Seismic Pore Pressure Research Articles

Centrifuge modeling of seismic response of slope on partially replaced soft soil ground

The slope on coastal soft soil ground creates a significant risk of landslide and settlement during an earthquake, causing damage to overlying structures. Soil replacement is an effective method to improve the seismic stability of coastal soft soil ground. The seismic response and stability of soft soil ground and slopes before and after replacement are investigated by centrifuge shaking table tests. Results show that the acceleration response of the backfill slope increases after the soft soil is replaced. The instantaneous settlement increases while post-earthquake settlement significantly decreases. There is a significant hysteresis in the seismic pore pressure response of soft clay. The time required for the pore pressure to reach its peak value decreases after the soft soil is replaced, while the peak seismic pore pressure value decreases significantly. Replacing the soft soil foundation can improve the drainage boundary, accelerate the dissipation of seismic excess pore pressure, and greatly reduce post-earthquake settlement. When a seismic wave is applied, a slip occurs along the interface between gravel and clay; this slip range is also reduced after the soil is replaced. The results of this work indicate that soft soil ground replacement can enhance seismic stability, providing a workable reference for the treatment of coastal soft soil ground.

Effect of Field Drainage on Seismic Pore Pressure Buildup and Kσ under High Overburden Pressure

AbstractThis paper studies the effect of a high effective overburden pressure [σv0′= ∼600 kPa (6 atm)] under two drainage conditions on the field liquefaction behavior of saturated Ottawa sand. A s...

Application of in-situ anisotropic parameters in 3D seismic velocity analysis for improved pre-drill pore pressure prediction in the onshore Niger Delta basin, Nigeria

The inaccurate prediction and characterization of anisotropic properties of hydrocarbon reservoirs during seismic processing have been a major obstacle to optimal time-to-depth conversion, well-to-seismic ties, and a robust fit-for-purpose velocity model for accurate seismic pore pressure (SPP) imaging. Though some levels of exploration successes have been achieved using isotropic velocity models in parts of the Niger Delta Basin, serious challenges however abound at the deeper shale-dominated intervals of the basin because of the intrinsic anisotropic properties of shale. The application of in situ anisotropic parameters in 3D seismic velocity analysis for improved pore pressure prediction in the study area was carried out with the objectives of applying the appropriate anisotropic parameters for optimal processing of seismic velocity field for overpressure studies. Anisotropic parameters for vertical transverse isotropic (VTI) wave-field were estimated from available wire-line logs using standard empirical relationships and were used to correct for shale anisotropy on a 6.0-km offset seismic data. The migrated velocities were further converted to geological depth and calibrated to wells. Isotropic and anisotropic pre-stack depth migrated (preSDM) velocities were inverted to seismic pore pressure prediction. Results of the study revealed that the anisotropic preSDM velocities yielded higher resolution in the vertical and lateral positioning of geological features when compared with the isotropic velocity model. Also, the seismic velocity fields generated from the anisotropic preSDM seismic velocities were observed to optimally tie to the well and accurately imaged the reservoir pore pressures at deeper intervals. These findings, therefore, revealed the appropriateness of the applied anisotropy parameters (−0.05 ≤ δ ≤ 0.05 and 0.08 ≤ η ≤ 0.48) in predicting elasto-thermal properties of the area. These parameters can therefore be incorporated into anisotropic velocity modeling and processing of long offset seismic data for accurate prediction of pore pressure, lithological, and fluid properties of reservoirs in the area.

