A Shear Experiment Over the Natih Field in Oman-pilot Seismic and Borehole Data

8/19/2019 A Shear Experiment Over the Natih Field in Oman-pilot Seismic and Borehole Data

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A shear experiment over the Natih field in Oman:pilot seismic and borehole data1

J.H. Hake,2

E.C.A. Gevers,2

C.M. van der Kolk3

and B.W. Tichelaar3

Abstract

An experimental multicomponent three-dimensional (3D) seismic survey has been

carried out over the Natih field in Oman. This paper describes the small-scale two-

dimensional experiment carried out beforehand, and how the results obtained from this

pilot were used to assess the feasibility of a nine-component three-dimensional (9C3D)

operation as well as to determine the field parameters for the field-scale 3D survey. It

also describes the two VSPs and a wireline shear log, acquired in conjunction with the

pilot experiment, and the importance of such borehole data for establishing the correct

time-to-depth relationship for the seismic data and for providing an independent check

on the seismic interpretation. The observation of cusps in the offset VSP indicated the

strong anisotropy of the Fiqa shales overlying the Natih reservoir.

Introduction

The main reservoir of the Natih field is formed by a fractured limestone. The anticlinal

structure has gentle dips (at most 4), is shallow, and is overlain by the Fiqa shales. The

fractures are nearly vertical. Extensive knowledge has been acquired throughout 30

years of production from a large number of wells. This makes it an ideal candidate for a

seismic experiment aiming at establishing the presence of shear-wave anisotropy and

its relationship to reservoir fracturing. Nevertheless, before making the final decision

on an expensive nine-component three-dimensional (9C3D) survey, a pilot experi-ment was carried out involving some 9C2D recording with minimal mobilization of

special equipment. To establish unambiguous identification of the major horizons and

to obtain an independent measure of anisotropy, a zero-offset VSP, an offset VSP and a

dipole wireline shear log were also acquired. The implications of these experiments are

described in detail below.

The pilot 9C2D experiment

The pilot experiment consisted of the acquisition of a short 2D seismic line, utilizing

three-component receivers in combination with both conventional vertical (P) and

1998 European Association of Geoscientists & Engineers 617

Geophysical Prospecting , 1998, 46 , 617–646

1 Received August 1997, revision accepted July 1998.2 Nederlands Aardolie Maatschappij, PO Box 28000, 9400 HH Assen, The Netherlands.3 Shell Research and Technology Centre, PO Box 60, 2280 AB Rijswijk, The Netherlands.


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shear (S) vibroseis sources. The geophones used were in the Gal’perin orientation,

which has the advantage of identical sensors in a symmetric configuration. A

disadvantage is that in-line and cross-line panels can only be made by combining data

from all three components. Noise or distortion originally present on one trace is then

present in all three orthogonal components.The first step in determining the optimal geophone arrays is the identification of the

various wave modes recorded in the data. For this purpose a noise spread was

recorded. In order to obtain sufficient offset we recorded the noise spread in four parts.

Each of the recordings has a different near-offset. A ‘moving-source’ noise spread was

recorded with a maximum offset of 1500 m. By moving the source two effects could be

observed: firstly, on soft gravel the surface waves dominated the seismic record; and

secondly, the Rayleigh wave could be seen scattering back from the side of a wadi. A

wadi is a dry river-bed; it is more consolidated and is often covered with larger rock

fragments than the surrounding soil.

Surface waves were compared in detail for shot records acquired with 12-element

and 24-element areal arrays. As very little difference could be seen, it was

recommended that a 12-element geophone array should be used in the 9C3Dsurvey. A cross-line receiver pattern was found to be effective in attenuating this noise,

attributed to scattering. The best results were obtained with a receiver pattern cross-

line extent of 40 m and a 25 m shot spacing. The parameters were similar to those used

for conventional 3D seismic acquisition in this area.

On the source side, an obvious requirement is that the amplitude and phase of the

shear signals are repeatable for the two perpendicular source directions used. By

controlling the ground force a known seismic wavelet is obtained that is independent of

the acquisition direction. For this purpose, force control was installed on the (Prakla

VVCA/SH17) shear vibrator. Various baseplate designs were considered. On the

survey gravel plain, a baseplate design with teeth was preferred over a flat baseplate to

improve the ground coupling.

An important parameter is the vibrator’s output force required to obtain sufficientreflection signal back from the target reflections. In the absence of any shear data for

comparison, there was some apprehension regarding the penetration of shear energy

into the ground. A sweep was therefore designed which gave the maximum possible

output force at the proposed location. This resulted in a 24 s, 6– 48 Hz linear sweep at a

70 kN output force. Thus a ‘production spread’ was recorded which simulated the

output force of four vibrators (the maximum envisaged in the 9C3D survey) by

sweeping in total 16 times per vibration point. A source pattern was simulated by

dividing the 16 sweeps equally over four locations 12.5 m apart. In order to obtain

sufficient offsets with the limited number of recording channels available for this

experiment, we recorded a production spread in four parts. The spread was kept at its

original location but the source was moved three times to give a maximum offset of

1521 m. A simulation of two vibrators was clearly inferior to the results of a simulationof four vibrators. It was therefore recommended that a group of four shear vibrators

should be used in the 9C3D experiment.

