# Fracturing Fluids and Their Application in the Republic of Croatia
## 4 Fracturing Fluid Design, Properties and Testing  
### Fracturing Fluid Design and Properties  
For the successful performance of hydraulic fracturing, the ideal fracturing fluid should meet the following requirements [6]: compatibility with the reservoir rock and the reservoir fluid, proppant maintenance in suspension and their transfer from the surface to the fracture, a sufficient value of viscosity for fracture creation, the generation of enough of a pressure drop along the fracture to create a wide fracture, low fracturing fluid loss, the exhibition of a controlled break to a low viscosity fluid for cleanup after the treatment (effective flow back), low friction pressure losses during injection, to be formulated using chemical additives that are approved by the local environmental regulations (environmentally friendly), to be easy to mix, safe for work and be cost-effective. If a fracturing fluid is not compatible with the reservoir rock or fluid, a chemical reaction between the fracturing fluid and the reservoir rock or fluid can occur. The consequences of those reactions are clay swelling or migration, emulsion or residue formation and matrix dissolving [6]. Another important feature of a fracturing fluid is the ability to carry the proppants from the surface to the fracture. The function of a proppant is to keep the fracture open after the hydraulic fracturing is completed. The traditional concept relies on viscosity to enhance proppant transport during treatments. Fluid viscosity should be high enough (from 100 to 1000 mPa\(\cdot\)s) to create the fracture and to enable proppant entrance into the fracture. After the fractures are created and the proppants are placed, viscosity should decrease in order to accomplish the effective flow back of the fracturing fluid [20]. Optimal proppant placement is critical to maintaining productivity from stress-sensitive reservoirs, in which gas conductivity depends on the connectivity of the network of secondary fractures to the wellbore [42]. [PERSON] et al. (2020) proposed a novel numerical approach for predicting proppant transport through a secondary fracture system, with far-reaching applications to porous media particulate transport [42]. Fracturing fluid rheology is used to evaluate its ability to carry a proppant. The fracturing fluid experiences different shear rates when it is pumped downhole. It is subjected to a large shear when flowing through the tubing, and a small shear when flowing through the fractures. Shear thinning properties of fracturing fluids are preferred since they can provide suspension stability and reduce friction during pumping. However, the ability of the fracturing fluid to perform shear recovery quickly as it transitions from a high shear regime in tubing to a low shear regime as it enters the fracture is crucial to avoid wellbore screenout [43]. The fracturing fluid viscosity and injection rate can influence the HF natural fracture and thus the complexity of the fracture geometry [44]. Density is an important property of a fracture fluid because it affects the surface injection pressure and the ability of the fluid to return to the surface after treatment. Water-based fluids generally have densities near 1006 kg/m\({}^{3}\), which are 20% to 30% higher than the densities of the oil-based fluids, and substantially higher than the densities of foam-based fluids. Low-density fluids can be used in low-pressure reservoirs to assist in fluid cleanup, but in certain deep reservoirs it is necessary to use fluids of a higher density (even over 1440 kg/m\({}^{3}\)). In the HF of the high permeability formation, a fluid loss additive is often added to the fracturing fluid to reduce leakoff and improve fluid efficiency. The fluid efficiency is the percentage of fluid that is still in the fracture at any point in time, when compared to the total volume injected at the same point in time. Ideally, a fluid efficiency of 40% to 60% will provide an optimum balance between creating the fracture and having the fracture close down after the treatment.