Ocean Bottom Cable Research Articles

Time-lapse 4D full-waveform inversion for ocean-bottom cable seismic data with seawater velocity changes

Time-lapse (4D) full-waveform inversion (FWI) is a valuable technology for high-resolution imaging of reservoir changes caused by hydrocarbon production and CO$_2$ storage. However, it still faces challenges in dealing with non-repeatability issues due to changes in seawater or near-surface velocity between baseline and monitor surveys. Despite recent advances, the ability of 4D FWI to address this problem has rarely been demonstrated. {\color{red}We} investigate the effectiveness of current 4D FWI strategies, including parallel, double-difference, sequential, and common-model strategies, in resolving non-repeatability issues for 4D OBC (ocean-bottom cable) seismic data. Additionally, a three-stage 4D FWI strategy is developed for 4D OBC seismic data, involving the estimation of seawater velocities in the baseline and monitor models, obtaining a good common starting model, and final convergence to obtain subsurface 4D changes. The synthetic data tests conducted with varying levels of seawater velocity changes indicate that among the current strategies tested, the common-model strategy performs the best, and that our new strategy can further enhance the 4D inversion results.

Non-Repetitive Time-Shifted Seismic Monitoring Study Based on Ocean Bottom Cable and Towed Streamer Data

Non-Repetitive Time-Shifted Seismic Monitoring Study Based on Ocean Bottom Cable and Towed Streamer Data

New technologies for seismic resolution enhancement and bandwidth expansion: Applications in SE Asian Basin

Over the last few years, the petroleum giant and its partner have effectively implemented new marine seismic technologies in this part of the world. During the acquisition phase, the industry has observed the introduction of novel broadband acquisition techniques such as dual-sensor cable, multiple towed streamers (over/under), slanted and variable depth streamers, and isometric areal recording. The application of these techniques has resulted in a noteworthy enhancement of the recorded data’s bandwidth. The focus of this paper is on the improvement of seismic bandwidth expansion resulting from advancements in marine broadband acquisition, innovative processing techniques, and inversion methodology. The outcome leads to a noteworthy enhancement in the vertical resolution, which proves to be valuable in the identification of thin stacked pay beds and the creation of a reservoir model that is constrained by seismic data. This paper focuses on the imaging aspect, particularly the complex structures. The utilization of wide and full azimuth (Circular) recording and the resolution of Gas wipeout issues through seabed acquisition with Ocean Bottom Cables (OBC) have been advantageous. By utilizing de-ghosted or far-field signature modeled techniques on the processing side, it is possible to increase the bandwidth for both legacy and modern high-quality data, such as Q-marine. A novel de-convolution technique has been developed, which can enhance signal frequency to a considerable extent while minimizing or eliminating any impact on noise. The article presents various wavelet transform techniques that can effectively enhance the signal-to-noise ratio (S/N). The methods are noteworthy due to their ability to be swiftly executed post-stack while incurring minimal expenses. Further, the inversion techniques, with a focus on stochastic elastic inversion, generates broadband data, enabling the identification of pay sand that falls below the quarter of wavelength criteria. The developed techniques are novel and have been validated using authentic Offshore Malaysia data.

OBC shallow water de-multiple based on the principle of Fresnel diffraction

Abstract In shallow-water ocean-bottom cable (OBC) seismic data, the ineffectiveness of conventional surface-related multiple elimination (SRME) methods due to poor seabed records is addressed. This research utilizes the seismic wavefield received by multiple cables from a single shot gather to predict shallow water multiple models for that shot gather. Initially, the seismic data within a finite aperture around a seismic trace in the time domain shot gather is treated as the known seismic wavefield. This seismic wavefield is then extrapolated along the water layer to this seismic trace, following the Fresnel diffraction principle. The extrapolated data becomes the shallow water multiple model for this seismic trace. This process is repeated for each trace in the shot gather to obtain the shallow water multiple model of the entire shot gather. Forward modeling tests have shown that smaller data apertures can effectively avoid the impact of spatial aliasing on multiple model prediction. To address the overlap of primary waves and shallow water multiples in deep seismic data, which have lower dominant frequencies, the multiple model data is used as a known seismic wavefield and extrapolated along the water layer again. This produces second-order and higher-order multiple models. Applying this model to suppress multiple waves can minimize primary waves loss. This entirely data-driven approach necessitates solely water depth information, imposing no additional conditions. Both forward modeling and real seismic data testing validate the efficacy of this method in shallow water.

