S037 Recent Low Frequency Passive Seismic Experiments in Abu Dhabi M.Y. Ali* (The Petroleum Institute), K.A. Berteussen (The Petroleum Institute), J. Small (The Petroleum Institute), B.T. Anjana (The Petroleum Institute), B. Barkat (The Petroleum Institute) & O. Pahlevi (The Petroleum Institute) SUMMARY Low frequency passive seismic experiments using array of 3-component broadband instruments have been conducted over two sites in Abu Dhabi, over a producing oilfield and around deep exploration dry well, to understand the characteristics and origins of microtremor signals (~2.5-3.0 Hz) that has been reported above several hydrocarbon reservoirs in Abu Dhabi. The results of the experiments show that a narrow-band of microtremor signal is present in both sites. Variations in the character of the signals recorded during day and night were observed. The analyses of the data indicate that the observed signals are probably caused by surface waves which have been modified by metrological and cultural noises particularly at higher frequencies. The V/H spectral ratios of the data from both experiments show a peak at ~2.5-3 Hz. The analyses of the data suggest that the V/H spectral ratios do not depend of presence or lack of hydrocarbons. Therefore, the method should not be used as a direct hydrocarbon indicator. Furthermore, the analyses show no direct evidence that the earthquakes can stimulate the hydrocarbon reservoir to modified the spectral amplitude of the microtremor signal.
Introduction For the past several years a narrow band of microtremor (~2 6 Hz) signals with a peak ~ 3 Hz has been observed over a number of hydrocarbon reservoirs, including some oilfields in Abu Dhabi (Dangel et al., 2003; Holzner et al., 2005; Akraw and Blocki, 2006; Walker, 2008; Hanssen and Bussat, 2008). The observations suggest that the low frequency signature diminishes towards the rim of the reservoir and is absent above non-reservoir locations. Furthermore, it has been suggested (e.g. Holzner et al., 2005; Saenger, et al. 2007; Graf et al. 2007) that low frequency can be used as a method for reconnaissance in frontier exploration areas, optimization of borehole placement, reservoir monitoring as well as complementary tool to structural imaging to reduce drilling risk and assist well positions. Some studies (e.g. Dangel et al., 2003; Holzner et al., 2005; Akrawi and Bloch, 2007; Walker, 2008) have even suggested a linear relationship between the observed signal and the total thickness of hydrocarbon-bearing layers. It has been suggested (e.g. Holzner et al., 2005; Saenger, 2007; Graf et al., 2007; Walker, 2008) that these signals are produced from resonance amplification or resonance scattering of the hydrocarbon reservoirs. In these models it is assumed that non-linear behaviour of the interaction between liquid hydrocarbons, water, and the pore-rock materials in the reservoirs distorts the normal signature of the Earth s natural ambient vibration spectra. Recently, Steiner et al. (2008) applied time reverse modelling and suggested that the hydrocarbon reservoir zone is the origin of the low-frequency spectral anomalies. However, the possible causes of this low frequency energy are not well understood. Berteussen et al. (2008) have shown that the observed signals are surface waves. In another study, Hanssen and Bussat (2008) analysed low frequency ambient noise over a field in Sahara desert of Libya and suggested that the noise caused by human activities can produce a similar signal as the low frequency signal presumably generated by the hydrocarbon reservoir. In this paper we present the results of microtremor recorded over two sites, a producing oilfield and around a deep exploration dry well in Abu Dhabi. The aim of the experiments is to understand the origins of the microtremor signals. 1. Data Acquisition and Processing Experiment 1: The experiment was carryout over an oilfield located ~50 km SW of Abu Dhabi city (Figure 1a). The oilfield was selected as a suitable site for the experiment because it has a clear and well-defined oil-water contact (OWC) mapped from 3D seismic and well data. Experiment 2: The experiment was conducted over and around a deep exploration well located 80 km SE of Abu Dhabi city (Figure 1a). The well was drilled on a seismically mapped structural closure to investigate the hydrocarbon potential and reservoir development of the Mishrif, Shuaiba (Middle-Lower Cretaceous) and deeper Upper Jurassic Formations. The drilling data show that the reservoir characteristics of all prospective reservoirs from Mishrif down to and including Upper Jurassic Formations are insignificant and these reservoirs were found to be water bearing by open hole DST s and production tests. The first experiment was carried out in May-June 2007 and consisted of a 2D profile running from location A to location B, and detailed studies around both locations (Figure 1b). Location A is situated over the maximum oil column (>120ft) of the reservoir, whereas location B was positioned over an area that presumably contained no oil. The second experiment was carried out in January 2009 and included acquisition of five arrays of varying aperture (25 m to 500m) around the dry well. The signals were recorded using 6 ultra sensitive 3-component seismometers (Guralp CMG- 6TD for the first experiment and CMG-3EX for the second experiment). The seismometers were placed on concrete slabs in pits of a ~50 cm deep and oriented to the geographic north. The sensors are then covered and buried for firm ground contact and wind shielding. Various signal analysis techniques were applied to the data including time series, power spectral density, and time frequency analyses. The processing window of 60 seconds and 5%
