Time-elApSe heTerogeneiTy meASuremenT in Two SedimenTAry roCkS: impliCATionS For Co2 SequeSTrATion

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Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel

Time-elApSe heTerogeneiTymeASuremenT in TwoSedimenTAry roCkS:impliCATionS For Co2SequeSTrATion

Ali Osman ÖNCELAli Osman ÖNCEL

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ÖzldV sistemi ile farklı gözeneklilik özelliklerine

sahip kayaç numunelerinin, suya ve petrole doygunluk düzeylerinin zaman içinde değişimine bağlı olarak sismik hız değişimleri belirlenmiştir. uygulanan yöntem kayaçların petrofizik özelliklerinde ki (doygunluk, gözeneklilik, sismik hız) değişimlerinin zaman içinde ki değişimlerinin izlenmesi ile ilgili olarak, ldV sisteminin ilk olarak uygulanması ile ilgili bir test çalışmadır. ortaya konan çalışma, üretim sahalarında ki kayaçların laboratuar ortamında doygunluk değişimlerinin zaman içinde sismik hız parametresi ile incelenmesi ile ilişkilidir. Sismik hız değişimleri, yeraltında farklı akışkan (örn., su ve petrol) hareketlerinin sürekli incelenebileceğini gösterdiği için, 4 boyutlu Saha Sismolojisi çalışmalarından önce Fizibilite risk Analizlerinin daha doğru yapılmasında kullanılabileceğini göstermektedir.

AbstractTime-lapse changes of ultrasonic seismic velocity

changes versus saturation changes are determined for two different rocks based on the ldV system. ldV system for the monitoring system for time-lapse changes of petro-physical properties (saturation, porosity, seismic velocity) has been firstly tested through this study. results of the present work show that the use of the time-lapse monitoring seismic velocity changes is highly sensitive to the changes in the level of the petrophysical parameters. Therefore, the method presented in this work can enhance the quality of Feasibility risk Analysis before starting 4d field seismology work.

Istanbul University Faculty of Engineering Department of [email protected]

Time-elApSe heTerogeneiTymeASuremenT in TwoSedimenTAry roCkS:impliCATionS For Co2SequeSTrATion

Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel, Türk Coğrafya Kurumu Yayınları, No:5, 105-114, İstanbul 2011

Ali Osman ÖNCEL

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

1. IntroductionTime-lapse study of the seismic tomography

in the area of fluid injection is used to image spatial extension of plume caused by fluid injection, CO2 saturation both in the field (e.g., Daley et al., Environ. Geol, 2007) and in the laboratory (e.g., Lei and Xue, 2009). Therefore, time-lapse seismic studies in the field appear to be an efficient tool to examine some details of the fluid injection. In this paper, I want to show the initial results of a testing work for the time-lapse heterogeneity measurement by laser ultrasonic, which is based on the measurements of petro-physical parameter, i.e. saturation, and geophysical parameter, i.e. ultrasonic velocity, caused by injected fluids, i.e. water and oil, into the sedimentary rocks.

2. Experimental Design and ResultsThe LDV (Laser Dopler Vibrometer) has

been used to study wave propagation of the surface rock heterogeneities by Nishizawa et al. (BSSA, 1997), and proved as one of the successful experimental tool to investigate rock heterogeneity. Seismic attributes, e.g. velocity and amplitude, are some reflectors of the

heterogeneity changes. A laboratory model that is used to monitor time-elapse velocity changes compose of waveform generator; LDV and recorder (see Fig. 1).

The detail of the experimental design is firstly introduced by Nishizawa et al., (1997) for the study of 2D rock surface heterogeneity. A higher number of stacking, i.e. 2000 times, applied to increase the signal-to-noise ratio of the waveform while the PZT is repeatedly being driven by the same input signal. Time-elapse seismic experiment in this paper is a first example of a testing study of the LDV following the previous works of the surface heterogeneity characterization of the rocks. In this experiment, we used two sandstone samples, i.e. Izumi and, Shirahama, from the onshore sedimentary sequences in Japan, and their porosity of Izumi and Shirahama is 6.5% and 13%, respectively.

