hydraulic fracturing has been seen as one of the key methods of extracting unconventional oil and unconventional gas resources. according to the international energy agency, the remaining technically recoverable resources of shale gas are estimated to amount to , tight gas to , and coalbed methane to . as a rule, formations of these resources have lower permeability than conventional gas formations. therefore, depending on the geological characteristics of the formation, specific technologies such as hydraulic fracturing are required. although there are also other methods to extract these resources, such as conventional drilling or horizontal drilling, hydraulic fracturing is one of the key methods making their extraction economically viable. the multi-stage fracturing technique has facilitated the development of shale gas and light tight oil production in the united states and is believed to do so in the other countries with unconventional hydrocarbon resources.