cosine taper was applied to the data to reduce spectral leakage. Fourier amplitude spectra were analysed without smoothing and with the smoothing procedure of Konno and Ohmachi (1998) using a b-value of 40. The mean was removed and excessively noisy sections of the signal were excluded from the analyses before stacking the data. Figure 1: (a) Regional location map (a) Location map of the oilfield. Microtremor: Diurnal variation and influence of anthropogenic and wind noises Figure 2a shows the spectral amplitude of the 2D profile running from location A to location B of the first experiment. The figure shows a definite microtremor that peaks at ~2.5 Hz. The microtremor signal is observed on all stations both over the oil reservoir (location A) and over water saturated zone (location B). In addition, all three components (N-S, E-W and Z) of all stations recorded the signal. This observation is confirmed by the second experiment over dry exploration well which shows a distinct microtremor signal at ~2.5 Hz on all three components (Figure 2b). The microtremor signal exhibits strong diurnal variations. During the day time it appears that all three sensors (vertical, north-south, and east-west) had recorded strong microtremor signal, whereas all sensors detected only weak signals over night (Figures 2a and 3). This observation suggests that the source responsible for the microtremor signals is possibly correlated with metrological, anthropogenic noises or solar heating. The recorded microtremor signal cannot be a P-wave wavefront travelling directly below the sensors. The signal is interpreted as a surface waves or a combination of shear and surface waves (Berteussen et al., 2008). However, the coupling of anthropogenic (e.g. traffic, production installations) and local meteorological energy (e.g. wind effects) into the earth may have affected the signal particularly at higher frequencies. Figures 3 and 4 show that the ambient noise is contaminated with high amplitude wind or cultural noise in the 6 Hz frequency range as well as low frequency noise generated from regional earthquakes. In a study carried out on an oilfield in Libya Hanssen and Bussat (2008) correlated the microtremor signal with surface waves caused by anthropogenic noises (e.g. production facilities, traffic), resonance frequency of the unconsolidated material in the area, and topography (height of sand dunes). Figures 2: (a) Spectral amplitude of the sensors located along a profile from location A to B, experiment 1. (b) Spectral amplitude of a sensor located over the dry exploration well, experiment 2. (For location see Figure 1b).
Figure 4: Spectral amplitudes showing various noises, experiment 1. Figure 3: Time frequency display for data recorded from vertical component, experiment 1. V/H spectral ratio Lambert et al. (2007) suggested that the dominant amplitude peak of the ratio of vertical to horizontal components in the microtremor range is higher over hydrocarbon reservoirs. As a result, high values of V/H are used as direct hydrocarbon indicator of hydrocarbons (Lambert, et al., Walker, 2008). The assumption is that the hydrocarbon reservoirs emit P-waves which cause an increased vertical polarization of the ambient noise wavefield at surface. For each station we calculated the H/V of 60 minutes of continuous ambient noise recording following Nakamura approach (1989). Fourier amplitude spectra are smoothed following Konno and Ohmachi (1998), with a b b-value of 40. The V/H spectral ratios of the first experiment (Figure 5) indicate a peak (>1) at ~3 Hz on most of the stations, both on oil saturated and water bearing areas. The peak is slightly weaker during night. This observation has been confirmed by the second experiment of the dry well, which showed strong V/H peak at ~2.5 Hz and a trough at ~1.5 Hz which could be the resonance frequency of the site. Therefore, the observations suggest that the V/H method cannot reliably be used as a hydrocarbon indicator. Figure 5: V/H spectral ratios of data from experiment 1. Earthquake triggered modification During the time of the experiments several regional earthquakes were recorded (Fig. 4). Figure 6 illustrates the time-frequency analysis of the Iran earthquake. The onset of the event is more apparent with dramatic increase of the amplitude on all three components. However, there is no clear indication whether spectral amplitude of the microtremor signal is modified during or after the earthquake. Both vertical and N-S components indicate a slight decrease of spectral amplitude after the earthquake. This contradicts with Nguyen et al. (2008) who reported that hydrocarbon reservoirs can be stimulated by an earthquake which results spectral increase of the microtremor signal above the reservoir after the earthquake. Conclusions We draw the following conclusions from this paper: A narrow microtremor (2.5 3 Hz) signals are observed over two sites (above oilfield and above dry well). During the day all sensors recorded strong microtremor signal, whereas all sensors detected only weak signals over night. The microtremor signal is interpreted as surface waves which have been modified by metrological and cultural noises.
