Changes In Seismic Response Research Articles

Seismic amplification of Late Quaternary paleovalley systems: 2D seismic response analysis of the Pescara paleovalley (Central Italy)

Robust site characterization and ground response analysis require a thorough understanding of subsurface features, including geophysical properties and geometries of sediment bodies. Late Quaternary paleovalley systems, often overlooked in seismic hazard assessments, represent a potential threat due to their unconsolidated infill (with shear wave velocities <200 m/s) and sharp contrast with the adjacent substrate. Through an integrated approach that combined geophysical and stratigraphic data, we characterized the subsurface of the Pescara paleovalley system. Geostatistical interpolation of microtremor measurements enabled mapping resonance frequencies, highlighting abrupt changes and delineating the paleovalley boundaries. High-resolution core descriptions were then correlated with resonance frequencies, enabling the reconstruction of a 3D geophysical depth model of the buried paleovalley morphology. Furthermore, analyzing velocity profiles from down-hole tests led to the identification of five main seismic/stratigraphic layers within the valley fill. The geometry and facies architecture were reconstructed through a cross-section transversal to the paleovalley axis and then implemented into a 2D finite element model. Seismic response was computed, revealing significant amplification factors at frequencies closely matching the direct observations. Amplification factors peaked at frequencies between 0.9 and 1.1 Hz in the paleovalley center and up to 5.5 Hz towards the flanks, reaching a factor of 4.6. These findings suggest a notable increase in amplification amplitude compared to simpler geological contexts and emphasize the potential impact on common building types. Response spectra show strong amplifications in the paleovalley system, potentially leading to an underestimation of spectral accelerations compared to NTC18 guidelines. The comparisons of 1D and 2D modeling approaches revealed minimal differences, indicating that the generally flat geometry of the valley may not exhibit clear 2D effects. However, local subsurface stratigraphy strongly influences lateral changes in seismic response, emphasizing the importance of detailed subsurface knowledge for realistic seismic response estimates.

The Behaviour of Building Frames on Raft Foundation using Dynamic Analysis

When designing building frames for seismic reasons, the effect of soil flexibility is typically disregarded, and the design is executed using the outcomes of dynamic analysis with a fixed base condition. Because the overall lateral stiffness of the structure decreases as a result of soil flexibility, the lateral natural period lengthens. The building frames situated on the Raft foundation may experience a significant change in seismic response due to this extension of the lateral natural period (T). Therefore, it is imperative to consider the soil's flexibility, also known as soil structure interaction, when doing analysis on the foundation's supporting layer.&#x0D; This paper examines how asymmetric building frames with raft footings behave dynamically when subjected to seismic forces that involve soil-structure interaction. The analysis is performed with SAP 2000*V21 FEM software. The structure is idealized as a three-dimensional space frame, with slabs modeled as a thin shell with four noded plate elements and six degrees of freedom at each node, and beams and columns modeled as two noded line elements. The soil is represented as equivalent springs with one (Winkler) and six (Modified Winkler) degrees of freedom; the stiffness of these springs varies depending on the type of soil and is determined by its dynamic shear modulus and poissons ratio. The Modified Winkler and Winkler raft foundations are modeled as thin shells with four noded plate elements, each with six degrees of freedom, and are criticized so that the element aspect ratios are equal to one.&#x0D; &#x0D; To assess the impact of soil structure interaction on building frames, the response is compared for a range of building frames with and without consideration of soil flexibility in terms of fundamental Natural Period, Seismic Base Shear, and Max. Lateral Displacement. The parametric study for Zone V takes into account the influence of various parameters, including the number of bays, stories, span lengths, and soil types (i.e., soft, medium, and stiff).&#x0D; &#x0D; It is discovered that the fundamental lateral natural period and seismic base shear of the system are significantly altered by the influence of soil flexibility on building frames. As soil stiffness decreases, the lateral natural period and seismic base shear increase as a result of soil flexibility. Additionally, it has been noted that as the number of bays increases, so do the building's base shear and lateral period. As the number of bays and stories increases, so does the maximum lateral displacement. &#x0D;

