1. Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel
Time-elApSe heTerogeneiTy
meASuremenT in Two
SedimenTAry roCkS:
impliCATionS For Co2
SequeSTrATion
Ali Osman ÖNCEL
51

3. Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel, Türk Coğrafya Kurumu Yayınları, No:5, 105-114, İstanbul 2011
Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel
Time-elApSe heTerogeneiTy
meASuremenT in Two
SedimenTAry roCkS:
impliCATionS For Co2
SequeSTrATion
Ali Osman ÖNCEL
Istanbul University Faculty of Engineering Department of Geophysics
oncel@istanbul.edu.tr
Öz Abstract
ldV sistemi ile farklı gözeneklilik özelliklerine Time-lapse changes of ultrasonic seismic velocity
sahip kayaç numunelerinin, suya ve petrole doygunluk changes versus saturation changes are determined
düzeylerinin zaman içinde değişimine bağlı olarak for two different rocks based on the ldV system.
sismik hız değişimleri belirlenmiştir. uygulanan ldV system for the monitoring system for time-lapse
yöntem kayaçların petrofizik özelliklerinde ki changes of petro-physical properties (saturation,
(doygunluk, gözeneklilik, sismik hız) değişimlerinin porosity, seismic velocity) has been firstly tested
zaman içinde ki değişimlerinin izlenmesi ile ilgili through this study. results of the present work show
olarak, ldV sisteminin ilk olarak uygulanması ile ilgili that the use of the time-lapse monitoring seismic
bir test çalışmadır. ortaya konan çalışma, üretim velocity changes is highly sensitive to the changes in
sahalarında ki kayaçların laboratuar ortamında the level of the petrophysical parameters. Therefore,
doygunluk değişimlerinin zaman içinde sismik hız the method presented in this work can enhance the
parametresi ile incelenmesi ile ilişkilidir. Sismik hız quality of Feasibility risk Analysis before starting 4d
değişimleri, yeraltında farklı akışkan (örn., su ve petrol) field seismology work.
hareketlerinin sürekli incelenebileceğini gösterdiği
için, 4 boyutlu Saha Sismolojisi çalışmalarından önce
Fizibilite risk Analizlerinin daha doğru yapılmasında
kullanılabileceğini göstermektedir.
53

4. Time-elapse heterogeneity measurement in two sedimentary rocks... Ali osman Öncel
1. Introduction heterogeneity changes. A laboratory model that
Time-lapse study of the seismic tomography is used to monitor time-elapse velocity changes
in the area of fluid injection is used to image compose of waveform generator; LDV and
spatial extension of plume caused by fluid recorder (see Fig. 1).
injection, CO2 saturation both in the field (e.g.,
Daley et al., Environ. Geol, 2007) and in the The detail of the experimental design is
laboratory (e.g., Lei and Xue, 2009). Therefore, firstly introduced by Nishizawa et al., (1997)
time-lapse seismic studies in the field appear for the study of 2D rock surface heterogeneity.
to be an efficient tool to examine some details A higher number of stacking, i.e. 2000 times,
of the fluid injection. In this paper, I want to applied to increase the signal-to-noise ratio
show the initial results of a testing work for the of the waveform while the PZT is repeatedly
time-lapse heterogeneity measurement by laser being driven by the same input signal. Time-
ultrasonic, which is based on the measurements elapse seismic experiment in this paper is a first
of petro-physical parameter, i.e. saturation, and example of a testing study of the LDV following
geophysical parameter, i.e. ultrasonic velocity, the previous works of the surface heterogeneity
caused by injected fluids, i.e. water and oil, into characterization of the rocks. In this experiment,
the sedimentary rocks. we used two sandstone samples, i.e. Izumi
and, Shirahama, from the onshore sedimentary
2. Experimental Design and Results sequences in Japan, and their porosity of Izumi
The LDV (Laser Dopler Vibrometer) has and Shirahama is 6.5% and 13%, respectively.
been used to study wave propagation of the
surface rock heterogeneities by Nishizawa et We used a fluid container and filled the fluid
al. (BSSA, 1997), and proved as one of the inside at first, and then the rock sample was set
successful experimental tool to investigate rock in the container (see Figure 2). Therefore, the
heterogeneity. Seismic attributes, e.g. velocity bottom of the rock was sank into fluid container
and amplitude, are some reflectors of the while the top of which that is dry was over the
Figure 1 - A waveform generator generates a signal which is converted to the elastic waves by a Piezoelectric
Transducer, which is the source site on the rock sample. Elastic waves propagate through the rock sample and
transmitted by the Optical unit of the LDV through reflection sheet through the LDV, which are finally recorded
in the computer
54

5. Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel
5 minutes 30 minutes
Travel Time (ms)
Elapsed Time in Hours
Figure 2-The data is an example of the water breakthrough experiment for the Izumi sandstone. Since the
porosity of the rock was lower, the experiment has continued longer. Thus, the record interval of the data is
considered to be 5 minutes at first nine hours, and 30 minutes for the rest of the experiment.
fluid container. We usually kept the level of the magnitude of the saturation causes a higher
water the same as well as keeping the room reduction of the capillarity strength by Hawkins
temperature being constant. Since there is a and McConnell, (Q.Eng. Geol., 1992).
contrast between the water and the air, it caused
the existence of capillary pressure (Pc), which Saturation rate is expressed by the time that
is a driven force of the saturation throughout the fluid fills the pore or weight changes of the
the rock. The empirical relationship between rock in time and shown in the following way:
capillary pressure and saturation is given in
the following form (see e.g., Bear and Verruijt, Saturation Rate = Weight / Time (2)
Kluwer Academic Pub, 1987): Initial condition of the water (t=0) in which the
rock is dry is 100 % while the later phase of the
Pn- Pw = Pc = f (S) (1) saturation is about the zero, but we stopped the
experiment in case the changes of the saturation
where Pn and Pw are the average pressures of air is slower much in which it is probably related
and fluid respectively. Pc is capillary pressure, to the decrease of the capillary force which is
and S is the saturation. getting closer to zero (see Table 1). In fact,
a model work of Han and Dusseault (2005,
3. Time-lapse Fluid Saturation Changes IJRMMS, 2005) suggested that the all capillary
Changes in the water saturation, caused by strengths become zero around a saturation
capillary pressure, are a well-defined factor value of 0.34.
affecting the rock capillarity strength that
generally decreases slower than the changes
of the capillary pressure as shown by model
works of Han and Dusseault, (IJRMMS, 2005).
In fact, another relation based on the samples
of the sandstone shows a negative exponential
relation between the rock capillarity strength and
water saturation, which means that the higher
55

6. Time-elapse heterogeneity measurement in two sedimentary rocks... Ali osman Öncel
Table 1-The details for the measurement of the weight and calculation of the saturation rate are given
a) Shirahama- Oil b) Shirahama-Water c) Izumi-Water
Time Rock Oil Saturation Time Rock Water Saturation Time Rock Water Saturation
Weight Rate Rate Weight Rate Rate Weight Rate Rate
Change Change
Change
Hours g g/hour %
Hours g g/hour % Hours g g/hour %
0.00 461.05
0.00 461.75 0.20 463.26 11.05 34
0 646.9
0.23 462.91 4.97 54 0.40 464.16 4.50 14 1 647.72 0.82 40
0.42 463.14 1.25 14 0.69 465.15 3.49 11 1.6 647.9 0.30 15
0.60 463.21 0.38 4 1.12 466.34 2.75 9 3.1 648.35 0.30 15
1.27 463.84 0.94 10 1.57 467.31 2.16 7 4.1 648.57 0.22 11
2.02 468.13 1.82 6 5.5 648.75 0.13 6
2.23 464.32 0.50 6
2.57 469.02 1.62 5 6.8 648.94 0.15 7
3.68 464.83 0.35 4 3.10 469.76 1.39 4
4.70 465.04 0.21 2 23.7 650.29 0.08 4
3.62 470.48 1.39 4
6.23 465.42 0.25 3 25.8 650.39 0.05 2
4.34 471.28 1.12 3
7.97 465.94 0.30 3 4.89 471.8 0.95 3
I show a plot to show the time-variability of The relation between the effective porosity
the saturation rate for two different kinds of fluid, (neff) and the loss of the capillary strength (σ) is
i.e. water and oil (Figures 3). suggested by Vásárhelyi and Ván (2006, Eng.
The time- changes of the rock saturation rate Geol) based on the field data of the sandstones
due to infiltration of the water and oil takes (Hawkins and McConnell, 1992, Q. Eng. Geol.)
five to eight hours in which the rocks are about as given in below:
closer to the full saturation. The saturation b = 6.0259/ neff (3)
rate of the rock, regardless of the fluid content, In present data of the rock saturation for varying
decreased sharply from about % 50 at first by degree of the porosity, the porosity is observed
20 minutes to about % 10, but then slightly as a significant factor effecting period of the
changed later throughout the end of the period. saturation (Figure 4). Higher porosity rock
In general the oil saturation is faster (%54) shows the lost of the capillary strength within
than the water saturation (% 34), which is five hours, whereas the lower pososity of which
probably related to their density changes. has taken about the 26 hours. The saturation
rate decreased faster of the higher porosity
rock (34%) than the lower porosity rock (40%),
which can show the effect of the porosity on the
capillary pressure. For example, the elapse-time
for the saturation rate of 11% is 0.69 hrs, while it
is 4.1 hours for the higher porosity rock (Figure
4), which shows the period of the rock capillarity
strength is effected by the posity change of the
sandstone.
4. Time-lapse Seismic Velocity Changes
In this part, we examine the changes of
seismic velocity caused for changes of porosity
changes, i.e. 6.5 - 13 %, and fluid content,
i.e. water and oil. The graphs show velocity
in mm/sec and time in hours. The seismic
velocity changes in time for lower porosity rock
slightly decreased until the end of a period of
Figure 3 - Rock weight changes of the Shirahama 25 hours (on the left of Figure 5). A couple
sandstone attribute to the changes of the saturation. of anomalous changes that is sharply decreased
Since the saturation is progressed at the beginning at about 12 hours, and increased by 25 hours,
of the experiment, the period of the weight and those changes may be related to either the
measurement is kept shorter.
56

7. Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel
Figure 4 - Water saturation for the rocks of high- and low porosity.
external factors of experimental set-up changes responses of capillary pressure changes seem
such as increased ratio of the noise or intrinsic the sensitivity of their porosity sizes. Since the
properties of the rock, i.e., the capillarity magnitude of the seismic velocity is well-known
strength. The slight velocity change through factor indicating the rock strength, therefore, the
the period – in case we ignore the anomalous velocity of the lower porosity is measured higher
changes though - are probably a smaller loss than the velocity of the higher porosity rock.
of the rock strength due to less water content of
the lower porosity. However, the velocity change We put the oil on fluid container for the
of the higher porosity rock that can be caused same rock to study time-lapse changes of rock-
by faster decrease of the capillary strength is capillarity strength following the weight of the
complex at the beginning, and shows slower rock came roughly up with the same value of
change (on the right of Figure 5). The velocity the dry state (see Table 1). The velocity chan-
caused by water-breakthrough went down from ges of the oil saturated rock increased faster at
2.47 mm/sec to 2.37 mm/sec at first, and then first, and rose up slightly later (see Figure 6).
rose sharply from 2.37 mm/sec to 2.71 mm/ The velocity decreased from about 0.250 mm/
sec by 0.4 hours. It fell sharply to 2.30 mm/ sec to the 0.247 at first. Then it rose up sharply
sec at first by first hour and then slightly went by 2.8 hours in which it fell to about 0.2465
down through the end of the experiment later. hours, and then it increased slightly until the end
Consequently, the changes of seismic velocity of the period. Not only is the velocity response
caused by injected water shows generally of the oil saturation that increased in time is
a decrease within the time, though their different, but also the velocity change made
Figure 5 - The velocity changes of the water saturated rock. On the left: lower porosity rock, Izumi. On the
right: higher porosity rock, Shirahama.
57

