# Fracturing Fluids and Their Application in the Republic of Croatia
## 1 Introduction  
Reservoir stimulation and enhanced hydrocarbon recovery using hydraulic fracturing was first applied in 1947 to Kelpper Well No. 1, located in Hugoton field, Grant County, Kansas, and since then, hydraulic fracturing has become a common treatment to boost the productivity of oil and gas wells [1, 2]. Gasoline-based napalm was used as a fracture fluid in this stimulation treatment. The first treatment of hydraulic fracturing in Croatia was carried out only 11 years later, in 1958 [3]. In recent years, hydraulic fracturing has become a very common and widespread technique, especially in North America, due to technological advances (such as directional drilling, high-volume fracturing, fracture divergence systems, slickwater, multi-well pads) that have allowed shale gas production to be technically and economically feasible [4]. In Europe, experience to date has been focused on low volume hydraulic fracturing in some conventional and tight gas reservoirs, mostly in vertical wells [2]. According to [PERSON] (2012) extensive shale gas resources are present in Europe but only Poland and the UK have performed high-volume hydraulic fracturing for shale gas extraction until 2012. However, some member states have already been active in developing shale gas resources, for example Poland, Germany, the Netherlands, the UK, Spain, Romania, Lithuania, Denmark, Sweden and Hungary [5]. Hydraulic fracturing (HF) is a well stimulation technique that involves injecting a fracturing fluid (typically water, proppant, and chemicals) under high pressure into an oil and gas reservoir through the well for the purpose of creating new fractures in the rock aswell as increasing the size, extent, and connectivity of the existing fractures. The fracture that develops is narrow, usually 2 to 3 mm in width and grows upward and outward, widening slightly until a barrier is encountered or there is sufficient leakoff into side fractures or a permeable formation to stop the fracture from growing [4]. HF is commonly used in low permeability rocks to increase oil and/or gas flow to a well from conventional hydrocarbon reservoirs (e.g., sands, sandstones, and fractured limestones and dolomites) as well as from unconventional reservoirs (with a permeability lower than \(10^{-3}\) um\({}^{2}\), in rocks such as tight gas sands, gas and oil shales, coalbed methane, heavy oil and tar sands, and gas-hydrate deposits) [6; 7; 8]. Hydraulic fracturing can increase the flow rate of oil and/or gas from low-permeability reservoirs and from wells that have been damaged, as well as connect the natural fractures in a formation to the wellbore [9]. A low permeability reservoir has a high resistance to fluid flow, and it is normally an excellent candidate for stimulation by hydraulic fracturing. Regardless of the permeability, a reservoir rock can be damaged during drilling and cementing when drilling and/or completion fluids leak into the reservoir causing the plugging of pores, a reduction in reservoir rock permeability, and a substantially reduced fluid flow through the damaged reservoir (the skin factor is positive). Damage can be especially severe in naturally fractured reservoirs. To stimulate damaged reservoirs, a short, conductive hydraulic fracture is often the desired solution [2]. An increasingly important segment of the industry is currently stimulating naturally fractured formations (e.g., tight sands and shales); where the assumptions of linear elasticity, simple fluid leakoff, and planar geometry fail to hold [10; 11; 12; 13; 14; 15]. Temporary plugging during fracturing operations has become an efficient method to create a complex fracture network in tight reservoirs with natural fractures. An accurate prediction of the network propagation process plays a critical role in the optimization of the plugging and fracturing parameters [13]. Hydraulic fracturing uses different kinds of fracturing fluids to increase bottom hole pressure.