# Fracturing Fluids and Their Application in the Republic of Croatia
## 8 Discussion  
Hydraulic fracturing has been used in the oil and gas industry for more than 70 years (since 1947) and in Croatia, for more than 60 years (since 1958). Hydraulic fracturing involves mostly fresh water and a proppant that is injected into wells at high pressure to crack the reservoir formation so hydrocarbons can flow more freely. The proppant holds the cracks open. The process is designed to only affect the target reservoir formation. In addition to water and a proppant, a small proportion of chemical additives in hydraulic fracturing fluid are needed to reduce friction, remove bacteria, dissolve some minerals and enhance the fluid's ability to transport a proppant.  
The emphasis in this article is on the fracturing fluids and hydraulic fracturing of naturally fractured conventional hydrocarbon reservoirs. However, for the sake of completeness, the article provides a brief overview of high-volume hydraulic fracturing, although it has not been implemented in Croatia so far and will not be allowed by transposing the recommendations of the European Commission on minimum principles for the exploration and production of hydrocarbons (such as shale gas) using high-volume hydraulic fracturing in Croatian legislation.  
Four wells have been selected for HF studies in order to cover different lithology (limestone, quartzite, siltstone), permeability, well completion (case/open hole), well status (production/exploration) and well trajectory (vertical/directional).  
Data determination for HF jobs in selected wells identified values of parameters specific to the formation and the well that were critical to the optimal fracture treatment design. The pre-frac tests were executed before the main treatment, in order to collect more information about formation properties. The main job design was adjusted and confirmed based on tests and further recommendations were made as a result of the analysis. The job was re-designed for a certain quantity of proppant based on the reservoir characteristics and fracture modeling. The aim is to generate as much reservoir coverage without excessive growth of fracture length due to the proximity to the water zone. The fracture geometry was evaluated after the treatment. A suitable FCD parameter is achieved for all selected wells in order to design a well and formation properties which indicated a successful fracture and job design. The maximum HF pressure and slurry rate were expected and depended on well depth, reservoir pressure and permeability.  
The W-1 well achieved a lower drawdown and higher gas production with stable formation water production. Significant gas production and wellhead pressure was achieved by HF in the W-2 well and the W-3 well. Unfortunately, despite the gas production and wellhead pressure increasing by HF in the W-4 well, formation water production is too high to put the well in production.  
Limitations in this paper mainly relate to the small number of wells analyzed due to the limited amount of available data on case studies and focusing only on the technical-technological aspects of hydraulic fracturing. Further research should focus on the economic and environmental aspects of hydraulic fracturing and, in particular, water consumption, produced water disposal and induced seismicity.