A33 The Challenge for Surface Passive Seismic Measurements in Kuwait

Transcription

1 A33 The Challenge for Surface Passive Seismic Measurements in Kuwait G.R. Rached* (Kuwait Oil Company) SUMMARY Surface passive seismic is a direct hydrocarbon saturation indicator technology. It detects the presence of subsurface hydrocarbons by recording and spectroscopically analyzing the low-frequency acoustic/seismic background noise (between Hz) which is actively emitted by the earth. The measurements after filtering and analysis can be calibrated with well data and integrated with other geophysical data to provide information which can lower the uncertainty level for hydrocarbon exploration, appraisal and field development decisions. Surface passive seismic measurements were carried out in Kuwait to evaluate the suitability of the technology to detect and the map the extent of hydrocarbon occurrences. The main concern was the suitability of the technology to differentiate between various reservoirs in carbonate stacked-reservoir environment. The level of certainty in the result of the measurements could not be accurately assessed as there is a possibility that some deep reservoirs have not been penetrated by the wells in the area of the experiment. It was concluded that the technology (at that point) could not indicate the distribution of the hydrocarbon occurrences among the stacked reservoirs since the measured indicators are the summation of the responses of all the underlying reservoir layers. Nevertheless, the areas with highest hydrocarbon potential indication are considered the most interesting ones. A number of advancements in surface passive seismic have been recently reported and research results are indicating that the distribution of the hydrocarbon occurrences among stacked reservoirs is becoming achievable. However, It remains a challenge for the surface passive industry to demonstrate convincing successful case histories.

2 Introduction A surface passive seismic measurements experiment was carried out in Kuwait. 328 surface observation points along four traverses in an area straddling three oil fields and passing through 14 wells of different hydrocarbon potential were acquired. At well locations, a recording time of 90 minutes was applied. At all other locations a recording time of 40 minutes was applied. The objectives of these surface passive seismic measurements were to test the suitability of this technology in directly detecting the presence and extension of subsurface hydrocarbons by recording and analyzing the low-frequency acoustic/seismic background noise and to indicate the distribution of the hydrocarbon occurrences among the stacked reservoirs. The survey was carried out as a blind test, that is, the well data were not made available to the contractor. Theory and method The possible causes of this low frequency energy and the type of waves that are being observed as well as the physical behavior of the multiphase fluid system in the reservoir are not yet well understood (Ali et al., 2007). Martakis et al., (2006) stated that a passive seismic project depends mainly on the microearthquake activity. Rhei et al., (2004) stated that the signal source for passive seismic surveys is the ever-present background noise of the earth (earth s hum) and concluded that the earth s hum is generated by the interaction between atmosphere, ocean and sea floor, probably through the conversion of storm energy to oceanic infragravity waves that interact with seafloor topography. A recently developed time-reverse modeling method successfully locates reservoir positions in the subsurface suggesting at least part of the anomalous signal originates in the reservoir (Walker, 2008). This signal/background noise, irrespective of origin, is ideally suited for spectroscopy. It follows, in an undisturbed case, a 1/f distribution. An oil reservoir can be described as a multifluid system in porous material. In a very low frequency spectral window (0.5 8 Hz) such system has a non-linear transfer function and is able to convert the higher frequency part of this noise into lower frequency components, which leads to a deformation of the earth noise. Thus, seismic waves originating from the background noise of the earth are modified in a different way when they penetrate geological structures containing hydrocarbons compared with interaction with similar structures not containing hydrocarbons (Holzner et al., 2005). The spectral power of that deformation (characteristic spectral lines) is utilized as an indicator for the presence of hydrocarbons. Since the nature of the signal is chaotic and also the signalto-noise ratio is very low as the signal is embedded in environmental noise, very long time recording per station is essential. The spectral power of a certain number of spectral lines in the interval 1 to 6 Hertz is used to create a relative hydrocarbon indicator for the subsurface hydrocarbon potential.