Temperature-dependent models for wave velocity of oil sand

Knowledge of how temperature affects the oil–sand acoustic response is useful to exploit these reservoir rocks with seismic methods. We propose three models: double-porosity coherent potential approximation (CPA), lower-bound Hashin–Shtrikmann (HS-), and contact cement (CC), based on different spatial distributions of heavy oil and temperature and frequency-dependent empirical equations. The shear modulus and S-wave velocity are affected by temperature in all the cases. Moreover, the properties of oil sands with heavy oil as a continuous matrix and higher viscosity are more sensitive to temperature. The models can provide theoretical support for the establishment of rock physical models in heavy oil reservoirs so as to quantitatively characterize the seismic response changes caused by thermal mining.

Numerical modeling of seismic wave propagation in loosely deposited partially saturated sands: an application to a mine dump monitoring case

Extensive mine dumps consisting of loosely deposited sands have been created as a result of open-pit lignite mining, with a risk of soil liquefaction under high water saturation and a corresponding initiating event. Soil compaction is one of the feasible methods for reducing the probability of liquefaction. For the monitoring of liquefaction events and the evaluation of compaction work, seismic survey methods with sensitivity to changes in soil saturation and structure may thus complement other methods. Compared to exploration methods for deep systems, the shallow subsurface presents some unique challenges. To this end, an open-source, customizable code based on Biot’s theory was developed in the FEniCS library, which takes into account partial saturation and porosity dependence of stiffness, permeability, and other quantities. Following code verification, a comprehensive investigation of parameter studies is conducted, from which the effects of different factors on wave propagation characteristics were obtained. The numerical model was applied to simulate the expected changes in seismic response following soil compaction. Furthermore, the position of the high saturation area could be detected from the reflection and refraction P waves. The goal of this work is to provide an analysis framework for the assessment of compaction works and monitoring liquefiable soils in mine dumps under conditions of variable saturation due to rising groundwater tables.

Petrophysical Acoustic Characterization of Weathered Granite: A Case Study of Mesozoic Granites in the Coastal Area of Fujian Province, China

In recent years, with the exploration and development of granite buried-hill oil and gas reservoirs, petrophysics research has played an important role in the study of reservoir characteristics and fluid identification. Through analysis of the relationship between the fluid-bearing petrophysical parameters and the reservoir, the seismic response changes caused by reservoir fluid changes can be determined. Mesozoic granites in the coastal area of Fujian Province in eastern China were investigated as the research object of this project. The mineral composition, density, porosity, P-wave velocity, and S-wave velocity of the granite were measured and analyzed by X-ray diffraction, rock density, rock porosity, and rock acoustics methods. Therefore, the granite’s petrophysical properties, fluid response characteristics, and gas sensitivity parameters were analyzed. The result of the study shows that the granite is predominantly monzogranite. According to the type of reservoir space assemblage, the samples can be divided into two types: those containing fracture-dissolution pores and those containing only dissolution pores. All the samples were characterized by medium to high densities and low to extra-low porosity. There was a linear correlation between the P-wave velocity and S-wave velocity under gas and water-saturated conditions. Factors such as P-wave to S-wave velocity ratio, Poisson’s ratio, Lame coefficient, and other parameters of the samples were analyzed, and the threshold values that distinguished the water and gas-saturated states of the samples were measured and determined. In addition, there were negative correlations between the P- and S-wave velocities and porosity. The sensitivities of the petrophysical parameters to the gas capacity from high to low are Ip2 − 2.03 Is2, λ − 0.03 μ, λ, λ/μ, E − 2.03 μ, σ, K/μ, K, Ip, Vp/Vs, Vp, E, μ, Vs, and Is. For granite-buried hill reservoirs, the variation ranges of the parameters, such as the density, porosity, and P-wave velocity, of the fracture-dissolution pore granite samples were larger than those of the dissolution pore samples. The bulk parameters (Ip, Vp, K, λ) and combination parameters (Ip2 − 2.03 Is2, K/μ, λ− 0.03 μ, E − 2.03 μ, λ/μ) of the dissolution pore samples were more sensitive to the gas capacity. The results of this study provide a basis for the geophysical identification of granite-buried hill reservoirs.

How petrophysical properties influence the seismic signature of carbonate fault damage zone: Insights from forward-seismic modelling

Seismic imaging is the most common tool to detect subsurface structures in continental and marine settings. Despite technological advancement, seismic analysis of carbonates is still challenging because they are strongly influenced by petrophysical heterogeneities, being this even more difficult when further heterogeneities are added by the presence of faults. In this work, unmigrated forward-seismic models are developed to understand the seismic response changes related to carbonate-bearing fault systems and their deformation behaviour. We focus on the carbonate ramp of the Majella Massif (central Italy), that is characterised by the presence of porous and faulted carbonate lithologies. This carbonate system represents an analogue of carbonate reservoirs worldwide. Field and laboratory data of fault rocks sampled at increasing distances from the slip planes highlight a damage zone/fault core architecture with a decreasing porosity approaching fault planes. This observation is also confirmed by the thin sections analysis and the calculation of the shear modulus. Seismic images of fault rocks presenting lower porosity than the host rock show weak diffraction hyperbolas while diffraction hyperbolas are more evident in the forward-seismic models of fault rocks with greater porosity than the host rock. Moreover, factors such as the variation in the dip angle of the fault or the thickness of the damage zone can enhance or reduce the hyperbolas. Performing the migration process to stacked sections would not provide evidence of increased porosity in the damage zone due to the suppression of the diffractive component. The presence of weak hyperbolas in unmigrated seismic images is thus interpreted as evidence of a lower porosity damage zone with respect to the host rock. This can be related to deformation mechanisms leading to porosity loss in the damage zone for porous rocks as observed in the study area. However, this could also be related to the confining stress or fracture filling that counteract the fracture-related porosity increase when faults are hosted by tight rocks.

A Structural Design Comparison Between Two Reinforced Concrete Regular 6-Level Buildings using Soil-Structure Interaction in Linear Range

Structural engineers commonly design superstructures as fixed at the base and transmit the reactions to the infrastructure in order to design the foundation system and estimate the displacement of the soil while disregarding the change in seismic response that this induces. In this article, the foundation system was transformed into equivalent springs, and the seismic response in the linear range was compared and quantified, obtaining results such as increased periods, increased amounts of steel reinforcement in beams (between 7% and 25%) and columns (between 29% and 39%), an increase in the number of stirrups per linear meter (between 3% and 11% in columns and between 5% and 45% in beams) and drifts (between 1% and 14%), and a decrease in basal shear (up to 20%), which directly affects the design of the structure. This study concludes that the inclusion of the soil-structure interaction is necessary for structural design in the linear range.

Testing and Numerical Studies on Transverse Responses of Cable-stayed Bridge Full Model under Transverse Non-uniform Excitations

ABSTRACT A cable-stayed bridge full model was tested to study the travelling wave effects on the transverse acceleration frequency spectra, seismic responses, and pile-soil interactions under transverse non-uniform excitations. The results indicate that the non-uniform excitations affect the transverse acceleration frequency spectra of the girder. The non-uniform excitations increase the displacement at 3/4-span of the girder but decrease the displacements at 1/2- and 1/4-span of the girder. The pile-soil interaction effects of one tower increase, whereas those of another tower decrease due to the non-uniform excitations. The numerical model could generally capture the seismic response changes caused by the non-uniform excitations.

Shaking table test verification of traveling wave resonance in seismic response of pile-soil-cable-stayed bridge under non-uniform sine wave excitation

The traveling wave resonance phenomenon exhibits in the seismic response of the symmetric bridge structure. However, there is a scarcity of shaking table tests validating the traveling wave resonance. A 1/70-scale pile-soil-cable-stayed bridge model was fabricated following the preliminary design of a cable-stayed bridge with a main span of 1400 m. Shaking table tests were performed to verify the traveling wave and mode shape resonances in the seismic response of the bridge model when separately subjected to two specific sine waves longitudinally. The vertical acceleration frequency spectra and seismic response of the bridge model were compared between uniform and non-uniform sine wave excitation. The results showed that the traveling wave and mode shape resonances were examined in the seismic response of the pile-soil-cable-stayed bridge model under non-uniform excitation of the specific sine waves. The traveling wave excitation increased the symmetric response at the symmetric girder location but decreased the antisymmetric responses of the towers and girder, compared to the uniform excitation regarded as antisymmetric input. Finally, a finite element model was developed in OpenSees that could capture the seismic response changes caused by the traveling wave effects of the specific sine waves and proximately reproduce the experimental results. Therefore, the traveling wave and mode shape resonances were verified by the numerical simulations.

CO2 storage in the high Arctic: efficient modelling of pre‐stack depth‐migrated seismic sections for survey planning

ABSTRACTThe sequestration of CO2 in subsurface reservoirs constitutes an immediate counter‐measure to reduce anthropogenic emissions of CO2, now recognized by international scientific panels to be the single most critical factor driving the observed global climatic warming. To ensure and verify the safe geological containment of CO2 underground, monitoring of the CO2 site is critical. In the high Arctic, environmental considerations are paramount and human impact through, for instance, active seismic surveys, has to be minimized. Efficient seismic modelling is a powerful tool to test the detectability and imaging capability prior to acquisition and thus improve the characterization of CO2 storage sites, taking both geological setting and seismic acquisition set‐up into account. The unique method presented here avoids the costly generation of large synthetic data sets by employing point spread functions to directly generate pre‐stack depth‐migrated seismic images. We test both a local‐target approach using an analytical filter assuming an average velocity and a full‐field approach accounting for the spatial variability of point spread functions. We assume a hypothetical CO2 plume emplaced in a sloping aquifer inspired by the conditions found at the University of Svalbard CO2 lab close to Longyearbyen, Svalbard, Norway, constituting an unconventional reservoir–cap rock system. Using the local‐target approach, we find that even the low‐to‐moderate values of porosity (5%–18%) measured in the reservoir should be sufficient to induce significant change in seismic response when CO2 is injected. The sensitivity of the seismic response to changes in CO2 saturation, however, is limited once a relatively low saturation threshold of 5% is exceeded. Depending on the illumination angle provided by the seismic survey, the quality of the images of five hypothetical CO2 plumes of varying volume differs depending on the steepness of their flanks. When comparing the resolution of two orthogonal 2D surveys to a 3D survey, we discover that the images of the 2D surveys contain significant artefacts, the CO2‐brine contact is misplaced and an additional reflector is introduced due to the projection of the point spread function of the unresolvable plane onto the imaging plane. All of these could easily lead to a misinterpretation of the behaviour of the injected CO2. Our workflow allows for testing the influence of geological heterogeneities in the target aquifer (igneous intrusions, faults, pervasive fracture networks) by utilizing increasingly complex and more realistic geological models as input as more information on the subsurface becomes available.

Markovian modeling of seismic damage accumulation

SummaryAnalysis of civil structures at the scale of life‐cycle requires stochastic modeling of degradation. Phenomena causing structures to degrade are typically categorized as aging and point‐in‐time overloads. Earthquake effects are the members of the latter category this study deals with in the framework of performance‐based earthquake engineering (PBEE). The focus is structural seismic reliability, which requires modeling of the stochastic process describing damage progression, because of subsequent events, over time. The presented study explicitly addresses this issue via a Markov‐chain‐based approach, which is able to account for the change in seismic response of damaged structures (i.e. state‐dependent seismic fragility) as well as uncertainty in occurrence and intensity of earthquakes (i.e. seismic hazard). The state‐dependent vulnerability issue arises when the seismic hysteretic response is evolutionary and/or when the damage measure employed is such that the degradation increment probabilistically depends on the conditions of the structure at the time of the shock. The framework set up takes advantage also of the hypotheses of classical probabilistic seismic hazard analysis, allowing to separate the modeling of the process of occurrence of seismic shocks and the effect they produce on the structure. It is also discussed how the reliability assessment, which is in closed‐form, may be virtually extended to describe a generic age‐ and state‐dependent degradation process (e.g. including aging and/or when aftershock risk is of interest). Illustrative applications show the options to calibrate the model and its potential in the context of PBEE. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Developing a model discrete fracture network, drilling, and enhanced oil recovery strategy in an unconventional naturally fractured reservoir using integrated field, image log, and three-dimensional seismic data

In this study, we develop a model discrete fracture network (DFN) for the unconventional, naturally fractured Tensleep Sandstone oil reservoir at Teapot Dome, Wyoming. Reservoir characterization is based on three-dimensional (3D) seismic data, fracture image logs from Teapot Dome, and field observations of the Tensleep exposure in the Alcova anticline and Fremont Canyon areas. Image logs reveal that the dominant reservoir fracture set trends parallel to the present-day maximum horizontal compressive stress () inferred from drilling induced fractures. Analog field studies of the Alcova anticline and Fremont Canyon suggest fracture heights and lengths are power-law distributed, while the fracture spacing distribution is best described as log-normal. Image-log–derived fracture apertures are also log-normally distributed. These properties are incorporated into a model DFN. We assume subseismic folds, faults, and fracture zones control fracture intensity distribution and use composite 3D seismic attributes to locate subtle changes in seismic response interpreted to result from subseismic structure. Directional curvature defines aperture-opening strain normal to the dominant reservoir fracture set. Seismic attributes are scaled and combined to control fracture intensity variations in the model. Grid-cell porosity and permeability distributions derived from the DFN suggest the presence of northeast–southwest-trending reservoir compartments. We suggest that enhanced oil recovery operations may be optimized using lateral injection and production wells oriented along interpreted compartment boundaries at high angles to . This combination of injection and production laterals could help maximize storage and hydrocarbon recovery in depleted reservoirs and in down-dip residual oil zones.

Assessment of 4D seismic repeatability and CO2 detection limits using a sparse permanent land array at the Aquistore CO2 storage site

Two 3D time-lapse seismic surveys were acquired in 2012 and 2013 at the Aquistore [Formula: see text] storage site prior to the start of [Formula: see text] injection. Using these surveys, we determined the background time-lapse noise at the site and assessed the feasibility of using a sparse areal permanent receiver array as a monitoring tool. Applying a standard processing sequence to these data, we adequately imaged the reservoir at 3150–3350 m depth. Evaluation of the impact of each processing step on the repeatability revealed a general monotonic increase in similarity between the data sets as a function of processing. The prestack processing sequence reduced the normalized root mean squared difference (nrms) from 1.13 between the raw stacks to 0.13 after poststack time migration. The postmigration cross-equalization sequence further reduced the global nrms to 0.07. A simulation of the changes in seismic response due to a range of [Formula: see text] injection scenarios suggested that [Formula: see text] was detectable within the reservoir at the Aquistore site provided that zones of greater thickness than 6–13 m have reached [Formula: see text] saturations of greater than 5%.

Monitoring CO2 Storage at Cranfield, Mississippi with Time-Lapse Offset VSP – Using Integration and Modeling to Reduce Uncertainty

