Depth is arguably the most important measurement made during data acquisition. It is the index used to associate multiple data types both within a well and also amongst multiple wells in a field. The depth measurement is critical for the understanding of reservoir properties such as fluid contacts, reservoir thickness and dips. Therefore, errors in depth measurement have the potential to impact reserve calculations and the overall understanding of a field.
It is not difficult to understand wireline depth control in shallow and vertical wells; however, as wells' deviations and depths increase, it becomes challenging to quantify the extent to which multiple variables affecting wireline depth measurement interact. This also increases the uncertainty in the depth control.
Depth control has improved in recent years with the use of finite element models to predict wireline stretch in tortuous well paths. Modeling has helped to quantify wireline stretch and has become a reference to compare different vendors' approaches to account for stretch and make wireline depth stretch corrections appropriately.
Wireline stretch can be noticed when the wireline is moving in different directions and different speeds across a reference point. However, quantifying and even measuring the stretch when the wireline is static is challenging. Improper stretch correction for stationary measurements will affect data quality, particularly formation pressures or rotary sidewall coring since the stationary measurement depth may not be properly assigned to the same depth of other logs taken while the tools were moving. It has similar impacts on perforation operations when guns are shot stationary.
The wireline depth control concept is typically well understood by wireline service providers, but this isn't necessarily the case with oil companies who are in turn the final data users and integrators. This work is an effort to close that gap and will also introduce methods for wireline stretch correction. The study shows a review of the elements affecting wireline depth control, cable stretch, depth control methods to account for wireline stretch, and factors to consider when comparing wireline versus logging while drilling (LWD) depth.
Methods to calculate wireline stretch due to variations in cable tensions are presented. They are aimed to provide a consistent approach to correct for depth differences between continuous logs and stationary measurements, sometimes called "wireline creep". Examples of the stretch correction methods are shown with deep and deviated Gulf of Mexico well examples of stationary data collection.