03welvl
DESCRIPTION
A PRACTICAL GUIDE TO VELOCITIES AND DEPTH CONVERSIONTRANSCRIPT
3.1
Well Velocity 1
5000 10,000 15,0000
2
4
6
8
10
12
14
16 AverageVelocity
RMSVelocity
Velocity - feet/secD
epth
-x1
000
feet
Velocities aredirectly measured in
wells:
Well Velocity 1
Well Velocity Measurements
What velocity measurements are made in wells?
What velocities do we obtain?
What are some of the problems encountered with these velocity measurements?
3.2
Well Velocity 1
Schematic cross section
The various measurements required to correct the measured times and well depths to true vertical times below datum.
Derrick floorGround surfaceGun/shot depth
Base of weathering
MSL Datum
Sea bed
Well geophoneor hydrophone
Checkshot
Well Velocity 1
VelocitiesWe observe the travel-time from a source at a known position to a receiver at a known position.
We make a direct measurement of the average velocity.
The difference between two measurements gives us the interval velocity.
Checkshot
3.3
Well Velocity 1
Source
Source related problems with onshore surveys:
• Variable coupling between shots,
• Variable weathering layer velocities,
Checkshot
Well Velocity 1
Source Source related problems with onshore surveys:
• Variable coupling between shots,Vibroseis source compacts ground, digs in and has to be movedExplosives cannot reoccupy the same placeMud pits in the ground can loose filtrate affecting groundwater saturationTanks can offer poor coupling
• Variable weathering layer velocities and layer thicknesses
Checkshot
3.4
Well Velocity 1
Near SurfaceBillie Yates 18D
0
1000
2000
3000
4000
5000
6000
8000 9000 10000 11000 12000
Average Velocity ft/s
Dep
th b
elow
SR
D ft
vibroseis dynamite
Two checkshot surveys acquired in the same well from different surface locations, but assuming the same weathering layer and low velocity.
These data suggest that the near surface velocity is different at the two locations.
Marsden, 1999, Leading Edge, v. 18, no. 2.
Checkshot
Well Velocity 1
Ray Paths - Zero Offset
Ray paths obey Fermat’s principle.
The seismic energy takes the path of least travel-time rather than the path which is geometrically the shortest distance.
Check shot interval velocities and integrated logs will fail to agree and the calibrated log may be found to require a geologically unreasonably fast velocity.
Checkshot
3.5
Well Velocity 1
Well Velocity Curves5000 10,000 15,000
0
2
4
6
8
10
12
14
16 AverageVelocity
RMSVelocity
Interval Velocityevery 500 feet
Velocity - feet/sec
Dep
th -
x100
0 fe
et
Data Courtesy of Amoco
Checkshot
Well Velocity 1
Vertical Seismic Profile
How does a VSP differ from a checkshot survey?
What common VSP surveys are there?
VSP
3.6
Well Velocity 1
Geometry
1000 m
2000 m
SHOT
A
B
C
Raypaths
The field geometry for a VSP.
The borehole geophone, or hydrophone, station interval was typically 50 to 100 ft (25m) apart, now it is often 12.5m.
A full seismic trace is recorded to enable correlation between borehole depths and seismic events.
VSP
Well Velocity 1
RecordsThe first break on each of the traces gives the time-depth records which correspond to the checkshot records.
Data Courtesy of Amoco
VSP
DOWNGOING
P-WAVE
UPGOINGWAVE
DEPTH
TUBE-WAVES S-WAVES
TIM
E
3.7
Well Velocity 1
VelocitiesNotice the more detailed interval velocity curve compared to that obtained by checkshots
Velocity x1000 m/s
500
1000
Dep
th -
m.
0 3 4
AverageVelocity
RMSVelocityInterval
Velocity
2 4 6
Data Courtesy of Amoco
VSP
Well Velocity 1
Walk Above VSP• Lateral velocity variations above the well make it difficult to reconcile VSP, sonic and seismic velocities.• Source cannot always be located above receiver position.• The sonic log is affected by anisotropy as the well deviation increases.• With this type of survey it is difficult to get a tie between the sonic log and VSP data.
VSP
3.8
Well Velocity 1
CorrectionsIn situations where the ray-paths are not close to vertical (large offsets, deviated holes, large near-surface velocity contrasts) the travel-times should be corrected to the vertical using RMS velocity rather than average velocity.
This reduces the error in the time depth curve and in subsequent interval velocity estimates.
After Noponen, 1995, Geophysics, v. 60, no. 6.
2000 m/s
4000 m/s
0
1
2
3
4
5
0 1 2 3 4
DEP
TH m
x 1
000
OFFSET m x 1000
MODEL
VSP
Well Velocity 1
CorrectionsApply average velocity correction, compute values of VI, compute VRMS, apply RMS velocity correction, iterate.
Average velocity correction:to = t x z / (z2 + x2)1/2
RMS velocity correction:to = t x z / (z2 + x2/r2)1/2
where r = VRMS/VA
42/27-1 VSP
0
500
1000
1500
2000
2500
0 5000 10000 15000 20000 25000Interval Velocity (ft/sec)
Dep
th T
VD
SS
(ft)
Standard Actis
VSP
3.9
Well Velocity 1
Sonic Log
The sonic log approximates a log of instantaneous velocity in the direction of the borehole. In practice it measures the average velocity of refracted energy (a head wave) over a short interval.
Sonic
Well Velocity 1
Sonic Log
There are a number of different devices in use.
Name as many as you can:
Sonic
3.10
Well Velocity 1
Tool
Schematic sonic logging tool
From Schlumberger brochures
2 ft 3 ft 2 ft
8 ft 3.5 ft
Transmitters BHC sonic logReceivers
Full WaveformReceivers
MudMeasurement
Sonic
Well Velocity 1
BHC and LSSThe Bore Hole Compensated (BHC) sonic and Long Spaced Sonic (LSS) logs use the principal of differential measurements which are averaged in order to provide compensation for borehole rugosity.
Sonic
3.11
Well Velocity 1
BHC Log
example of log
Notice the scale, 40 to 190 µsec/ft, is a slowness scale.
Also notice the integrator curve; a small tick every 1 msec and a large tick every 10 msec. This is very useful for computing average interval velocity in the absence of checkshot or VSP.
Data Courtesy of Amoco
Sonic
µsec/ft
Well Velocity 1
The older BHC logs suffer from two main problems:
BHC Problems
Cycle Skip
Noise
Correct detection point
From a Schlumberger brochure
Transmittedpulse
Time
Time
Sonic
3.12
Well Velocity 1
BHC ProblemsLog example showing noise and cycle skip. It is advisable to edit these spikes out of the log or they may influence interval transit time calculations and the velocity log.
Data Courtesy of Amoco
Sonic
Well Velocity 1
Sonic Log
T R RR
Fractures Washout InvasionHydration
Problems occurring close to the borehole, as a result of drilling, which affect sonic logs:
Sonic
3.13
Well Velocity 1
Long Spaced Sonic LogExamples of Long Spaced Sonic log compared to BHC. The Long Spaced Sonic “sees” deeper into the formation than the BHC sonic and is therefore less affected by washouts, fractures, invasion and clay hydration.
Data C
ourtesy of Amoco (U
.K.) Ex. C
o.
From a Schlum
berger brochure
Sonic
Well Velocity 1
Full Waveform Sonic Log
Example of waveform
P-wave S-wave
Stoneley wave
Pseudo - Rayleigh wave
Time
Sonic
3.14
Well Velocity 1
Full Waveform Sonic Log
Sonic
Well Velocity 1
Full Waveform Sonic LogThis log provides a shear wave velocity curve as well as a compressional wave log. It also gives a log of VP/VS.Why do we get gaps in the shear wave log?
Data Courtesy of Amoco
Sonic
3.15
Well Velocity 1
Dipole and Quadripole SourcesThese are special tools for use in formations where the shear wave velocity of the formation is less than the velocity of the P-wave in the borehole fluid, i.e. when mode conversion does not occur.
Body waves in the formation and surface waves in the borehole wall are excited by the use of non axially symmetric acoustic sources which operate out of phase with one another.
Shear
Well Velocity 1
Dipole and Quadripole Sources
The out of phase sources cause the borehole walls to be flexed asymmetrically setting up shear and other wave types in the formation.
CANCEL
REINFORCE +-
--
+
+
Shear
3.16
Well Velocity 1
Dipole and Quadripole SourcesNote the shear wave velocity of about 4500 ft/sec in the shales which is less than the p-wave velocity in water!
