# Fracturing Fluids and Their Application in the Republic of Croatia
## 5 Fracturing Fluids--Composition and Additives
### Classification of Fracturing Fluids
#### 5.1.1 Water-Based Fracturing Fluids  
Water-based fracturing fluids include liner gel, crosslinked gel, slickwater and viscoelastic surfactant fluids (see Table 4). **Linear gel fluid** is water containing a gelling agent such as guar, hydroxypropyl guar (HPG), hydroxyethyl cellulose (HEC), carboxymethyl hydroxypropy1 guar (CMHPG) or carboxymethyl hydroxymethyl cellulose (CMHEC). Polymers expand and form a high viscosity gel. Other possible additives are buffers, biocide, surfactant, breaker, and clay control. This fluid has a medium viscosity of 10-30 mPa-s, which results in improved proppant transport and wider frac compared to water frac fluid. However, low viscosity linear gels and friction reducers cannot offer efficient suspending characteristics within the fracture under static conditions, which may lead to the early settling of the proppant, so high-viscosity fluids have been the preferred method for increased proppant suspension and transport [43]. A **crosslinked gel fluid** is made to enable gelling polymers to have higher performance without raising their concentration. The polymers that are mainly used are guar, carboxymethyl hydroxypropy1 guar (CMHPG) or carboxymethyl hydroxymethyl cellulose (CMHEC) and crosslinkers such as zirconate and titanate complexes, borate and aluminum ions to crosslink the gelling polymers and provide a higher viscosity than that achieved by a linear gel. This crosslink process is reversible and is accomplished by changing the pH value of the fracturing fluid, which results in it having a good cleanup property for recovered permeability and conductivity. Crosslinked gels are very stable for good proppant carriage at high temperatures and have more expectable rheology. Using systems such as borate-crosslinked fluid with the downside of considerable damage to the proppant pack, typically results in about 85% regain conductivity. While this may still be acceptable, the major limitation of these systems is the additional loss of needed fracture length [43]. For high temperature wells, guar-based fracturing fluids need to be formulated with higher polymer loading and at a high pH, that leaves insoluble residue and a tendency to form scales with divalent ions [50]. A thermally stable acrylamide-based polymer with a reduced polymer loading of 30-40% less than guar-based fracturing fluid was considered to minimize formation damage concerns. With a polymer loading of 3 kg/m\({}^{3}\), the fluid viscosity stayed above 300 mPa-s at 100 s\({}^{-1}\) shear rate for 2 h at 143.3 \({}^{\circ}\)C [50]. The crosslinked guar and derivatized guar fluids have been successfully formulated with seawater and used in wells at a bottom hole temperature (BHT) up to about 149 \({}^{\circ}\)C and the Zr-crosslinked carboxymethyl hydropropy1 guar (CMHPG) fluids have been successfully formulated with produced water containing total dissolved solids (TDS) up to about 280,000 mg/L and a hardness up to about 91,000 mg/L and used in wells at a BHT of about 121 \({}^{\circ}\)C [51]. [PERSON] et al. (2020) studied the influence of the zirconium crosslinker chemical structure on the rheological properties of both biopolymer (carboxy methyl hydroxypropyl guar, CMHPG) and synthetic polymer-based fracturing