Four questions you need to answer before shooting a 4D Seismic (Part 1) – Why 4D? Why this reservoir?
iResGeo has considerable experience in the integrated 4D feasibility studies through the integration of petrophysics, reservoir engineering, and geophysical expertise. In this series of articles, we go through key elements of best practices for 4D feasibility studies.
4D feasibility studies determine the scope of application of the 4D technology in the development of the field under study. The decisions that are made at this stage could have a long-term impact on the future management of the field. Questions to be considered in 4D feasibility include:
- Assessing the detectability of the expected 4D signal from production induced changes in the reservoir over time.
- How frequent should we use seismic monitoring?
- How repeatable should the surveys be, suggestions for survey design?
- Can the existing seismic surveys serve as the baseline?
- What is the added value of the 4D technology?
Why shoot 4D on this reservoir?
If 4D seismic technology has not been implemented by your company yet, a careful high-level screening and ranking of the several assets managed by your company are very important. Managers’ perception of the role of each technology in the reservoir development plan could be highly influenced by the value gained from its first application. Therefore, prior to a detailed 4D feasibly analysis, you have to answer these four questions!
1) What are the key challenges in the development of each of the reservoirs? Rank them based on the usefulness of the 4D seismic in addressing each challenge.
2) Has 4D technology been successfully applied to a similar reservoir? Points to consider include:
- Is the reservoir located onshore or offshore?
- What is the reservoir geology? Is it a clastic or carbonate? And what are the reservoir quality and level of heterogeneity? Are both original GOC and OWC present?
- Reservoir production mechanism (see below)
- Reservoir depth and seismic data quality? Are there any specific imaging challenges such as salt/gas chimneys/complex structures in overburden?
3) What stage of the field development are you at?
This is a very important question, as each stage requires its specific treatments. Having a pre-production baseline survey is always extremely valuable in tying the 4D signal to production data and cross-checking the simulation models against the 4D seismic data. The reservoir could undergo notable pressure variations at the beginning of the production, and the 4D signal is pressure dominated. In the later stages (secondary and tertiary recovery mechanisms), depending on the field development strategy, a complex mixture of pressure, saturation, geomechanical effects and rock and fluid interactions could form the 4D signal. The 4D repeat surveys frequency and repeatability required should specifically be addressed for each of these phenomena at different stages of the life of a field.
4) What are the reservoir drive mechanism and the drainage strategy? Which of the following do you foresee as a result of the production?
- Any water/gas/steam injection?
- Bottom water drive or edge water drive?
- Gas breakout or secondary gas cap expected?
- Fluid contacts movement? The expected variations of GOC versus OWC movements and relative thickness of gas cap/ oil rim versus the reservoir thickness?
- How large are the variations in pore pressure?
- Any geomechanical effects expected?
The items above are closely related to the earlier question on different stages of the life of reservoir and will define the range of scenarios that should be included in the detailed modelling exercise during the next stage of the feasibility study.
In the next part of this series, a modular approach for such analysis will be shared and we will give a practical checklist for all 4D feasibility exercise!
Author: Dr. Hamed Amini,
Formerly Lead Reservoir Geophysicist, LR Senergy, Reservoir Geophysics Adviser, Ph.D. in Reservoir Geophysics, Heriot-Watt University, Edinburgh, UK
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