DEPTH – Wellbore Placement
Depth is one of many factors in defining the wellbore placement/trajectory. Other factors that need to be considered are how the survey is acquired and its measured depth converted to true vertical depth. Better understanding of how wireline and drill pipe depth are measured and other associated uncertainties is critical in making the right decision for calculating the reserve.A majority of surveys are acquired with a magnetic surveying tool while drilling utilizing drill pipe depth compared to the Log that utilizes both wireline and/or drill pipe depth. Issues arise when converting measured depth to true vertical depth for gas oil contact and oil water contact for different wells that are not at the same true vertical depth. As an industry standard, it is common to convert true vertical depth by combining drill pipe survey and wireline Log that produces a significant error and large vertical uncertainties. It is important to apply environmental corrections to both measured drill pipe and wireline depth to minimize the error and acquire the “best” survey possible to minimize vertical uncertainties.