V/H spectral ratios of 2.5 3 Hz are observed on both sites. The V/H peak is not related to the location of the hydrocarbon. Analyses of regional earthquake data show no evidence of earthquake triggered spectral amplitude modification of hydrocarbon reservoir. Figure 6: Time frequency display of Iran earthquake on location A, experiment 1. Acknowledgements We are grateful to the Oil-Subcommittee of the Abu Dhabi National Oil Company (ADNOC) and its operating companies (OpCos) for sponsoring this project. We thank Mr. Marwan Haggag for his logistical support of the fieldwork and in coordinating the project. References Akrawi, K. and Bloch, G. [2006] Application of passive seismic (IPDS) surveys in Arabian Peninsula. EAGE Workshop on Passive Seismic: Exploration and Monitoring Applications, Dubai, Extended Abstract, A28. Berteussen, K., Ali, M. Y., Small, J. and Barkat, B. [2008] A Low Frequency, Passive Seismic Experiment over a Carbonate Reservoir in Abu Dhabi Wavefront and Particle Motion Study. 70 th EAGE Conference & Exhibition, Rome, Extended Abstract, B046. Dangel, S., Schaepman, M.E., Stoll, E.P., Carniel, R., Barzandji, O., Rode, E.D. and Singer, J.M. [2003] Phenomenology of tremor-like signals observed over hydrocarbon reservoirs. Journal of Volcanology and Geothermal Research, 128, 135-158. Graft, R., Schmalholz, S.M., Podladchikov, Y. and Saenger, E.H. [2007] Passive seismic low frequency spectral analysis: Exploring a new field in geophysics. World Oil, January, 47-52. Hanssen, P. and Bussat, S. [2008] Pitfalls in the analysis of low frequency passive seismic data. First Break, 26, 111-119. Holzner, R., Eschle, P., Zurcher, H., Lambert, M., Graf, R., Dangel, S. and Meier, P.F. [2005] Applying microtremor analysis to identify hydrocarbon reservoirs. First Break, 23, 41-46. Konno, K. and Ohmachi, T. [1998] Ground-motion characteristics estimated from spectral ratio between horizontal and vertical components of microtremor. Bulletin of the Seismological Society of America, 88(1), 228-241. Lambert, M., Schmalholz, S.M., Saenger, E.H., Podladchikov, Y. [2007] Low-frequency anomalies in spectral ratios of single station microtremor measurements: Observations across an oil and gas field in Austria. 77 th SEG Annual International Meeting, Expanded Abstracts, 1352-1365. Nakamura, Y. [1989] A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Quarterly Report Railway Tech. Res. Inst., 30(1), 25-30. Nguyen, T.T., Saenger, E.H., Schmalholz, S.M.,Artman, B. [2008] 70 th EAGE Conference & Exhibition, Rome, Extended Abstract, B026. Saenger, E.H., Schmalholz, S.M., Podladchikov, Y., Holzner, R., Lambert, M., Steiner, B., Quintal, B. and Frehner, M. [2007] Scientific strategy to explain observed spectral anomalies over hydrocarbon reservoirs generated by microtremors. 70 th EAGE Conference & Exhibition, London, Extended Abstract, A033. Steiner, B., Saenger, E.H. and Schmalholz, S.M. [2008] Time reverse modelling of low-frequency microtremors: Application to hydrocarbon reservoir localization. Geophysical Research Letters, 35, L03307. Walker, D. [2008] Recent developments in low frequency spectral analysis of passive seismic data. First Break, 26(2), 69-77.