Abstract
Depth is a critical measurement in the economic development of a hydrocarbon asset. Almost all downhole activities, from making petrophysical measurements to setting packers, are performed remotely from surface. The common reference for all such activities is depth. A depth error of as little as one foot vertically can have a financial impact counted in millions of dollars. However, despite the Industry’s heavy reliance on depth, its accuracy is poorly specified.
This paper describes an algorithm which allows proper quantification of along-hole depth uncertainty for all commonly used measurement systems. Additionally, the algorithm includes correlation coefficients that allow quantification of the relative uncertainty between two competing measurements.
Although the physical measurement that is made at the rig site is normally along-hole depth, it is vertical depth that defines the relationship between sub-surface features. The quantification of along-hole measured depth uncertainty is therefore only a partial solution; it is also necessary to estimate vertical uncertainty.
The directional survey of the wellbore defines vertical depth for any along-hole depth, and directional surveys are routinely accompanied by an estimate of positional uncertainty. A method is described for combining the directional survey’s estimate of the wellpath’s vertical position uncertainty with the along-hole depth uncertainty associated with another downhole operation, resulting in a valid vertical uncertainty for that operation.
Adoption of the techniques described in this paper will result in valid estimates of depth uncertainty, which it is hoped will encourage better depth management practices, and result in more productive wells.
Introduction
There are frequent calls from the end users of formation evaluation (FE) logs for improved depth accuracy (1). Zones of interest within the wellbore identified from FE logs (e.g. zones targeted for production, injection, etc.) are subsequently exploited using tools and procedures that are also applied at specified depths. It is therefore desirable that improvements made to the measurement and management of FE depths are applied to all other depth measurements.
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Quantification of Depth Accuracy