3D seismic responses of a long lined tunnel in layered poro-viscoelastic half-space by a hybrid FE-BE method

This paper aims to study the seismic responses of a long lined tunnel embedded in a water-saturated poro-viscoelastic half-space, with focus on wave passage effects. The poro-viscoelastic half-space is modeled by the Biot's theory and the soil nonlinearity is considered via an equivalent linear approach. The seismic responses of the tunnel structure are simulated by a 2.5D hybrid FE-BE method. A total of 72 cases of different incident angles for plane SV- and P1-waves are simulated, in order to conduct a systematic study on the tunnel seismic responses, in terms of longitudinal internal forces and strains, as well as seismic pore pressure and soil stresses. It is shown that the spatial variation of earthquake ground motion due to wave passage effects results in considerable longitudinal internal forces of the tunnel lining, which cannot be overlooked for seismic design. The responses of the tunnel depend on both the horizontal and vertical incident angles of the seismic waves. It is also shown that the 2D scenario with perpendicularly incident SV-waves, which is commonly considered in engineering practice, may lead to nonconservative seismic design, especially for tunnels in saturated soil.

Seismic Velocity Analysis for Improved Geopressure Modelling in Onshore Niger Delta

In this study, an improved evaluation of pore pressure using a model based seismic inversion technique has been carried out. Across six wells in the Onshore Niger Delta Basin, post stack seismic volume, angle stack gathers, seismic horizons, check shot, wireline logs, drilling and pressure data were analysed and interpreted. The model based inversion technique was applied to improve the seismic resolution as well as derive acoustic impedance using well velocities along with stacking velocities from velocity analysis of the 3D seismic data. Bowers’ Vp-VES coefficients of 7.43 and 0.77 were used to transform the derived seismic acoustic impedance velocity into seismic pore pressure cube. The seismic acoustic impedance interval velocity reveals much of the geology and resulted to a high resolution seismic pore pressure cube when compared at well location with direct pressure data. The Derived Seismic Pore Pressure (DSPP) also revealed that pore pressure and overpressure can reach or exceed 4000 and 1000psi respectively in the field. The results obtained have demonstrated that seismic acoustic impedance volume can offer high resolution seismic pore pressure cube in both time and space.

Pore Pressure Prediction in Onshore West Niger Delta Using Inverted Seismic Velocity and Derived Velocity (Vp) - Vertical Effective Stress (VES) Coefficients

In this study, pore pressure has been predicted using seismic data and derived compressional wave velocity (V p ) - Vertical Effective Stress (VES) coefficients. Post Stack Time Migration (PSTM), angle stack gathers, seismic horizons, checkshot, wireline logs, drilling and pressure data from six wells in the Onshore West Niger Delta, Nigeria were analysed and interpreted. Using generated velocity and density crossplots, the active overpressure generating mechanisms for the studied area were deduced. The V p -VES coefficients were modelled using the direct pressure data and the overburden profile computed from density log. Post stack seismic inversion was performed to improve the seismic resolution as well as derive acoustic impedance using well velocities and stacking velocities from velocity analysis of the 3-D seismic data. The derived V p -VES coefficients were used to transform the seismic acoustic impedance velocity into seismic pore pressure volume. Pore pressure profiles were accordingly extracted along well paths so as to test the accuracy of the model. Interpreted density-velocity crossplots revealed a decrease in velocity at constant density of 2.4 g/cc, an indication that unloading mechanisms contribute to overpressure in the field. The Bowers’ V p -VES coefficients of 7.43 and 0.77 were determined for A and B parameters respectively. Based on the results obtained, the top of overpressure occurred at a depth of 3750 ft and 3800 ft in UMO-001 and UMO-002 wells respectively with a corresponding average pore pressure gradient of 0.47 psi/ft for both wells, indicating that the wells are mildly overpressured. Onsets of unloading were observed in UMO-001 and UMO-002 wells at depths of 6250 ft and 6800 ft with pore pressure gradients of 0.51 psi/ft and 0.60 psi/ft respectively. The Derived Seismic Pore Pressure (DSPP) matched the measured pressure value (kick) of 5300 psi at a depth of 7450 ft and this validated and further increased confidence on the values of the V p -VES coefficients derived. These results show that the derived seismic acoustic impedance volume, vertical effective stress and overburden model produce high resolution seismic pore pressure cube in both time and space. The derived models when applied especially, with seismic acoustic impedance volume can be used to plan and drill future wells with great successes in the studied area. Keywords: pore pressure, vertical effective stress, seismic velocity, acoustic impedance DOI : 10.7176/JEES/9-10-07 Publication date :October 31 st 2019