Robust and efficient waveform-based velocity model building by optimal transport in the pseudotime domain: An ocean-bottom cable case study in the North Sea

Full-waveform inversion (FWI) in the North Sea has demonstrated its imaging power starting from low-resolution models obtained by traveltime tomography, enriching them with geologically interpretable fine-scale details. However, building a traveltime-based kinematically accurate starting model for FWI is a time-consuming and rather subjective process requiring phase identification and selection. The two main problems faced by FWI starting from noninformative initial models are the susceptibility to cycle skipping and a lack of sensitivity to low wavenumbers in the deep subsurface not sampled by turning waves. On a North Sea ocean-bottom cable 3D data set, a novel [Formula: see text] building methodology is applied that addresses those issues by jointly inverting reflections and refractions (joint full-waveform inversion [JFWI]) using a robust misfit function in the vertical traveltime domain (pseudotime). Pseudotime addresses reflectivity-velocity coupling and attenuates phase ambiguities at short offsets, whereas a graph-space optimal transport (GSOT) objective function with dedicated data windowing averts cycle skipping at intermediate-to-long offsets. A fast and balanced reflectivity reconstrution is obtained prior to JFWI thanks to an asymptotic-preconditioned impedance waveform inversion ([Formula: see text]WI). Starting from a linearly increasing one-dimensional model, GSOT-pseudotime JFWI is effective at obtaining a meaningful P-wave velocity macromodel down to depths sampled by reflections only, without phase identification and picking. P-wave FWI, starting from the JFWI-based model, injects the high wavenumbers missing in the JFWI solution, attaining apparent improvements in shallow and deep model reconstruction and imaging compared with the previous studies in the literature, and a satisfactory prediction of the ground-truth logs.

Dynamic positioning method and precision analysis of marine seismic vertical cables

AbstractSimilar to conventional ocean bottom seismometers and ocean bottom cables, the attitude and position of vertical cables deployed on the seafloor should be calibrated. However, due to the effect of ocean currents, the cable attitude is not stationary. Therefore, a new method for the dynamic repositioning of vertical cables is required. The dynamic repositioning of the hydrophones on vertical cables plays an important role in accurately describing the structure of the stratum and establishing an accurate velocity model. To realize the dynamic positioning of marine vertical cables, the shot lines are divided according to time, the first break time objective equation is established and the receiver position in each time period is obtained using the inversion methods outlined in this work. By analysing the trace spacing, first break time, shot point coordinate accuracy, temporal resolution, shot line spacing and other factors after repositioning, it is found that the first break time pickup accuracy and shot line spacing are the main factors that affect the repositioning accuracy. Furthermore, the resolution is limited by the timing of a single shot line. Additionally, the trace spacing after repositioning is consistent with the actual situation, which verifies the correctness and feasibility of this method.

Geophysical Characterization and Attenuation Correction Applied for Hydrate Bearing Sediments in Japan Sea