618 J.H. Hake et al.

1998 European Association of Geoscientists & Engineers, Geophysical Prospecting , 46 , 617–646


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With this equipment in place and the field parameters decided, the seismic pilot line

was orientated east– west (Fig. 1), at 45 to the main open fracture direction in the

reservoir. At the test site the top of the Natih reservoir lies at a depth of 700 m and

dips about 4 to the west.

A shear experiment over the Natih field 619


Figure 1. Surface location of well Natih-85, vibration points of VSPs and pilot line.


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Results

Figure 2 shows displays of some of the horizontal-component data acquired. Note that

by this terminology we refer consistently to the four components of data recorded by

the horizontal components of the geophones and generated by two perpendicular

directions of the shear vibrator. The displays contain the four horizontal components

of a shot gather with an offset range between ¹1200 m and þ1200 m. The following

processing was applied:

1 statics (the same for all components),

2 k – f filtering for ground-roll removal, and

3 NMO correction with the so-called EE stacking velocities.

A precise nomenclature is adopted to identify unambiguously the various components.

Each data component is identified by a two-letter label to specify the source and

receiver orientation, respectively. We use N for North, E for East and Z for vertical. For

example, data component EE was generated with an east–west polarized source and

recorded by an east–west orientated geophone. For our pilot line, component EE thus

has shots and receivers orientated parallel to the seismic line whereas component NN

has shots and receiver orientated perpendicular to the seismic line.

The residual moveout on the NN component for all reflections can be seen in Fig. 2.

Data components NE and EN are those with orthogonal source–receiver orientation.

These contain very little coherent reflection signal in the shallow part, but at 2.40 s, and

particularly around 2.65 s, significant reflection energy can be seen. This is a clear

indication of anisotropy in this time window.

A criterion for determining the shear-wave polarization is the maximum linearity of

the particle motion. This is based on the principle that a linearly polarized signal

preserves its linear particle motion if the displacement vector coincides with that of one

of the shear polarization eigenmodes. We calculated the linearity of the particle motion

in the shot gather within the time windowbetween 2.60s and 2.70s for a full 180 range

of orientations. The result is shown in Fig. 3, and the results are clearest for the negativeoffsets. At small offsets, maximum linearity (red colour) occurs in the directions NW

and NE. At the large offsets, maximum linearity is seen with the data orientated north–

east. This means that for far-offsets the largest shear-wave splitting is seen with the data

orientated north–east. Another illustration of this phenomenon is shown in Fig. 4, in

which components NN and EE are displayed next to each other. Notice the absence of

splitting at the small offsets in the 2.40–2.65 s time interval. The observed variation of

the shear eigenpolarization with offset, and with shot-to-receiver bearing, has

consequences for the processing of 9C3D data. Velocity analyses and stacking, for

example, will be inferior if shot and receiver orientations are fixed (i.e. not varying with

offset and bearing). A possible solution is to limit the data to small offsets.

Figure 5 shows the result of rotating the data into the NE–NW direction. Down to

2.40 s there is still no shear-wave splitting at the small offsets, but at 2.65 s a clear timesplitting has occurred. Also notice the inverted polarity at 2.50 s. At the far-offsets the

data appear more noisy than in Fig. 4, a consequence of the interference of the two




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Figure 2. Four-component prestack data set in [N,E] coordinates (NN, NE, EN, EE), NMO

corrected with the EE stacking velocities.


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F i g u r e

3 .

L i n e a r i t y o

f p a r t

i c l e m o t i o

n a s a

f u n c t i o n o

f s o u r c e – r e c e

i v e r o

f f s

e t a n

d r o t a t i o n a n g

l e ( i n t e r v a

l 2 . 6 –

2 . 7

s ) .


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Figure 4. Shot gathers for components NN and EE (NMO corrected with EE stacking

velocities). No splitting at small offsets, but residual moveout at larger offsets for component

NN.


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Figure 5. Shot gathers Y0Y0 and X 0X 0 with Y0¼ NE and X 0¼ NW (NMO corrected with EE

stacking velocities). Note the splitting for the small offsets at 2.6 s.


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shear modes in this particular coordinate system. The observed variation of the shear

polarization eigendirections with offset and shot-to-receiver bearing has consequences

for the processing of 9C3D data. Steps such as velocity analysis and stacking, with

source and receiver orientations independent of offset and bearing, will give inferior

results because of the mixing of the shear modes. A solution is to limit the processing tosmall offsets only.

As we have not yet established a time–depth relationship for the major horizons, it is

not possible at this point to identify the Natih interval unambiguously. However, this

will be revealed by the VSP data acquired after the pilot experiment; this is the subject

of the next section.

Acquisition of the shear VSPs

Two VSPs were recorded with Schlumberger’s Combinable Seismic Imager (CSI, also

known as CSAT) in well Natih-85 at offsets of 90 m and 614 m. For brevity we refer to

the former as the ‘zero-offset VSP’. The well position with respect to the pilot surface

seismic data and the coverage of the 9C3D survey is shown in Fig. 1(b). Shear datawere generated with one shear vibrator moving forwards and backwards over a quarter

circle with a radius of 5 m. With this technique the two zero-offset shear vibration

points are almost 5 apart, as seen from the well head, but no correction has been made

for this effect. Compressional data were acquired with a separate vertical vibrator. Data

were acquired in two open-hole sequences. First the well was drilled down to the top of

the Natih formation after which the zero-offset VSP was acquired and recorded from

top Natih up to a depth of 280 m, with a nominal depth decrement of 15 m. The offset

VSP was acquired up to a depth of 600 m. After deepening the well down to about

1125 m the deepest parts of both VSPs were acquired in the same manner. At most

levels the shots were repeated several times.

