Overburden 4D time shifts — Indicating undrained areas and fault transmissibility in the reservoir

We reformulate the original model of Hatchell and Bourne and Røste, Stovas and Landrø that couples fractional velocity change to subsurface strain via a fundamental constant R. The new model combines elastic compressibility of a dual‐porosity system for a sand–shale mixture with horizontal planes of inter‐granular weakness. The majority of observed R‐factor magnitudes from post‐stack 4D seismic data in both the reservoir and overburden can thus be explained. R is predicted to depend strongly on lithology and also initial strain state. The model is also extended to predict the observed angle‐dependence of time‐lapse time‐shifts from pre‐stack data. An expression for the gradient of time‐shift with incidence angle is obtained in terms of the background VP/VS, and also the ratio of tangential to normal compliances BT/BN representing loss or creation of inter‐granular coupling. If accurately estimated from data, this compliance ratio can be used as an additional parameter to assess the post‐production state of the overburden. It is concluded that whilst R remains the over‐arching parameter controlling the magnitude of time‐shifts measured from 4D seismic data, BT/BN is a subtler parameter that may also prove of future value.

Section: Key Conclusion From Vertical Time-shifts Observed In 4d Seimentioning

confidence: 97%

Review Paper: A semi‐empirical model of strain sensitivity for 4D seismic interpretation

MacBeth

Kudarova

Hatchell

2018

“…Signature is between the 2006 and 2009 surveys. Figure adapted by Thomas Røste from Røste and Ke ().…”

Section: The Magnitude Of 4d Seismic Time‐shiftsmentioning

confidence: 99%

“…). Indeed recent work by Røste and Ke () has suggested that overburden factors of up to 17 may be possible. An additional factor that might affect this result is the presence of saturation‐related time‐shifts in the reservoir – this is discussed further in the next section.…”

Section: The Magnitude Of 4d Seismic Time‐shiftsmentioning

confidence: 99%

“…However compressional responses in the overburden are predicted above elevated changes of pore pressure at injectors or from the complex deformations arising due to stress-arching at the edges of the reservoir. In one of the few published examples to date, Røste and Ke (2017) observe evidence for such stress arching effects on the Snørre field (see Fig. 4b).…”

Section: Strain-related Time-shifts Observationsmentioning

confidence: 99%

“…This observation supports the growing conclusion that R-factors in the overburden must be large -it is observed that they must be around 5 or greater (MacBeth et al 2018). Indeed recent work by Røste and Ke (2017) has suggested that overburden factors of up to 17 may be possible. An additional factor that might affect this result is the presence of saturation-related time-shifts in the reservoir -this is discussed further in the next section.…”

Section: Time-shifts Related To Reservoir Pore Pressure Changesmentioning

confidence: 99%

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Review Paper: Post‐stack 4D seismic time‐shifts: Interpretation and evaluation

MacBeth

Mangriotis

Amini

2018

A review and analysis of post‐stack time‐lapse time‐shifts has been carried out that covers published literature supplemented by in‐house datasets available to the authors. Time‐shift data are classified into those originating from geomechanical effects and those due to fluid saturation changes. From these data, conclusions are drawn regarding the effectiveness of post‐stack time‐shifts for overburden and reservoir monitoring purposes. A variety of field examples are shown that display the range and magnitude of variation for each class of application. The underlying physical mechanisms creating these time‐shifts are then described, and linked to a series of generic and field‐specific rock physics calculations that predict their magnitudes. These calculations serve as a guide for practitioners wishing to utilize this information on their own datasets. Conclusions are drawn regarding the reliability of this attribute for monitoring purposes, and the extent to which further development is required and how it should be reported by authors.

Offset dependence of overburden time‐shifts from ultrasonic data

Bakk

Holt

Bauer

et al. 2020

Depletion or injection into a reservoir implies stress changes and strains in the reservoir and its surroundings. This may lead to measurable time‐shifts for seismic waves propagating in the subsurface. To better understand the offset dependence of time‐shifts in the overburden, we have systematically quantified the time‐shifts of three different overburden shales in controlled laboratory tests. These experiments may be viewed as an analogue to the time‐shifts recorded from seismic field surveys. For a range of different stress paths, defined as the ratio between the horizontal and the vertical stress changes, the changes of the P‐wave velocities in different directions were measured such that the offset dependence of time‐shifts for different stress paths could be studied. The time‐shifts are stress path dependent, which is particularly pronounced at large offsets. For all stress paths, the time‐shifts exhibit a linearly decreasing trend with increasing offset, that is, a negative offset gradient. At zero offset, for which the ray path is normal to the bedding, the time‐shifts are similar for all investigated stress paths. The isotropic stress path is associated with the smallest offset gradient of the time‐shifts. In contrast, the constant‐mean‐stress path shows the largest gradient with a flip in the polarity of the time‐shifts for the largest offsets. The separate contributions from the strain and velocity changes to the time‐shifts were also quantified. The time‐shifts for the isotropic stress path are dominated by the contribution from velocity changes at all offsets. In contrast, the strain contributes significantly to the time‐shifts at small offsets for the constant‐mean‐stress path. This shows that the offset dependence in pre‐stack seismic data may be a key to understand the changes of subsurface stresses, pore pressure and strain upon depletion or injection. To utilize this knowledge from laboratory experiments, calibrated rock physics models and correlations are needed to constrain the seismic time‐shifts and to obtain an adequately updated geological model reflecting the true anisotropic nature of the subsurface. This may have important implications for improved recovery and safety, particularly in mature fields.