Sensitivity of time-lapse seismic data to pore pressure changes: Is quantification possible?

A B S T R A C TAn approximation is developed that allows mapped 4D seismic amplitudes and timeshifts to be related directly to the weighted linear sum of pore pressure and saturation changes. The weights in this relation are identified as key groups of parameters from a petroelastic model and include the reservoir porosity. This dependence on groups of parameters explains the inherent non-uniqueness of this problem experienced by previous researchers. The proposed relation is of use in 4D seismic data feasibility studies and inversion and interpretation of the 4D seismic response in terms of pore pressure and water saturation changes. A further result is drawn from analysis of data from the North Sea and West Africa, which reveals that the relative interplay between the effects of pore pressure and saturation changes on the seismic data can be simplified to the control of a single, spatially variant parameter C S /C P . Combining these results with those from published literature, we find that C S /C P = 8 appears to be a generality across a range of clastic reservoirs with a similar mean porosity. Using this C S /C P value, an in situ seismic-scale constraint for the rock stress sensitivity component of the petroelastic model is constructed considering this component carries the largest uncertainty.

Section: Numerical Examplementioning

confidence: 99%

An insightful parametrization for the flatlander's interpretation of time‐lapsed seismic data

Alvarez

MacBeth

2013

“…; Khatiwada et al . ; Mikhaltsevitch, Lebedev, and Gurevich ), pressure (Eiken and Tøndel ; Tura and Lumley ; Grude, Landrø, and Osdal ), or both (Landrø ; Grude et al . ).…”

Section: Introductionmentioning

confidence: 99%

“…In most cases, observed time-lapse seismic signatures are associated with reservoir compaction of poorly consolidated rocks (Landrø and Stammeijer 2004;Hatchell and Bourne 2006) and changes in fluid saturation (Wang et al 2000;Daley et al 2008;Khatiwada et al 2012;Mikhaltsevitch, Lebedev, and Gurevich 2014), pressure (Eiken and Tøndel 2005;Tura and Lumley 1998;Grude, Landrø, and Osdal 2013), or both (Landrø 2001;Grude et al 2013). A recent study in Ketzin (Germany) has shown that timelapse seismic changes could be attributed to fluid saturation, pressure change, and mineral dissolution of the rock in a sandstone reservoir (Ivandic et al 2015).…”

mentioning

confidence: 99%

Are changes in time‐lapse seismic data due to fluid substitution or rock dissolution? A CO₂ sequestration feasibility study at the Pohokura Field, New Zealand

Sim

Adam²

2016

Seismic methods are commonly used to monitor the subsurface when carbon dioxide (CO2) is injected into a reservoir. Besides fluid saturation and pressure changes, CO2–water mixtures may cause rock alteration. In this petrophysical study, we compare the elastic property changes due to fluid replacement and those due to mineral dissolution for carbonate‐cemented sandstones at the Pohokura Field, New Zealand. We quantify the effects of fluid substitution from fully brine to fully supercritical CO2 saturation and carbonate cement dissolution on the seismic signatures of the reservoir rocks by combining laboratory results, petrographic analyses, and geophysical well log data. We conclude that elastic property changes due to mineral dissolution are significantly greater than those due to fluid substitution alone. The northern part of the Pohokura Field has coarser‐grained sandstones, which experience the largest changes in wave speeds. Our hypothesis is that these changes result from carbonate cement dissolution in the presence of CO2–water–sandstone reactions. If time‐lapse seismic data were to be acquired in this field, the northern area could show P‐wave velocity reductions of up to 20% and a 131.7% increase in seismic amplitude from a brine‐saturated rock to an altered, fully CO2‐saturated rock. In comparison, the southern part of the field, where sandstones are mostly fine‐grained, we expect a P‐wave velocity decrease of 6% if such dissolution process took place. Finally, we show that the elastic properties of the reservoir rocks can be described with the constant‐cement model. The model is used to predict that the dissolution process reduces the volume of grain contact cement, on average, from 2.5% to 1.75% of the total rock mineral volume. Our analysis suggests that changes to the rock frame, which includes carbonate minerals, cannot be ignored for a CO2 injection scenario.

“…2006). It has also long been acknowledged that pore pressure changes can have a significant impact on the seismic response (Eiken and Tøndel 2005; Hatchell and Bourne 2005; Tura et al 2005). Such pore pressure changes can influence seismic properties both within the reservoir and in the surrounding formations.…”

Section: Introductionmentioning

confidence: 99%

“…Historically, focus was on the most easily recognizable effects, such as steam injection (Eastwood et al 1994;Lumley 1995) or straight fluid replacement, observed for example at the Gullfaks and Heidrun fields * E-mail: akafu@statoilhydro.com (Landrø et al 1999;Furre et al 2006). It has also long been acknowledged that pore pressure changes can have a significant impact on the seismic response (Eiken and Tøndel 2005;Hatchell and Bourne 2005;Tura et al 2005). Such pore pressure changes can influence seismic properties both within the reservoir and in the surrounding formations.…”

Section: Introductionmentioning

confidence: 99%

Deriving effects of pressure depletion on elastic framework moduli from sonic logs

Furre

Andersen

Moen

et al. 2009

A B S T R A C TWe have developed a method to determine the effect of pore pressure depletion on elastic framework moduli, by utilizing sonic logs from wells drilled at different locations through a reservoir at varying depletion stages. This is done by first inverting the sonic logs for elastic framework bulk and shear moduli, thus carefully removing pressure dependent fluid effects. By crossplotting these elastic framework moduli against an increase in net stress (which is directly related to depletion), we derive the stress sensitivity of the elastic framework moduli. We found that the observed stress sensitivity was consistent with time-lapse seismic results and that the sensitivity derived from the sonic logs was much smaller than predicted by hydrostatic measurements on core samples. This method is applicable to depletion scenarios in mature fields and can be used both for modelling and inverting time-lapse seismic data for effects of pore pressure depletion on seismic data.