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If the reservoir appears impermeable there are remedial measures, called stimulation, which may be employed to induce flow. In the 19th and early years of the 20th century simple dynamite charges were used onshore to accomplish this task. It was later found that the walls of a borehole tend to rupture when the pressure of fluids in the hole exceeds a certain critical value. When a rupture occurs a crack is formed that is held open by fluid pressure and such fractures usually extend radially away from the borehole. If additional fluid is pumped in, the fractures can propagate over hundreds of meters.
In a process first used in 1946, low permeability formations were deliberately hydraulically fractured. This now involves rapidly pumping large volumes of fluid containing propping material (for example, sand or small ceramic pellets) into the well bore at pressures great enough to open fractures. The proppants then keep the fractures open. These fractures enormously increase the effective surface of the well bore, hence increasing production rates. However, hydraulic fracturing is not successful in formations that behave plastically. Furthermore if there is a water-bearing bed close to the oil reservoir it may break a path for the water to flow into the well. Fracturing must therefore be carefully planned.
Matrix acidising removes near wellbore formation damage by dissolving or bypassing drilling mud or other restrictions. Such stimulation techniques can be used on their own by simply washing the formation or in conjunction with fracturing. In general, hydrofluoric acid is used to dissolve clay and fine particles in sandstone reservoirs whilst hydrochloric acid etches wormholes that bypass damage in carbonates. Acidising also requires considerable practical experience for a successful job. The correct application of fracturing, acidising or both can change a sub-commercial well into a commercial one.
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