Velocity modelling and depth conversion uncertainty analysis of onshore reservoirs in the Niger Delta basin

Alexander Ogbamikhumi,Olatubosun Temitope Aderibigbe

doi:10.4314/jcas.v14i3.7

Abstract

Depth uncertainty is one of the major uncertainties associated with hydrocarbon field development. This uncertainty mostly arises due to the complexity of the subsurface, paucity of data, time-to-depth conversion, seismic picks, fault positioning and well ties. These uncertainties explain the non-uniqueness of models built and can have a significant impact on fluid contact and hydrocarbon in-place evaluation. To manage depth uncertainty, The Polynomial and Vo_K method were adopted to build velocity models for depth conversion and residual analysis for several reservoir levels to determine the method that will give the best depth residuals. Depth conversion residual analysis result of both velocity models for the reservoirs studied gave average depth residual of less than 50ft for reservoir levels below 9000ft. As the depth increases, the polynomial method derived average residual becomes unreliable with depth uncertainty of over 100ft for the deeper MOT reservoir, compared to 11. 65ft of the Vo_K method for the same reservoir. This was expected at depth since the polymonial method adopts average velocities while the Vo_K method uses instantaneous velocity. Hence, the latter is expected to give a better result at great depth during depth conversion and should be preferably employed for velocity modeling and depth conversion study of reservoir in the Niger delta Basin. Key Words : Depth-Conversion, Velocity-Modelling, Polynomial-Function, Vo_K -Function, Niger- Delta.

Highlights

Résumé L’incertitude de la profondeur est l’une des principales incertitudes associées au développement des champs d’hydrocarbures
The measured depth (Md) to the well top in the sand, the true vertical depth (Z) to the sand top and the depth recovered from each well top from the depth converted seismic horizon are displayed alongside the depth residual (The difference between the depth values of the well top from each well and the depth value to the top from the depth conversion result)
A negative depth residual indicates that the depth conversion process displaces the reservoir to a greater depth than where it occurs in the subsurface, while a positive depth residual signifies that the depth converted result has placed the reservoir at a shallower depth

Summary

Introduction

Résumé L’incertitude de la profondeur est l’une des principales incertitudes associées au développement des champs d’hydrocarbures. Mot-clé: Conversion de profondeur, Modélisation de la vitesse, Fonction_mini_ polynomiale V0_KFunction, Niger-Delta, Introduction Assessment of uncertainties is critical for a field development decision making process (Thore et al, 2002; Azeke et al, 2009 and Euan et al, 2011). Singh et al, 2009 identified Depth uncertainty associated with velocity model building and depth conversion of features interpreted from seismic time as critical and impact on volumetric estimation. In the absence of the more reliable calibrated depth migrated seismic velocity data (which is always the case), two well based velocity modeling techniques (The Polynomial and the V0-K methods) were examined in the present study. The study is a comparative one that practically compares both techniques to highlight the shortcoming of the polynomial method for depth conversion in the Niger Delta Basin especially at great depth

Methods

Results

Conclusion