Embracing the Depth Residual, a Pragmatic Approach to Depth Uncertainty

The accurate estimation of subsurface depths is vital in subsurface resource extraction, but the inherent uncertainty associated with depth measurements presents significant challenges. Herein, we propose a pragmatic solution to address the problem’s impact on decision-making, risk assessment, and resource evaluation.

Our work suggests a top-down depth calibration methodology using residuals generated from time to depth exercises involving velocities. By separating velocities into stratigraphic intervals and correcting them interval-by-interval, depth uncertainties can be derived for each layer. The method also considers the possibility of errors in the seismic interpretation.

We further discuss the estimation of depth uncertainty in undrilled regions using multiple calibrated velocity models and blind tests. By examining the residuals at excluded well locations, a 3D appreciation of depth uncertainty can be created.

In conclusion, our work acknowledges the challenge of accurately predicting depths outside known data regions but advocates for robust and reliable methods that utilize available information to create depth uncertainty models. As new wells are drilled and new well tops become available, there is an urgency to be integrated into the existing depth calibration workflow, providing more accurate ‘reference case’ depth maps for well planning and reserve estimates. The methods described herein have been proven in practice by drilling wells in areas of lower well densities. The depth outcomes based on the depth uncertainty study have been shown to be within the ranges calculated by these methods.