12.02.2018

5 minutes of reading

FacebookTwitterLinkedInImprimer

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. This orientation method  is detailed in the presentation entitled  VSP orientation using inclinometers,  with field examples, courtesy of NIOC, IRAN.

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.

An example of a substantial scientific VSP survey aimed to the study of the azimuthal anisotropy in the granite basement of the deep geothermal site of Soultz, Alsace, France, was conducted in 1993: The field equipment comprised a fleet of TWO vertical vibrators and TWO horizontal vibrators placed in orthogonal directions, activated successively in the same run for each station of the TWO downhole VSP tools simultaneously descended in TWO separate neighboring deviated wells, in order to improve the field operation efficiency. A field seismic processing unit implemented with the CGG seismic software was present on the field site, with a experiences processing geophysicist. At least one vertical vibrator was located in offset location from the wells, with the intention of using the direct P-wave arrival of the O-VSP to correctly orientate the 3 components of the VSP tools to enable a successful academic S-wave anisotropy study: the oriented 3C VSP results using this procedure are shown in the presentation entitled : Soultz 1993 VSP-3C orientation matters; it exhibits the unfortunate failure of obtaining consistently oriented 3 Component VSP datasets, due to the intense seismic propagation heterogeneities  induced by the pervasive, regional basement faulted –tilted  block geometry, and seriously alters the reliability of all subsequent processing procedures on horizontal/off well axis components. Although imperfect, the remediation procedure of the orientation of 3C VSP signals exposed in  VSP orientation using inclinometers  was successively tested on the elliptically polarized direct Shear wave arrivals, in order to enable reprocessing these 1993 academic VSP datasets with increased benefit, mainly on the horizontal components, thus 3C components, although the 1C vertical component processing results have been quite interesting.  

As a consequence of the difficulties encountered in the 1993 VSP campaign in Soultz, the following 2007 VSP campaign survey conducted in deeper deviated wells GPK3 and GPK4 was carried out using HT analog VSP tools with geophones mounted on gimbals, as commercial HT-VSP tools with fix 3C geophone settings combined with inclinometer and magnetometer sensors were not available at this time. 

The 2007 VSP field operation and preliminary results have been exposed Paris in Presentation-SAID-VSP 2007 Soultz, and at a specific conference in Soultz: Paper EHDRA Cuenot 2007

Definitely, the constantly interfered 3C and 4C oriented VSP signals of the 2007 campaign relate to a stain glass structure of faulted blocks in the granite basement, and require innovative seismic processing and seismic modeling advances to extract valuable information for the structural geologists and reservoir engineers…

For the homogeneous sedimentary propagation media, 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.

Oriented 3C VSP data observations in a sedimentary context, and a joint evaluation of the most reasonable seismic propagation path in the vicinity of a major normal fault, surrounded by highly attenuating geological bodies and expected diffracting fault edges, were achieved by the young geophysicist Moez AMRI, in a trial and error approach. The different interpretation hypotheses were immediately confronted with 2D elastic and acoustic Finite Difference modeling results quickly obtained with the Tesseral commercial modeling software. The 6 VSP dataset recorded in the Aigion-10 well, Greece, with orientable 3C component downhole tool and two source positions per run was made available for this study. Ref:  Aigion Fault case study; 2D finite difference modeling applied to VSPs.
The whole VSP field survey and basic data processing were  part of an EC research project called  “ DG-LAB-CORINTH”,  (EC project n° EVR1 – CT – 2000 – 40005).

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 seismic 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: