**6. Conclusions**

 Advanced horizontal drilling and hydraulic fracturing have been widely applied for unlocking a huge amount of hydrocarbon reserves from tight shales. Conventional water-based polymeric solutions have been commonly utilized for fracturing jobs due to their ability to transport proppants deep into the reservoir. However, the use of polymer tends to cause plugging of nanopores and detrimentally affects the shale productivity. Moreover, many environmental constraints are associated with the use of a huge amount of fresh water and the disposal of contaminated water during flow back period is continuously reported, driving a need of waterless fracturing fluid system.

CO2 has been known as alternative fracturing fluid with various added benefits including releasing the adsorbed gas, water flow back improvement, and carbon sequestration. However, the use of single CO2 system has limitations affected by the low viscosity of gas, limited proppant carrying ability, and limited possibility to operate at depth. An innovation to have waterless fracturing fluid has also been attractively developed resulting in less water consumption, less formation damage, and less liquid to recover during the fracturing process.

The combination of surfactant with CO2 generates foam which is considered as a highly attractive and unique solution to all the above associated concerns during fracturing operation. CO2 foam has high viscosity, good thermal stability, better proppant transport and placement ability, stable rheological performance as compared to polymers, ability to reduce clay swelling and fine mitigation issues, and increased flow back due to gas expansion. Additionally, surfactants in the base fluid reduce capillary forces and alter shale wettability, which assists water flowback, and increases gas production. Therefore, selection of an appropriate surfactant is of prime importance. However, available literature studies on the evaluation of surfactant and foam performance as fracturing fluid are limited.

In spite of all exceptional benefits, a good understanding of foam rheology is required for the design of optimum foam fracturing treatment. The foam fracturing process highly depends on foam viscosity and it is highly desirable that the foam

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

should provide sufficient viscosity for efficient job completion under reservoir design and operating conditions. According to the previous studies explained in this chapter, besides the evaluation based on foam stability, an analysis of the applicability of foam-based fracturing fluid could be derived from several experimental investigations including foam viscosity measurement using flow loop foam rheometer which also could provide the information of foam texture at different foam qualities and fracturing conditions. A thorough screening and optimization of foam considering different variables under fracturing conditions could effectively improve the efficiency of fracturing job. Studies also have implied that the foam rheological property is challenging to estimate due to numerous variables involved.