Abstract A time-lapse Offset Vertical Seismic Profile (OVSP) data set was acquired as part of a subsurface monitoring program for geologic sequestration of CO2. The storage site at Cranfield, near Natchez, Mississippi, is part of a detailed area study (DAS) site for geologic carbon sequestration operated by the U.S. Dept. of Energy's Southeast Regional Carbon Sequestration Partnership (SECARB). The DAS site includes three boreholes, an injection well and two monitoring wells. The project team selected the DAS site to examine CO2 sequestration multiphase fluid flow and pressure at the interwell scale in a brine reservoir. The time-lapse (TL) OVSP was part of an integrated monitoring program that included well logs, crosswell seismic, electrical resistance tomography and 4D surface seismic. The goals of the OVSP were to detect the CO2 induced change in seismic response, give information about the spatial distribution of CO2 near the injection well and to help tie the high-resolution borehole monitoring to the 4D surface data. The VSP data were acquired in well CFU 31-F1, which is the ∼3200 m deep CO2 injection well at the DAS site. A preinjection survey was recorded in late 2009 with injection beginning in December 2009, and a post injection survey was conducted in Nov 2010 following injection of about 250 kT of CO2. The sensor array for both surveys was a 50-level, 3-component, Sercel MaxiWave system with 15 m (49 ft) spacing between levels. The source for both surveys was an accelerated weight drop, with different source trucks used for the two surveys. Consistent time-lapse processing was applied to both data sets. Time-lapse processing generated difference corridor stacks to investigate CO2 induced reflection amplitude changes from each source point. Corridor stacks were used for amplitude analysis to maximize the signal-to-noise ratio (S/N) for each shot point. Spatial variation in reflectivity (used to ‘map’ the plume) was similar in magnitude to the corridor stacks but, due to relatively lower S/N, the results were less consistent and more sensitive to processing and therefore are not presented. We examined the overall time-lapse repeatability of the OVSP data using three methods, the NRMS and Predictability (Pred) measures of Kragh and Christie (2002) and the signal-to-distortion ratio (SDR) method of Cantillo (2011) . Because time-lapse noise was comparable to the observed change, multiple methods were used to analyze data reliability. The reflections from the top and base reservoir were identified on the corridor stacks by correlation with a synthetic response generated from the well logs. A consistent change in the corridor stack amplitudes from pre- to post-CO2 injection was found for both the top and base reservoir reflections on all ten shot locations analyzed. In addition to the well-log synthetic response, a finite-difference elastic wave propagation model was built based on rock/fluid properties obtained from well logs, with CO2 induced changes guided by time-lapse crosswell seismic tomography ( Ajo-Franklin et al., 2013 ) acquired at the DAS site. Time-lapse seismic tomography indicated that two reservoir zones were affected by the flood. The modeling established that interpretation of the VSP trough and peak event amplitudes as reflectivity from the top and bottom of reservoir is appropriate even with possible tuning effects. Importantly, this top/base change gives confidence in an interpretation that these changes arise from within the reservoir, not from bounding lithology. The modeled time-lapse change and the observed field data change from 10 shotpoints are in agreement for both magnitude and polarity of amplitude change for top and base of reservoir. Therefore, we conclude the stored CO2 has been successfully detected and, furthermore, the observed seismic reflection change can be applied to Cranfield's 4D surface seismic for spatially delineating the CO2/brine interface.

Transient and Long-Term Changes in Seismic Response of the Natural Resources Building, Olympia, Washington, due to Earthquake Shaking

The Natural Resources Building (NRB) in Olympia, Washington, was shaken by three earthquakes (Mw = 5.8, 6.8, and 5.0) between 1999 and 2001. Building motions were recorded on digital accelerographs, providing important digital recordings of repeated strong shaking in a building. The NRB has 5-stories above grade with 3 sub-grade levels and a ductile steel-frame elongated in the E-W direction. The upper two floors extend significantly beyond the lower 3 on the southern and eastern sides. N-S motions dominate the fundamental modal vibrations of the building system. In the 1999 Satsop M5.8 earthquake, the frequency of this fundamental system mode was 1.3 Hz during motions of 10% g. The frequency dropped to 0.7 Hz during the 2001 M6.8 Nisqually strong motions. Moreover, the Nisqually recordings reveal both numerous high-frequency transients of up to 0.18 g, several of which are visible on widely spaced sensors, and long-term tilts of some of the sensors. The weaker 2001 M5.0 Satsop earthquake motions showed the frequency remained depressed at less than 1 Hz for the eastern side of the structure, although the western side had recovered to 1.3 Hz. An ambient noise survey in 2008 showed the fundamental frequency of N/S vibrations remains about 1.0 Hz for the eastern side of the building and 1.3 Hz for the western side. These results suggest that in the Nisqually earthquake, the east side of the NRB suffered a permanent reduction in fundamental mode frequency of 37% due to loss of system stiffness by undetermined mechanism.