Data Courtesy of Amoco
Shear
Well Velocity 1
Shear Wave Logs FROM A SCHLUMBERGER BROCHURE
Shear
3.17
Well Velocity 1
Sonic Log IntegrationIntegration of the sonic log gives the travel time over the integration interval.
Travel time & interval thickness gives the average interval velocity.
This will usually show a discrepancy with the checkshot value for the same interval.
Sonic log not recorded from TD to surface
Velocity Log
Well Velocity 1
DiscrepanciesWhat are the sources of the discrepancies between integrated sonic logs and checkshots?
Velocity Log
3.18
Well Velocity 1
Absorption and Dispersion.Absorption and Dispersion of body waves in rocks are frequency dependent and therefore affect the VSP and sonic logs differently.
0.05
0.10
1 2 3 4 5 6
1 / Q
Log (frequency)
(After Neep et al EAEG 1993) also see Sams et al Geophysics 1997
ABSORPTION
2900310033003500370039004100
1 2 3 4 5 6P-
wav
e ve
loci
ty
m/s
ec
Log (frequency)
DISPERSION
Velocity Log
Well Velocity 1
Absorption and Dispersion.
Stiffpore
spaceSoft thinfracture
Squirt in fully saturated rocks -pore fluid is exchanged between the soft and stiff pore space.
Absorption and dispersion depend on: -
After Dvorkin et al, Squirt flow in fully saturated rocks, Geophysics 1995.
After Batzle et al, Fluids and frequency dependent velocity of rocks, Leading edge 2001.
Permeability and fluid viscosity
Velocity Log
Most recently: see Batzle et al, Fluids and frequency dependent velocity of rocks, Geophysics 2006.
3.19
Well Velocity 1
Sonic Log and CheckshotsTo overcome the discrepancies we calibrate the sonic log.
Velocity Log
TIME
DEPTH
DEPTH
µsec/ft
CHECKSHOTS SONIC INTEGRATED SONICWITH CHECKSHOTS
TIME
DEPTH
Well Velocity 1
Drift Curve
In calibrating the sonic log the integrated time is made to match the checkshot time.
More than one estimated time-depth pair is required from checkshots.
A curve is constructed of the differences in travel time which is called the drift curve.
Velocity Log
DEPTH
msec
DRIFT
TIME
DEPTH
INTEGRATED SONICWITH CHECKSHOTS
3.20
Well Velocity 1
Drift Curve
Velocity Log
At this point it is necessary to recognise that poor sonic logs (cycle skipping problems) and poor checkshots will produce large, possibly erratic, shifts in the drift curve.
Missing sonic logs or poor quality logs can be modelled from either resistivity logs or from other porosity logs.
Erratic checkshot values must be edited out.
Where dispersion is a problem it may be corrected for.
Well Velocity 1Velocity Log
Modelling the SonicWhen the sonic log is missing there are a number of formulae that have been proposed for modelling the log.
Faust’s formula for use in shallow clastic sequences:
Vmod = K1 x RS1/K2 x z1/K3
Where Rs is the shallow reading resistivity log, z is the depth and K1, K2 and K3 are constants determined by regression analysis for the project area from other wells.
Smith formula:
1/ Vmod = K4 x RSK5
K4 and K5 are constants determined by regression analysis for the project area from other wells.
3.21
Well Velocity 1Velocity Log
Modelling the SonicThe sonic-porosity log relationship involves the clays in the rock matrix as well as a temporally varying matrix transit time. The sonic log, δt, is normally used to determine the porosity, Φ,according to: -
Φ = [(δt- δtm) - Fsh*(δtsh-δtm)] / [(δtf -δtm)]
Where the matrix transit time is the weighted transit times of the mineral constituents: - δtm = Σ(Fi * δti)
F = fraction, m = matrix, sh = shale and f = fluid. The shale fraction comes from the gamma ray or SP curve and Φ from the neutron or density porosity curve
Well Velocity 1
Erratic Checkshots
Unrealistic Interval Velocities
Velocity Log
Recognising erratic checkshot values should be part of the process.
3.22
Well Velocity 1Velocity Log
DispersionThe sonic log and checkshot data are used to find interval velocities over the same interval, we usually integrate the sonic log to derive estimates of the interval velocity for the intervals over which the checkshots were taken. The ratio of these two measures of interval velocity is plotted against depth. Any trend is indicative of dispersion.