Seismic pore pressure prediction at the Halten Terrace in the Norwegian Sea

Pre-drill pore pressure prediction is essential for safe and efficient drilling, and is a key element in the risk-reducing toolbox when designing a well. On the Norwegian Continental Shelf, pore pressure prediction commonly relies on traditional 1D offset well analysis, whereas velocity data from seismic surveys are often not considered. Our work with seismic interval velocities shows that the velocity field can provide an important basis for pressure prediction and enable the construction of regional 3D pressure cubes. This may increase the confidence in the pore pressure models and aid the pre-drill geohazard screening process. We demonstrate how a 3D velocity field can be converted to a 3D pore pressure cube using reported pressures in offset wells as calibration points. The method is applied to a regional dataset at the Halten Terrace in the Norwegian Sea; an area with a complex pattern of pore pressure anomalies which traditionally has been difficult to predict. The algorithm is searching for a velocity to pore pressure transform that best matches the reported pressures. The 3D velocity field is a proxy of rock velocity and is derived from seismic surveys, and is verified to checkshot velocities and sonic data in the offset wells.

Seismic Pore Water Pressure Relief Wells for Gravel Column–Bed System

Liquefaction mitigation can be achieved by dissipating seismic pore pressures. The research reported in this paper elaborates the effectiveness in dissipating seismic pore pressures of a gravel bed and relief well system using gravel columns in a case study in Cilacap, Indonesia. Seismic pore pressure generation was analyzed using commonly available methods in liquefaction analysis. The evaluated pore pressures in the sand layer and gravel columns were used in a 2D dissipation analysis using finite-difference consolidation equation solutions. The results of this study showed that a simple and cost-effective relief well and gravel bed or strip system can effectively dissipate excess pore pressures in the sand layer and gravel columns to a maximum residual pore pressure below 40%, thus reducing liquefaction potential as well as protecting the foundations in the sand.

A Database for the Experimental Study of Earthquake-Induced Liquefaction and Lateral Spreading in Sands

Centrifuge and large-scale testing in geotechnical engineering are very useful tools for modeling soil behavior under different loading conditions, particularly under earthquake loading. The paper presents an extensive database of nine centrifuge and large-scale liquefaction experiments performed both at the geotechnical centrifuge testing facility at Rensselaer Polytechnic Institute (RPI) and the large-scale testing facility at the University at Buffalo (UB). The database described herein was generated using the NEEShub online DataStore tool under the name “CENSEIS: Centrifuge and Large (Full)-Scale Modeling of Seismic Pore Pressures in Sands” (DOI: http://dx.doi.org/10.4231/D3GF0MX4F ). The paper discusses the tools and materials used in the experiments along with an explanation of each item in the database. Sample analyses are also presented in the paper to give an insight on the capabilities of the database for numerical and analytical applications. The paper is concluded with some possible applications along with tips and limitations of the database.

Centrifuge and Large-Scale Modeling of Seismic Pore Pressures in Sands: Cyclic Strain Interpretation

AbstractCentrifuge modeling of pore pressure buildup in a sand deposit as a result of shaking is evaluated by comparison with a large-scale experiment. In large-scale Test SG-1, a 5.6-m-thick, mildly sloping deposit of hydraulic fill clean Ottawa sand of Dr=40%, was subjected to 5 s of low-intensity base shaking (<0.02g) that induced excess pore pressures short of liquefaction. Three centrifuge experiments using various soil deposits and saturation fluids were conducted and compared with the large-scale test. One of these centrifuge simulations used the same Ottawa sand and Dr=40% of the prototype, a viscous pore fluid, and dry pluviation deposition, which created a soil fabric stiffer than the prototype. The other two centrifuge simulations used silty sand saturated with water. The pore pressure buildup in one of the silty sand tests was in good agreement with the prototype, while the other two centrifuge deposits did not develop any excess pore pressure. The various responses in the four tests are expla...