Geophysical characterization of the Devonian Nisku Formation for the Wabamun area CO2 sequestration project (WASP), Alberta, Canada

Geophysical characterization of the Devonian Nisku Formation in the Wabamun area, Alberta, Canada for large-scale CO 2 sequestration project has been successful in revealing two primary groups of anomalies. The first group is interpreted to be due to contrasts in lithology of the Nisku Formation. This interpretation is supported by constraints provided by well control, seismic modeling and petrophysical data. The second group is interpreted to be footprints of geological discontinuities which are induced by dissolution and karsting in a geologic formation shallower in the stratigraphy. Even though there is no evidence to indicate that the integrity of the Nisku Formation or the overlying caprock has been compromised, such geologic discontinuities should be taken into consideration if supercritical CO 2 were to be injected into the Nisku Formation. The analysis has identified favorable low-impedance high-porosity locations that could be developed for a CO 2 injection site. Those locations exhibit a good correlation with relatively high porosity-permeability zones of the Nisku Formation on maps derived from wireline data and core analysis using petrophysical analysis. Finally, fluid replacement modelling was undertaken to predict the feasibility of time-lapse seismic monitoring for detecting an injected CO 2 plume. The results suggest that changes in seismic response will most likely be subtle and that the plume will probably be at the lower threshold of seismic detectability.

Real-Time Reservoir Management in a High-Cost Deepwater Environment - Greater Plutonio

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 128542, ’Greater Plutonio - Real-Time Reservoir Man age - ment in a High-Cost Deepwater En vironment,’ by Dave Booth, SPE, and Pedro Sebastiao, SPE, BP plc, prepared for the 2010 SPE Intelligent Energy Conference and Exhibition, Utrecht, The Netherlands, 23-25 March. The paper has not been peer reviewed. Greater Plutonio is a large subsea development with a planned first phase of 43 high-rate subsea wells. A relatively low level of classical field appraisal was performed because of high costs. Innovative subsurface management was required to reduce substantial uncertainty during the development phase. Remote access to real-time data enabled making better decisions faster and reducing the number of field staff. Introduction The Greater Plutonio fields (Plutonio, Cobalto, Galio, Cromio, and Paladio) were discovered offshore Angola between 1999 and 2000 and are Upper Oligocene turbidite reservoirs. Changes in fluid and lithology in the semiconsolidated reservoir rocks manifest themselves as distinct changes in seismic response. Water depths are between 1200 and 1400 m, and a relatively dispersed resource base resulted in the most viable development concept being a 100% subsea-well development with multikilometer-scale well spacing produced through a large floating production, storage, and offloading (FPSO) vessel. The initial development phase of 43 wells, plus up to 25 additional infill and sidetrack wells, was the largest subsea development undertaken by the operator. Of the initial 43 wells, 23 are injection wells, and most are completed across multiple reservoirs with full sand control. At the time of writing this paper, 34 wells had been drilled, and subsurface outcome was largely as expected. All reservoirs are waterflooded, with approximately one-half the injection wells having downhole flow control (DHFC) to improve injection management. The high-porosity rocks often have permeability exceeding 1 darcy, which, combined with high-gravity oil, provide a favorable mobility ratio and very efficient displacement. Recovery factors in some reservoirs should exceed 50%. The associated gas is injected through two wells in the Plutonio field to provide pressure support. This process will continue for 3 to 4 years after field startup, until gas export is available after commissioning of the Angola liquefied-natural-gas project. All of the production wells have openhole-gravel-pack (OHGP) completions, some with openhole completion lengths of up to 800 m. Although confidence to sanction the project was provided by the high-quality seismic data, a high degree of subsurface uncertainty remained (e.g., reservoir connectivity). Appraisal drilling was conducted in only one of the five fields. Therefore, a significant amount of subsurface appraisal was loaded into the development stage. The understanding and management of subsurface uncertainty has been crucial for the development, with early and effective data collection by use of wireline, logging-while-drilling (LWD), and coring programs; preproduction well-interference testing; and real-time data acquisition during the ramp-up period.

