Rayleigh wave inversion for the near-surface characterization of shallow targets in a heavy oil field in Kuwait

Transcription

1 Rayleigh wave inversion for the near-surface characterization of shallow targets in a heavy oil field in Kuwait Near surface complexity can be particularly challenging in the case of shallow targets, such as the Kuwait Lower Fars heavy oil field. Claudio Strobbia, 1* Adel El Emam, 2 Jarrah Al-Genai 2 and Jürgen Roth 1 discuss the results of a joint study with a presentation of new approaches for point-receiver data acquisition and processing. T he distortions induced by the near surface and its heterogeneities present major challenges for land seismic explorationists. Some standard techniques with intrinsic limitations can fail in the presence of a complex near surface structure, for instance when velocity inversions are involved. The correction of near surface perturbations is more important for low relief structures, and for shallow and thin targets. In northern Kuwait, the Lower Fars field is an important heavy ( API) oil reservoir. It consists of two sandstone layers separated by a shale bed at a depth that ranges from 100 m to 225 m (Dusseault et al., 2008). Detailed knowledge of the reservoir, especially in terms of stratigraphy and structure would enhance the optimization of production technology. High quality seismic data are particularly valuable for reservoir characterization and monitoring. Exploration in this unconventional environment is challenging because: 1) The required high resolution implies the need for high frequencies. Despite the shallow target depth, the highly attenuating nature of the nearsurface requires the use of powerful sources to achieve an adequate signal to noise ratio (S/N) at a high frequency. On the other hand, to image a shallow target, short offsets are needed. Coherent and source generated noise, in the proximity of the source, must be addressed during data processing. 2) The horizons of interest are well above the first clear refractors, and are embedded in a complex sequence of layers with velocity inversions. In this situation, refractionbased statics solutions can lead to ambiguous results. In order to evaluate and test the potential of new near surface characterization approaches with point receiver data, a test was designed, acquired, and processed. The aim of the test was to evaluate the impact of Rayleigh wave inversion on near surface characterization and statics, near offset noise attenuation, and point receiver processing. Data acquisition Considering the apriori information about the area and the target, two sites, about 10 km apart, were selected for acquisition. A 2D geometry was used. One of the test lines was located on an existing shallow well. The surface consisted of a flat gravel plain, without significant elevation changes. For each of the tests, a macroline with four sublines of point receivers was deployed. Low frequency geophone accelerometers (GAC) were used as receivers. Different sources were tested including a single DX-80 Desert Explorer vibrator emitting maximum displacement sweep (Bagaini, 2008) at 50% of the peak force, and an accelerated weight drop. Vibrator sweep data were found to provide the best signal strength even at shallow depth. An example shot gather is depicted in Figure 1 showing a strong, highly dispersive, multimodal Rayleigh wave, with high lateral continuity. Heterogeneities of different types are present in the shallow subsurface. They cause significant lateral velocity variations that can be observed to the right of the gather in Figure 1. In Figure 2 a zoom in the shot gather is depicted, together with the graph of the residual phase against the offset at 10 Hz. The figure represents the phase after removal of the average linear trend (at each frequency) for a single mode, and indicates lateral variations. The local phase velocity at 10 Hz is indicated on the phase plot for four segments of the line. Rayleigh wave inversion Surface waves, commonly referred to as groundroll, are traditionally regarded as simple coherent noise that must be attenuated as early as possible, either in the data processing, or in the acquisition phase by using receiver arrays. With 1 WesternGeco. 2 Kuwait Oil Company (KOC). *Corresponding author, cstrobbia@slb.com 2010 EAGE 103