Estimation of rock properties from seismic data is important for exploration and production activities in the petroleum industry. Considering the compressional velocity—the speed of propagating body waves in formations—and the quality factor (Q)—a measure of the frequency-selective energy losses of waves propagating through formations—both properties are usually estimated from multichannel seismic data. Velocity is estimated during multichannel processing of seismic reflection data in either the time or depth domain. In marine seismic acquisition, Q can be estimated from the following sources: Vertical Seismic Profile (VSP) surveys, where sources are located near the sea surface and geophones are distributed at depth along a borehole; and multichannel reflection data, where sources are also located near the sea surface and receivers are distributed either at the sea surface (conventional seismic survey with streamers) or on the sea floor (use of nodes or Ocean Bottom Cables (OBC)). The aforementioned acquisition devices, VSP, conventional streamers, nodes, and OBCs are much more expensive than single-channel acquisition with one receiver per shot due to the cost of operation. There are numerous old and new datasets from academia and the oil industry that have been acquired with single-channel acquisition devices. However, there is a paucity of work addressing the estimation of velocity and Q from this type of equipment. We investigate the estimation of Q and velocity from single-channel seismic data. Using the windowed discrete Fourier transform for a single seismic trace, we calculate the peak and dominant frequency that changes with time. In the geologic environment, higher frequencies are attenuated at shallow depths (time), while lower frequencies remain at deeper positions. From the rate at which higher frequencies are attenuated with time, we estimate the effective quality factor (Qeff). However, when using Kirchhoff migration to process single-channel seismic data, events far from the vertical projection of the receiver contribute to the trace at a given time. Then, an underestimation of the effective quality factor occurs. To compensate for the effects of more distant events with lower-frequency content contaminating the shorter events, we propose a linear equation to correct the effective quality factor estimated from migrated seismic data. Effective Q and its correction are estimated in five single-channel seismic lines surveyed along the Joetsu Knoll, a SW-NE anticline structure on the eastern margin of the Sea of Japan. These results are linked to geomorphological and geological features and the velocity field. Joetsu Knoll is a known site of massive gas hydrates (GH), which occur in the first hundred metres of Neogene sediments and, together with gas chimneys, play an important role in seismic wave absorption. Qeff estimated from migrated seismic data maintains the spatial relationship between high and low Q regions. The region of low Q, which is below 124 and has an average value of 57, occurs near the anticlinal hinge and tends to coincide with the region in which the Bottom Simulating Reflector (BSR) resides. The coexistence of GH and free gas coincides with the very low P velocity gradient of 0.225 s−1. BSR occurrence, Qeff and the geometry of the Joetsu anticline testify to progressive gas hydrate depletion northward along the dome.

First-Break Picking Method Based on the Difference Between Multiwindow Energy Ratios

First-break picking is an important step during processing of both passive and active seismic data. Many automated algorithms have been developed to detect first-break points in large volumes of seismic data. However, it remains difficult to determine precise first-break points in seismograms with low signal-to-noise ratios. Therefore, we present a new approach based on the differences between multi-window energy ratios (DERs) that minimizes the effects of noise. First, the DER is defined and a thresholding method detecting first-break points using the DERs is proposed. Thresholding can be varied depending on the DER parameters, which ensures reliable results even if the parameters change. We use two types of seismic data to establish and verify the DER method: big data derived via global earthquake monitoring (STanford EArthquake Dataset) and ocean bottom cable (OBC) data acquired offshore of Pohang, Republic of Korea. We investigated the effects of parameter changes on the DER picking results. Good picking performance was verified under low signal-to-noise conditions and compared to conventional first-break picking methods. The DER accuracy was higher than that of conventional methods and outliers were rare.

Active-Source Interferometry in Marine and Terrestrial Environments: Importance of Directionality and Stationary Phase

ABSTRACT We utilize active-source seismic interferometry with dense seismic arrays both offshore and onland to explore the utility of this method to create virtual sources and reveal body-wave reflections in these two different environments. We first utilize data from an ocean-bottom cable (OBC) array in the Gulf of Mexico with equal numbers of sources (160 airgun shots) and receivers (160 ocean-bottom four-component sensors). We next use data from a geophone array across the Bighorn Mountains of Wyoming with many receivers (1300 vertical-component geophones) but a small number of sources (14 borehole active-source shots). We find that the OBC virtual source results, which produce strong reflections from sub-seafloor structures, are far superior to the onland results which lack usable reflections, and we explore reasons for these differences through a set of selective stacking approaches. We present techniques to account for the direction the seismic waves travel (directionality) and stationary phase and show that improvements can be made when incorporating these corrections. Although interferometric methods are based on assumptions of large numbers of widely distributed actual sources, we find that selective exclusion of potentially problematic source–receiver pairs can yield improved results. These geometric adjustments to active-source interferometry methods have utility for dense-nodal-array surveys that are now common in academic studies, but that often suffer from sparse source geometry.

