At South Arne a highly repeatable time-lapse seismic survey (normalized root-mean-square error or NRMS of less than 0.1) allowed us to reliably monitor reservoir production processes during five years of reservoir depletion. Time-lapse AVO (amplitude v. offset) inversion and rock-physics analysis enables accurate monitoring of fluid pathways. On the crest of the field, water injection results in a heterogeneous sweep of the reservoir, whereby the majority of the injected water intrudes into a highly porous body. This is in contrast to a pre-existing reservoir simulation model predicting a homogeneous sweep. On the SW flank, time-lapse AVO inversion to changes in water saturation DS w reveals that the drainage pattern is fault controlled. Time-lapse seismic data furthermore explain the lack of production from the far end of a horizontal producer (as observed by production logging), by showing that the injected water does not result in the expected pressure support. On the highly porous crest of the reservoir compaction occurs. Time-lapse time shifts in the overburden are used as a measure for compaction and are compared with predictions of reservoir compaction from reservoir geomechanical modelling. In areas where compaction observations and predictions disagree, time-lapse seismic data give the necessary insight to validate, calibrate and update the reservoir geomechanical model. The information contained in time-lapse seismic data can only be fully extracted and used when the reservoir simulation model, the reservoir geomechanical model and the time-lapse seismic inversion models are co-visualized and available in the same software application with one set of coordinates. This allows for easy and reliable investigation of reservoir depletion and gives deeper insight than using reservoir simulation or time-lapse seismic individually.

Reservoir production causes subsurface deformations and changes in seismic velocity. These deformations and velocity changes can be monitored using time-lapse seismic data. A fundamental challenge in the interpretation of time shifts observed in time-lapse data is the decomposition of the time delay into a spatial compaction component and a velocity change component. Several authors (Hatchell and Borne, 2005; Janssen et al., 2006) have published the application of pragmatic linear relationships between overburden stretching and velocity changes which, have proved applicable in a wide range of geological settings. In this study, we predict velocity changes through coupled reservoir and geomechanical modeling of the subsurface stress state and subsequent application of a rock physics model that relates changes in subsurface stress and strain to velocity changes. We expand on previously published work by investigating the anisotropy of the velocity changes and relate these to field observations of offset-dependent timelapse time-shifts from the South Arne Field. Our modeling shows that in the overburden, vertical seismic velocities decrease over time, causing traveltimes in a monitor survey to increase compared to traveltimes in a base survey. We furthermore predict that horizontal seismic velocities in the deep overburden increase and for seismic waves propagating at intermediate angles (approx. 20°-30°), the velocity changes are minimal. This suggests, that time-shifts between base-and monitor surveys are largest for zero-offset data and will gradually decrease as a function of offset. We test this prediction on a 4D field data set at South Arne, North Sea. For zero-offset data, we find a maximum traveltime increase in the overburden of 6ms. Time-shifts show a dependence on observation angle, with large offset time-shifts in the overburden being up to 50% smaller than near-offset time-shifts.

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Ocean-bottom seismic (OBS) is experiencing a resurgence of popularity in the North Sea. This is due in part to recent advances in acquisition equipment and operational efficiency as well as advances in geometry design and processing algorithms. Using two recent case examples, we review the key stages of the evaluation process and share the derived conclusions. The primary goal of these studies was to investigate and validate the image improvement associated with an OBS survey. The studies helped establish the optimal OBS geometry design by benchmarking it against legacy data as well as a number of alternative towed streamer and ocean-bottom acquisition solutions. The secondary goal of these studies was to increase the understanding of OBS acquisition options that is, ocean-bottom node (OBN) acquisition and ocean-bottom cable (OBC) acquisition. Using a multiphase feasibility study, which integrates geometry design, illumination analysis, and finite-difference modelling, we were able to successfully evaluate the suitability and value of OBS for a number of seismic acquisitions in the North Sea. By investigating the natural sampling, illumination characteristics, and processing considerations of each geometry, we were able to design and optimise an OBS geometry that met the imaging and operational challenges of each area.

A seismic recording system has been developed in which seismic sections, printed photographically in variable‐density form, simulate geologic cross‐sections. The seismic signals recorded from geophone stations arranged for continuous reflection center‐point control are presented in a sequence of vertical tracks. The tracks are adjusted for known corrections and reproduced at a selected horizontal scale. Prints of the variable‐density section exhibit all events and their interrelationships for ready recognition and appraisal. The printer mechanism reduces seismic information to a uniform time basis, with weathering and elevation corrections to datum, and with stepout corrections in which account is taken of changing wave‐front velocity with vertical travel‐path time. The original field records comprise variable‐density tracks on strips of 70‐mm film. In the printer, individual record tracks are successively scanned and transferred to a continuous sheet of film by contact printing. Punched cards supply correction data to mechanisms which displace each record track according to the desired corrections. Normal and special printing functions are automatically performed.