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


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  • 1. Fiziki Coğrafya Araştırmaları; Sistematik ve BölgeselTime-elApSe heTerogeneiTymeASuremenT in TwoSedimenTAry roCkS:impliCATionS For Co2SequeSTrATionAli Osman ÖNCEL 51
  • 2. Fiziki Coğrafya Araştırmaları; Sistematik ve Bölgesel, Türk Coğrafya Kurumu Yayınları, No:5, 105-114, İstanbul 2011 Fiziki Coğrafya Araştırmaları; Sistematik ve BölgeselTime-elApSe heTerogeneiTymeASuremenT in TwoSedimenTAry roCkS:impliCATionS For Co2SequeSTrATionAli Osman ÖNCELIstanbul University Faculty of Engineering Department ofÖz Abstract ldV sistemi ile farklı gözeneklilik özelliklerine Time-lapse changes of ultrasonic seismic velocitysahip kayaç numunelerinin, suya ve petrole doygunluk changes versus saturation changes are determineddüzeylerinin 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-lapseyöntem kayaçların petrofizik özelliklerinde ki changes of petro-physical properties (saturation,(doygunluk, gözeneklilik, sismik hız) değişimlerinin porosity, seismic velocity) has been firstly testedzaman içinde ki değişimlerinin izlenmesi ile ilgili through this study. results of the present work showolarak, ldV sisteminin ilk olarak uygulanması ile ilgili that the use of the time-lapse monitoring seismicbir test çalışmadır. ortaya konan çalışma, üretim velocity changes is highly sensitive to the changes insahalarında 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 theparametresi ile incelenmesi ile ilişkilidir. Sismik hız quality of Feasibility risk Analysis before starting 4ddeğişimleri, yeraltında farklı akışkan (örn., su ve petrol) field seismology work.hareketlerinin sürekli incelenebileceğini gösterdiğiiçin, 4 boyutlu Saha Sismolojisi çalışmalarından önceFizibilite risk Analizlerinin daha doğru yapılmasındakullanılabileceğini göstermektedir. 53
  • 3. 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 computer54
  • 4. 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 higherwater the same as well as keeping the room reduction of the capillarity strength by Hawkinstemperature being constant. Since there is a and McConnell, (Q.Eng. Geol., 1992).contrast between the water and the air, it causedthe existence of capillary pressure (Pc), which Saturation rate is expressed by the time thatis a driven force of the saturation throughout the fluid fills the pore or weight changes of thethe rock. The empirical relationship between rock in time and shown in the following way:capillary pressure and saturation is given inthe 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 thePn- Pw = Pc = f (S) (1) saturation is about the zero, but we stopped the experiment in case the changes of the saturationwhere Pn and Pw are the average pressures of air is slower much in which it is probably relatedand fluid respectively. Pc is capillary pressure, to the decrease of the capillary force which isand 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 saturationcapillary pressure, are a well-defined factor value of 0.34.affecting the rock capillarity strength thatgenerally decreases slower than the changesof the capillary pressure as shown by modelworks of Han and Dusseault, (IJRMMS, 2005).In fact, another relation based on the samplesof the sandstone shows a negative exponentialrelation between the rock capillarity strength andwater saturation, which means that the higher 55
  • 5. 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
  • 6. 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 seemsuch as increased ratio of the noise or intrinsic the sensitivity of their porosity sizes. Since theproperties of the rock, i.e., the capillarity magnitude of the seismic velocity is well-knownstrength. The slight velocity change through factor indicating the rock strength, therefore, thethe period – in case we ignore the anomalous velocity of the lower porosity is measured higherchanges though - are probably a smaller loss than the velocity of the higher porosity rock.of the rock strength due to less water content ofthe lower porosity. However, the velocity change We put the oil on fluid container for theof 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 thecomplex at the beginning, and shows slower rock came roughly up with the same value ofchange (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 at2.47 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 sharplysec at first by first hour and then slightly went by 2.8 hours in which it fell to about 0.2465down through the end of the experiment later. hours, and then it increased slightly until the endConsequently, the changes of seismic velocity of the period. Not only is the velocity responsecaused by injected water shows generally of the oil saturation that increased in time isa 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
  • 7. 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 between58
  • 8. 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 andKawahara titled as “ Laboratory studies ofseismic wave propagation in inhomogeneousmedia using a Laser Doppler Vibrometer,BSSA, pp.809-823, vol. 87” for getting in theinformation of the LDV.Acknowledgement: The author than for thesupport of Japan Cooperation Center, Petroleumfor getting in touch with the laboratory of theAIST in Japan to conduct the present work.Thanks to Osamu Nishizawa and Xinglin Leifrom the AIST for their great help to conduct thetime-elapse experiment. 59
  • 9. Time-elapse heterogeneity measurement in two sedimentary rocks... Ali osman Öncel60