Hydrofracture, also known as frac-out, is the unintended reversal of drilling fluid to the surface during trenchless operations such as horizontal directional drilling (HDD). This problem commonly occurs during HDD projects and usually takes place near the entry or exit points of the bore. Hydrofracture should be avoided because the drilling fluid, though not toxic, contains chemicals that are harmful to plant and animal life, especially where the drilling operation is being conducted close to environmentally sensitive areas.
Trenchlesspedia Explains Hydrofracture
Hydrofracture takes place in cohesive soils that are non-fissured when the overburden pressure is exceeded by the down hole mud pressure. Sometimes, the drilling fluid may find a pathway to seep through cracks, fractures and fault lines or through loose soil. Frac-outs can be detected by observing the surface near the borehole for unnatural accumulation of fluid or by loss of pressure of the drilling fluid itself during the operation.
Drilling fluid functions to stabilize the borehole by maintaining the down hole pressure. Though nominal loss of drilling fluid does occur, excessive loss can be harmful. Hydrofracture can be avoided by conducting thorough geotechnical investigation before the drill path is planned, and the risk can be mitigated at prone areas by deepening or shortening the bore, boring relief wells, selecting better soil strata and using conductor casings when boring through loose, gravelly or rocky soils to prevent the drilling fluid from escaping.