Importance

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Abstract

The invention provides a method for obtaining a balanced, pressurized core sample from an underpressurized geological reservoir formation. A pressure core sampling well is drilled to a preselected depth and the bottom hole reservoir pressure therein is determined. A computer is programmed to compute predicted bottom hole foam pressures at the bottom of the well produced by a stable foam coring fluid, having a preselected composition, which is introduced into the well under preselected control pressures. Measurements are than made of actual bottom hole foam pressures at the bottom of the well produced by the foam when it is introduced into the well at preselected control pressures and under core sample drilling conditions. A comparison is made between the measured bottom hole foam pressures and the computer predicted bottom hole foam pressures to derive a correlation function and to select a correlated core pressure. The selected correlated control pressure produces a foam balance pressure at the bottom of the well which substantially balances the bottom hole reservoir pressure. A core sample is then drilled while said foam is introduced into the well under the correlated control pressure. The core sample is encapsulated while the reservoir pressure within the sample is balanced by the bottom hole foam balance pressure to produce the balanced, pressurized core sample.

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First Claim