Rock strength and elastic properties are primary rock parameters. Many rocks are inherently anisotropic. Knowledge of strength anisotropy and elastic anisotropy has great practical application in stimulation design. Twelve oriented pyrophyllite core plugs were tested to analyze the effects of anisotropy. The orientation angles between core axes and the anisotropy axis range from 0° to 90° with 15° increments. Ultrasonic pulse transmission technique is employed to measure P-wave velocity and S-wave velocities. Uniaxial compression tests give rock strength and static modulus from stress-strain data. Static moduli are compared with dynamic moduli calculated from wave velocities.
The results indicate that compressive strength, wave velocities and elastic moduli are affected significantly by anisotropy. Pyrophylllite has an asymmetrical U-shaped strength anisotropy behavior. The greatest strength is measured parallel to fabric, 108.7 MPa. The lowest strength is 88.3 MPa found for samples cored at 45° orientation to the fabric. The strength anisotropy behavior of pyrophyllite can be predicted by knowing the strength values at 0°, 90° and the orientation angle of the minimum strength (approximately 45° in this study). P-wave and S-wave velocities, dynamic and static Young’s moduli are strongly anisotropic. The variation of measured P-wave and S-wave phase velocities agree with the predicted model (Thomsen 1986). Dynamic Young’s modulus varies from 38.6 GPa to 53.6 GPa while the static modulus varies from 13.9 GPa to 32.5 GPa. Dynamic modulus is generally higher than the static for all the orientations. The ratio of dynamic to static moduli ranges from 1.65 to 2.83. A cross-plot of strength anisotropy ratio versus static modulus anisotropy ratio for pyrophyllite and other rock types reported in the literature is presented. No obvious correlation is found between these two parameters, which may be due to the limited number of published data..
Today, the problem of surface-related multiples, especially in shallow water, is not fully solved. Although surface-related multiple elimination (SRME) method has proved to be successful on a large number of data cases, the involved adaptive subtraction acts as a weak link in this methodology, where primaries can be distorted due to their interference with multiples. Therefore, recently, SRME has been redefined as a large-scale inversion process, called estimation of primaries by sparse inversion (EPSI). In this process the multi-dimensional primary impulse responses are considered as the unknowns in a large-scale inversion process. By parameterizing these impulse responses as spikes in the space-time domain, and using a sparsity constraint in the update step, the algorithm looks for those primaries that, together with their associated multiples, explain the total input data. As the objective function in this minimization process truly goes to zero, the tendency for distorting primaries is greatly reduced. An additional advantage is that imperfections in the data can be included in the forward model and resolved simultaneously, such as the missing near offsets. In this paper it is demonstrated that the ghost effect can also be included in the EPSI formulation after which a ghost-free primary estimate can be obtained, even in the case the ghost notch is within the desired spectrum.
Full wavefield migration (FWM) is an inversion-based imaging algorithm that utilizes the complete reflection measurements: primaries as well as all multiples, both surface and internal. Using multiples in the imaging can extend the illumination of the subsurface. In this paper we concentrate the study on the internal scattering that can be helpful in imaging structures from below that are otherwise difficult to image by primaries, i.e. an undershooting setting. This can be fruitful in the case of the obstacles, like oil-production facilities, or in the case of poor illumination, like sub-salt imaging or near-surface complexities. We demonstrate such approach on two synthetic examples.
Tomographic full waveform inversion (TFWI) provides a framework to invert the seismic data that is immune to cycle-skipping problems. This is achieved by extending the wave equation and adding an offset axis to the velocity model. However, this extension makes the propagation considerably more expensive because each multiplication by velocity becomes a convolution. We provide an alternative formulation which computes the backscattering and the forward scattering components of the gradient separately. To maintain high resolution results of TFWI, the two components of the gradient are first mixed and then separated based on a Fourier domain scale separation. This formulation is based on the Born approximation where the medium parameters are broken into a long-wavelength and short-wavelength components. The inversion setup includes two steps that maintain the high resolution results of TFWI. First, the linearized residual are updated in a nested inversion scheme. This step corrects for the underlying assumption that the data contain primaries only without multiples. Second, the two components of the gradient are first mixed and then separated based on a Fourier domain scale separation to allow for a fully simultaneous inversion of model scales. After deriving the equations, we test the theory with two synthetic examples. The results of both the Marmousi and BP models show that convergence is possible even with large errors in the initial model that would have prevented convergence to conventional FWI.
Han, Jianfa (Tarim Oilfield Company, CNPC) | Sun, Sam Zandong (China University of Petroleum) | Zhang, Xinchao (China University of Petroleum) | Zhang, Yuanyin (China University of Petroleum) | Chen, Jun (Tarim Oilfield Company, CNPC) | Pan, Yangyong (Tarim Oilfield Company, CNPC) | Liu, Xin (Tarim Oilfield Company, CNPC) | Zhao, Haitao (Tarim Oilfield Company, CNPC)
In order to improve the SNR of seismic data, the multi-coverage technique is developed and it is a common practice to stack all trace information of a CRP as zero-incident data. In this process, although the SNR is greatly enhanced, yet the resolution is inevitably lost since the AVO effects are smeared. Moreover, the traditional post-stack seismic data is not the true pure P-wave data. By contrast, the pre-stack inversion can obtain the true P-wave data, and the resolution is much higher than the post-stack data since this method removes the effects of S-wave information. The study in this paper finds that feasible and reliable evaluations on the reservoir quality could be realized, based on which we can predict the high quality reservoirs. What's more, the mud-filled area at the top of Yingshan Formation is successfully identified, and could provide a unique and new insight for studying the development rules of carbonate reservoir.
