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In page Fracking:

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Hydraulic fracturing (fracking) and acidising (acid fracking) are two of the most common methods for well stimulation. The flow chart shows that hydraulic fracking and acid fracking, highlighted in yellow, are two categories of unconventional hydraulic methods. But acidising is complicated by the fact that matrix acidising is considered conventional. Note that it takes place below the fracture gradient of the rock.[citation needed]