Integrated Fracture Characterization of Thamama Reservoirs in Abu Dhabi Oil Field, United Arab Emirates

Summary Volumetric maximum curvature attribute computed from 3D ocean bottom cable (OBC) seismic data, production logging tool (PLT), inorganic chemical tracer data, and fractures observed from core and full-bore formation microimager (FMI) logs were integrated to characterize fractured carbonate reservoirs of an offshore oil field in Abu Dhabi, United Arab Emirates (UAE). The extracted maximum curvature anomalies are predominantly orientated in NNE-SSW and NE-SW, a trend perpendicular to the dominant fault direction in the oil field and similar to the dominant strike directions of fractures measured from core data and FMI logs. Because the fracture strike directions of well data mimic the strike directions of curvature anomalies at corresponding reservoir levels, we interpreted the maximum curvature anomalies to represent dilatational fractured zones or fracture corridors. Integration of dynamic data, such as PLT and chemical tracers, and maximum curvature anomalies demonstrate that the inferred fracture zones can determine water breakthroughs as well as inter- and intrareservoir communications. As a result, this study highlights possible fracture zones and their internal architecture, as well as their potential flow capabilities. These results play a key role in reservoir management and monitoring of water movement through structural pathways.

Imaging through gas clouds: The application of CSI and FWI in Bohai, China

Oil field A, situated in Bohai Bay, was discovered in 1999 and has been developed as one of the most productive oil assets in China. It continues to hold significant growth potential for the future. Though the field contains a large amount of resources remaining to be developed, seismic imaging has been challenging in area 5, resulting in structural uncertainty for reservoir interpretation and well planning. In the past three decades, several 2D and 3D seismic surveys have been acquired, processed, and reprocessed in this area. However, due to the existence of complicated gas clouds, which are shallow, multilayered, and extensive, obscured sub-gas-cloud images appear in all legacy seismic results, making fault interpretation under the gas clouds almost impossible. To improve the sub-gas-cloud image and overall structural interpretability, a narrow-azimuth full-field ocean-bottom cable (OBC) acquisition was conducted in field A during 2018 and 2019, and later, a compressive seismic imaging (CSI)-based full-azimuth and large-offset OBC infill survey was acquired in area 5, covering the widest gas cloud. Through high-fidelity signal processing, full-waveform inversion (FWI)-driven velocity model building, and imaging using both Kirchhoff migration and reverse time migration (RTM), the seismic image quality beneath complicated gas clouds is improved significantly. It is the first time that sub-gas-cloud faults and the Base of Guantao event have been imaged by seismic without significant dim zones. CSI acquisition, FWI, and RTM are the key elements to resolve gas-cloud-related challenges in area 5.

Full-waveform inversion strategies using common-receiver gathers for ocean-bottom cable data

Full-waveform inversion (FWI), which is among the most powerful seismic data processing techniques for imaging subsurface geological structures, has a huge computational cost in proportion to the number of sources. To increase the speed of FWI, we explored the use of common-receiver gathers (CRGs) as an alternative to common-shot gathers (CSGs) as observed data. This approach has the potential to reduce the computational cost of FWI significantly, particularly for ocean-bottom cable (OBC) acquisition. As we match modelled and observed CSGs in CSG-based FWI, we match observed CRGs with modelled CSGs of switched source–receiver geometry in CRG-based FWI. According to the reciprocity principle, CRG FWI based on the steepest descent or Gauss–Newton method yields the same inverted velocity models as CSG FWI for an identical and known source signature applied over whole source positions. However, when the source wavelet is unknown, and changes from one source position to another, each trace of a CRG contains a different source signature, making it difficult to apply conventional source estimation or source-independent methods to CRG FWI. Therefore, in this study, we proposed inversion strategies for CRG FWI in both the time and frequency domains to deal with errors arising from different source wavelets between CRG traces. Then, we used a synthetic example for the Marmousi-II model and a real data example from the North Sea to demonstrate that our strategies for CRG FWI provide very similar inverted velocity models to those obtained from CSG FWI, at a lower computational cost.

