How Often Should you Acquire 4D Seismic Surveys to Optimise Value ?

Watts, Giles; Marsh, Monica Miller

doi:10.3997/2214-4609.20145217

Cited by 2 publications

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“…In the reservoir, a range of independent activities such as injector or producer well shut‐off or re‐start/start, alteration of a rate/choke setting, well tests, bumping/slugging and squeeze treatment (Watts and Marsh ) are known to cause reservoir pressure fluctuations. Pressure is also affected by turnaround, production optimization, workovers, evolving water cuts and the introduction of new wells or re‐introduction of old wells.…”

Section: Seismic Acquisition Versus Production Time‐scalesmentioning

confidence: 99%

Intra‐survey reservoir fluctuations – implications for quantitative 4D seismic analysis

Omofoma

MacBeth

Amini

2018

Geophysical Prospecting

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A B S T R A C TDuring the time taken for seismic data to be acquired, reservoir pressure may fluctuate as a consequence of field production and operational procedures and fluid fronts may move significantly. These variations prevent accurate quantitative measurement of the reservoir change using 4D seismic data. Modelling studies on the Norne field simulation model using acquisition data from ocean-bottom seismometer and towed streamer systems indicate that the pre-stack intra-survey reservoir fluctuations are important and cannot be neglected. Similarly, the time-lapse seismic image in the post-stack domain does not represent a difference between two states of the reservoir at a unique base and monitor time, but is a mixed version of reality that depends on the sequence and timing of seismic shooting. The outcome is a lack of accuracy in the measurement of reservoir changes using the resulting processed and stacked 4D seismic data. Even for perfect spatial repeatability between surveys, a spatially variant noise floor is still anticipated to remain. For our particular North Sea acquisition data, we find that towed streamer data are more affected than the ocean-bottom seismometer data. We think that this may be typical for towed streamers due to their restricted aperture compared to ocean-bottom seismometer acquisitions, even for a favourable time sequence of shooting and spatial repeatability. Importantly, the pressure signals on the near and far offset stacks commonly used in quantitative 4D seismic inversion are found to be inconsistent due to the acquisition timestamp. Saturation changes at the boundaries of fluid fronts appear to show a similar inconsistency across sub-stacks. We recommend that 4D data are shot in a consistent manner to optimize aerial time coverage, and that additionally, the timestamp of the acquisition should be used to optimize pre-stack quantitative reservoir analysis.

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Section: Seismic Acquisition Versus Production Time‐scalesmentioning

confidence: 99%

Intra‐survey reservoir fluctuations – implications for quantitative 4D seismic analysis

Omofoma

MacBeth

Amini

2018

Geophysical Prospecting

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The interpretation of amplitude changes in 4D seismic data arising from gas exsolution and dissolution

Falahat

Obidegwu

Shams

et al. 2014

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This study examines the four-dimensional (4D) seismic signatures from multiple seismic surveys shot during gas exsolution and dissolution in a producing hydrocarbon reservoir, and focuses in particular on what reservoir information may be extracted from their analysis. To aid in this process, hydrocarbon gas properties and behaviour are studied, and their relationship to the fluid-flow physics is understood using numerical simulation. This knowledge is then applied to interpret the seismic response of a turbidite field in the UK Continental Shelf (UKCS). It is concluded that for a repeat seismic survey shot 6 months or more after a pressure change above or below bubble point (as in our field case), the gas-saturation distribution during either exsolution or dissolution exists in two fixed saturation conditions defined by the critical and the maximum possible gas saturation. Awareness of this condition facilitates an interpretation of the data from our field example, which has surveys repeated at intervals of 12–24 months, to obtain an estimate of the critical gas saturation of between 0.6 and 4.0%. These low values are consistent with a range of measurements from laboratory and numerical studies in the open literature. Our critical gas-saturation estimate is also in qualitative agreement with the solution gas–oil ratios estimated in a material balance exercise using our data. It is not found possible to quantify the maximum gas saturation using the 4D seismic data alone, despite the advantage of having multiple surveys, owing to the insensitivity of the seismic amplitudes to the magnitude of this gas saturation. Assessment of the residual gas saturation left behind after secondary gas-cap contraction during the dissolution phase suggests that small values of less than a few per cent may be appropriate. The results are masked to some extent by an underlying water flood. It is believed that the methodology and approach used in this study may be readily generalized to other moderate- to high-permeability oil reservoirs, and used as input in simulation model updating.

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How Often Should you Acquire 4D Seismic Surveys to Optimise Value ?

Cited by 2 publications

References 0 publications

Intra‐survey reservoir fluctuations – implications for quantitative 4D seismic analysis

Intra‐survey reservoir fluctuations – implications for quantitative 4D seismic analysis

The interpretation of amplitude changes in 4D seismic data arising from gas exsolution and dissolution

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