Zero-offset shear VSP data quality and interpretation

The best shots were selected for further processing. Subsequently the data were rotated

in a fixed N–E –Z coordinate system which is necessary in view of the different

coordinate systems of the vibrators and the orientation of the tool in the hole. In view of

later rotation operations applied to the data, we use primed characters to indicate

transformed data; for such a new coordinate system we will use X 0 and Y0, and this is

related to the N–E–Z system as follows: X ¼ north and Y¼ west. The zero-offset pure

shear data are shown in Fig. 6, with an automatic volume control (AVC) of 200 ms

applied for display purposes. The data quality is obviously good and direct waves as

well as reflected upcoming waves are clearly visible. The fact that coherent energy is

present on the two cross-components is evidence of anisotropy. Note that on all

components tube waves are visible, with a velocity of 1050 m/s.From Fig. 6 it is possible to establish the expected P- and S-wave arrival times from

top Natih on the surface seismic. At the well location, top Natih is at a depth of about




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F i g u r e

6 .

Z e r o - o

f f s e t s

h e a r

V S P d a t a a

f t e r c o r r e c t i o n

f o r t o o

l o r i e n t a t i o n

( A V C a p p

l i e

d ) .


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900 m, therefore the P-wave two-way reflection time will be about 835 ms and the

S-wave two-way reflection time will be about 2550 ms. For the NN component the

arrival times of the direct P- and S-waves were measured and plotted (Fig. 7).

From these we derive V p/V s¼ 2.2 for the shallow part of the data and V p/V s¼ 2.8

for both the Fiqa shales and the Natih formation. The low V p/V s for the shallowpart may be affected by the lack of data over this interval.

Comparison between seismic and VSP velocities

In Tables 1 and 2 the vertical interval velocities derived from the zero-offset VSP are

compared with the seismic interval velocities obtained from the pilot data using

stacking velocities with the Dix equation. The stacking velocities are measured from the

moveout on the far-offsets, for which the eigendirections are NN and EE.

The seismic velocities of the shear waves are significantly higher than the vertical

velocities from the VSP. Based on the vertical velocities the event at 2.40 s on the pilot

shear data could be identified as the top Natih reflection. This unambiguous event

identification is one of the most important objectives of the VSP data. Significantly, itproves that the observed shear-wave anisotropy at small offsets in the pilot experiment

commences at the Natih level, which is consistent with an interpretation of vertical



Figure 7. Picked direct arrival times for P- and S-waves.


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T a b l e

1 .

C

o m p a r i s o n o

f s h e a r v e

l o c

i t i e s

f r o m

V S

P a n

d p

i l o t d a t a .

T w o - w a y s h e a r

V e r t

i c a

l s h e a r

S e

i s m

i c v e

l o c

i t y

S e

i s m

i c v e

l o c

i t y

V e

l o c

i t y r a t i o

V

e l o c

i t y r a t i o

t r a v e

l t i m

e i n t e r v a

l

v e

l o c

i t y

E E

N N

E E / v e r t

i c a

l

N N / v e r t

i c a

l

0 – 1

5 0 0 m s

5 7 5 m

/ s

1 4 2 0

1 1 5 m

/ s

1 1 6 5

1 1 5 m

/ s

2 . 4

7

0 . 2

2 . 0

3

0 . 2

1 5 0 0 – 2 0

0 0 m s

6 9 0 m

/ s

1 7 6 0

2 1 0 m

/ s

1 3 1 5

2 7 5 m

/ s

2 . 5

5

0 . 3

1 . 9

1

0 . 4

2 0 0 0 – 2 4

0 0 m s

8 4 0 m

/ s

2 0 6 5

3 3 5 m

/ s

1 5 8 5

3 3 5 m

/ s

2 . 4

6

0 . 4

1 . 8

9

0 . 4


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fractures in the reservoir. This first indication of consistency between the observations

on the shear data and the expected behaviour in the presence of reservoir fractures was

crucial for the entire 9C3D project.

Zero-offset VSP anisotropy

In order to arrive at the direction of anisotropy and the associated shear-wave time

splitting, a continuous coordinate transformation was carried out on all four (pureshear) components to find the angle at which the time splitting between the rotated NN

and EE components shows a maximum. This maximum is taken as the time splitting,

the corresponding angle as one of the symmetry axes of the anisotropy.

An example for the zero-offset VSP is shown in Fig. 8 for a depth of 780 m where the

downgoing waves show a maximum time splitting of 2.89 ms at an angle of

approximately N30E. This procedure was carried out for both downfields and

upfields of the zero-offset VSP, and the results are shown in Fig. 9.