2 first break volume 28, May 2010 Figure 1 Sample shot gather with strong Rayleigh waves. The ground-roll consists of multiple modes, is dispersive and indicates the presence of lateral variations. Figure 2 Shot gather containing significant lateral variations of the Rayleigh waves (top). The local phase velocity at 10Hz is indicated on the residual phase plot (bottom). finely spaced point receivers, the groundroll reveals its nature and complexity. It consists of a composite wave train, containing multiple modes of Rayleigh wave, superimposing one on another. The wavetrain exhibits strong dispersion, different attenuation, amplitude spectra, and beatings due to offset dependent interference. The propagation properties of the Rayleigh waves directly depend on the near surface properties. It is therefore possible to come to a change of perspective on the surface waves, in the domain of reflection seismic. If data are acquired properly, surface waves can be identified, analyzed, and inverted to characterize the nearsurface down to their propagation depth. Surface waves can be considered signal and not noise. Once the information that the surface waves carry has been extracted, they can be effectively removed as unwanted signal (Strobbia et al., 2009). Surface wave methods, which are popular as stand alone techniques in other fields of applied geophysics, can be implemented robustly as part of the land workflow, provided some requirements on the data are met. The physical principle on which surface wave methods are based is related to the different penetration of different wavelengths. This is the reason for geometric dispersion (Figure 3): different frequencies have different phase velocity, but also different intrinsic attenuation. The dispersion, the attenuation, and the amplitude spectra are strictly related to the site properties, and hence can be inverted to a near surface velocity and attenuation model. In Figure 3, the steps of the method are schematically illustrated. Data are processed in order to extract the propagation properties, that are inverted for near surface models. The aforementioned data requirements are based on the fact that the wavefield has to be properly sampled in order to suitably constrain the near surface model in the inversion. In practice, the propagation properties must be extracted over a wide wavelength range. To have long wavelengths, low frequencies are generated and propagated. To observe short wavelengths, fine spacing is needed. Fine spacing is needed also to have high lateral resolution. Both test lines were acquired with point-receivers and a point source vibrator employing a maximum displacement sweep from 3.5 to 120 Hz, therefore sampling from low frequency to short wavelength. For a given observation position along the line, it is possible to extract the propagation properties of Rayleigh waves over a wide frequency (wavelength) range. In Figure 4, a high resolution f-k analysis confirms the presence of multiple modes. The importance of the low frequency part of the fundamental mode is crucial, as shown later. The propagation properties can be inverted for a near surface velocity model, notably a shear wave velocity profile. These methods, used in shallow engineering applications for site investigation, are powerful tools in seismic reflection surveys for the characterization of the near surface. For this purpose the focus is on the near surface heterogeneities, the lateral variations, and the lateral resolution of the analysis. A robust workflow for the surface wave analysis, based on a continuous and adaptive surface wave processing, has been developed for land data (Strobbia et al., 2009). The objective of the processing is the extraction of the local Rayleigh wave properties. For each given location within a 3D survey, or along a 2D line, the properties are estimated considering a frequency-dependent aperture, which takes into account the EAGE

3 frequency-dependent near field effects, intrinsic attenuation, and variable lateral resolution. Stacking over multiple offsets and azimuths enhance linear events over other phenomena, providing high quality dispersion images. For both test lines, the application of the Rayleigh wave analysis allowed the extraction of their dispersive properties over a wide frequency range. The high S/N in the low frequency range (remembering that, at this stage, Rayleigh waves are signal) is of particular importance enabling sufficient investigation depth with active data. The fundamental mode dispersion, depicted in Figure 5 as phase velocity versus wavelength highlights the importance of low frequencies. The decrease of velocity for the longer wavelengths is related to a velocity inversion, and is observed only below 7 or 8 Hz. With the adaptive aperture, the lateral resolution of the high frequencies can be completely exploited, without sacrificing the accuracy on the long wavelength. The analysis was run with a spacing of 25 m along the line, with a frequency-dependent overlap between adjacent locations. The result of the analysis is a 2D dispersion pseudosection (or a 3D pseudovolume with 3D geometries) for each mode. The fundamental mode phase velocity pseudosection for the test line 1 is shown in Figure 6. The horizontal axis represents the location along the receiver line, the vertical axis the wavelength (related to depth), and the colour scale is the phase velocity of the fundamental mode. Each column of the pseudosection (A in Figure 6) represents the local dispersion curve, for the considered mode. Figure 6 depicts raw data without any preconditioning: the high data quality is reflected in the smoothness of the image.the dispersion volume or section is inverted using a linearized least squares algorithm, based on locally one dimensional forward modelling. Figure 7 shows the shallow part of the inverted model. This is the first 70 m, after conversion from shear to compressional velocity. The compressional velocity is estimated from the inverted Vs after calibration with the velocity of first arrivals of shallow, discontinuous refractors. A first velocity inversion is present in the very shallow near surface: the discontinuous thin fast layer is embedded in low velocity material, above a faster continuous layer. The complexity of the shallow near surface is evident looking at the nearoffset first arrivals. Two shot gathers from the weight drop acquisition are plotted on the velocity section. Shallow velocity inversions produce shadow zones and jumps in the first breaks. Data processing After the analysis of the Rayleigh wave is complete, this component of the wave field becomes undesired for further data processing. The simple processing flow described consists of digital group forming (DGF) followed by post- Figure 3 Surfacewave propagate with a penetration function of the wavelength, and are therefore dispersive. In the bottom the steps of the method are schematically represented: acquisitioj, processing and inversion. Figure 4 Example of single f-k spectrum and picked modes. Figure 5 Dispersion curve, as phase velocity versus frequency, for one location along line EAGE 105

