*CO2 Foam as an Improved Fracturing Fluid System for Unconventional Reservoir DOI: http://dx.doi.org/10.5772/intechopen.84564*

 Unconventional reservoirs have been significantly important due to their huge amount of stored hydrocarbon. Unconventional reservoirs are defined as those hydrocarbon reservoirs that require stimulation techniques including the alteration of rock permeability or fluid viscosity in order to develop them economically and to produce the hydrocarbon at commercial rates [3]. Unconventional reservoirs, however, come with various challenges that include a complex system having hydraulically induced fractures, natural fractures, and a complex matrix system comprising of different minerals and kerogen [4]. It is of great interest to develop and refine new and existing techniques to recover more oil and gas from these types of unconventional reservoir.

Tight gas is the natural gas present in sandstone or limestone having very low matrix permeability, less than 0.1 millidarcy (mD), and porosity of less than 10% [5]. Shale gas is the trapped natural gas produced from the shale formation with minimal migration. Moreover, coalbed methane is methane gas trapped in coal beds or seams which is stored on the coal internal surface during the coalification process. Natural gas has been called "sweet gas" because of the absence of hydrogen sulfide content which makes it different from the typical conventional gas reservoir [6]. According to the estimate of Energy Information Administration (EIA), recoverable gas in the world is about 7299 trillion cubic feet (Tcf) [7]. The production of natural dry gas produced from shales in the US in 2015 was about 9.96 Tcf which is about 43% of the total US gas production. A continuous increasing trend in shale gas production was observed from 1999 to 2015 as shown in **Figure 2** [8].

The permeability range of shale gas and tight gas reservoir is usually between nano-Darcy (nD) and micro-Darcy (μD). The size of pore throat in shale is in nanometer and there are some cracks present which assist connection between pores [9]. The shale gas reservoir possesses high capillary pressure, high irreducible wetting phase saturation, low porosity, and extremely low permeability [9]. In order to produce gas commercially from these extremely low permeability reservoirs, horizontal drilling and hydraulic fracturing stimulation technology are required to execute [7].

**Figure 2.**  *Production of natural gas from shales in United State from 1999 to 2015 (in trillion cubic feet) [8].* 
