As seen in the above photo, the water tanks, sand mover, and chemical trucks contain the principal constituents of hydraulic fracturing fluid. The hydraulic fracturing flow back tank contains so-called produced water; that is, the portion of the hydraulic fracturing fluid injected underground that returns to the surface. Note the private residence in the background, highlighting one environmental controversy associated with hydraulic fracturing. To fully appreciate a hydraulic fracturing operation, however, it is necessary to first understand the development of a natural gas well. Central to a gas well is its wellbore, the conduit through which various fluids and, eventually, gas flow.