Precision Depthing for Improved Well Planning, Prospecting and Structural Modeling
Peter Wang, Technical Sales Advisor
Featured Technologies: Explorer™
|Seismic data from the Taranaki basin before/after well-tie tomography
Since the beginning of the reflection seismic method for oil & gas exploration, initiated by John C. Karcher in a farm field three miles north of Oklahoma City ninety-five years ago, the predominant method for resolving well marker mis-ties between true well depths and seismic map depths has been to accommodate them using simple math, usually by subtracting a gridded error map. In other words, we have historically applied simple “fudge factors” when converting from seismic time to depth.
If simple methods work reliably for an operator in a specific operating area, then an argument can be made for leaving well enough alone. However, if operators are experiencing unpredictable, large and costly errors in depth conversion of seismic time maps, they may want to consider precision depthing techniques which are governed by 3D seismic ray paths and rock physics, not simple heuristics.
After all, the costs to operators of poor depthing can be significant – incorrect depth maps will create structural models which yield incorrect volumetrics. This will lead to wrong sizing and mispositioning of deepwater megaproject investments, as well as increased dry hole risk due to structural factors; or, in the case of onshore operations, failure to properly land a horizontal well within the productive zone.
What exactly do we mean by 3D seismic ray paths and rock physics? In the field, seismic rays sample a large 3D cone of the earth; most of them do not travel straight down to the target and back up vertically. From rock physics, we know that the earth is highly anisotropic, especially in a vertical sense. Therefore, our depth conversion velocity model needs to be calculated and applied to take account of the 3D anisotropic nature of the earth. Fortunately, there is an accurate and efficient way to do this.
Paradigm Well Marker Mis-tie Tomography updates the 3D subsurface anisotropic velocity model in an accurate, efficient and flexible way. It still uses mis-tie data, but only as input. It then uses 3D ray tracing to invert for anisotropic updates to the earth model. Well Marker Mis-tie Tomography uses the same mathematics as full-fledged Depth Imaging Tomography, but it makes the simplifying assumption of initially flat imputed depth gathers. Therefore, actual field gathers and picking of residual image gather moveouts are not needed. With this large reduction in the compute task, Well Marker Mis-tie Tomography runs on a desktop computer, and does not need a large data store or Linux cluster.
This tomographic precision depthing approach is an excellent solution for interpreters carrying out final depth adjustments of horizon maps interpreted from either time or depth migrated seismic. Examples of mis-tie correction for different scenarios will be presented.
Peter Wang is a Technical Sales Advisor with Paradigm. He has an MS in Geophysics and an MBA from the University of Houston, and is a Texas Licensed Professional Geoscientist (“PG”). He has a thirty-year history in the geophysical industry, having worked for Schlumberger, Amoco Production Company, and Kelman Seismic Processing.