You want to obtain a subsurface fluid samples from using a wireline formation tester (WFT) tool like RCI and MDT from your well for PVT analysis, reservoir studies, and reserve booking.
The bad news is you couldn’t do it because of the operational constraints like:
1. Wellbore conditions: severe fluid/mud losses, high differential pressures or bad hole condition.
2. Reservoir properties: using normal probe might not work in heterogeneous formation and low fluid mobility, tight formation.
3. Cost: if you need to test in several layers at the same time, it will be costly.
4. Operational constraints: Not allowed to stay stationary in one place for long time. Or in DST, you can’t flare due to environmental concerns.
5. Uncertainty in fluid types: You need physical evidence to prove that you have the fluid in hole for reserve booking.
Regardless of the conditions, you still need that fluid sample, but you just can't get it in the open hole.
You don't have to give up -- yet. There's still hope. All the limitations that were mentioned above could be managed with this:
Try fluid sampling in cased hole using dual probe configuration.
When the borehole is cased, the borehole environment could be managed better. Pressure could be control. The well wont be risky as in the open hole.
To do this, take these seven steps.
1. Your check cement quality needs to be good to get good isolations across your zones of interest.
2. Perforate target zones: Either perforate and sample each zone individually or perforate all zones and then sample each zone individually.
3. Run scrapper: Perforates zones might produce burrs. You need to clean this up so the inflatable packers won’t get damaged from the burrs.
4. Run WFT with dual packers: Test packer seals by performing short drawdown and build up test. Then, pump out the fluid from the test interval into the wellbore to create pressure drawdown in formation. Clean up while pump out until fluid contamination is low enough by observing the fluid analyzer, before opening the sampling bottles.
5. If pressure transient test is planned (like mini DST or VIT), continue to pump out for about 15 minutes and perform pressure build up. Look for radial flow and validate build up test.
6. Deflate and move to next stations.
7. Once done, patch the holes on the casing.
Here are a few tips to make the operation smoother:
1. Try to use mud with minimal overbalance.
2. Perforated zones should be less than 1 meter, so packers can sufficiently isolate the zones.
3. Depth control is crucial since the perforated zone is less than 1 meter. Running CCL or pip tags before it might help.
In theory, it looks simple. In practice, there's a chance that it might not work. But, in a cased hole environment, you can stay longer in hole safer. Planning and close monitoring would help a lot during this job.
- Kyi, K. et all, “Cased Hole Reservoir Fluid and Rock Evaluation – What Can Be Done if This Was the only option Available?”, paper OTC-26601-MS was presented at Offshore Technology Conference Asia held in KL, Malaysia 22-25 March 2016.