We used a fluid container and filled the fluid inside at first, and then the rock sample was set in the container (see Figure 2). Therefore, the bottom of the rock was sank into fluid container 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

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fluid container. We usually kept the level of the water the same as well as keeping the room temperature being constant. Since there is a contrast between the water and the air, it caused the existence of capillary pressure (Pc), which is a driven force of the saturation throughout the rock. The empirical relationship between capillary pressure and saturation is given in the following form (see e.g., Bear and Verruijt, Kluwer Academic Pub, 1987):

Pn- Pw = Pc = f (S) (1)

where Pn and Pw are the average pressures of air and fluid respectively. Pc is capillary pressure, and S is the saturation.

3. Time-lapse Fluid Saturation ChangesChanges in the water saturation, caused by

capillary pressure, are a well-defined factor 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

magnitude of the saturation causes a higher reduction of the capillarity strength by Hawkins and McConnell, (Q.Eng. Geol., 1992).

Saturation rate is expressed by the time that the fluid fills the pore or weight changes of the rock in time and shown in the following way:

Saturation Rate = Weight / Time (2)Initial condition of the water (t=0) in which the rock is dry is 100 % while the later phase of the saturation is about the zero, but we stopped the experiment in case the changes of the saturation is slower much in which it is probably related to the decrease of the capillary force which is getting closer to zero (see Table 1). In fact, a model work of Han and Dusseault (2005, IJRMMS, 2005) suggested that the all capillary strengths become zero around a saturation value of 0.34.

Elapsed Time in HoursElapsed Time in Hours

Trav

el T

ime

(ms)

5 minutes 30 minutes

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.

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I show a plot to show the time-variability of the saturation rate for two different kinds of fluid, i.e. water and oil (Figures 3). The time- changes of the rock saturation rate due to infiltration of the water and oil takes five to eight hours in which the rocks are about closer to the full saturation. The saturation rate of the rock, regardless of the fluid content, decreased sharply from about % 50 at first by 20 minutes to about % 10, but then slightly changed later throughout the end of the period.In general the oil saturation is faster (%54) than the water saturation (% 34), which is probably related to their density changes.

The relation between the effective porosity (neff) and the loss of the capillary strength (σ) is suggested by Vásárhelyi and Ván (2006, Eng. Geol) based on the field data of the sandstones (Hawkins and McConnell, 1992, Q. Eng. Geol.) as given in below:

b = 6.0259/ neff (3)In present data of the rock saturation for varying degree of the porosity, the porosity is observed as a significant factor effecting period of the saturation (Figure 4). Higher porosity rock shows the lost of the capillary strength within five hours, whereas the lower pososity of which 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 ChangesIn 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 25 hours (on the left of Figure 5). A couple of anomalous changes that is sharply decreased at about 12 hours, and increased by 25 hours, and those changes may be related to either the

Table 1-The details for the measurement of the weight and calculation of the saturation rate are given

Time Rock Water Saturation

Weight Rate Rate Change

Hours g g/hour %0 646.91 647.72 0.82 40

1.6 647.9 0.30 153.1 648.35 0.30 154.1 648.57 0.22 115.5 648.75 0.13 66.8 648.94 0.15 7

23.7 650.29 0.08 425.8 650.39 0.05 2

Time Rock Water SaturationWeight Rate Rate Change

Hours g g/hour %0.00 461.050.20 463.26 11.05 340.40 464.16 4.50 140.69 465.15 3.49 111.12 466.34 2.75 91.57 467.31 2.16 72.02 468.13 1.82 62.57 469.02 1.62 53.10 469.76 1.39 43.62 470.48 1.39 44.34 471.28 1.12 34.89 471.8 0.95 3

Time Rock Oil SaturationWeight Rate Rate Change

Hours g g/hour %0.00 461.750.23 462.91 4.97 540.42 463.14 1.25 140.60 463.21 0.38 41.27 463.84 0.94 102.23 464.32 0.50 63.68 464.83 0.35 44.70 465.04 0.21 26.23 465.42 0.25 37.97 465.94 0.30 3

Time Rock Oil SaturationWeight Rate Rate Change

Hours g g/hour %0.00 461.750.23 462.91 4.97 540.42 463.14 1.25 140.60 463.21 0.38 41.27 463.84 0.94 102.23 464.32 0.50 63.68 464.83 0.35 44.70 465.04 0.21 26.23 465.42 0.25 37.97 465.94 0.30 3

a) Shirahama- Oil b) Shirahama-Water c) Izumi-Water

Figure 3 - Rock weight changes of the Shirahama sandstone attribute to the changes of the saturation. Since the saturation is progressed at the beginning of the experiment, the period of the weight measurement is kept shorter.