The splitting at the shallowest level (300 m) is ¹4 ms, which indicates that in the top

layers some anisotropy must already be present. The direction of anisotropy relates to

the fast component and hence results in a positive splitting. In the case of a negative

splitting the associated direction relates to the slow component. For the shallow part

(down to 550 m) the direction for the downgoing wave is found to be between N30Eand N40E. Thereafter, down to top Natih, the direction changes from approximately

north at a depth of 550 m to approximately N30E at about 830 m. From 300 m to top

Natih the anisotropy is about 1%. The splitting of the upcoming wave from top Natih

exhibits an increase in the splitting to some 10 ms at 500 m; the increase seems to be the

continuation of the downgoing wave. However, the direction of the maximum splitting

stays constant at about N10E. At present the reason why the downfield gives a

different direction from the upfield is not known. Unfortunately the quality of the data

obtained between depths 800 m and 900 m is such that no meaningful splitting can be

extracted.

Zero-offset VSP time splitting and anisotropy direction with strippingBefore analysing the Natih interval we have to eliminate the anisotropy effects of

the Fiqa formation. This process is called stripping (Winterstein and Meadows 1990).



Table 2. Comparison of compressional velocities from VSP and pilot data.

Two-way P-wave Vertical P-wave Seismic P-wave Velocity ratio

traveltime interval velocity velocity seismic/vertical

0–800 ms 1975 m/s 2075 75 m/s 1.05 0.04


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The data at a depth of 780 m were selected to carry out the stripping as they offer the

best-quality data above the reservoir with very similar cross-components (X 0Y0

and

Y0X 0). The data were first (parallel) rotated over 34 (corresponding to the direction of the fastest shear wave at this level; see Fig. 8) and subsequently 2.9 ms was subtracted

from the time of the slowest components Y0X 0

and Y0Y0. A pictorial proof the validity of

shifting the data of the slowest components with the established time shift is shown in

Fig. 10(a), where the arrivals of two perpendicular shots into two perpendicular

receivers are constructed. The waves have travelled through two layers with different

directions and magnitudes of anisotropy. As can be seen, a shot in the X-direction

arrives at both receivers at the same time. Receivers from the Y-shot show the same

arrival times but these are retarded. This method of stripping is valid only for one-way

wave propagation as in the downfield of VSPs. For reflection seismics, however, the

slowest component should be corrected with the time splitting obtained and the cross-

component with half the time splitting. This is illustrated in Fig. 10(b) where, for

the same model, the reflected waves at the surface are analysed. For upfields inVSPs the situation is more complicated: the downgoing waves down to the

stripping level should be corrected according to Fig. 10(a) and the remaining part



Figure 8. Example of time picks for shear components X 0X 0 and Y0Y0 as a function of rotation

angle.


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of the wave path with an (as yet unknown) combination of the one-way and the

two-way corrections.

The time splitting and the direction of the fastest shear wave over the Natih intervalafter stripping at a level of 780 m are shown in Fig. 11. The splitting is now very small:

almost zero with a possible increase to about 3 ms towards the bottom of the well,



Figure 9. Time splitting and main direction of anisotropy as a function of depth as measured

from direct arrivals of downgoing waves and from the reflection of top Natih in the upfield.


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Figure 10. (a) Direct arrival times of two shots (X and Y) after travelling through two

layers with different anisotropy; direction of top layer in X-direction. Signals arrive at the same

time for individual shots. (b) Reflected arrival times of two shots (X and Y) after travelling

through two layers with different anisotropy; direction of top layer in X-direction. Signals on

cross-components arrive at the same time.


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indicating that hardly any anisotropy is present over the Natih interval at the location of

well Natih-85. Because the splitting is so small the direction of the fastest shear wave is

ill defined, as is apparent from the scatter in the azimuth plot. For this reason we havenot attempted to correct the upfield of the deeper reflection via stripping. The bottom

of the Natih interval (Natih E) yielded a splitting of some 6 ms with an azimuth for the



Figure 11. Time splitting and main direction of anisotropy as a function of depth as measured

from direct arrivals in zero-offset VSP after stripping at 780 m.


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fast wave between N45E and N50E. This is consistent with the pilot experiment

results. A deeper event, although not penetrated by well Natih-85, was identified as

corresponding to the top Shuaiba reflection and yielded an upfield splitting of some

6 ms with an azimuth of about N50E.

Observation of cusps in the offset VSP

The VSP acquisition programme in well Natih-85 also included an offset shear VSP.

With the seismic sources located 614 m offset to the east of the well head, recordings in

the well were made between depths of 600 and 1100 m. We now show and explain an

interesting wave phenomenon observed on this VSP and prove its recognition to be of

paramount importance for the determination of the elastic properties of the

overburden.

Figure 12 shows three data components from this offset VSP. The downgoing

P-wave can be seen on components EE and EZ around 0.5 s. For clarity we point out

that component EE for the offset VSP data implies shots and receivers orientated

parallel to the source-location–well-head direction, as the shot location is to the east of the well head (Fig. 1). The downgoing shear wave should arrive near 1.4 s at 600 m

depth in the offset VSP if it propagates with the vertical velocity. However, the data

show arrivals near 1.1 s, indicating the strong anisotropy of the Fiqa shale. Instead of a

single arrival, four arrivals are observed with distinct traveltimes and distinct

polarizations. Branches 1 and 2 have a particle motion predominantly horizontal in

the east–west direction, branch 3 has a true vertical particle motion and branch 4 is

polarized north–south. The corresponding hodograms, depicting the particle motions

in a short time gate around the points indicated on the branches, are given in Fig. 13.

