What Causes Depth Errors in Surface-Based Drilling Sensor Systems?

Drill string depth is a fundamental measurement performed by surface sensor systems. However, the depth reading on an electronic drilling recorder (EDR) often diverges from the pipe tally and requires periodic adjustment to remain aligned with the pipe tally total. In this paper, we investigate what causes depth errors and quantify the error magnitude in land-based wells. The main results show that the depth adjustment workflow varies significantly from driller-to-driller, and workflow errors are a major contributor to total depth error. The investigation is performed with two main efforts. First, depth error is characterized by analyzing the corrections made to EDR depth in 29 onshore wells. Scatterplot analysis reveals that errors can be broadly classified into 3 main types: offset errors, accumulation errors, and workflow errors. Workflow errors can be further categorized into 7 subtypes. Statistical analysis of correction data shows that 75% of depth corrections are caused by offset and accumulation errors. However, workflow errors, such as human error and casing error, account for over 50% of depth corrections when measured on a per-footage basis. In the second effort, a user experience study is conducted to understand the depth correction workflow currently used by drillers. Interviews with 24 rig personnel reveal that workflows are not entirely consistent from driller-to-driller. Drillers tend to adopt the depth calibration methodologies taught by their mentors. A more consistent workflow is possible with increased training and a best practice guideline.