**7. Summary comments**

Stimulation of unconventional hydrocarbon reservoirs to enable or improve production is inexorable. There are a plurality of reasons for this; the primary ones are encapsulated in the constraints that hinder the access of the target reservoir and/or source rock and the peculiarity of both formation rocks and fluids. The distinctive nature of unconventional formations are manifested through, for instance, rocks with ultra-low permeability and porosity, the presence of heavy oils as constituent reservoir fluids and the multiphase and multicomponent composition of the formation. Stimulation approaches used in practice are wide ranging. Some of these—e.g. hydraulic fracturing—are age long and have evolved into well-developed methods, whereas others, e.g. spontaneous imbibition, are advancing at a fast pace.

Hydraulic fracturing is traditionally used to artificially create additional flow channels by injecting fluids at high pressures; however, aspects of these techniques are adopted or used in tandem with other stimulation methods. Acid fracturing, for example, is one of the two major acidisation techniques and involves the injection of acids at pressures high enough to generate fractures while dissolving and etching their surfaces. The central objectives of each stimulation method and its limitation are determinants of the choice of fracturing fluid or fluid system. Obviously, it is expected that acid-based fluids would be used for acid fracturing operations. Likewise, either foam-based or oil-based fracturing fluids are superior options for water-sensitive formations. The behaviour of in situ reservoir fluids including their interactions with injected fluids (in terms of mixing, diffusion, etc.) influences the effectiveness of the recovery process and the recyclability and reusability of the introduced fluids. Pivotal drivers of reservoir fluid behaviour include the properties of the injected/resident fluid and formation rock, the reservoir condition, gravity, capillarity and diffusion.

 Other important aspects regarding the exploitation of unconventional hydrocarbon formations are health, safety and the environmental effect. This is generally considered in terms of occupational and public health and safety and the environmental impact of drilling and production activities. Studies on occupational health and safety are fairly established; there seem to be sufficient evidence to substantiate correlations linking health and safety hazards with incidences of accident in the industry. Also, standardised environmental impact assessments have made it possible to identify and measure changes in surrounding and far-reach areas through, for instance, the use of indicators. Conversely, there are several grey areas with respect to threats to public health and safety, since the validity of many investigative studies is disputable because they are apparently subjective, incredible and therefore inconclusive.

*Exploitation of Unconventional Oil and Gas Resources - Hydraulic Fracturing...* 
