**4. Summary and conclusion**

The imperative of reservoir stimulation is borne out of the need to maximise exploitation of hydrocarbon reserves. Candidate reservoir formations that benefit from stimulation operations span across both conventional and unconventional reservoirs. Stimulation is necessary in conventional reservoirs to enhance the productivity of depleted oil and gas formations, which is accomplished through enhanced oil/gas recovery (EOR & EGR) strategies. EOR/EGR is even more pertinent to the exploitation of unconventional reservoirs (i.e., heavy oil, oil shales, tight sandstones, tight limestone formations, etc.). The advent of the exploration of these peculiar hydrocarbon formations has revolutionised the oil and gas industry, driving down energy prices and revealing potential opportunities for cleaner fuels. It is also essential for coal bed methane (CBM) reservoirs to be stimulated in order to instigate and/or improve productivity.

Unconventional reservoirs are complex, distinctive and diverse. They greatly differ from conventional reservoirs in terms of their structure, composition, and rock and fluid properties. Due to these features, there are increased challenges in producing from this type of reservoirs. The stimulation of oil and gas unconventional reservoirs has been ongoing for many decades and over the years, the employed techniques have evolved to become more effective, economical, sustainable and environment-friendly. The diversity of unconventional reservoirs with respect to their structural layout, rock type, hydrocarbon content, proximity to conventional formations, etc., brings to the fore the impracticability of applying a single set of stimulating techniques across board.

Heavy oil reservoirs consist of high-viscosity and high-density hydrocarbon fluids. They are generally produced via two methods: cold production and thermal stimulation. Cold production is carried out either by injecting a diluent to decrease the viscosity of the reservoir fluid or by 'cold heavy oil production with sand' (CHOPS). Alternatively, thermal stimulation is typically implemented in any of the following two ways: cyclic steam injection and steam flooding. Oil shale reservoirs are normally produced by any or a combination of methods, including the use of horizontal wells, hydraulic fracturing, creating transverse vertical fractures
