ABSTRACT
In the last four years, horizontal drilling using many fractures along a horizontal wellbore has been used commercially to access the deepest shale gas (over 1800 m below the surface) in the United States [1,2]. Horizontal drilling employs turning a downward-plodding drill bit to continue drilling within a layer underneath the ground. Accompanying horizontal drilling is hydraulic fracturing, a well-stimulation technique that maximizes extraction of oil and natural gas in unconventional reservoirs such as shale, coalbeds and tight sands. Hydraulic fracturing involves injecting specially engineered fluids consisting of chemicals and granular material into the wells at incredible pressure to break up the fuel stores and stimulate the flow of natural gas or oil to the surface [1]. Once the well has been fractured, the pressure forces out some of the injection fluids containing chemicals, brines, metals, radionuclides and hydrocarbons [3]. For some wells, the toxic flowback fluids are removed and later injected into class II injection wells [4]. In other situations, the fluids are recycled or are transported to local wastewater treatment facilities. As a result of horizontal drilling, there has been a significant increase in the natural gas supply and a reduction in wholesale spot price of natural gas by nearly 50% [5].
