ORCID Profile
0000-0002-2158-6413
Current Organisation
Inpex (Japan)
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Publisher: Wiley
Date: 11-11-2022
Abstract: Pore‐pressure depletion causes changes in the triaxial stress state. Pore‐pressure depletion in a flat reservoir, for ex le, can be reasonably approximated as uniaxial compaction, in which the horizontal effective stress change is smaller than the vertical effective stress. Furthermore, the stress sensitivity of velocities can be angle‐dependent. Therefore, time‐lapse changes in reservoir elastic anisotropy are expected as a consequence of production, which can complicate the interpretation of the 4D seismic response. The anisotropic 4D seismic response caused by pore‐pressure depletion was investigated using existing core velocity measurements. To make a direct comparison between the anisotropic 4D seismic response and the isotropic response based only on vertical velocities, pseudoisotropic elastic properties were utilized, and the two responses were compared in terms of a dynamic rock physics template. A comparison of the dynamic rock physics templates indicates that time‐lapse changes in reservoir elastic anisotropy have a noticeable impact on the interpretation of 4D seismic data. Changes in anisotropy as a result of pore‐pressure depletion cause a time‐lapse litude variation with offset response as if there is a reduction in V P / V S (i.e., pseudoisotropic V P / V S decreases), although the vertical V P / V S increases. The impact of time‐lapse changes in anisotropy on the litude variation with offset gradient was also investigated, and the time‐lapse anisotropy was found to enhance changes in the litude variation with offset gradient for a given case.
Publisher: Society of Exploration Geophysicists
Date: 2021
Abstract: Amplitude variation with offset (AVO) inversion of an anisotropic data set is a challenging task. Nonnegligible differences in the anisotropy parameters between the various lithologies make the seismic data AVO response completely different from the isotropic synthetic seismogram. In this case, it is difficult to invert for V P /V S and density consistent with well-log data. AVO inversion using pseudoisotropic elastic properties is a practical solution to this problem. Verification of this method was performed using data from an offshore Western Australia field. It was found that wavelet extraction and density inversion are improved significantly by replacing the isotropic elastic properties with the pseudoisotropic properties. Inverted density shows reasonable quality and therefore can be included in the reservoir characterization study. Postinversion analyses can be performed effectively on the pseudoisotropic elastic properties because crossplot analysis shows the increased separation of different lithofacies due to contrasts in anisotropy parameters. This result could have significant implications for other fields, as shale constitutes most of the overburden in conventional oil and gas fields and often shows strong elastic anisotropy.
Publisher: American Geophysical Union (AGU)
Date: 09-2021
DOI: 10.1029/2021JB021993
Abstract: Shales often show strong elastic anisotropy that originate from the alignment and platy nature of its constituent minerals. Despite its impact on litude variation with offset response and seismic time‐shifts, elastic anisotropy of shales is, however, often ignored since it is difficult to measure enough parameters in the field. Being able to correctly estimate anisotropy parameters of shales can therefore significantly improve seismic reservoir characterization. A predictive model is developed by combining existing theories. Properties of locally aligned clay platelets, called domains, are calculated using a rock physics model based on the anisotropic Hashin‐Shtrikman estimates. The effect of domain orientation is then accounted for by the orientation distribution function of domains. The applicability of the model was investigated using existing core measurements. Interesting findings include: (a) most of modeled anisotropy parameters are consistent with the measured values even though only limited information was used for the model parameters optimization, and (b) most of optimized interplatelet medium properties are consistent with the saturated fluid and small interplatelet medium shear modulus suggested by existing studies. These findings imply that the model can be used to predict anisotropy parameters from limited information.
