Formation strength testing

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Limit or leak-off tests in development wells may be omitted if no hydrocarbon bearing and/or overpressured formations are to be penetrated. Information obtained from leak-off tests in straight holes is not applicable to deviated holes in the same field (and vice versa). Only measurements in the deviated hole themselves should be used.

It is not necessarily true that deeper horizons always have a higher strength than shallower ones. If less competent formations have been drilled through, another test can be made immediately upon penetrating a transition zone or a cap rock which could overlay higher pressured zones. However, the practice of repeating limit or leak-off tests needs to be applied with much caution. In general during kick control the breakdown pressure at the shoe will remain the critical pressure, unless much weaker zones, close to the shoe, are penetrated.

Data obtained from leak-off tests should be treated with some caution. It is considered acceptable to use the values obtained to calculate the maximum pressure to which the formation can be temporarily subjected, such as while circulating out a kick. The capability of the formation to support pressure continuously may, however, be adversely affected by changes in the hole profile or local damage to the borehole wall. Therefore, it should not be presumed that a mud gradient can be increased up to that of the formation strength, as indicated by a leak-off test. If available, information on the fracture propagation pressure should be used to determine the maximum allowable mud gradient in the hole.

All leak-off tests should be carried out with the lowest mud weight necessary to overbalance the expected reservoir pressure at the shoe. Weighting up mud to combat anticipated higher pressure gradients further down the hole should be done after the leak-off test.

When good zonal isolation behind the casing is imperative, the leak-off/limit test should be carried out with a retrievable packer to avoid pressurising the casing which may cause possible permanent damage to the cement bond by the creation of micro-annuli in the cement.

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