Abstract: Hydraulic fractures are initiated or extended into fluid-producing earth formations from a cased well by filling a space within the casing adjacent the formation zone of interest with liquid which extends into a tubing string and forming a pressure gas charged portion of the tubing string by introducing pressure gas or liquid into the tubing string to compress a column of gas therein to a pressure which exceeds the formation fracture breakdown pressure. By perforating the casing or, if perforations already exist, releasing a frangible disk type closure interposed in the tubing string, the gas charge in the tubing string forces fluid into the formation at sustained pressure and flow conditions not attainable by surface pumped fracture fluids.