The apparent velocities of these arrivals are clearly distinct from the tube-wave velocity

and the P-wave velocity. The apparent velocity of the branch seen on the EZ

component is very large.

These four arrivals can be explained as an exceptional manifestation of anisotropywhich is known to occur in a medium with elastic parameters such that the wave

surface of one of the shear modes contains cusps. The three branches observed on

components EE and EZ belong to this shear mode, with the upper arrival on

component EE being the reverted branch of the wavefront. This interpretation of the

data is supported not only by traveltime modelling (see discussion below), but also by

analyses of the wavelets. The wavelet on the reverted branch should, in a homogeneous

medium, be the Hilbert transform of the wavelet on the normal branches (White 1982).

There is indeed a remarkable agreement between the Hilbert transform of the wavelet

of what is interpreted as a normal branch and the wavelet of the interpreted reverted

branch (Fig. 14).

Modelling of the cusps

We undertook an anisotropic ray-trace modelling exercise to match the observed




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F i g u r e

1 2 .

B a

s i c

d a t a f r o m

o f f s e t V S P ( c o m p o n e n t s

E E

, E Z a n

d N N ) .


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traveltimes and polarizations, and in this way retrieve the elastic medium parameters of

the Fiqa shales covering the Natih reservoir. First the time picks were corrected forchanges in wavelet shape as described above. The matching was done at two depth

levels, namely 645 m and 780 m. In the modelling the interval from the surface down to

645 m was assumed to be homogeneous. This implies that the inhomogeneity observed

in the zero-offset VSP is explained in terms of anisotropy. While this is obviously not

correct, the neglect of inhomogeneity simplifies the inversion; furthermore we feel that

the inhomogeneity is too small to warrant a more robust approach.

An obvious choice for the anisotropy system is that of hexagonal symmetry with a

nearly vertical symmetry axis. This is not only because of the limited complexity of

such a system but also because this matches the measured polarizations of the fast and

slow shear modes. The fast mode is polarized in the vertical plane through source and

receivers and exhibits the cusps, whilst also having the larger stacking velocity.

When we use the conventional notation, parameter c 33/ r is the squared verticalP-wave velocity and is computed in a straightforward manner from the zero-offset

shear VSP. Similarly c 44/ r is easily determined as this is the squared vertical shear



Figure 13. Hodograms at different positions along the cusp. X ¼ east ¼ in-line. Y¼ north ¼

cross-line, Z¼ vertical.


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velocity. Branch 4 in the offset shear VSP corresponds to the shear wave with anelliptical wave surface, defined by c 44/ r and c 66/ r. With c 44/ r known, c 66/ r is then

determined from the time pick on this branch. The remaining elastic parameters

are c 11/ r and c 13/ r. They can be found in a trial-and-error procedure by scanning the

c 11/ r – c 13/ r space for combinations that result in a match with the four observed

traveltimes, i.e. the offset P-wave traveltime plus the three time picks for the cuspoidal

shear mode. One additional variable is the orientation of the symmetry axis. With an

exactly vertical symmetry axis, no combination of elastic constants could be found that

fitted the observed times. Even the best fit in this orientation still gave time errors of up

to 20 ms which is far outside the measurement accuracy. A much better fit (virtually no

difference between measured and modelled traveltimes) was obtained with the

symmetry axis tilted 3 to the east. In this orientation the symmetry axis is

perpendicular to the local structural dip of the Fiqa shales. The azimuth of the tiltedsymmetry axis was not well resolved: variations between N45E and E45S affected the

traveltimes by less than 5 ms.



Figure 14. Wavelets of direct arrivals for different branches. The wavelet of branch 1 is almost

identical to the Hilbert transform of the wavelet of branch 3.


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The results thus derived for this first layer (i.e. from the surface to depth 645 m)

are:

c 11/ r¼ 5.15 km2

/s2

,

c 13/ r¼ 3.00 km2/s2,

c 33/ r¼ 3.69 km2/s2,

c 44/ r¼ 0.39 km2

/s2

, and

c 66/ r¼ 1.17 km2

/s2

.

With these parameters for the first layer and a plane interface at depth 645 m, dipping

4 to the west, the modelling was continued for the 780 m depth level. Again,

traveltimes could be matched only when this second interval also possessed strong

anisotropy of hexagonal symmetry with a nearly vertical symmetry axis. For the best fit

the modelled times were within 5 ms of the picked times, except for the branch shown

on component EZ on the offset VSP, which was 7 ms in error. The parameters obtained

are:

c 11/ r¼ 7.0 km2

/s2

,

c 13/ r¼ 4.0 km2

/s2

,c 33/ r¼ 5.3 km

2/s2

,

c 44/ r¼ 0.72 km2

/s2

, and

c 66/ r¼ 1.9 km2

/s2

.

Again the symmetry axis is approximately perpendicular to the structural dip. A wider

range of symmetry axis orientations was possible without affecting traveltimes by more

than 5 ms: from true vertical up to 10 tilt for azimuths ranging from north through east

to south.