This study assessed possible application of the friable sand model for shallow mechanically compacted overconsolidated sands from experientially derived velocity and porosity relations of six brine saturated sand aggregates and three published sand compaction datasets. The results showed that the friable sand model established with the assumption of the sediments normally consolidated can also be used for overconsolidated sands caused by stress released as long as the pore pressure is hydrostatic during unloading. We also found that the friable sand model used for overconsolidated sands not only describes change in depositional sorting but also variation in preconsolidation stress related to the amount of uplift. The study outcomes expand the rock physics diagnostic approach to predict seismic properties of shallow overconsolidated sands that have undergone complex burial history (i.e., loading-unloading-reloading) in uplifted basins like the Barents Sea.
The experience of integration of a broadband seismic dataset on CLOV project is presented. The fields to be developed are based in turbiditic reservoirs of Oligocene (Cravo, Lirio and Orquidea) and Miocene ages (Orquidea-Violeta) trapped within faulted structures in the passive margin transitional domain.
The survey acquired early 2012 with Geostreamer1 technology, mainly as 4D baseline, is replacing a reference 3DHR dataset in the geoscience field evaluation process. Hence in the optimization of the development wells and operational follow up, drawing benefits from the quality improvements associated with bandwidth broadening. Further S/N enhancement around the challenging areas results from more coherent low frequency content and more elaborated processing sequence.
As it proves to be valuable in terms of 3D seismic interpretation, too, the CLOV broadband dataset appears reliable as baseline dataset for the planed 4D reservoir surveillance seismic.
We develop a gradient computation for recovering anisotropic parameters associated with tilted-transverse isotropy (TTI) in full-waveform inversion (FWI). Our treatment is based on adjoint state theory and pseudo-analytic TTI wave propagation. The method provides reasonable estimates for anisotropic parameter updates and we observe that the migrated images for both the synthetic and real data show improvements with the inverted anisotropic parameters. However, due to the huge null space associated with the inversion we occasionally obtain parameters that are not in line with our understanding of the geology. To overcome this, we introduce a priori information within our inversion via dip annihilation filters. The introduction of this model styling goal within our inversion not only makes the results aesthetically pleasing but also in accordance with our physical intuition about the model. We illustrate our method with the help of a synthetic 3D example.
The purpose of this work is to evaluate the geo-potentials of graptolitic argillite in northern Estonia from the point of geology and even more from economic perspective, i.e. to understand spatial features of the shale layers including changes in thickness and compositional dynamics. The Late Cambrian to Ordovician crustal section of Estonia contains kukersite oil shale as well as graptolite argillite. Kukersite has been predominantly mined because of its higher calorific value (9–11 MJ/kg). On the other hand, graptolite argillite has a lower calorific value (4.2–6.7 MJ/kg); consequently it has not been mined as a source of fuel. However, graptolite argillite is characterized by significant amounts of U, Mo, Pb and other metals; thus can be treated as metal ore and two-fold energy source including U and shale oil. Sequel to the identified objectives, spatial analysis tools in ArcGIS have been used to integrate geologic, geochemical and environmental information to evaluate, better understand and spatially analyse the graptolite argillite in northern Estonia. A relatively easy pancake geology as induced from the bedrock geological map of Estonia indicates more or less even pattern of resource distribution but spatial analysis of available well data revealed that relatively high contents of microelements: U (>200 ppm), Pb (> 200 ppm), Mo (> 300 ppm), Zn (> 200 ppm), V (> 1000 ppm), Th (> 11 ppm) and Ag (> 0.7 ppm) occur in northeastern Estonia. More so, thickness of graptolitic argillite ranges from 0–2 m in northeastern Estonia and reaches up to 8 m in the northwestern area. Depth of the upper and lower surfaces of graptolitic argillite in northern Estonia increases from the northeastern area with values from 0.1 m and 0.4 m, respectively to the northwestern area where it reaches 266.2 m. From the results of cost-benefit analysis, it can be adduced that the microelements in graptolitic argillite hold highly scientific as well as significant economic value and possibly form future energy/ore resources.
Pre-salt targets in Brazil’s Santos Basin have become the focus of much exploration in recent years, creating a need for higher resolution images over these complex structures. Variable-depth streamer acquisition is an emerging technology in the Santos Basin that can increase the usable bandwidth in both the low and high frequency ranges, gain stronger low frequency penetration, and reduce acquisition related noise. We examine the benefits of variable-depth streamer data for producing high-resolution pre-salt images and the potential for higher resolution velocity models in the Santos Basin. We utilize a real field data set to demonstrate that variable-depth acquisition, combined with advanced processing techniques, provides improvements to the pre-salt imaging in this region.