Effect of core composition on seismic stability of earth dams

It is current practice to employ composite clay –a mixture of clay and aggregates that floats within the clayey matrix – as the core of embankment dams, which were previously constructed from pure clays. Experience has shown that significant pore pressure can build up during cyclic loading in composite clays. In this paper, the results of dynamic analyses performed on the Karkheh dam in Iran, incorporating different core materials, are presented. Pure clay and composite clay are used as the core materials and the results of the analyses are compared. It is shown that significant seismic pore pressure build-up in composite clays can significantly reduce the factor of safety against sliding for the fragment of the slip surface passing through the core, even though the overall factor of safety falls within a reasonable level.

Geological constraints of pore pressure detection in shales from seismic data

ABSTRACTMethods for detection of pore fluid overpressures in shales from seismic data have become widespread in the oil industry. Such methods are largely based on the identification of anomalous seismic velocities, and on subsequent determination of pore pressures through relationships between seismic velocities and the vertical effective stress (VES). Although it is well known that lithology variations and compaction mechanisms should be accounted for in pore pressure evaluation, a systematic approach to evaluation of these factors in seismic pore pressure prediction seems to be absent. We have investigated the influence of lithology variations and compaction mechanism on shale velocities from acoustic logs. This was performed by analyses of 80 wells from the northern North Sea and 24 wells from the Haltenbanken area. The analyses involved identification of large‐scale density and velocity variations that were unrelated to overpressure variations, which served as a basis for the analyses of the resolution of overpressure variations from well log data. The analyses demonstrated that the overpressures in neither area were associated with compaction disequilibrium. A significant correlation between acoustic velocity and fluid overpressure nevertheless exists in the Haltenbanken data, whereas the correlation between these two parameters is weak to non‐existing in the North Sea shales. We do not presently know why acoustic velocities in the two areas respond differently to fluid overpressuring. Smectitic rocks often have low permeabilities, and define the top of overpressures in the northern North Sea when they are buried below 2 km. As smectitic rocks are characterized by low densities and low acoustic velocities, their presence may be identified from seismic data. Smectite identification from seismic data may thus serve as an indirect overpressure indicator in some areas. Our investigations demonstrate the importance of including geological work and process understanding in pore pressure evaluation work. As a response to the lack of documented practice within this area, we suggest a workflow for geological analyses that should be performed and integrated with seismic pore pressure prediction.

Overview: Seismic Applications (August 2005)

The age of digital technology has definitively arrived. As an industry, we now are reaping the benefits of many years of technology development as well as becoming mature in our ability to understand and evaluate the value of the data. In reviewing papers from this past year, I noted several interesting case studies and methodologies for integrating seismically derived attributes with geological and engineering data to improve the characterization and management of fractured reservoirs. Nevertheless, I am primarily struck by the variety of papers related to the development and application of real-time data transmission from seismic while drilling, through crosswell seismic, to methods for acquisition at the seabed. Much of the push has come from the exploration and development of deepwater reservoirs and the need for real-time integration of data to make well-informed decisions. Although driven by the high-end projects, the advantages of the technology are seen across the industry at large. We are realizing the benefits of implementing digital technology in life-of-field studies, in the development of green fields, in intelligent drilling and completions, and in automation of decision support processes. The “digital” toolbox is expanding. The expertise and domain knowledge required to further develop the technology extend across the entire range of classic sciences and will affect the way we work and communicate. Challenges still remain in our ability to integrate data fully and in how we use the information effectively. From the technical point of view, there still is work to do on fully integrated data-management platforms. On the human-resource side, we need great improvements in our ability to effectively manage communication across disciplines and to bridge the gap between onshore and offshore personnel. The articles selected here represent a taste of the trends and developments in digital technology from drilling and completion, to seismic at the drill bit, to seabed- and marine-seismic acquisition, to life-of-field development scenarios. Available from the SPE eLibrary: www.spe.org SPE 89994 - “Seismic Pore-Pressure Imaging in Deep Water Offshore West Africa,” by Banik, N.C., SPE, WesternGeco, et al. Available at the OTC Library: www.otcnet.org OTC 17284 - “Remote Reservoir-Resistivity Mapping—Breakthrough Geophysics for the Upstream,” by Srnka, L.J., ExxonMobil Upstream Research Co., et al. OTC 17669 - “Defining Fault-Exclusion Zones at Proposed Suction-Anchor Sites Using an AUV Micro-3D-Seismic Survey,” by Campbell, K.J., Fugro GeoServices Inc., et al. OTC 17646 - “LWD for Imaging, Wellbore Placement, and Formation Evaluation,” by Rasmus, J.C., Schlumberger, et al.