Publisher: Society of Exploration Geophysicists
Date: 12-2018
Abstract: Routinely applied methods in seismic reservoir characterization such as forward modeling, wavelet extraction, litude-variation-with-offset (AVO) analysis, AVO inversion, and interpretation of seismic data usually assume that the earth can be modeled by a stack of isotropic layers. This assumption may cause significant problems where there are nonnegligible differences in the anisotropic parameters between the various lithologies that cause vertical profiles of V P /V S and the anisotropy parameters to be dissimilar. In this case, a significant mismatch between the seismic data and the isotropic synthetic seismogram AVO response will occur, making far-angle stack interpretation difficult. In some cases, the mismatch might be misinterpreted as a data quality issue. In an offshore Western Australia field, three lithofacies (volcanic rock, sandstone, and shale) need to be correctly identified for detailed reservoir characterization. Here, the AVO response of the actual seismic data is significantly affected by velocity anisotropy. Originally, it was thought that the far-angle stack could be used to detect volcanic rock in the field however, after accounting for the velocity anisotropy effect, it was found that the far-angle stack enables us to identify sandstone. A proper understanding of the anisotropy effect allows the interpreter to use seismic data more effectively, which leads to a more robust estimation of the distribution of lithofacies in the target area.
Publisher: European Association of Geoscientists & Engineers
Date: 2020
Publisher: Wiley
Date: 29-03-2023
Abstract: Seismic litude variation with offset projection is widely used in the oil and gas industry to extract information contained in litude variation with offset responses. Traditional two‐term litude variation with offset projection, based only on intercept ( A ) and gradient ( B ), has been used in various ways, including hydrocarbon detection and the estimation of target properties such as water saturation. Three‐term litude variation with offset projection, in which A , B and curvature ( C ) are blended using two projection angles ( χ and ψ ), can also be used for the same purposes and has some advantages over the two‐term methods. The effectiveness of the three‐term litude variation with offset projection in the detection of sandstone has been investigated using field data. A sandstone detection workflow is proposed, which includes (1) defining the target attribute, (2) optimizing projection angle in the χ – ψ analysis plane painted with the target attribute and (3) confirming/refining the selected projection angle using the two‐term litude variation with offset projection‐ C cross‐plot. The impact of anisotropy is taken into consideration, and the sandstone index is introduced as the target attribute. The sandstone detection workflow was applied for both synthetic and field data, which illustrates that three‐term methods offer an important perspective on noise in the litude variation with offset attributes: Noise in the third term is correlated with noise in the other terms so that including C can actually suppress noise to some extent, even if C looks unusable. Moreover, two‐term litude variation with offset projections are a subset of three‐term litude variation with offset projections therefore, if C adds no information, this fact is revealed as the worst‐case outcome in the three‐term analysis inclusion of C will never degrade results and there is no reason to exclude C a priori.
Publisher: Society of Exploration Geophysicists
Date: 09-2018
Abstract: Core velocity measurements are an essential part of any 4D seismic feasibility study. During recently conducted core velocity measurements, we found some interesting results regarding velocity anisotropy and hysteresis. These findings include: (1) the stress sensitivity of velocity varies depending on the propagation direction, (2) velocities measured during loading have a significantly larger stress sensitivity than those measured during unloading, and (3) horizontal effective stress has a noticeable impact on velocity anisotropy. We conducted rock physics analysis and 1D seismic forward modeling, incorporating velocity anisotropy, and found that the estimated 4D seismic signal is largely affected by velocity anisotropy and hysteresis. These findings suggest the importance of considering the velocity measurement direction and the nature of the stress change to obtain a realistic 4D seismic signal. Neglecting these considerations may lead to a significantly underestimated or overestimated modeled seismic response.
Publisher: European Association of Geoscientists & Engineers
Date: 2022
Publisher: Springer Science and Business Media LLC
Date: 03-11-2020
DOI: 10.1007/S00603-020-02283-0
Abstract: Shale formations are the main source of borehole stability problems during drilling operations. Suboptimal predictions of borehole failure may partly be caused by neglecting the anisotropic nature of shales: Conventional wellbore stability analysis is based on borehole stresses computed from isotropic linear elasticity (Kirsch solution) with the assumption of no induced pore pressure. This is very convenient for a practical implementation but does not always work for shales. Here, anisotropic wellbore stability analysis was performed targeting an offshore gas field to investigate in particular the impact of elastic anisotropy on borehole failure predictions. Stress concentration around a circular borehole in anisotropic shale was calculated by the Amadei solutions, and induced pore pressure was obtained from the Skempton parameters based on anisotropic poroelasticity. Borehole failure regions and modes were then predicted using the effective stresses and those are apparently consistent with observations. A comparison with the conventional approach suggests the importance of accounting for elastic anisotropy: Predicted failure regions, modes, and also the associated mud weight limits can be completely different. This observation may have significant implications for other fields since shale often show strong elastic anisotropy.
