Establishing confidence in the vertical thickness of the hydrocarbon column (HC) is a critical driver in development project economics. True vertical depth (TVD) uncertainty associated with the HC, in the absence of vertical appraisal well data, may be dominated by the propagation of inclination error in non-vertical or horizontal pilot wells. The challenges associated with establishing reliable hydrocarbon column height can be extreme if high-stepout extended reach (ER) wells are the primary source of appraisal data. Moreover, thin reservoirs targeted by ER wells often have a limited vertical "driller’s target" based on standard TVD error propagation models.

Historically, TVD error improvements over current industry standards have been difficult to document reliably for various reasons: down-hole equipment limitations, bandwidth constraints on measurement-while-drilling (MWD) data, sparse data sets with limited resolution, accounting for variable bottomhole assembly (BHA) geometries, changing borehole environment with time, and little scope for independent validation in a field environment.

This paper outlines the integration of three complementary elements to significantly improve on the TVD uncertainty associated with ER well paths and demonstrates the application in a field setting. The methodology leverages the capabilities of an advanced system incorporating MWD tools and a rotary steerable system (RSS) in combination with a high-quality, dynamically updated borehole sag-correction model to account for axial misalignment of the inclination sensors within the wellbore. Results were corroborated initially with independent third-party surveys and validated with a high-quality formation pressure while drilling (FPWD) data-set from seven ER wells drilled in a hydraulically continuous reservoir prior to production.