Deepwater hazard avoidance in a large top-hole section using LWD acoustic data

Drilling expensive and challenging wells in deepwater requires significant prewell planning to mitigate risks. Understanding the regional drilling hazards that nearby operators have encountered and correlating them to the current project must be done to gain a clear appreciation of potential problem hole sections. This paper describes a deepwater well drilled by Agip where significant efforts were taken to fully evaluate the potential for a shallow water flow in the top-hole section. Another operator in a nearby block had recently lost well to a shallow water flow. This heightened the concern of the Agip asset team so an extensive pore-pressure analysis of the surface seismic data was undertaken. The results of this analysis indicated that there was also a strong potential for a shallow water flow on this well. In order to build the best possible understanding of the pore pressure profile the surface seismic pressure analysis was taken much deeper than what is customary, in this case to the top of the salt. Prewell pore pressure models confirmed Agip's surface seismic pore pressure assessment. As a result, real-time pore pressure monitoring services were deemed essential to closely monitor the progress of the well and, with that information, make real-time adjustments to the drilling parameters (i.e. mud weight) as they became necessary. An extremely important and sometimes overlooked aspect of real-time interactivity is the definition of the role each team member has and how each team member interacts with the others. Assembling a team and defining each member's roles and responsibilities in the planning phase streamlines the time-critical decision-making process and allows for a decisive, unambiguous, work flowpath. Figure 1 shows the communication and workflow organization assigned by Agip before the well was drilled. Rig-based personnel are highlighted in green and office-based personnel in blue. The operations geologist was the …

Analysis of Seismic Settlements and Pore Pressures in Centrifuge Tests

An analytical approach has been developed to predict permanent settlements of foundations subjected to earthquake shaking. The validity of this approach is evaluated through comparison with results from centrifuge tests simulating the response of a tank foundation resting upon saturated sand and subjected to earthquake‐like shaking. Analyses are carried out for different values of soil permeability to back‐calculate the in situ permeability of the sand and demonstrate the effect of partial drainage. Good correlation exists between predicted and measured response for in situ permeability within the range of laboratory measured values. The results also demonstrate that partial drainage may occur even during high‐frequency dynamic loadings, with significant effect on settlements and excess pore pressures.

Erratum “ Chimney Drains to Control Seismic Pore Pressures ” by Arnoldo J. L. Bolognesi (September, 1984)

Erratum “ <i>Chimney Drains to Control Seismic Pore Pressures</i> ” by Arnoldo J. L. Bolognesi (September, 1984)

Chimney drains to control seismic pore pressures : Bolognesi, A J L J Geotech Engng Div ASCEV110, NGT9, Sept 1984, P1342–1364

Chimney drains to control seismic pore pressures : Bolognesi, A J L J Geotech Engng Div ASCEV110, NGT9, Sept 1984, P1342–1364