Publisher: Society of Exploration Geophysicists
Date: 25-07-2022
Abstract: A seismic litude-variation-with-offset (AVO) analysis is an important element in seismic reservoir characterization and has been successfully applied in many fields. Traditionally, AVO studies are based on AVO intercept ([Formula: see text]) and gradient ([Formula: see text]) only curvature ([Formula: see text]) is neglected because it is very sensitive to noise and avoiding noisy [Formula: see text] is very difficult in practice. The recently introduced concept of three-term (3T) AVO projections, combined with continuous improvements in seismic data acquisition and processing, allows interpreters to effectively use [Formula: see text] by optimizing projection angles based not only on rock physics but also on seismic noise. The effectiveness of the 3T AVO projection, in which [Formula: see text], [Formula: see text], and [Formula: see text] are used as the triplet of reflectivities that are summed in the projection, and the impact of random noise on the 3T AVO projection have been investigated using data from an offshore Western Australian field. The 3T AVO projection is used in the framework of the extended elastic impedance. Because the study area is known to have nonnegligible differences in anisotropy parameters between the various lithologies, its impact on [Formula: see text] and [Formula: see text] is taken into account. First, the 3T projection angle targeting lithology fraction logs is optimized using well-log data. Analyses using synthetic seismograms are then performed to investigate the impact of seismic noise. Finally, the method is applied to field data. The results find that (1) [Formula: see text] is an important component of seismic reservoir characterization even if it appears unusable and (2) seismic noise must be taken into account when optimizing the projection angle.
Publisher: Society of Exploration Geophysicists
Date: 08-2016
Abstract: Velocity-model building with a good understanding of anisotropy is one of the important elements in a prestack depth migration (PSDM) project. During a recently conducted PSDM project, we observed that (1) the application of negative delta/epsilon is necessary to achieve reasonable depthing and gather flatness, and (2) azimuthal variation in gather flatness exists in the interval where negative delta/epsilon are necessary. A geologic explanation of these interesting observations was necessary to justify the velocity-model-building result. Investigation of stress magnitude/orientation and theoretical anisotropy of unconsolidated sandstone under nonhydrostatic stress revealed that these observations can be explained by stress-induced anisotropy. This concept may be applicable to other fields, and the existence of azimuthal anisotropy and value of anisotropy parameters can be roughly estimated by the method described.
Publisher: Springer Science and Business Media LLC
Date: 14-02-2023
DOI: 10.1007/S40948-023-00536-0
Abstract: Time-delayed wellbore failures are often observed in shale formation. In some cases, cavings indicating bedding plane failure are observed a few days after initial drill-out with a gradual increase, eventually leading to wellbore collapse. Possible causes of time-delayed wellbore failure include pore pressure diffusion, and its impact can be assessed by traditional poroelastic modeling in which coupled hydraulic-mechanical processes are taken into account. Externally applied loads induce pore pressure that is dissipated according to a diffusion law. During recently conducted wellbore stability analysis targeting a particular field within a strike-slip regime, we found: (1) time-dependent bedding plane failures are significant for highly inclined wells subparallel to the maximum horizontal stress direction (bedding plane failure region gradually increases with time), (2) the Kirsch solution and the assumption of no induced pore pressure (elastic nonporous model) does not give proper mud weight limits to avoid bedding plane failures, and (3) failure regions and modes predicted by the poroelastic model are different from those based on the elastic nonporous approach, under the given situation. Results given by the poroelastic model are apparently consistent with filed observations, demonstrating pore pressure diffusion as a possible cause of observed time-delayed bedding plane failure.
No related grants have been discovered for Michinori Asaka.