# Fracturing Fluids and Their Application in the Republic of Croatia
## 6 Hydraulic Fracturing in Croatia
### Case Study
#### 6.2.2 Hydraulic Fracturing Common Service and Fracture Design Steps  
The service typically consists of two (2) tests: the closure test which determines closure pressure or the minimum in situ rock stress, which is essential for all fracture analysis and the calibration test which is an injection/shut-in/decline procedure. A viscosified fluid without proppant (pad fluid) is pumped into the well at a rate proposed for the fracturing treatment. The well is then shut in and pressure decline is monitored and analyzed using fracturing design and evaluation software. The selected case studies included three (3) main stages for fracture design: breakdown injection, a step rate test (SRT) and a calibration injection test. After hard shutdown on each of the stages, the decline pressure was observed to identify closure pressure and fluid efficiency. All stages will be explained by using data for the W-3 well because the procedure is the same for all the selected wells. First, breakdown injection was performed followed by an SRT to identify near wellbore damage (NWB) restrictions and friction losses. A calibration injection test was performed to analyze the reservoir geomechanical model and calibrate pumping fluid properties before the main fracturing treatment. At shut-in, a total friction pressure drop of 256 bar and an instantaneous shut-in pressure (ISIP) at 231 bar (surface pressure) were observed (see Figure 5). The FracCAT* fracturing computer-aided treatment system comprises hardware and software for monitoring, controlling, recording and reporting all types of fracturing treatments. ISIP is defined as a final injection pressure reduced by a pressure drop due to friction in the wellbore and perforations or slotted liner. Breakdown decline analysis indicates an efficiency of 36% for treated water, with a closure pressure estimated at 521 bar BHP with a frac gradient of 0.14 bar/m [3]. An SRT was pumped after performing a breakdown injection test (see Figure 6). An ISIP of 227 bar was observed, lower than in the breakdown injection. The calibration injection test was carried out after the SRT (see Figure 7). An ISIP of 252 bar was observed. As a result, 91 bar of total friction (Pfric) were estimated for the linear fluid. The main fracturing treatment was redesigned based on previous test analysis results and collected information. The main frac execution and the hydraulic fracturing was carried out according to the designed parameters. The fracture geometry was evaluated after the treatment execution, according to the actual pumping data. The simulation model was adjusted to match the simulated and actual treating pressures. The final simulation results of the matched model are considered to reflect the treatment results. Based on simulation results, the fracture geometry was estimated as per Figure 8.  
Figure 5: Breakdown injection test diagram [3]. ISIP: instantaneous shut-in pressure; Pfric: total friction. Figure 6: Step rate test (SRT) test diagram [3]. The success of fracturing depends on the value of the dimensionless fracture conductivity (FCD). Fracture flow capacity is a measure of how conductive the fracture is or how easily fluid moves through a fracture. It is defined as the product of fracture permeability and fracture width. When the value of fracture flow capacity is divided by the product of formation permeability and fracture half-length, the result is the FCD dimensionless value. The treatment must be designed to create a fracture wide enough, and pump proppants at concentrations high enough, to achieve the conductivity required to optimize the treatment. However, in many low permeability reservoirs, the FCD must be 50 to 100 for the fracture fluid to clean up after the treatment. As such, the value of FCD = 10 is considered a minimum value [76]. Figure 8: Fracture geometry profile as executed [3]. Figure 7: Calibration injection attempt execution [3].