Multiparameter full-waveform inversion of 3D ocean-bottom cable data from the Valhall field

Full-waveform inversion (FWI) applications on 3D ocean-bottom cable (OBC) data from the Valhall oil field in the North Sea have demonstrated the importance of appropriately accounting for attenuation. The Valhall field contains unconsolidated shallow sediments and a low-velocity anomaly in its center — indicative of gas clouds — which have a significant attenuation imprint on the data. Our challenge is to perform time-domain viscoacoustic 3D FWI, which requires more sophisticated tools than in the frequency domain wherein attenuation can be incorporated in a straightforward manner. The benefit of using a viscoacoustic, instead of a purely acoustic, modeling engine is illustrated. We have determined that, in the frequency band used (2.5–7.0 Hz), it is better to reconstruct the velocity only keeping the attenuation fixed because simultaneous inversion of the velocity and quality factor [Formula: see text] does not provide reliable [Formula: see text] updates. We develop an efficient time-domain workflow combining a random source decimation algorithm, modeling using standard linear solid mechanisms, and wavefield preconditioning. Our results are similar to those obtained from state-of-the-art frequency-domain algorithms, at a lower computational cost compared to conventional checkpointing techniques. We clearly illustrate the improvement in terms of imaging and data fit achieved when accounting for attenuation.

P- and S-wave separated elastic reverse time migration for OBC data from fluid-solid coupled media with irregular seabed interfaces

In marine seismic exploration, elastic reverse time migration (ERTM) uses both compressional (P-) and shear (S-) waves of ocean-bottom cable (OBC) data to produce multi-component images with adequate amplitude information and illumination of the subsurface. If the fluid-solid medium has an irregular seabed interface, the conventional rectangular-grid-based finite difference (FD) method cannot obtain accurate source and receiver wavefields. In addition, the conventional ERTM applies Helmholtz decomposition to separate P- and S-wavefields, suffering from a change of the wavefield dynamics properties and polarity reversals in PS- and SP-image components. To overcome these problems, we propose a curvilinear-coordinate marine separated-wavefield elastic reverse time migration (CM-SERTM). In this method, the fluid-solid velocity models are meshed into orthogonal grids and mapped to the curvilinear coordinates. A curvilinear-coordinate acoustic-elastic coupled forward modelling algorithm is then developed by introducing boundary functions to combine the acoustic and elastic equations together. The decoupled P- and S-wave equations in the curvilinear coordinates and vector imaging conditions are introduced to implement the CM-SERTM. In numerical examples, we use a simple fluid-solid model and a modified Marmousi2 model to verify that our CM-SERTM method has great superiority over three conventional ERTMs.

Semipermanent ocean-bottom seismic node: Toward practical reservoir monitoring

Seismic reservoir monitoring has been found to be effective with recent ocean-bottom seismic techniques. The technology of ocean-bottom nodes can be extended to a semipermanent ocean-bottom-node (SPN) acquisition type, which is fixed on the seabed and activated on demand for reservoir monitoring. A newly developed SPN method allows a high-speed underwater optical to communicate with a remotely operated vehicle or an autonomous underwater vehicle to stably harvest seismic data on multiple repeat surveys. Long-term capabilities of deployment life, cumulative recording time, and internal clock characteristics are also key technologies for practical use of SPN acquisition. This is an alternative to fixed ocean-bottom cables that are currently a major tool of permanent reservoir monitoring. SPN acquisition will present more flexibility for receiver spreads, low installation cost, and fewer risks for asset management compared with existing seabed technologies.

Micro-Seismic Monitoring Data Analysis System Based on Sequentially Discounting Autoregressive and its Application to Offshore CO2 Storage Safety Operation

Micro-seismic monitoring is widely used as an effective way in detecting the seismic events during injection to ensure the safety during the injection of CO2 into geologic formations and is beneficial for increasing the public acceptance. At the offshore site, the challenges in monitoring involve processing high noise record and distinguishing the induced seismic events from the background natural earthquakes. We introduce the Sequentially Discounting AutoRegressive (SDAR) learning method to identify seismic events against the background noise. The SDAR method has a higher detection rate than traditional methods (e.g., STA/LTA), even for low signal to noise (< 1.0) events. We apply the SDAR method to synthetic record and the Tomakomai Ocean Bottom Cable (OBC) baseline data to test the method effectiveness. The detected background natural earthquakes will then be used to construct the seismic model and identify the induced seismic events during the injection. Based on the SDAR method, an Advanced Traffic Light System (ATLS) is being built for induced seismicity monitoring and injection safety management at the Tomakomai storage project. Our results suggest that the SDAR-based ATLS will be a promising tool for safety management, especially for injection projects in offshore storage project.

