Opening of natural fracture and its effect on leakoff behavior in fractured gas reservoirs

Fluid leakoff is one of the most important issues frequently encountered in hydraulic fracturing, especially in naturally fractured gas reservoirs. Opening of natural fractures will bring about excessive fluid loss and restrain the propagation of major hydraulic fractures. A mathematical model of natural fracture dominant leakoff in fractured gas reservoirs has been developed in this paper. Besides, the opening mechanism of natural fractures has been discussed and a new criterion is proposed. Simulation results indicate that open natural fractures have a dominant effect on fracturing fluid leakoff. The wider the opening of the natural fractures, the larger the leakoff of fracturing fluid. The opening of natural fractures depends not only on stress but also the hydrodynamic size of polymer molecules. Due to the formation of filter-cake, the solid phase in fracturing fluid, consisting of polymer residues, formation fines and fluid loss additives plays an important role in the fluid leakoff behavior, which could be divided into two regions, solid dominant region and reservoir dominant region. Moreover, the leakoff behaviors between slick water and cross-linked fluid are compared for shale gas reservoirs, demonstrating that it is easier for slick water to seep into natural fractures and produce a fracture network.