Problems and Solutions of Velocity Modeling in Natural Gas Exploration

Aiqun Liu,Qianwei Hu,Caiwei Fan,Yong Deng,Peiyuan Zhu,Peng Song

doi:10.4236/ojms.2017.71005

Abstract

Recently, velocity prediction and modeling have been focus of the geophysical exploration in the high temperature and high pressure of the south China sea area, especially for new offshore exploratory areas. The error is large with great difficulty owing to fewer exploratory wells and misunderstanding. Firstly, on the basis of three typical velocity prediction and modeling examples in Ying-Qiong basin, it’s easy to put forward the corresponding not common but urgent problem in each instance, then combined with the velocity problem and misunderstanding to expand method discussion and solution, which include geological model to guide the velocity interpretation and analysis, the establishment of forward velocity of the auxiliary model explaining and constructing high precision velocity model. This research basically solves existing velocity problems in gas exploration of south China sea in recent years, and proposes corresponding solution and application, which is of great significance to the further exploration and productive practice.

Highlights

The west coast of South China Sea is rich in natural gas resource and has been an important natural gas exploration area for CNOOC
It’s very difficult to predict the velocity of favorable targets in a basin with low overall prospecting degree like Ying-Qiong basin
Combined with problems above and the misunderstanding, method research is carried out and corresponding solution is put forward for every problem, which lays a solid foundation for accurate velocity prediction before drilling

Summary

Introduction

There are three main problems: how to do velocity reprocessing and determine its accuracy, the system error existing between logging VSP velocity and velocity spectrum interpretation, and larger error occurs when predicting depth of new drilling well with the time-distance relationship of drilled wells nearby. The assumptions that formation has no large dip angle and seismic data is in good quality are difficult to meet, and there exists misunderstanding of velocity prediction [8]. Stacking velocity is obtained by velocity spectrum interpretation, and the accuracy of interpretation is judged by flattening effect of phase axis. We can see that the flattening effect of phase axes in three different solutions is basically consistent, but large errors (about 10% between low/high velocity and medium velocity) occur.

Typical Case Analysis

Solution and Method Discussion

Application Analysis

Conclusion and Understanding