Overview of Investigation of Potential CO2 Storage Sites Offshore Japan

Japan CCS Co., Ltd. (hereinafter “JCCS”) has been conducting the investigation of potential CO2 storage sites in Japan undercontract to the Ministry of the Environment (hereinafter “MOE”) and Ministry of Economy, Trade and Industry (hereinafter“METI”) of Japan since 2014. We have been conducting a number of 2D/3D seismic surveys and geological interpretation at pre-selected areas in offshore Japan, in order to identify prospective sites from among these areas. Several candidate sites will be selected from these prospective sites, and exploratory wells will be drilled to confirm the presence of geological conditions necessary for storage. The drilling will be followed by the construction of a geological model and flow simulation. The final goal of the current scope of work is to recommend three potential sites for CO2 geological storage to MOE and METI by around 2021. The main selection criteria of a potential storage site are as follows: > The potential sites should be located in offshore areas > Capacity of each site is to store over 100 million tonnes of CO2 > There are no active faults close to the site > The existence of a CO2 emission source near the site is not taken into consideration > The method of CO2 transportation to the site is not taken into consideration > Producing oil and gas fields are excluded (site is not for EOR purpose) > There is no environmental protected area around the site In addition, a preferable condition is whether the site can be reached by directional drilling from onshore, as is the case for the Tomakomai CCS Demonstration Project. In this project, two directional CO2 injection wells were drilled with horizontal reach of over 3,000 meters, saving considerable construction cost as it was not necessary to bring in an offshore drilling rig, or to lay expensive subsea pipelines and related offshore facilities. The 2D/3D seismic survey conducted in the investigation of potential sites is the most effective method to delineate the geological features of the subsurface. 2D seismic surveys are conducted to grasp the regional geological features over a wide range such as 50 x 100 km, and if prospective structures for CO2 geological storage are identified, 3D seismic surveys over a smaller range (e.g., 10 x 10 km) will be executed to delineate the geological features more precisely. When necessary, we employ state-of-art seismic acquisition and processing technology utilized in the oil and gas industry to accomplish our objectives. In one of the 3D seismic surveys, we have conducted a seamless 3D seismic survey across the transition zone between shallow marine and onshore. As the possible presence of active faults was suspected in the transition zone, there was a need to collect data from the shallow marine continuously to the onshore area. Although the survey has taken time and effort, we were able to fill the gap in the existing seismic data between offshore and onshore with data of reasonable resolution utilizing an Ocean Bottom Cable (OBC). The geological interpretation found that the subsurface of the transition zone was a narrow synclinal valley along the current coastline with no active faults present, and therefore that it was possible to store CO2 in the offshore area as it was safely isolated from the populated onshore area. At another prospective site, we have conducted a 3D seismic survey utilizing broadband technology; the first application of this state-of-art technology in Japan. Because the target sandstone reservoirs in the survey area are composed of deep marine sediments such as turbidites and submarine fans, high-resolution 3D seismic data is required to differentiate the sandstone types. The high resolution data may be further utilized for special processing such as acoustic impedance (AI) inversion and amplitude versus offset (AVO) analysis which is help to evaluate rock properties in order to reduce geological risk. The high resolution near surface data acquired in broadband technology is also useful in identifying shallow gas hazards in the area. In BroadSeisTM(*), the broadband seismic technology we adopted, the slanted streamer cables are deployed deeper than conventional seismic surveys, mitigating the effects of sea surface swells and wave noises under rough sea conditions. As a result, we were able to reduce drastically the downtime due to rough weather during the survey period. (*: trademark of CGG) In summary, the application of state-of-art seismic acquisition technology in the investigation of potential CO2 storage sites is expected to reduce geological risk as well as contribute towards saving acquisition costs.

