Hydraulic fracturing field testing is a two-step procedure. Sequentially, it consists of (a) pressurization of the selected borehole segment until fracturing occurs, and (b) delineation of the induced fracture. Typically, a complete test will consist of two trips downhole to the test interval, each with different equipment assemblies.

The most common tool for fracturing is a straddle packer system; for fracture tracing on the borehole wall the oldest and still the most reliable is the impression-orienting tool. In its simplest form the straddle packer system includes two inflatable rubber packer elements straddled by an interval spacer. The packer elements are used to seal off a segment of the hole so as to enable its leak-free hydraulic pressurization. The impression packer-orienting tool assembly comprises of a compass-like (or gyroscopic) device and a packer element covered with a partially cured thin rubber sleeve for taking an imprint of the borehole wall when pressurized against it. A common method has been described in ASTM standard D4645-87 (1989).

Wireline Straddle Packer System (Haimson and Lee, 1984)