The porosity of gas-bearing formations is determined using empirical models. These models use the porosity values displayed by the density and neutron logs, assuming that both tools investigate the same gas saturation profile. They also assume that the variation of gas density and mud filtrate saturation and salinity has little effect on porosity values. These assumptions could introduce a high degree of uncertainty, specially in case of tight gas sands.

A new model is proposed which uses the density porosity value and a neutron porosity value derived from the reading of the near detector. The model incorporates a parameter depending on the gas density and drilling fluid saturation and salinity. A forward model establishing the neutron and density tool responses under a known set of conditions was used to assign values to this parameter. The inverse model derives the needed porosity values.

In addition to providing a more representative porosity value for gas-bearing formations, the model can also be used to determine the bypassed gas saturation in the flushed zone of the formation. The value of the bypassed gas saturation is used as an indication of the displacement efficiency in water- drive gas reservoirs.

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