Darcy flow is creatively established on the above mentioned basis. Quantitative evaluation of natural fracture and matrix's contribution to the individual well production rate has been conducted by using this model and the numerical reservoir simulation technique. The contribution rate of natural fractures obtained from the two methods to the average individual well production is 30%–40%. Research results show that the understanding of influences of natural fractures on development results of the ultra-low permeability reservoirs is deepened. Quantitive description of effective natural fractures and their assessment method of the contribution rate to the productivity have been preliminarily formed at the same time, which can be used for reference in the development of the same kind of reservoirs.

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