The Implications of Erroneous Estimate of Local Anomalous Velocities in Field Development
Authors: Eudorah Ochai-Audu , Glukstad Miguel Mora , Tope Jegede , Deborah Uraechu , Steph Engel 
Published: July 2017

Misinterpreting subsurface lateral changes in apparent structure due to overlying anomalous velocities created by incised channels and canyons, filled with either fast or slow velocity materials is one of the simple-to-make errors or pitfalls that confront every seismic interpreter.

Seismic events that directly underlie an anomalous velocity can be seen to be either pulled-up or pushed-down on two-way time seismic sections. Because of the localized nature of the anomalies, even high resolution seismic data may not properly image the lateral velocity changes within the channel system. These anomalous events are estimated to contain velocities of up to 30% more or less than the surrounding rocks and impact the resulting depth structure in various magnitudes. It becomes trickier when the local anomalous velocities overlie the structural crest of pay zones in a field with low well count. These velocity anomalies if not taken into account and properly corrected, can lead to an over or under estimation of the hydrocarbon in-place volumes, sub-optimal development Well placement and erroneous Well maker prediction that can cause well integrity issues during drilling.

There are various methods that have been used to account and correct for local anomalous velocities. In this paper, we will show two examples and three methods that have been applied to correct for the local anomalous effect identified just below the channel subsets of a complex channel system or canyons. In the first example, a well was drilled with totally unexpected results. Careful review of the seismic identified localized high amplitude channels within a major channel complex directly overlying a series of seismic pull-up events with no clear seismic definition. Two methodologies were used to address this problem. Time shifts were estimated from sonic logs and applied to the time events to correct for the anomalies. The other method was to update the velocity model within the top and bottom of the channel complex before migrating the seismic volumes. Both methods show that once the pull-up effect was corrected about 20-30% of the in-place volume could have been overestimated.

In the second example, a series of channels in a major canyon have developed above one of the culminations of a known field where appraisal and development wells are being planned. A channel correction methodology was applied within the depth conversion process to correct for the abnormalities. The resulting maps by default showed good fit of amplitudes to structure; then compared using the difference in residual as a proxy for improvement. The impact of this approach is crucial for development Well placement and volume estimates.

The results of the local velocity anomaly correction methods applied for both fields showed an overall reduction in erroneous hydrocarbon in-place volume estimate by over 20% and saved about $25 mln that would have been spent to drill a development well on velocity pull-up event.