4D time-lapse seismic data analyses
The general objective of time lapse 4D seismic monitoring is to track production related
changes in the reservoir and determine areas of bypassed production, or inefficiencies in
the production process. This is accomplished through the comparison of 3D seismic
surveys that have been recorded at various points in time over the life of the field.
The analysis of time-lapse seismic data generally includes the following steps:
1) Estimate the types of velocity and density changes that will occur in the
reservoir during production.
2) Create synthetic seismic traces that represent the reservoir conditions for a
range of production scenarios.
3) Analyze the synthetic traces to determine the types of changes that may occur
in the seismic data.
4) Compare the seismic surveys that were recorded at various times in the
field’s production history.
5) Calibrate the 3D seismic volumes to remove spurious differences related to
seismic acquisition and processing as well as changes in the near surface.
6) Subtract the calibrated seismic surveys and map the differences.
7) Interpret the calibrated 3D surveys and the difference surveys to
determine the areas of the field that have been changed during production.
8) Compare the seismic differences to the synthetic traces to analyze the types of
changes that have occurred in the reservoir (pressure, temperature, saturation,
etc.).
9) Estimate the area and volume of these produced areas and compare to the
known production of the field.
In this class we will mainly deal with steps 4 – 7 of this work flow. We will use the
tutorial data example of the software package PRO4D from Hampson&Russell to
illustrate the general procedures.
This example will illustrate the procedure of how to compare, calibrate and analyze two
3D seismic volumes that were recorded before and after gas injection. The locations of
several wells and one actual set of well logs are included to help interpret and map the
differences in the reservoir where production induced changes have occurred.
Lumley, 1995:
“The most important feature of time-lapse seismic monitoring data is the opportunity to
compare seismic images as a function of elapsed time. Careful attention to data
processing issues is needed to ensure that images obtained at one time are validly
comparable to subsequent images. This is especially true for seismic difference sections,
in which one seismic image is subtracted from a second seismic image acquired at a
different time.”
“Small artifacts in amplitude, phase, time, and depth of imaged seismic
events can lead to disastrously noisy difference sections which may
completely obscure seismic fluid-flow anomalies.”
The following is a list of processing parameters, which should be considered for seismic
monitoring data:
(1) experimental repeatability (source/receiver characteristics)
(2) survey position accuracy (3D binning)
(3) wavelet shape (source wavelet)
(4) spectral (frequency) content
(5) amplitude preservation (gain functions)
(6) velocity accuracy and depth accuracy (target location in depth)
(1)
It is most desirable to conduct the 3D surveys with identical equipment. If this
cannot be achieved, the risk of producing artifacts is high as the processing
sequences (3) – (5) are not always a guarantee for successful removal of those
artifacts.
(2)
Survey positing is critical. The 3D surveys to be compared should follow
identical source and receiver positions. However, if the time between surveys
is considerably large, terrain conditions may have changed or new production
facilities were build obscuring data acquisition in the repeat survey(s). In this
case the first survey geometry can be used as a reference grid (or any other of
the repeat survey if they are of superior quality), and the other surveys are regridded on top this reference. This generally involves some kind of
interpolation to either fill in gaps or averaging if the reference grid contains
larger bins than the repeat survey.
(3)
Wavelet shape is related to the source. If identical survey parameters were
used, the wavelet shape should not have changed. However, in case of
significant differences, wave-shape filters can be designed to overcome this
problem.
(4)
Frequency content can be equalized by deconvolution methods if there are
significant differences in the source and receiver characteristics between
surveys.
(5)
Amplitude preservation is relatively simple. Some overall gain may have been
applied and the surveys to be compared need to be on average in the same
amplitude range. Global gain factors can be applied to boost or diminish
amplitude in the surveys.
(6)
If the depth to the target is a critical issue, care has to be taken in the velocity
ranges used for migration. If the production of oil/gas has altered the velocity
field of the overburden, this needs to be taken into account very carefully as
otherwise vertical shifts are introduced, amplifying the difference plot
artificially.
Suggested literature:
Lumley, D.E., 2001. Time-lapse seismic reservoir monitoring, Geophysics, Volume 66,
Issue 1, pp. 50-53 (January-February 2001)
Eastwood, J. E., Johnston, D., Huang, X., Craft, K., and Workman, R., 1998, Processing
for robust time-lapse seismic analysis: Gulf of Mexico example, Lena Field: 68th
Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 20–23.
Gan, L., F. Yao, Y. Hu, Y. Liu, and Du, W. 2004. Applying 4D seismic to monitoring
water
drive
reservoir,
SEG
Expanded
Abstracts
23,
2553
(2004),
doi:10.1190/1.1839705
Altan, S. 1997. Time-lapse seismic monitoring: Repeatability processing tests: 67th Ann.
Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 866–867.
Harris, P. E., and Henry, B. 1998. Time lapse processing: A North Sea case study: 68th
Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 1–4.
Naess, O.E. 2006. Repeatability and 4D seismic acquisition, SEG Expanded Abstracts 25,
3300 (2006), doi:10.1190/1.2370217.