What Does Hydraulic Fracturing Test Mean?
Hydraulic fracturing test is a geotechnical investigative method used to determine in-situ stresses in boreholes. The test is conducted at near-surface formations or underground formations in a test hole which is inherently free from fractures. In situ stress test is carried out to determine the stresses in rocks through which drilling operations may have to be carried out because stress conditions in faults, dykes, and joints often change significantly.
Hydraulic fracturing is used in conjunction with horizontal directional drilling (HDD) to extract oil and gas from tight reservoirs. Hydraulic fracturing is also used for applications such as tunnelling, geothermal energy, groundwater remediation, and water well development.
Trenchlesspedia Explains Hydraulic Fracturing Test
In hydraulic fracturing tests, thick-walled hollow cylinder samples are used. The specimens are placed in a triaxial cell and subjected to predetermined stress conditions i.e. confining pressure, vertical stress and pore pressure. The specimen is pressurized by injecting a fluid into the cavity hole at a constant flow rate. The change in the volume of the cavity hole corresponding to the injection pressure is measured.
The fluid is injected till peak pressure is reached and cracks begin to appear. At this stage, the fluid injection is stopped causing a decline in fluid pressure. In a curve on the plotted graph, the significant point associated with minimum horizontal in-situ stress indicates the shut-in pressure. The breakdown pressure and shut-in pressure are measured during the shut-in stage.
Even though injection is stopped at the shut-in stage, enough energy is present to propagate cracks until the fluid pressure drops and becomes constant in the crack by leaking off from the fracture and borehole.