4D Seismic Pilot Successfully Interprets Carbonate Reservoir

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 193063, “A Successful 4D Seismic-Monitoring Pilot in a Middle East Carbonate Reservoir Context,” by F. Cailly, T. Al-Romani, C. Hubans, and A. Lafram, Total, and A. Al Kaabi, ADNOC, prepared for the 2018 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11–14 November. The paper has not been peer reviewed. This paper describes interpretation results of a 4D seismic-monitoring program in a challenging Middle East carbonate reservoir. The program consists of a 4D pilot [oceanbottom cable (OBC)] over a giant field divided into two phases. The authors discuss the difficulties faced by both phases of the pilot, and prove that a reliable 4D signal can be extracted over a Middle East carbonate reservoir. Introduction The time-lapse, or 4D, seismic method consists of repeating seismic survey acquisitions over the production time of a field. Newer vintages are compared with previous ones, and the signal differences are interpreted to characterize effects of the production spatially. Though this technique is mature and has been widely used in clastic reservoir environments, it is still used rarely, in operational terms, to monitor carbonate fields. An opportunity for a 4D proof-of-concept program in a Middle East marine carbonate environment was identified during a new OBC seismic acquisition in 2013. While the survey was intended for 3D imaging purposes, the operator decided to acquire a 4D pilot over a test area of approximately 25 km2 (Phase 1) while repeating as best as possible the 1994 acquisition design (baseline for this 4D study). The goal was, first, to process this test, and then, if a reliable 4D signal (above noise level) could be established, to propose interpretation in accordance with the field-production mechanisms. Field-Production and 4D Monitoring Objectives The principal oil-producing reservoir belongs to the Upper Jurassic and is a mixture of limestones, dolomite, and anhydrite. It contains the primary 4D monitoring target for this pilot. First oil was produced in the 1960s, followed by peripheral water injection in the 1970s and crestal gas injection in the 1990s. Thus, the 1994 4D baseline survey is an OBC seismic acquired after a rich and complex production history. Four-dimensional monitoring is justified by the fact that many expensive wells are drilled every year and their positioning is not straightforward. In particular, the crucial fluid-movement prediction (gas and water) is challenging and, thus, is considered the main monitoring objective. Phase 1 Acquisition Status Repeating baseline acquisition parameters during the monitoring survey is crucial. Any deviation from this principle is likely to generate a 4D noise that will be difficult to remove. In this pilot, taking advantage of a seismic crew mobilization for a modern wide-azimuth OBC acquisition, the field operator decided to try to repeat, in a small patch, an older OBC survey as a 4D pilot. In fact, this legacy vintage, which has a very different narrow-azimuth design, only was repeated over three swaths. The geometrical repeatability was good, except in the obstruction areas.

3D inversion of transient electromagnetic data from the Alvheim field, North Sea

SUMMARY We present a case study leading to the 3D inversion of transient electromagnetic (EM) data for delineating reservoir extent at the Alvheim field in the Norwegian sector of the North Sea. The survey was conducted in July and August 2008 using a two ship operation and ocean bottom cables. One ship laid a receiver cable with 30 receiver nodes on the sea floor. The second ship placed a source cable used to generate a coded transient signal on the sea floor. The configuration of the source and receiver spread was analogous to 2D seismic acquisition, as the system was rolled along to obtain multi-fold coverage of the subsurface. The survey spanned 20 km, resulting in measurements of 1270 source-receiver locations. The electric fields for each source-receiver pair were measured and deconvolved with the source current to determine the impulse response function. Preliminary inversions were made for each source-receiver pair using a 1D model, and the results were stitched to a 2D image. Having defined a background model, all data were then simultaneously inverted in 3D with focusing regularization. This revealed high resistivity volumes corresponding to the known hydrocarbon-bearing reservoirs of the Alvheim field.

A Processing Workflow for Ocean Bottom Cable Dual-Sensor Acquisition Data: A Case of 3D Seismic Data in Bohai Bay

The technique of ocean bottom cable (OBC) dual-sensor acquisition is an effective method to suppress the ghost wave and the reverberation at the receiver. With the advent of this technique, the processing method has become the key to the effective use of the OBC dual-sensor data. This paper has developed a new set of processing workflow based on the principle of combining the hydrophone and geophone data. This new process was applied to the OBC data acquired in Bohai area. The actual processing results show that the ghost and the reverberation are attenuated effectively. The frequency energy of the first notch point of the hydrophone data increased from −22 dB to −13 dB, and the frequency energy of the first notch point of the geophone data increased from −18 dB to −10 dB. The spectral characteristics of the dual-sensor data are more reasonable. The frequency spectrum is broadened and richer, and the resolution of the stack profile is improved greatly.