# Fracturing Fluids and Their Application in the Republic of Croatia
## 2 Relationship between Hydraulic Fractures and Natural Fractures  
Commonly, a fractured reservoir presents a matrix with a low permeability but with a high storage capacity of fluid. The networks of fractures within the reservoir vary in aperture, length-scale and density of fractures. The production of naturally fractured reservoirs is highly influenced by these features, which increases the permeability and alters the flow trajectory and the pressure in the reservoir [36]. [PERSON] et al. (2020) conducted a numerical simulation using a complex fracture development model where the interaction between one single hydraulic fracture within a temporary plugging segment and multiple natural fractures was observed. The numerical results demonstrated that the opening and development of a natural fracture are mainly dominated by its approaching angle and relative location [13].  
The physics of the interaction between the hydraulically induced fractures and the natural fractures in the rock is often disregarded [10]. Understanding hydraulic fracture interaction with natural fractures aids in optimizing fracture treatments in permeable reservoirs that need dominant fractures which accept a high proppant concentration. It is then beneficial to initiate a fracture with a high rate from a small perforation cluster and preferably with the circulation of crosslinked gel to the perforations [11].  
During hydraulic fracturing, the interaction of hydraulic fractures with natural fractures can result in the formation of complex fracture networks in carbonate reservoirs and influence well productivity [12]. To solve this problem, the researchers applied different models, but in this paper, we briefly mention only two of them, developed by [PERSON] et al. (2018) and [PERSON] et al. (2020). [PERSON] et al. (2018) studied this interaction in three-dimensions (3-D) and presented a hydraulic fracturing simulator that solves the coupled fluid flow and geomechanics problem for 3-D fractures and captures the physics of fracture growth and the intersection of a hydraulic fracture with a pre-existing discrete fracture network. They observed that as a hydraulic fracture approaches a natural fracture, the natural fracture simultaneously experiences compressive (the region of the natural fracture shadowed by the hydraulic fracture) and tensile (the region of the natural fracture in front of the hydraulic fracture) stresses which can further result in the partial failure of the natural fracture and in both the crossing and the intersection of the hydraulic fracture with the natural fracture [14]. The presence of natural fractures in carbonate reservoirs, which are candidates for acid fracturing, tend to reduce the productivity of acid fractured wells, because the natural fractures increase leakoff, which causes the main hydraulic fracture to be shorter [15; 37; 38; 39; 40; 41]. [PERSON] et al. (2020) developed a model to explain acid (reactive fluid system) distribution in hydraulic and intersecting natural fractures and then used it to explain the complex interactions of acid fractures in naturally fractured carbonate formations and to optimize acid fracture design parameters. They found that naturally fractured reservoirs with a low permeability (\(10^{-5}\) um\({}^{2}\)) were significantly different from hydraulic fractures not intersecting with natural fractures, and because of the presence of natural fractures, the acid fractured well productivity index (PI) is lower. This negative impact of the natural fractures becomes less significant as the reservoir permeability increases, and for high reservoir permeability (\(10^{-2}\) um\({}^{2}\)), the difference in productivity index between high permeability reservoirs with or without natural fractures is negligible. As for the impact of the acid injection rate on well productivity in naturally fractured reservoirs, a high acid injection rate is recommended in low permeability reservoirs, but it should be moderate in high permeability reservoirs [15].