Estimation of over-burden / pore / effective stress
Estimation of settlement / consolidation
The classical concept was published by Geertsma:
Geertsma, J., 1973. Land subsidence above compacting oil and gas reservoirs. JPT, Journal of Petroleum Technology 25, 734-744.
If you look for citing papers, there is plenty of models/methods that address subsidence above HC reservoirs.
I hope this is what you were looking for.
Regards, Andreas
Thanks Dr. Andreas.
Well, I am actually looking for some simple techniques, whereby the following issues could be addressed?
1. How to delineate - whether the computed reservoir compaction - for a particular site - becomes significant or not?
2. If the reservoir compaction is significant, is there a simple method in order to estimate the land subsidence with minimum data ? - as securing, the field-scale stress details are quite cumbersome.
3. Is it possible to quantify the magnitude of land subsidence using groundwater pumping test data alone?
4. Why does the magnitude of land subsidence measured from laboratory (always) tend to over-estimate - with reference to the actual filed measured data? Where is the catch?
I would be glad to receive responses on the above points.
Thanks in anticipation.
Suresh Kumar.
http://www.doe.iitm.ac.in/gskumar/
Geertsma approaches reservoir compaction as a poro-elastic problem, i.e. the material is assumed to behave elastically and compacts as the pore pressure drops and the effective vertical stress increases. This requires knowledge of the compressibility of the rock.
This is where usually issues arise, as laboratory tests are generally performed (under in-situ conditions) on samples retrieved from core, which may have undergone some damage by being brought up to the surface, or from coring itself. Small (induced) cracks in the rock could then lead to higher compressibility values measured in the lab.
Though the Geertsma method is a good way to achieve a first approximation on elastic reservoir compaction. However, it does neglect time-dependent mechanisms, which may induce additional compaction, i.e. by grain cracking/failure or by dissolution - the Ekofisk field in the North Sea is perhaps the most telling example of this. Perhaps this is not an issue for your reservoir, but it's good to keep in mind/look into it's possibility.
Concluding if reservoir compaction leads to 'significant' surface subsidence is quite subjective, as it is determined by your definition of significant. Simplest assumption to make is that all reservoir compaction is translated to the surface. It's up to you to determine what is acceptable.
Long story short: first estimates for (elastic) reservoir compaction can be 'fairly easy' obtained, but it might as well be that reservoir deformation becomes a complicated interplay between elastic and plastic deformation... Have a look at 'Subsidence Delay: Field Observations and Analysis' by Hettema et al. (2002).
Cheers,
Suzanne
Thanks Dr. Suzaane.
Yes..I understand that there might be a delay in the subsidence response (transient ground deformation).
However, I am not sure whether both continued compaction and delayed compaction - are time-dependent?
Also, whether both continued and delayed compaction pertain to the particular reservoir of interest (or the delayed compaction occurs outside the reservoir zone of interest) ?
Thanks for your time and reply.
Suresh Kumar.
http://www.doe.iitm.ac.in/gskumar/
- Badges
- Science topic
- Similar topics
- Engineering
- Petroleum Engineering
- Oil