**2. Hydraulic fracturing technology**

 Hydraulic fracturing is classified as formation stimulation technique in which reservoir rock is fractured by pumping fracturing fluid with high pressure to create a fracture networks in order to increase the hydrocarbon production rate. This stimulation process has been used in 9 out of 10 gas wells in the United States [10, 11]. Waterbased fracturing fluid has been widely used for fracking rock formation. In particular, shale reservoirs are characterized by their extremely tight rock formations with very low pore connectivity. The matrix permeability of a typical shale gas reservoir is about 1–100 nano-Darcy (nD) [12, 13], whereas the porosity is mostly less than 10%. To develop such ultra-low permeability formations, hydraulic fracturing is proven to be a successful method. In applications for shales, millions of gallons of water as the base fluid and sands in combination with a small amount of chemical additives are pumped into the reservoir [1, 14]. The injected fluid breaks the rock at high pressure and releases the free and adsorbed gas as shown in **Figure 3**. An immensely high permeability can be achieved by applying hydraulic fracturing that also aids in connecting the fracture networks [1]. During fracture generation and propagation, the sand or other coarse materials, the so-called proppant, is employed to expand the fractures as it holds the fractures open when the pressure is eventually relieved. The proppants can be classified into three types which are silica sand, resin coated sand, and ceramic proppant [15]. The utilization of proppant must be appropriate and its selection is strongly based on type and characteristic of well and reservoirs which will be hydraulically fractured. Proppant selection including its type, size, and shape is a critical element for the stimulation process whereby proppant characteristics such as weight, strength, consistency in size, and inert nature must be taken into account for effectively maintaining cracks from fracturing operation [16].

 Furthermore, different types of fluids and treatments have been used and continuously developed for fracturing application. The effectiveness of fracturing fluids such as water, micellar solution, crosslinked-gel, polymer foam, and polymer-free foam has been studied and its selection is generally based on various factors including pressure gradient, reservoir temperature, formation Young's modulus, fracture half-length requirements, the presence of natural fracture, and

**Figure 3.**  *Hydraulic fracturing of shale gas reservoir [1].* 
