Hydrofracturing

Old boreholes drilled into low-yield formations, such as Precambrian crystalline rocks, can be stimulated by a process known as hydrofracturing. The technique can be applied only to open, uncased sections such as might occur towards the bottom of a hole.

First, inspection by down-hole camera or down-hole geophysical log must be run to assess the suitability of the borehole to such treatment. The section to be worked should already be fractured to a certain extent, and must be isolated using some kind of packer.

This might consist of a series of rubber seals that can be expanded in the borehole by a hydraulic ram or by compressed air from the surface. An injection pipe runs down the center of the packing system. High-pressure water is injected into the borehole in order to create or enlarge the fractures.

Sand can be added to the water to keep open (‘prop’) newly developed fractures. Reports indicate that yield increases of 20 to 80% have been achieved using hydrofracturing. Depending upon the nature of the formation, injection pressures of 35 (soft) to 140 (hard) bar are used. After treatment, water and debris are air-lifted out in the normal way.