# Fracturing Fluids and Their Application in the Republic of Croatia
## 1 Introduction
When bottom hole pressure (BHP) reaches the fracturing pressure of the reservoir rock, the fracturing fluid starts to penetrate it, thus causing the creation of fractures. Fracture initiation and propagation are mainly affected by rock properties, confining pressure, pressurization rate and fluid type [16]. In theory, hydraulic fractures should propagate in the direction of maximum principle stress, but due to the inhomogeneities of rock strength and pre-existing fractures in rocks, the direction of hydraulic fractures does not always follow that path [17]. After the pump pressure is released, a proppant should be set in the created fracture to prevent fracture closure [18]. Therefore, the selected fracturing fluid is a crucial component of hydraulic fracturing and its composition must be altered to meet the specific reservoir and operational conditions. According to the base fluid, the following fracturing fluids are distinguished: water-based fluids, fluids based on produced water, oil-based fluids, acid-based fluids, alcohol-based fluids, foam-based fluids, emulsion-based fluids and energized (cryogenic) fluids (CO\({}_{2}\), N\({}_{2}\), etc.) [2; 6; 7; 19; 20; 21]. Each of these fluids has its advantages and disadvantages and a specific area of application. Since the first hydraulic fracturing treatment, the composition of fracturing fluid is being improved in order to achieve better proppant transportation and the creation of the desired fracture geometry to enhance the productivity of low permeability reservoirs. Besides those two fluid functions, great attention is focused on developing new environmentally acceptable additives which will increase fluid thermal stability and reduce reservoir damage after fractures are made [22; 23; 24; 25]. Aqueous fluids such as acid, water, and brines are now used as the base fluid in approximately 96% of all fracturing treatments employing a propping agent [2]. For conventional reservoirs, crosslinked guard-based fluids are mainly used as fracturing fluids. Borate crosslinkers are the most commonly used crosslinker in fracturing fluids. However,they exhibit a lower performance at high temperature, high pressure, high water salinity, and low pH applications. These problems are solved by using zirconium crosslinkers [26]. N\({}_{2}\)-energized fluid, CO\({}_{2}\)-energized fluid, N\({}_{2}\) foam and CO\({}_{2}\) foam have been widely applied, especially in water-sensitive formations [27]. These energized fracturing fluids are able to help control fluid leakoff and assist load fluid recovery during flowback [27]. The amount of frac fluid recovered on flowback in shales may range from 5% to 50% [4]. Recently, researchers have been looking for a fracturing fluid that fulfills their requirements in deep, high temperature wells [24; 25; 28]. An ultra-high temperature hydraulic fracturing fluid has been developed by [PERSON] and [PERSON] (2016) [25]. The existing conventional hydraulic fracturing technology faces problems, such as reservoir damage, equipment abrasion, low effective propped area, and early screenout. [PERSON] et al. (2020) proposed a novel self-propped fracturing fluid (SPFF), which remains in the liquid phase before entering the fracture and forms solid proppart particles when stimulated by the reservoir temperature after entering the fracture (chemical proppart, CP). The test results showed that the leakoff volume and core damage level of the SPFF were less than that of a conventional fracturing fluid, thereby effectively reducing damage to the reservoir permeability [29]. Hydraulic fracturing for natural gas extraction from unconventional reservoirs has raised public concerns over its potential environmental impacts on human health, drinking water, air quality, landscape, etc., and especially on the contamination of shallow groundwater aquifers [4; 18; 30; 31]. A couple of research projects have investigated groundwater contamination resulting from shale gas development.