4 first break volume 28, May 2010 DGF coherent noise attenuation, static corrections, several iterations of velocity analysis with residual reflection statics, CMP sorting, and stacking. The full DGF workflow involves attenuation of incoherent and coherent noise, intra-array static correction, and spatial antialias filtering and down-sampling. The noise attenuation in the near-offset is achieved by a combination of K filtering and FX coherent noise attenuation. The identified properties of the Rayleigh waves (velocity range, frequency range, number of modes) are used in the parameterization of the noise attenuation workflow. The velocity variations in the shallow near surface are expected to produce time perturbations. As mentioned earlier, a standard refraction static solution presents some concerns. A deeper refractor can be identified and picked at farther offsets. The refractor, however, is well below the zone of interest, and the refraction static solution, valid for deeper reflections, has obvious limitations for a shallow target. In the near-offset other limitations are present, due to the source noise degrading the first arrivals and the complexity of the wave propagation. A hybrid static solution has hence been generated and applied, merging long period statics computed from the velocity model (Figure 7) with standard short period statics. The processing results show that near surface characterization provides an effective static solution for the shallow target. In particular, for the first test line, shallow velocity variations produce long wavelength statics which have to be corrected. In Figure 8 two static solutions are compared. Figure 6 Phase velocity pseudo-section for line 1. Figure 7 Velocity section from the Rayleigh wave inversion for line 1, after conversion to Vp. Two gathers are plotted, with the indication of the shot position. Figure 8 Two stacks comparing a standard refraction static solution (A) and the hybrid static solution (B) computed from the shallow near surface velocity model EAGE

5 The first panel (A) is obtained by applying the refraction statics on the deeper refractor, while the second (B) uses the hybrid static solution computed from the shallow velocity model inferred from Rayleigh wave inversion. The top stack shows some residual dip, that is not consistent with the regional geology, and which is corrected by the used static solution. For near surface compensation only the shallow portion of the inverted model, shown in Figure 7, was used. The deeper part presents smaller and smoother lateral velocity variations in terms of compressional wave velocity, because of saturtation. In this respect, the deeper subsurface, below a depth of m, cannot be defined as the near surface, because it hosts the target. The heavy oil reservoir is a member of a clastic sequence of unconsolidated, pebbly to gravelly sands. The heterogeneities within this zone (the vertical and lateral velocity variations, the discontinuities and faults) should not be considered as the source of perturbations. The heterogeneities can be related to variation of the reservoir properties and to its compartmentalization. This information is important for choosing the optimal production technology. Results The Rayleigh wave inversion provides a shear wave velocity section that indicates a strong velocity inversion at a depth of about 100 m. The shear velocity decrease from over 1000 m/s, at a depth of 80 m, to very low values, locally almost down to 500 m/s at a depth of 120 m. This velocity drop is related to a lithological boundary associated with variations of compaction or cementation of the sediments. The corresponding variation of the compressional velocity is much smaller, due to the fluid saturation of the deeper layers. Figure 9 Sonic (Vp and Vs) and gamma ray log along line 2. The velocity values are confirmed by a sonic log available close to the center of the line (Figure 9). In the logged interval, the shear velocity doesn t show major variations, with an average value of 590 m/s and a standard deviation of 30 m/s. The inverted Vs section for the test line 2, which decreases to a depth of about 200 m, is depicted in Figure 10. The shear wave velocity profile from the log is overlain on the velocity section. The vertical resolution of the log is higher, and several small-scale velocity variations are identified. The agreement of the inverted velocity is good, within 5% difference. The vertical resolution of Rayleigh wave inversion, at the depth of interest, is not sufficient to identify thin beds. However, the lateral shear wave velocity variations identified by the Rayleigh wave inversion can be of interest for the stratigraphic characterization and for mapping attributes in 3D, and for the petrophysical characterization of the formations. Figure 10 Shear wave velocity section for line 2, with a shear sonic log in the deepest 100 m EAGE 107

