Description:

"First Break том 26, Февраль 2008", "Наземная Сейсмика". Особая тема. Последние разработки в низкочастотном спектральном анализе данных пассивной сейсморазведки. Recent developments in low frequency spectral analysis of passive seismic data from the first industrial work published by First Break journal in 2005 have led to rapid progress. David Walker1 from Spectraseis describes recent advancements and explains how large volumes of data contribute to new methods of processing and analyzing signals. Low-frequency anomalies in wideband receiver signals indicate the presence of hydrocarbon reservoirs. Combining attribute analysis with methods developed during this research provides a more accurate correlation between signal and hydrocarbons compared to previous correlations using simple but often erroneous low-frequency data. The origin of observed signals, whether due to emission within the basin or as a low-frequency effect from acoustic anomalies in hydrocarbon basins inside another relatively homogeneous medium, is subject to investigation. Recently developed time-reversal modeling methods successfully locate reservoirs assuming that a small part of the anomalous signal originates in the reservoir. In 2004, Brazilian company Spectraseis analyzed low-frequency seismic data. In 2005, Holzner's first report led to an 85% correlation between signals and hydrocarbons (Macedo, 2005). By late 2007, Spectraseis received five low-frequency seismic works from a major oil company client and new datasets from known fields in the Middle East, North Africa, Europe, and South America. The ongoing scientific program increased understanding of these low-frequency signals and their relation to hydrocarbon basins, enriching Spectraseis' growing database. A scientific explanation for the empirical phenomenon is sought: why do hydrocarbon reservoirs in porous media exhibit low-frequency signal anomalies? Models of resonant amplification (RAM) and resonant scattering (RSM) are introduced. RAM occurs when specific frequencies from the natural energy spectrum of the earth are captured by a multiphase liquid system within hydrocarbon reservoirs, producing distinguishable energy levels. Data from developing fields also show that low-frequency energies associated with productive reservoirs result from dynamic processes in the productive medium. However, even in such environments, low-frequency anomalies can indicate hydrocarbons. Without production activity and during exploration, the location of hydrocarbon reservoirs was confirmed by drilling. Figure 1: Spectral passive wave field (vertical surface velocity) over a productive area (70139) and an area without known hydrocarbon reserves (70575). Figure 2: Analysis parameter IZ. 1 David Walker, Spectraseis; E-mail: david.walker@spectraseis.com. Generally, RSM can be explained as reservoirs having different acoustic properties from the host medium. Wave fields are scattered or altered due to hydrocarbon presence, manifesting in low-frequency seismic signals. RAM and RSM likely influence received signals since neither explanation contradicts this. For decades, it has been known that hydrocarbon basins appear as low-frequency anomalies on seismic data (Goloshubin, 2006; Chapman, 2006). Chapman (2006) writes about reservoir attenuation affecting AVO characteristics: "anomalous high attenuation in the reservoir is observed when controlling surface data." Given this fact, hydrocarbon reservoirs significantly affect scattered waves due to their high complex impedance contrast compared to host rocks with low or no attenuation. Thus, reservoirs can be detected through scattering effects, such as standing waves. However, since these micro-seismic events are not the sole source of observed anomalies on seismic data, it may be incorrect simply to correlate low-frequency extrema with hydrocarbons' presence. Additionally, the influence of hydrocarbons on anomalous signal values might "hide" in any frequency range or seismic wave energy; thus, detecting hydrocarbon reserves requires more complex processing using three-component wideband sensors. For various fields, a combination of seismic signal attributes is more reliable than one parameter to determine hydrocarbons' presence. RAM suggests that hydrocarbon reservoirs act as energy converters, accumulating and subsequently releasing some signal energy. The origin and mechanism of this accumulation are subjects of current research. Possible origins include ocean waves interacting with coastal structures or the seafloor. The so-called maximum ocean wave frequency is around 0.1-0.2 Hz in background seismic noise (Berger, 2004), which is globally recorded. Surface waves travel through entire continents and can be used to determine seismic velocities down to depths of 20 km (Shapiro, 2005). Interestingly, Rayleigh waves with a frequency around 0.1 Hz reach much greater depths than the productive layer depth, primarily vertically (Aki and Richards, 2002). To detect hydrocarbons in Earth's subsurface, vertical motions are predominantly used in several seismic attributes. The mechanism of energy release from reservoirs involves the transmission of vibrational movements of unsaturated fluids in porous media to surrounding medium. Inhomogeneities such as voids in an elastic medium and partial saturation in a porous medium can cause vibrational movements. For instance, Graham and Higdon (2000a; 2000b) and Dvorkin (1990) studied vibrational movements of unsaturated fluids confined in capillary tubes and idealized pore space. The main result was vibrational movements under applied external force (Hilpert, 2000). The frequency of these movements lies at the lower boundary of the spectral range (Holzner, 2007; Hilpert, 2007). The restoring force for such a vibrational movement is surface tension acting on the interface between wetting and non-wetting liquid phases. These results were used to study oil recovery using methods called "wave-enhanced production" or "vibratory mobilization" (Beresnev, 2005; Hilpert, 2007; Iassonov and Beresnev, 2003; Li, 2005). These methods and their application were discussed by Beresnev and Johnson in 1994. Another example is voids or gas bubbles in the solid phase that can vibrate with resonant frequency (e.g., Landau and Lifschitz, 1997; Meyer, 1958). Although the physical mechanism of vibration differs from that found in partially saturated pores, the mathematical description of the process and its effect on surrounding solid phases are similar. Anderson and Hampton (1980a and b) provided a detailed description of this phenomenon. A third example of vibrational movement is layered media. Urquizu and Correig (2004) show that seismic wave propagation through layered media can be described by various mathematical equations. The discovery that it may stimulate fluid vibrations in multiphase systems has important implications. A single-phase porous fluid can absorb energy but will not vibrate because there are no restoring forces acting between fluids and causing vibration. Since the source of energetic anomaly is limited to the reservoir's fluid system, detecting vibrations in three directions might directly indicate productive layers. The vibrational process may be an interpretation of low-frequency (<10 Hz) anomalous movement." 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