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external factors of experimental set-up changes such as increased ratio of the noise or intrinsic properties of the rock, i.e., the capillarity strength. The slight velocity change through the period – in case we ignore the anomalous changes though - are probably a smaller loss of the rock strength due to less water content of the lower porosity. However, the velocity change of the higher porosity rock that can be caused by faster decrease of the capillary strength is complex at the beginning, and shows slower change (on the right of Figure 5). The velocity caused by water-breakthrough went down from 2.47 mm/sec to 2.37 mm/sec at first, and then rose sharply from 2.37 mm/sec to 2.71 mm/sec by 0.4 hours. It fell sharply to 2.30 mm/sec at first by first hour and then slightly went down through the end of the experiment later. Consequently, the changes of seismic velocity caused by injected water shows generally a decrease within the time, though their

responses of capillary pressure changes seem the sensitivity of their porosity sizes. Since the magnitude of the seismic velocity is well-known factor indicating the rock strength, therefore, the velocity of the lower porosity is measured higher than the velocity of the higher porosity rock.

We put the oil on fluid container for the same rock to study time-lapse changes of rock-capillarity strength following the weight of the rock came roughly up with the same value of the dry state (see Table 1). The velocity chan-ges of the oil saturated rock increased faster at first, and rose up slightly later (see Figure 6). The velocity decreased from about 0.250 mm/sec to the 0.247 at first. Then it rose up sharply by 2.8 hours in which it fell to about 0.2465 hours, and then it increased slightly until the end of the period. Not only is the velocity response of the oil saturation that increased in time is different, but also the velocity change made

Figure 4 - Water saturation for the rocks of high- and low porosity.

Figure 5 - The velocity changes of the water saturated rock. On the left: lower porosity rock, Izumi. On the right: higher porosity rock, Shirahama.

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some dramatic changes, i.e. peak and through, during entire period of experiment.

4. 1. Porosity effecting seismic velocitySaturation rate (%) is a useful parameter to

study time-variability changes of saturation and velocity (see on the left of Figure 7), which is significant to understand the response of the seismic attributes to the change of the capillary pressure since those physical parameters are used to study changes of injected fluids injection, e.g. CO2 sequestration. Since time-elapse changes of the petro-physical and geophysical attributes are exponentially decreased within the time – similar to change of the capillary pressure versus the water content- the decrease of physical parameters is controlled by the rock porosity and type of the fluid. For example, the change is observed normally faster for the higher porosity rock and vice-versa. The velocity changes at the beginning of the experiment that is faster than the saturation changes at first, but they remain closer to the change of the saturation until the end of experiment, which can show the sensitivity of velocity to the loss of the rock strength is significant.

The statistical relation between velocity and time is less significant (R=0.6) than the relation (R=0.9) between saturation and time for the lower porosity rock (top plot on the left, Figure 7). However, the statistical relation between

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 the saturation (on the right of the Figure 7). For example, the saturation change is slower than the velocity change of the water saturated rock, whereas it is higher than the velocity change for oil breakthrough. The modeled curves for the change of both the velocity and the saturation show significant change of their correlation with time. For example, the correlation between time and saturation is 0.97 that is higher than the correlation of R=0.91 between time and velocity.

The correlation for oil-saturated rocks is about 0.7 for saturation/velocity versus time, since it can be related to the higher sampling rate due to longer period of the experiment.

5. Conclusion Initial results based on a testing LDV system

are promising since present experiment showed a new perspective for the measuring time-lapse changes of the high-resolution ultrasonic velocity. Simultaneous measurements of both rock saturation and the rock velocity may increase the accuracy of the relation between the velocity and the saturation since lack of interruption for the measuring rock weight might enhance the quality of the measurement.

Since the petro-physical changes, e.g. porosity and saturation, are factors effecting time-lapse changes of geophysical properties, time-lapse experimental study can be used for Feasibility Risk Analysis before starting 4D instrumented oil field studies of the CO2 sequestration.

Figure 6 - The velocity changes of the oil-saturated rock, Shirahama. The velocity intermittently changed, while which of change shows an increase in time.

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Suggested ReadingSee 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.

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.

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Time-elApSe heTerogeneiTy meASuremenT in Two SedimenTAry roCkS: impliCATionS For Co2 SequeSTrATion