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This paper describes the theory behind how high accuracy pressure gauges could be used, and how they were attempted used to verify the True Vertical Depth (TVD) of a 8 1/2" 800 m long horizontal section.

The paper also describes the successful use of resistivity as an alternative method for the verification of TVD in the same horizontal section.

The Troll field has accumulated oil as "thin" layers (12-26 m thick) in the order of 600 x 106 Sm3 OIP beneath the giant gas reservoir. Due to the anticipated severe gas coning problem, horizontal wells very accurately positioned relative to the OWC, are looked upon as a feasible way of commercial development of the oil.

Well 31/5-4AS was drilled with a 800 m long horizontal section within the intended target tolerance of + 2 m. By using real time resistivity and resistivity in memory mode, as a correction for MWD inaccuracy, the well was drilled within + 1.5 m relative to the Target horizon. Pressure surveys used as a third independant TVD indicator could not improve the perfect correlation between the two others (MWD and Resistivity). Pressure surveys were, in order to ensure a safe operation, postponed until the 6 5/8" production liner was run. The results from the pressure survey confirmed the validity of the theory but revealed that technical problems still exist.

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