Following partial implementation of gas/oil gravity drainage as the primary recovery process in the Natih Field, Petroleum Development Oman undertook a gas saturation monitoring campaign to determine its effectiveness. Evaluation based on thermal decay tool measurements produced secondary gas saturations lower than anticipated from material balance considerations.

It was reasoned that the limited depth of investigation of the nuclear tools may have been insufficient to sample formation representative of the reservoir. In order to resolve this uncertainty a number of wells were surveyed using the borehole gravimeter as a deep reading formation density tool. Comparison with original porosity measurements obtained using conventional devices enabled quantitative gas saturations to be evaluated. Confidence in the method was obtained by additionally running the tool across the same formation in a gas reservoir.

Results indicate that the gravimeter can be used to quantify gas saturations to within a 15% range. The gravimeter has proven effective in situations where conventional neutron and thermal decay gas monitoring tools are limited by their depth of investigation.

The accuracy of the method is constrained by the control of distance between discrete gravity stations, the borehole and cable noise and the confidence with which the reservoir porosity is known. In low porosity systems the accuracy is degraded.

As a consequence of the higher gas saturations evaluated using the Borehole Gravimeter, implementation of full scale gas/oil gravity drainage in the Natih Field will continue.

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