6 first break volume 28, May 2010 In addition, the processing of point receiver data allows the identification of several reflections in the near surface, above the Rus formation. The top Rus is a strong continuous reflector over almost all Kuwait, and marks the top of a tertiary anhydrite. It has a regional dip towards northeast, and in the areas of interest is at a depth of over 500 m. Above the Rus, three main formations are found in the near surface: Dibdibba, Fars and Ghar, and Dammam. Under quaternary sediments, the Dibdibba formation has an upper sandy member and a lower member consisting of a coarse-grained sandstone. The Fars and Ghar formations consist of interbedded, well-sorted sand and sandstone, silty sand, and sandstone with clay. The Lower Fars, as mentioned before, consists of two sand units separated by a shale bed and capped by a regional shale layer. The Dammam is a limestone and is unconformable at the top. The lithostratigraphic column is shown in Figure 10. The top Rus is often considered the base of the near surface, despite the presence of shallower interfaces and seismic reflectors. The aim of the test was the evaluation of the practicalities of imaging above the Rus, in what has often been considered an anonymous near surface. The results of a simple processing sequence are shown in Figure 12. The unmigrated stack section for line 1 is plotted (B) together with the shear wave velocity section (A) for the same line. Some of the formations can be identified and picked, and structural elements can be identified. Figure 11 Lithostratigraphic columns for the Kuwait near-surface (after Al Sulaimi and Al Ruwaih, 2004). Conclusions The shallow subsurface can be complex, even in areas with mature topography. Some of the standard methods used Figure 12 Stack section and shear wave velocity section for line EAGE

7 for near surface characterization have intrinsic limitations. When high resolution is required at shallow depth, the near surface characterization is of primary importance. For very shallow targets, as well as for deeper ones, the heavy vibrator data were superior compared to weight drop or vibrator pulse data. The weight drop, however, added low frequencies from 2 to 4 Hz to the Rayleigh waves which increased the depth penetration. The Rayleigh wave analysis and inversion can provide reliable images of the subsurface. The velocity values have been validated by direct log data. The Rayleigh wave inversion has a dual value. First, it is a tool for the near surface characterization, providing a model for the computation of near surface compensation, for instance statics. Secondly, it provides an estimate of the formation properties in the sequence hosting the reservoir. For the second application, the shear sensitivity of the Rayleigh wave is an advantage, bringing additional information for the petrophysical characterization. The test has also proven that source and coherent noise in the near offset can be properly handled with point receiver data, leading to a high quality shallow stack section. Further processing and interpretation are in progress, but the test shows that it s possible to characterize and image the near surface above the usual first seismic marker. Besides the obvious use of this information for shallow targets, these results can be beneficial for deeper targets, in the presence of a complex near surface. Acknowledgements The authors acknowledge Kuwait Oil Company and WesternGeco for permission to publish this work. The contribution of Anna Glushchenko, Amr El Sabah, Frederico Melo, and Ehab Metwali of WesternGeco is also acknowledged. References Bagaini, C. [2008] Low-frequency vibroseis data with maximum displacement sweeps. The Leading Edge, 27, Dusseault, M.B., Oskui, R.P., Farhad, B., Al Sammak, I. and Al Naqi, A. [2008] Production technologies for Lower Fars Heavy Oil, Kuwait. World Heavy Oil Congress, Edmonton, Canada. Strobbia C., Laake A., Vermeer, P.L. and Glushchenko, A. [2009] Surface waves: use them, then lose them. 71 st EAGE Conference & Exhibition, Extended Abstract. Al Sulaimi, J.S. and Al Ruwaih, F.M. [2004] Geological, structural and geochemical aspects of the main aquifer systems in Kuwait. Kuwait J. Sci. Eng., 31(1) November 2010 Dubai, UAE Geosteering & Well Placement is no longer in its youth. In recent years we have witnessed new developments extending the capabilities of well placement far beyond expectations and enabling oil and gas operators to maximize the return on investment. Experience on reservoir production history is building up and best practices have been documented, but there is still a long way to go to ensure things are done as well as possible. The aim of this EAGE workshop is to bring together key experts from oil companies and service companies to discuss, through case studies, the following themes: State of the Art: Value articulation, success & challenges, technology Operations: Processes, remote operations, people Integration: Geological models, drilling, well completion & reservoir management For further details or to submit a paper please go to Call for Papers deadline 31 May EAGE 109