3 Figure 1: Specification of seismometers and layout per station. The survey was carried out with high sensitivity seismometers which were arranged in a quadruple per station with a distance of 300 meters between two opposite seismometer, connected with the recording station by cable, Figure 1. Station spacing was about 800m. The seismometers were placed in 60 cm deep pits and covered to reduce surface noise as success in surface measurements appears to be highly dependent on the signal-to-noise ratio. Filtering The raw signal recorded is strongly dependent on environmental noise and on stochastic variation of the earth noise spectra. For this reason, the raw signals have to be filtered before and after transformation in the frequency domain. Various enhancing techniques in both the time and frequency domains are applied in order to improve the signal-to-noise ratio. Different kinds of filters were tested. The selection of the filter to be used is mostly a process of iteration and comparing with existing data. The filtered signals are then spectroscopically analyzed. Hydrocarbon reservoirs produce a unique spectral signature which is used as a direct hydrocarbon indicator. Data evaluation was based on the selection of 3 characteristic spectral lines from the interval 1 to 6 Hertz. Figure 2 shows the typical appearance of such spectral lines. The relative strengths of the spectra of the spectral lines at two different locations are shown in Figure 3. Micro Event Figure 2: Typical appearance of spectral lines and spectrogram of a micro event.

4 8500 EA077 Signal 7300 EA089 Signal Figure 3: Typical appearance of spectral lines and the relative strength of their spectra. Results The results from the blind test survey are shown in Figure 4. Based only on the surface passive seismic along four traverses, the results showed that the surveyed area could contain three main hydrocarbon bearing structural bodies and three structurally low relief areas. These structures are heavily affected by hydrocarbon barriers, possibly affected by faults (sealed or partially sealed). The map indicates that 10 wells have hydrocarbon potential and 4 wells have low hydrocarbon bearing. These results were arrived at prior to any geological or geophysical information as the survey was carried out as a blind test. The level of certainty in the resulting map could not be assessed as there is a possibility that some deep reservoirs have not been penetrated by the wells in the area of the experiment. In addition, the current understanding of the area of the experiment suffers from uncertainties which prevent from objective evaluation of the results.

5 Figure 4: Interpretation map showing hydrocarbon potential in relative units. However, attempting to directly correlate simple low-frequency amplitude maxima anomalies to hydrocarbons is likely to be misleading because hydrocarbon micro-tremors are not the only source for low frequency amplitude maxima on seismic records. Additionally, the presence of a hydrocarbon related signal anomaly may be masked in any frequency by ambient or local environmental seismic energy (Walker, 2008). The way forward in stacked reservoir environment is through developing the following: - 3D measurements using three component broad band sensors, - Modeling studies to establish acquisition layout and implementation techniques, - Sophisticated processing for depth determination like the recently developed time-reverse modeling, - Techniques to rank the vertically stacked reservoirs such as numerical modeling. In addition, the integration of the results with all available geological and geophysical data could result in making surface passive seismic more useful in stacked reservoir environment. Conclusion Surface passive measurements were carried out in Kuwait to evaluate the suitability of passive surface seismic to detect and map the extent of hydrocarbon occurrences. The main concern was the suitability of the technology to differentiate between various reservoirs in stacked-reservoir environment. The level of certainty in the results could not be assessed as there is a possibility that some deep reservoirs have not been penetrated by the wells in the area of the experiment. It was concluded that the technology (at that point) could not indicate the distribution of the occurrence among the stacked reservoirs since the measured indicators are the summation of the responses of all the underlying reservoir layers. Although the areas with highest hydrocarbon potential indication are considered the most interesting ones, the technique did not deliver the expected results in a stacked-reservoir environment. The good news is that a number of advances in surface passive seismic have been recently reported and

6 research results are indicating that the distribution of the hydrocarbon occurrences among stacked reservoirs is becoming achievable. Convincing case histories are still a challenge for the passive seismic technology. Acknowledgments The author thanks Kuwait Ministry of Energy and Kuwait Oil Company for permission to publish this paper. References Ali, M. Y., Berteussen, K. A., Small, J. and Barkat B. [2007] A low frequency, passive seismic experiment over a carbonate reservoir in Abu Dhabi, First Break, 25, Holzner, R., Eschle, P., Zürcher, H., Lambert, M., Graf, R., Dangel, S. and Meier, P. F., [2005] Applying microtremor analysis to identify hydrocarbon reservoirs: First Break, 23, Martakis N., Kapotas S. and Tselentis G. [2006] Integrated passive seismic acquisition and methodology, Case Studies: Geophysical Prospecting, 54, Rhie J. and Romanowicz B. [2004] Excitation of Earth's continuous free oscillations by atmosphere-ocean-seafloor coupling: NATURE, 431, 552. Walker, D. [2008] Recent developments in low frequency spectral analysis of passive seismic data: First break, 26,