(see Box and Lowrey, Leading Edge, June 2003)
Well Velocity 1
Calibration
The sonic or transit time log is shifted so that the drift is minimised.
The integrated travel time is stretched or squeezed by making the instantaneous velocity proportionally faster or slower over an interval.
Velocity Log
DEPTH
msec
DRIFT
DE
PTH
µsec/ft
SONIC
3.23
Well Velocity 1
Calibration
The main danger from using a linear interpolation of the drift curve is that sudden shifts may be induced in the calibrated sonic log which will result in erroneous reflections being produced in the synthetic seismogram.
The problem is minimised by having checkshots at macrovelocity unit boundaries.
DEPTH
VELOCITY
Velocity Log
Well Velocity 1
CalibrationTo guard against the problem of inducing sudden shifts in the calibrated sonic log a spline or polynomial interpolation is used.
This gives a smooth drift curve rather than the angular drift curve of the linear interpolation.
Velocity Log
DEPTH
msec
DRIFT
DE
PTH
µsec/ft
SONIC
3.24
Well Velocity 1
Calibrated Sonic/Velocity Log.
The calibrated sonic log is displayed as a conventional log with velocity and depth scales.
The log is also displayed with a time scale so that it can be matched to the seismic data.
The velocity log is used to generated synthetic seismograms for effecting well-seismic ties.
Velocity Log
Well Velocity 1
Calibrated Sonic/Velocity Log
LinearTime
Scale
Non-linearDepthScale
Data Courtesy of Amoco
Velocity Log
3.25
Well Velocity 1
Pseudo VelocityThere are always occasions when we don’t have a sonic log, or checkshots, or we do but they cannot be reconciled.
In these circumstances we use Pseudo Velocity.
The well depth / seismic reflection time.
Not always a good approximation to average velocity, but better than nothing.
Will ensure that the depth converted seismic fits the well depth.
Pseudo Velocity
Well Velocity 1
Pseudo VelocityWhy is pseudo velocity not the average velocity?
Pseudo Velocity
3.26
Well Velocity 1
Datum
The driller’s datum is usually the Kelly Bushing or rig floor which is above topographic surface. Logs are originally measured with respect to this datum.
Seismic data uses a regional reference datum.
Before using well depths and seismic times it is necessary to ensure that they are referenced to the same datum.
In the marine environment this will be mean sea level.
Pseudo Velocity
Well Velocity 1
Composite ReflectionsIt is usually difficult to find a seismic reflection which is associated with a clean lithologic break. Most seismic events are band-limited composite events coming from a number of spikes in the earth’s wideband reflectivity series.
Consequently a geologic marker will tie to different positions in the seismic wavelet at different wells.
Pseudo Velocity
3.27
Well Velocity 1
The geological marker may be picked on the gamma-ray curve but the velocity log variations do not necessarily mirror the gamma ray log.
Depth
Pseudo Velocity
WELL 1 WELL 2
γ-ray sonic 1 γ-ray sonic 2
Geologicalmarker
Well Velocity 1
Travel Time ErrorSeismic events may be distorted by:
long period multiplesshort period multiplesfree surface ghoststhe phase of the dataAVO effects
Deconvolution should stabilize the first four effects.
Range limited stack is necessary to avoid AVO effects.
Pseudo Velocity
3.28
Well Velocity 1
Travel Time Error - AVOA model example of AVO
P-wave velocity, s-wave velocity and density all increase giving rise to an AVO anomaly which will distort the seismic event on the stacked section.
Model courtesy of Conoco (UK) Ltd.
Pseudo Velocity
Well Velocity 1
Travel Time Error - Position
Most interpretation is carried out on time migrated data which is represented by the image ray. In the presence of lateral velocity inhomogeneities the image ray will be bent. Thus the seismic event will come from a subsurface location which is different to that of the borehole.
Example after Larner et al, Depth migration of imaged time sections, Geophysics, May 1981
Pseudo Velocity
1 km
2 km
1 km
2900
2600
4350
4700
54005200
5000
51004550
3.29
Well Velocity 1
Travel Time Error - Stack
Seismic image distorted by overlying velocity anomaly.
In some situations pseudo velocities are more practical than checkshots and seismic velocities.
The zero offset reflection time, T0, of the seismic data does not always agree with the checkshot time.
