The Vp/Vs Inversion Procedure: A Methodology for Shallow Water Flow (SWF) Prediction from Seismic Analysis of Multicomponent Data.
Authors: Moreno Carlos , Castagna John , Huffman Alan , Bertagne Allen 
Published: October 2008

The petrophysical properties characteristic of the sands prone to shallow water flow (SWF) make them identifiable due to their abnormally high Vp/Vs ratios compared to the adjacent rocks. The Vp/Vs inversion procedure identifies anomalies that could be related to SWF sands and it is based on seismic inversion of multicomponent seismic data. The new approach for estimating the Vp/Vs ratio from multicomponent seismic data was evaluated on synthetic logs where the estimated Vp/Vs ratios matched very well the real values. For real ocean bottom cable data (OBC) from GOM, the Vp/Vs inversion procedure was used to estimate Vp/Vs sections that showed clear anomalies that could be related to sand bodies responsible for SWF. These sections are useful for identifying high Vp/Vs ratio zones and could be used to plan a drilling program to prevent SWF during perforation. Preliminary analyses indicate that the methodology has the potential of being a direct hydrocarbon indicator (DHI) as well.

Introduction

In the last few years prospecting for hydrocarbon in deepwater has increased enormously. With these new plays, new problems have emerged. Several articles have been written to describe the new "geohazards" in deep-water projects.

The most dangerous geohazard during drilling operations is known as Shallow Water Flow (SWF). It has been defined as water flowing to the ocean floor on the outside of structural casing (Alberty, 2001). This flowing water can erode the structural support of the well and lead to casing buckling and subsequent casing failure. This flow path can also compromise wellbore integrity, which can result in the loss of well control. SWF typically occurs when overpressured sandy zones are penetrated and shallow casing is not bonded to the formation.

When overpressured sands occur, the appropriate drilling technology should be used to prevent losses and future remediation requiring additional expenditures. However, the prediction of such sands is required to plan appropriately the option that will control and/or prevent shallow water flows.

Interpretation of two-dimensional and three-dimensional high-resolution seismic data is becoming the most used technology for identification of geohazards. However, this method is not a direct indicator of SWF sands. It has been hypothesized (Huffman and Castagna, 2000) that unconsolidated sands that flow during drilling operations have abnormally high Vp/Vs ratios. The use of multicomponent seismic data, previously recorded for petroleum exploration purposes, is used in this project to recognize shallow Vp/Vs ratio anomalies. A methodology that can be used as a direct indicator of overpressured sands, and presents the derived high-resolution Vp/Vs sections for a test line located in deep water in the Gulf of Mexico is described. It has been developed to identify potential Shallow Water Flows sands, however, it could be implemented in hydrocarbon prospecting. This new methodology, based on the appropriate processing and interpretation of pre-stack and post-stack multicomponent seismic data has the potential to be a direct hydrocarbon indicator (DHI) tool more powerful than AVO analysis. We will hereafter refer to the methodology described as the "Vp/Vs Inversion Procedure", as it is designed to estimate the Vp/Vs ratio at the seismic scale.