４Ｄ地震探査によるオイルサンド貯留層の動的挙動把握

Time-lapse three-dimensional (3D) seismic monitoring study was conducted in the JACOS Hangingstone steam assisted gravity drainage (SAGD) operation area, Alberta, Canada. The objective of the study was to delineate steam-affected areas using differences between two 3D seismic data acquired at different production stages for efficient reservoir management.The time-lapse surveys were acquired in February, 2002 and in March, 2006. As repeatability is important for the time-lapse seismic surveys, the two 3D seismic surveys were recorded with nearly identical field acquisition parameters and the data sets of both surveys were processed with identical processing flows.P-wave and S-wave velocities of oil sands core plugs from the field were also measured under various pressure and temperature conditions to understand the relationship between seismic velocities and reservoir conditions. The laboratory measurement results were combined and a rock physics model was proposed to predict velocity changes of the oil sands under reservoir conditions expected during SAGD operations.The two seismic volumes show significant differences in seismic character within the reservoir and time delays below the reservoir around the active SAGD well pairs. Synthetic seismic data based on the rock physics model were analyzed to evaluate seismic response changes of the time-lapse 3D seismic survey. From our analysis, the differences of the seismic responses between the two 3D seismic volumes can be quantitatively explained by P-wave velocity decrease of the oil sands layers due to the steam-injection. In addition, our result suggests that a larger area would be influenced by pressure than by temperature.In conclusion, the time-lapse 3D seismic monitoring along with the rock physics model is useful for qualitative and quantitative estimate of the rock property changes of the interwell reservoir sands in the field.

Modeling Time Lapse Seismic Monitoring of CO2 Sequestration in Hydrocarbon Reservoirs Including Compositional and Geochemical Effects

The sequestration of CO2 into geologic formations, specifically existing and depleted oil and gas reservoirs, is a promising solution for reducing environmental hazards from the release of greenhouse gases into the earth's atmosphere. A critical component of long-term sequestration will be our ability to adequately monitor the movement of CO2 fronts in the subsurface. In this article, we examine the viability of time-lapse seismic monitoring using an integrated modeling of fluid flow, including chemical reactions and seismic response. Modeling of CO2 injection is complicated by the various interactions between CO2, reservoir fluids, and the minerals in the formation. These interactions change fluid and bulk rock properties with time, which in turn impact the seismic signatures. We perform a comprehensive simulation of the gas injection process accounting for the phase behavior of CO2-reservoir fluids, the associated precipitation/dissolution reactions, and the accompanying changes in porosity and permeability. The simulation results are then used to model the changes in seismic response with time. The general observation is that gas injection decreases bulk density and wave velocity of the host rock system. Seismic amplitude attributes therefore change with time as well, and these effects provide a tool for tracking the movement of the CO2 front. Analysis of the results also confirms that much of the change can be attributed to chemical effects that should therefore be considered in studies of long-term sequestration projects.

A Rock Physics Simulator and its Application for CO2 Sequestration Process

Injection of CO2 into underground saline formations, due to their large storage capacity, is probably the most promising approach for the reduction of CO2 emissions into the atmosphere. CO2 storage must be carefully planned and monitored to ensure that the CO2 is safely retained in the formation for periods of at least thousands of years. Seismic methods, particularly for offshore reservoirs, are the primary tool for monitoring the injection process and distribution of CO2 in the reservoir over time provided that reservoir properties are favourable. Seismic methods are equally essential for the characterisation of a potential trap, determining the reservoir properties, and estimating its capacity. Hence, an assessment of the change in seismic response to CO2 storage needs to be carried out at a very early stage. This must be revisited at later stages, to assess potential changes in seismic response arising from changes in fluid properties or mineral composition that may arise from chemical interactions between the host rock and the CO2. Thus, carefully structured modelling of the seismic response changes caused by injection of CO2 into a reservoir over time helps in the design of a long-term monitoring program. For that purpose we have developed a Graphical User Interface (GUI) driven rock physics simulator, designed to model both short and long-term 4D seismic responses to injected CO2. The application incorporates CO2 phase changes, local pressure and temperature changes, chemical reactions and mineral precipitation. By incorporating anisotropic Gassmann equations into the simulator, the seismic response of faults and fractures reactivated by CO2 can also be predicted.We show field examples (potential CO2 sequestration sites offshore and onshore) where we have tested our rock physics simulator. 4D seismic responses are modelled to help design the monitoring program.