Pseudo Velocity
TIM
E
DISTANCE
Well Velocity 1
Travel Time Error - Stack
When there is non-hyperbolic moveout in the gather then the seismic energy can be stacked to a time that does not agree to either the vertical travel time or the zero offset travel time.
In this situation the pseudo velocity will accurately depth convert the seismic event.
Pseudo Velocity
CMP GATHER
Moveout
NMO HyperbolaSeismicevent
VSP
Offset
Tim
e
3.30
Well Velocity 1
Core MeasurementsWhat are the advantages
disadvantages
of velocity measurements from cores?
Core Velocities
Well Velocity 1
MeasurementsLaboratory measurements are usually made on 1 inch diameter plugs. These are cut perpendicular, parallel (2 orthogonal) and at 45º to the bedding. Vp and Vsh measurements are taken at atmospheric pressure and at a confining pressure to simulateformation pressure.
Core Velocities
3.31
Well Velocity 1
Velocities3000 3500 VP m/sec
DEPTH
4000Horizontal/ parallel
Vertical/ perpendicular
45 degrees
Data Courtesy of Amoco
Core Velocities
Well Velocity 1
Single Well
Unrealistic Interval Velocities
Quality Control
3.32
Well Velocity 1
Multiple WellsKimmeridge Shale
00.020.040.060.08
0.10.120.140.16
0 500 1000 1500
Isopach (ft)
Isoc
hro
n (s
ec) Corralian
y = 18554x0.2249
02000400060008000
1000012000140001600018000
0 0.1 0.2 0.3 0.4 0.5
Mid Point Tim e (sec)In
terv
al V
elo
city
(ft/s
ec)
AnomalousPoint
AnomalousPoints
Quality Control
Well Velocity 1Summary
Checkshot and VSP surveys give, for both P & S waves,Average Velocity,Interval Velocity.
Sonic Logs give, for both P & S waves,Instantaneous Velocity,Interval Velocity.
Core measurements give the anisotropy parameters.
Pseudo velocities provide a practical solution.
3.33
Well Velocity 1Summary
Checkshot and VSP surveys : -In land surveys – source related problemsRay pathsCorrection to vertical – weathered layer, refractions
Sonic/Velocity Logs : -Cycle skipping not recognised
Core measurements : -Frequency dependant velocitiesStress relaxationAlteration of sales
Pseudo velocities : -DatumsMarker/Event identificationMispositioning of events
Pitfalls
Well Velocity 1
Before starting to build a macrovelocity model it is essential to examine
Sonic / velocity logsCheckshot data (velocity - time)Seismic data
in order to determine
Macro layersVelocity distribution in layer(s)Discrete velocity anomalies
of the macrovelocity model.
Well Velocity 1
What is the average interval velocity of the formation based on the integrated sonic travel time?
Use the information from Definitions 1, Exercise 2.2 to estimate the drift.
Exercise 3.1
140 90 40 µsec/ft
100
ft
Data courtesy of ARCO British Ltd
FORMATION A
3.34
3.35
3.36
Well Velocity 1
The following sheets contain suggested answers to the class discussion topics and
form a summary for the different well velocity measurements.
Well Velocity 1
Vertical Seismic ProfileHow does it differ from a checkshot survey?
A full seismic trace is recorded rather than the first break
What common VSP surveys are there?Zero OffsetWalk awayWalk above3D
3.37
Well Velocity 1
Sonic Log
There are a number of different devices in use.
Borehole Compensated SonicLong Spaced SonicFull Waveform SonicDipoleQuadripole
Well Velocity 1
Sonic Log.Discrepancies between integrated sonic and checkshot due to:
Rugose boreholeMicrofracturingInvasionClay hydrationRaypath / Volume of investigationDifferent signal wavelengths / Backus averagingAnisotropy (transverse isotropy and intrinsic)Frequency dependent attenuationDispersionMeasurement errors
3.38
Well Velocity 1
Pseudo Velocity
Why is pseudo velocity not the average velocity?
1. Seismic datum and driller’s datum are different.
2. Composite reflection.
3. Which marker?
4. Seismic data contaminated by multiples or AVO effects.
5. Mispositioning of seismic events.
Well Velocity 1
Core MeasurementsWhat are the disadvantages of velocity measurements from cores?
At a few discrete points
Small rock volume sampled
Small S-R separation
Much higher frequency - Dispersion