Using VSP Data to Determine Anisotropy - Ikon Science

Using VSP Data toDetermine Anisotropy- with RokDoc-VSP and RokDoc-anisotropymodules.Phil WildIkon ScienceEAGE – Amsterdam 2009

Talk Outline• Discussion of the anisotropies that we might encounter• VSP configurations• Travel time inversions: Walk-away, away, walk-aroundaround• Shear waves: multi-componentVSPs• Inverting the VSP data for anisotropy• Overburden• Fracture anisotropy• Acquisition design• Live demo using RokDoc - VSP

Seismic Anisotropy• VTI - Overburden anisotropy• For example, fromhorizontally layered shales.XSymmetryaxis• Vp vertical < VpZYVp horizontalVTI• HTI – e.g. Vertical fractures• Such as in sandstone orcarbonate reservoirsXSymmetryaxisZYHTI• Azimuthal velocity variation• Shear-wave splitting©EAP

How might we measure it?LaboratoryBoreholeVSPWalk-AwayFMIShear-wavesplittingDipoleSonic

Shot and receiverlocationsWalk-Away VSPReceiverGatherShotGatherHodograms

Inverting data from the overburden1. Compute vertical andhorizontal slownessesCommon shot gather: verticalslowness (q) computed fromwave arrival travel time picks(Δt/ t/Δdepth) depth).2. Invert for ThomsenparametersCommon receiver gather:horizontal slowness (p)computed from wave arrivaltravel time picks (Δt/(t/Δoffset) offset).3. Repeat over a range ofdepth locations

Multi-component VSP1. Use P-arrivals Ptorotate data intoradial andtransversedirections.2. Identify shear-waves (bymoveout andsplitting).3. Rotation andtime-shift of splitshear-wavesallows data fromthe 2 componentsto be overlain.

Interpretation of Fracture Direction1. Fast and slowshear directionsfrom thehodogram.North3. Confirmation ofshear-wavedirection fromborehole image.TSlowshearShotpointWellheadReceiver to shot offset: 150mReceiver to shot azimuth: 258°R2. Registration ofshear-wavedirection withrespect to theacquisition.Fastshear

Acquisition Design for Fracture Imaging• For a VSP we want sufficient offset for P to Shear conversion and d forgeophone alignment.• Beyond VSPs, , e.g. OBC, there are two rules of thumb relating to theuse of seismic data to image fractures:• 1. Certain azimuthal directions give a stronger AVO responsethan others.• 2. In general, a reflected shear-wave has a larger amplitudechange in the presence of fractures than a corresponding P-Pwave.

Acquisition Design for Fracture ImagingThree steps to acquisition design for imaging fractures:1. Use existing fracture information(e.g. from FMI logs or VSPs) ) to selectanisotropic zones. Assign anisotropicparameters to the zones based onlaboratory data, empirical relationshipsor fracture compliances.2. Blocky modelling to investigateAVO trends, here showing PSv andPSh diffferences and an angulardependence.3. Syntheticseismograms showdifferencesbetween the signalwith and withoutfractures (shadedzones in the plot).Offset

Talk Outline• Discussion of the anisotropies that we might encounter• VSP configurations• Travel time inversions: walk-away, away, walk-aroundaround• Shear waves: near offset• Inverting the VSP data for anisotropy• Near surface• Fracture anisotropy• Acquisition design• Live demo using RokDoc - VSP