8. Time-elapse heterogeneity measurement in two sedimentary rocks... Ali osman Öncel
saturation and time for the higher porosity is
significant. For example, the time-saturation
correlation of R=0.98 while time-velocity
correlation is 0.92. The reason of the varying
degree of the significance for the correlation
can be related to the sampling rate of the
measurements. Thus, the higher frequency for
the measurement of the higher porosity rock,
which may reduce the loss of capillary strength
faster, may be a factor for the magnitude change
of correlation.
4. 2. Fluid content effecting seismic ve-
locity
Fluid content appears to be a factor affecting
the nature of the relation between the velocity and
Figure 6 - The velocity changes of the oil- the saturation (on the right of the Figure 7). For
saturated rock, Shirahama. The velocity example, the saturation change is slower than
intermittently changed, while which of change the velocity change of the water saturated rock,
shows an increase in time. whereas it is higher than the velocity change for
oil breakthrough. The modeled curves for the
some dramatic changes, i.e. peak and through, change of both the velocity and the saturation
during entire period of experiment. show significant change of their correlation with
time. For example, the correlation between time
4. 1. Porosity effecting seismic velocity and saturation is 0.97 that is higher than the
Saturation rate (%) is a useful parameter to correlation of R=0.91 between time and velocity.
study time-variability changes of saturation and
velocity (see on the left of Figure 7), which is The correlation for oil-saturated rocks is
significant to understand the response of the about 0.7 for saturation/velocity versus time,
seismic attributes to the change of the capillary since it can be related to the higher sampling
pressure since those physical parameters are rate due to longer period of the experiment.
used to study changes of injected fluids injection,
e.g. CO2 sequestration. Since time-elapse 5. Conclusion
changes of the petro-physical and geophysical Initial results based on a testing LDV system
attributes are exponentially decreased within are promising since present experiment showed
the time – similar to change of the capillary a new perspective for the measuring time-
pressure versus the water content- the decrease lapse changes of the high-resolution ultrasonic
of physical parameters is controlled by the rock velocity. Simultaneous measurements of both
porosity and type of the fluid. For example, the rock saturation and the rock velocity may
change is observed normally faster for the higher increase the accuracy of the relation between
porosity rock and vice-versa. The velocity the velocity and the saturation since lack of
changes at the beginning of the experiment interruption for the measuring rock weight might
that is faster than the saturation changes at enhance the quality of the measurement.
first, but they remain closer to the change of the
saturation until the end of experiment, which can Since the petro-physical changes, e.g.
show the sensitivity of velocity to the loss of the porosity and saturation, are factors effecting
rock strength is significant. time-lapse changes of geophysical properties,
time-lapse experimental study can be used
The statistical relation between velocity and for Feasibility Risk Analysis before starting
time is less significant (R=0.6) than the relation 4D instrumented oil field studies of the CO2
(R=0.9) between saturation and time for the sequestration.
lower porosity rock (top plot on the left, Figure
7). However, the statistical relation between
58

9. Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel
Figure 7-The changes of the velocity and saturation for various rocks and fluid contends are shown. On the
left: Changes of the geophysical attributes are related to the differences in the rock porosity. On the right: The
changes are caused by fluid content.
Suggested Reading
See the article of Nishizawa, Lei and
Kawahara titled as “ Laboratory studies of
seismic wave propagation in inhomogeneous
media using a Laser Doppler Vibrometer,
BSSA, pp.809-823, vol. 87” for getting in the
information of the LDV.
Acknowledgement: The author than for the
support of Japan Cooperation Center, Petroleum
for getting in touch with the laboratory of the
AIST in Japan to conduct the present work.
Thanks to Osamu Nishizawa and Xinglin Lei
from the AIST for their great help to conduct the
time-elapse experiment.
59

10. Time-elapse heterogeneity measurement in two sedimentary rocks... Ali osman Öncel
60