The velocities in the second interval are higher than those in the first layer, but the

anisotropy characteristics are quite similar. For both intervals the elastic parameters

result in cusps in a direction at about 45 with the symmetry axis. This occurs when the

condition (c 11¹ c 44)(c 33 c 44)¹ (c 13þ c 44)2>0 is fulfilled (Musgrave 1970). Because of

the similarity between the elastic tensors in the two layers the cusps generated in theshallow part of the Fiqa shale are sustained in the deeper part. Modelling shows that for

an isotropic second layer, for instance, ray-bending effects would have resulted in a

rapid shrinking of the surface of the triangle formed by the three branches (also known

as a ‘laguna’).

We are not aware of other reports of this phenomenon, which was recognized in

theory a long time ago. It is difficult to believe that the Fiqa shales are unique worldwide

in terms of anisotropy cusps. The absence of similar observations elsewhere might be

explained by the fact that it requires the combination of a suitable shear VSP

experiment and the circumstance of a large interval with uniform characteristics,

allowing the laguna to develop. In general, inhomogeneities causing ray bending or

reflection effects will rapidly deform the characteristic shape of a cuspoidal wavefront,

making it difficult to recognize. However, we can conclude that the presence of suchstrong Fiqa anisotropy over a large depth range has important implications for the

interpretation of the underlying Natih reservoir.




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Well log data

Several types of sonic log were run with the Schlumberger’s Dipole Sonic Imager

(DSI) tool in Natih-85. Figure 15 shows the raw data for the compressional-wave and

one of the shear-wave logs. The compressional data show a regular increase in velocityover the Fiqa shales in contrast to the shear velocities, which exhibit definite breaks in

the lithology. Two shear logs were recorded over most of the well trajectory with

perpendicular sources and receivers parallel to the sources. In multicomponent

nomenclature they can be compared with X 0X 0

and Y0Y0

components. From these

shear logs we have constructed a fast and a slow shear log by taking at every depth the

highest and lowest reading, respectively. The difference between these two integrated

shear logs shows some remarkable features (Fig. 16a).

In a qualitative sense the splitting derived from direct arrivals (Fig. 16b, which is a

copy of Fig. 9) is almost identical to the splitting of the two integrated shear logs. In a

quantitative sense the splitting between the two shear logs is about 3.5 times as large as

the splitting from the direct waves of the VSP data. This could be a result of the fact

that the layers (with different intrinsic anisotropic directions) are significantly smallerthan the wavelength of the seismic. The seismic data represent an average property as

opposed to the intrinsic anisotropy of each individual layer as measured by the sonic



Figure 15. P- and S-wave traveltimes as a function of depth as measured in Natih-85.


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tool. Consequently the seismic anisotropy can be at most the same as the borehole-

derived anisotropy. (However, higher-frequency borehole waves have the possibility of

avoiding any compliant or fractured zones by ‘fast-tracking’ whilst seismic waves donot. In that situation higher-frequency waves might underestimate any fracture-related

anisotropy.) The changes in the slope of both curves correspond to changes in



Figure 16. Comparison between time difference of the two integrated shear logs (a) with the

splitting of the direct arrivals (b).


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lithology. Although the lithology is reported to be shale from 100 m down to 900 m,

obvious changes are visible from the shear logs.

DSI data processing

It is, in principle, possible to detect aligned open fractures with borehole acoustic

methods since such fractures can cause the reservoir to be anisotropic for the

propagation of borehole flexural waves (Ellefsen, Chang and Toksöz 1991). Flexural

waves are dispersive surface waves (modes) excited by an acoustic dipole source in a

mud-filled borehole that propagate along the borehole wall. In anisotropic formations,

the flexural wave splits into a fast mode and a slow mode. We refer to the particle

motion of the fast and slow flexural modes measured on the borehole axis in the plane

perpendicular to the borehole as mode orientations or anisotropy orientations, which

are aligned with the polarizations of fast and slow quasi-shear waves, respectively, when

such body waves would travel through the formation parallel to the borehole. For

example, the orientation of the fast flexural mode in a vertical borehole penetrating a

formation which contains steeply dipping, aligned, open fractures is expected to beparallel to the strike of the fractures.

The flexural waves excited by the DSI tool originate from two acoustic dipole

sources orientated perpendicularly to each other and to the borehole wall. Flexural

waves from each source are recorded along two perpendicularly orientated receiver

arrays, resulting in four-component (4C) recordings. The processing method we used

is similar to the method presented by Esmersoy et al . (1994). For each source depth the

two perpendicular mode orientations are estimated from the recorded waveforms, the

two corresponding (quasi-)flexural modes are synthesized and their slowness at some

frequency is estimated. Thus a fast-mode orientation and a slow-mode orientation are

determined. All elements of the tool’s dipole receiver array are simultaneously used in

the processing.

Figure 17(a) shows flexural-mode orientations (red diamonds) for Natih C, togetherwith the gamma-ray log (black curve). Only one of the two perpendicular mode

orientations is shown; its azimuth ranges between about N30E and N60E. Figure

17(b) shows the same orientations together with the borehole geometry (see caption).

An important observation is that the borehole is elongated in the NW–SE direction and

that the mode orientations resemble the borehole geometry. Figure 17(c) shows the

slowness of the two flexural modes (red and green diamonds) together with the

standard error (red and green dashes), estimated at a frequency of 2.0 kHz. While a

lower frequency would yield slowness estimates closer to the formation (quasi-)shear-

wave velocities, as predicted by flexural-wave dispersion theory, flexural-wave

amplitudes recorded in Natih C are below the noise level for frequencies less than

2.0 kHz. The red diamonds in Fig. 17(c) belong to the orientations in Fig. 17(a). In

conclusion, while mode orientations (anisotropy orientations) are well resolved fromthe 4C flexural data, significant differences in slowness of the two flexural modes are

not well resolved in Natih C.




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Figure 17(d) shows mode orientations for Natih B together with the gamma-ray log.

Along most of the depth interval the borehole geometry (Fig. 17e) shows no significant

elongation. Owing to different formation characteristics and hole size, the flexural

waves recorded along Natih B contain higher frequencies than those recorded along

Natih C. Figure 17(f) shows the slowness of the two flexural modes determined at3.0 kHz. Between shots 550 and 630, the difference in slowness for the two flexural

modes is statistically significant, while outside this interval no significant slowness

difference is measured. The fast-mode orientation varies between about N30E and

N60E between shots 550 and 630.

Figure 17(g) shows mode orientations for Natih A together with the gamma-ray log.

The orientation varies between about N10W and N30E. The borehole is

predominantly elongated in a N– S direction (Fig. 17h). Compared with the Natih C

unit, the caliper and anisotropy azimuths in the Natih A interval do not resemble each

other as well; deviations are as large as 30. Figure 17(i) shows that a large slowness

difference is measured for the two flexural modes.

Discussion of the DSI results

The NE–SW flexural-mode orientations observed along Natih C and part of Natih B

are consistent with fast-mode orientations found in the downfield of the zero-offset

VSP survey as well as the pilot experiment and the geological expectations. It is

therefore attractive to explain these orientations in terms of the presence of open

fractures aligned in the NE–SW direction, but for completeness we need to offer an

alternative explanation for some intervals at least. Because the fractures are orientated

parallel to the present-day in situ maximum horizontal principal stress, they are likely to

be open (Mercadier and Mäkel 1991). According to this interpretation the phase

velocity of the NE–SW orientated flexural mode should be larger than the phase

velocity of the perpendicularly polarized mode, measured at some frequency. Because

we do not resolve such a slowness difference (Fig. 17c), the magnitude of flexural-waveanisotropy is apparently small, which is in line with the small time splitting found in the

zero-offset VSP.

The alternative interpretation of the observed mode orientations in Natih C is that



Figure 17. Estimated anisotropy orientations and borehole geometry for Natih C (top), B

(middle) and A (bottom). (a),(d),(g) Anisotropy orientations (red diamonds) with respect to

geographic north, together with the gamma-ray log (black curve); a reference line (dashed) is

shown at 0. (b),(e),(h) Borehole geometry; the red and green curves are the hole diameter in two

perpendicular directions, the blue line is the caliper orientation of the red curve; anisotropy

directions are also shown (red diamonds). (c),(f),(i) Slowness of the two synthesized flexural

modes. Red diamonds belong to the mode orientations shown in (a),(d),(g) and green diamonds

to the perpendicularly orientated flexural mode. The time window is 1.5–4.0 ms (Natih A), 1.0–

3.0 ms (Natih B), 1.5–4.5 ms (Natih C shot 1000–1052) and 1.5–3.5 ms (Natih C shot 1053–

1110).


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they are caused by the elongated borehole shape rather than formation anisotropy. A

flexural wave propagating in an elliptical borehole in a purely isotropic formation splits

into a slow mode and a fast mode with mode orientations parallel to the major and

minor axes (Randall 1991). The observed mode orientations may also result from

superposition of the effects of borehole shape and formation anisotropy. Note that theshort caliper (red dashed curve in Fig. 17b) is parallel to the proposed fast-mode

orientation for Natih C.

The borehole cross-section along the Natih B unit appears to be predominantly

circular (Fig. 17e). Furthermore, slowness estimates at 3 kHz are not significantly

different for the two modes along most of Natih B; the two flexural modes propagate at

equal slowness along most of the unit. The large variability in mode orientations is

therefore most likely to be a consequence of the absence of anisotropy and is noise

controlled. An exception is the depth interval between shots 550 and 630 (965 m and

978 m depth), where low gamma readings indicate that the rocks probably consist of

relatively clean carbonates; the NE–SW fast-mode orientation in this depth interval is

similar to Natih C.

The borehole along Natih A is elongated. Caliper azimuth and flexural-modeorientation differ by as much as 30 (Fig. 17h). This suggests that mode orientations

are not fully dominated by borehole shape in Natih A. Note that, in contrast with the

findings for Natih C, the long caliper (red dashed curve in Fig. 17h) is parallel to the

fast-mode orientation for Natih A. The processing indicates that the fast-mode

orientation for Natih A is not NE–SW, but is centred about a N–S direction with a

scatter of approximately15. This orientation is very close to the N10E direction

found from the upfield of the zero-offset VSP.

The difference in slowness between the two shear waves at 2.0 kHz over the Natih A

interval is about 40 ms/ft (130 ms/m). Integrating this value over the thickness of this

interval (some 40 m), we arrive at a splitting of about 5 ms one-way time which is com-

parable with the maximum time splitting of 3 ms derived from the zero-offset VSP.

Conclusions

The 9C2D seismic pilot, shear VSP and DSI log experiments discussed provided

significant underpinning of the large-scale Natih 9C3D survey in terms of expected

data quality, field effort, usable offsets for post-stack processing and interpretation,

feasibility of measuring anisotropy and local calibration of the later full-scale anisotropy

interpretation. With the exception perhaps of the offset shear VSP (which may only be

needed in special circumstances), we consequently recommend such feasibility and

calibration experiments for any full-blown 9C3D survey of this kind.

The pilot experiment confirmed the feasibility of acquiring shear reflection data of

sufficient quality with conventional receiver patterns and a conventional station

spacing. It revealed a substantial difference in stacking velocities for the two shear-wavemodes, an indication of strong anisotropy in the Fiqa shales that overlie the objective

Natih reservoir.




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The zero-offset VSP was essential in providing the vertical velocities required for

time–depth conversion. With this information the Natih interval could be identified in

the pilot data between 2.40 and 2.65 s. Analyses of this interval gave clear indications of

shear-wave splitting with the fast shear wave polarized N45E, which is consistent with

open fracture directions known from nearby wells. The arrival of the top Natih shearreflection later than originally expected was particularly important in relaxing the initial

16 m station spacing to the standard 25 m spacing. By showing that the ground roll

would cross the Natih reflections at a later time than had been earlier estimated, we

were confident that sufficient multiplicity for signal-to-noise suppression would be

retained within the blanking cone at standard spacing.

The offset VSP further confirmed the strong anisotropy of the Fiqa shales and

allowed the data to be inverted in terms of its elastic parameters. The anisotropy is such

that the wave surface of one of the shear modes has cusps. As a result of this the

downgoing shear wavefield in the offset VSP resulted in four clearly distinct arrivals.

The anisotropy could be modelled by hexagonal symmetry with a nearly vertical

symmetry axis. The Fiqa anisotropy complicates 9C3D data processing and its impact

has to be carefully considered. At small offsets the shear-wave eigendirections will bedetermined by azimuthal anisotropy, but at the larger offsets the strong Fiqa anisotropy

will dominate all azimuthal effects.

From the analysis of zero-offset VSP data, it is concluded that at the location of well

Natih-85 the anisotropy in the Fiqa gives rise to a time splitting of the order of 12 ms

two-way traveltime with an azimuth direction increasing from N10W to about N45E

at top Natih. The anisotropy in the reservoir is small at this well. The upfield at top

Natih is approximately N10E, whereas the splitting over the reservoir section is at

most 3 ms. This makes it difficult to apply layer stripping for the deeper reflections.

The deeper reflections (from Natih E and Shuaiba) show a splitting in the upfield of

some 6 ms with azimuths of N45E and N50E, respectively.

Anisotropy orientations for various units of the Natih reservoir have been obtained

by processing 4C borehole flexural waves recorded in well Natih-85. For the Natih Cunit the processing results are consistent with a NE–SW fast-mode orientation

obtained from the VSP survey, as is the case for the depth interval between about

965 m and 978 m in the Natih B unit. For most of the Natih A unit, fast-mode

orientations are centred about a N–S direction, in line with the findings of the upfield

of the zero-offset VSP. Also, the small differences between the two shear waves indicate

that at the position of well Natih-85 the anisotropy is rather small.

Acknowledgements

The authors gratefully acknowledge discussions with J.C. Hornman (then at Shell

Research in the Netherlands) and with D.C. DeMartini, P. Hatchell and S. Smith atShell Development Company in Houston. We thank the Oman Ministry of Petroleum

and Minerals for their permission to publish this paper.




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References

Ellefsen K.J., Cheng C.H. and Toksöz M.N. 1991. Effects of anisotropy upon the normal modes

in a borehole. Journal of the Acoustical Society of America 89, 2597–2616.

Esmersoy C., Koster K., Williams M., Boyd A. and Kane M. 1994. Dipole shear anisotropy

logging. 64th SEG meeting, Los Angeles, USA, Expanded Abstracts, 1139–1142.

Mercadier C.G. and Mäkel G.H. 1991. Fracture patterns of Natih formation outcrops and their

implications for the reservoir modelling of the Natih field, North Oman. SPE paper 21377.

Musgrave M.J.P. 1970. Crystal Acoustics. Holden-Day, Inc.

Randall C. 1991. Multiple acoustic waveforms in nonaxisymmetric boreholes and formations.

Journal of the Acoustical Society of America 90, 1620–1631.

White J.E. 1982. Computed waveforms in transversely isotropic media. Geophysics 47, 771–783.

Winterstein D. and Meadows M. 1990. Shear-wave polarizations and subsurface stress

directions at Lost Hills field. 60th SEG meeting, San Francisco, USA, Expanded Abstracts,

1435–1438.



Documents

A Shear Experiment Over the Natih Field in Oman-pilot Seismic and Borehole Data