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This paper presents state-of-the-art modeling of complex hydraulic fracture network evolution in a naturally fractured formation with pre-existing bedding and cross joints, in the Silurian Longmaxi formation, southeast of Sichuan Basin, China. A flow-stress-damage coupling approach has been used in an initial attempt toward how reservoir responds to perforation angle, injection rate, in situ stress, cohesive and frictional strength of natural fractures. A detailed sensitivity study reveals a number of interesting observations resulting from these parameters on the fracturing network evolution in naturally fractured system. When the perforation angle is 60°, it gets to the maximum stimulated reservoir area (SRA). Injection rate as an operator parameter, it strongly impacts the interaction between hydraulic fractures and natural fractures, and associated SRA. In addition, in isotropic in situ stress field, fracturing effectiveness is not the best, complexity of SRA is enhanced when pre-existing fractures are oriented at an angle to the maximum stress. Moreover, the morphology of fracturing network and SRA is closely related to the frictional and cohesive strength of natural fractures. This work strongly links the production technology, geomechanical evaluation and aids in the understanding and optimization of hydraulic fracturing simulations in naturally fractured reservoirs.

This study aims at investigating the flow characteristics of soil–rock mixtures (SRM) with different rock block percentage. A self-developed servo-controlled permeability testing system was developed and used to carry out the permeability testing. Cylindrical SRM specimens (50 mm diameter and 100 mm height) with staggered rock block proportions (20, 30, 40, 50, 60 and 70 % by mass) were produced via compaction tests with different hammer strike counts to roughly insure the same void ratio. From the test results, the non-Darcy flow characteristic of SRMs was first proposed. The relationship between hydraulic gradient and the seepage velocity obeys a power function with good correlation. The increasing trend of the seepage velocity gets much more obvious with increasing hydraulic gradient. With the increase of the rock block percentage, the average permeability coefficient decreases to a minimum at a rock block percentage of 40 %. As the rock block percentage continues to increase above 40 %, the permeability increases again. The critical hydraulic gradient decreases gradually with the increase of rock block percentage. The variation of permeability for SRM specimens is the result of soil matrix properties combined with rock blocks and rock–soil interfaces. The research results can be helpful to predict the subsurface erosion and piping hazards in soil–rock mixture stratum.