12.02.2018

5 minutes of reading

FacebookTwitterLinkedInPrint

The present booklet supports and amplifies the observations and methodological conclusions of the method of oriented 3 component VSP developed by IFP and CGG in the 1990's.

Poster presented at the 3rd EAGE Workshop on Borehole Geophysics
(Athens, Greece - 20-22 April 2015)

Around 75% of commercial borehole seismic operations are zero-offset vertical seismic profiles (VSPs), or “Rig-source VSPs”.

VSPs are recorded before final completion in low deviated holes, with a deep open hole interval, a single casing interval, then multiple casing intervals above.

A 3-component geophone recording is performed, with excellent mechanical tool coupling to the borehole wall. However, the VSP tool cannot generally be oriented into geographical coordinates.

Therefore only the Z-axis component is processed usually, or a near-vertical component computed from the three recorded components (3C), resulting in a marked reduction in the geological information derived from rig-source VSPs.

The authors suggest incorporating cost-effective fluxgates and inclinometers into current and future generations of VSP tools in order to improve the present situation.

Three examples of 3C VSP data orientations have been produced recently, demonstrating how common orientation hardware integrated to commercial VSP tools or toolstrings can be used to restitute the rig source 3C VSP dataset into a geographical system in a reliable and timely manner prior to processing:

  1. Poster WE P9 05: VSP tool orientation using Magnetometer and Inclinometer sensors, namely the GPIT / General Purpose Inclinometer Tool manufactured and operated by Schlumberger;
        
  2. Poster PO04: Orientation of a 3-C VSP dataset acquired by integrated geophone sensor and MEMS inclinometer devices; namely the High Side Indicator inclinometers developed by Avalon Sciences Ltd and the magnetometer-inclinometer orientation probe manufactured by Applied Physics Systems (ex: Model 850HT orientation sonde). A detailed illustration of the effect of successive rotations can be found in the ASL website. Where you can find a PDF version of the HSI guide to Orientating 3-C Geochain data using HIS devices". Additionally, an HSI orientation PowerPoint version can be downloaded for the reader who desires to examine the precise step by step effect of each of the 3 rotations to be applied at preprocessing stage in order to restitute the 3C VSP data into geographical system ( slides 6-9).
         
  3. Poster PO01: Orientation Method for onshore 3-C geophone VSP’s recorded with a Relative Bearing sensor in partially low deviated holes.

An example of processed and interpreted VSP using the full potential of the three oriented components is presented on the "SEISDIP"poster; a comparison of the VSP dip/ azimuth results with those obtained by borehole wall imaging logs demonstrates the complementarity of the methods, as explained in the Extended Abstract SEISDIP: the “VSP dipmeter” from oriented 3 components.

An early example of a complex VSP case study, entitled “Pre-seisdip case-study” processed in 1989, shows the unique structural solution of a 3C VSP seismic response after difficult orientation. Several ray-tracing results and corresponding VSPCDP stack images illustrate the different dipping structures considered in the borehole vicinity (Models 1 to 4); the O-VSP imaging process was initially achieved with the single vertical component, and performed according to several dip hypotheses (Models 1,2,3), as the interpreter was uncertain about the actual dipping trends of the overburden and of the deep reservoir interval from the available surface seismic images and other borehole data… in order to settle this dilemma, 3C processing was attempted by CGG: it successfully resulted in the image labeled “Model 4”, built as the unique dipping structural VSPCDP stack image compatible with the 3C polarization of observed P-P seismic reflections.

A comprehensive presentation of the method and objectives of 3C VSP processing and interpretation is summarized in the following “True Amplitude 3C-VSP method” document. Additional case study results will be presented in the near future, in order to familiarize the industrial VSP practitioners to the 3C VSP acquisition and processing for 3D structural seismic restitution in the borehole vicinity, and help deciding on the combination of an orientation hardware tool with the VSP tool prior to the field acquisition.

Tying the VSP results to other measurements carried out in the same borehole, or on surface, can be illustrated in the shallow geotechnical subsurface by examples collected in the following open access book, by  Jean-Luc Mari and Christophe Vergniault : 

Couverture-livre-Well-seismic-surveying-and-acoustic-logging Book: "Well seismis surveying and acoustic logging"
Authors: Jean-Luc Mari and Christophe Vergniault

>> Summary

>> Full book  Download

This book, as many others books on VSP, illustrates the large domain of applications of single component- near vertical VSP datasets, and how the use of 3 components of VSP’s after orientation, remains industrially undeveloped.
 

Scientific contact:
Charles Naville
Geosciences Division
charles.naville@ifpen.fr

 

References: