At a recent TWOP (Test Well on Paper) that we facilitated, the discussion got 'round to the measures of cleanliness in an Exploration well that is not going to be gravel-packed.
We were discussing NTU (Nephelometric Turbidity Units) and how relevant they are as opposed to solids content, especially in our case.
For sure, it can take a lot of circulation (and filtration!) to achieve notional NTU values that may have just have been pulled out of the book without understanding the
reason/consequences. Would solids content not be a more meaningful/relevant measure in this case?
It would be wonderful if at least twenty (20) of our members could contribute to this discussion on:
- NTU vs. solids content?
- Any war stories from choosing one over the other, bearing in mind that cow's milk would likely fail an NTU test
From a wellbore cleaning perspective, my thoughts were that NTU should be considered an indicator and solids content a more definite measure of well cleanliness.
NTU is a measurement of light refraction of suspended particles however discoloration will result in a high reading. Likewise a clear fluid with larger solid particles will give a low reading.
When displacing the well from mud to brine, it is common in the program to 'circulate and filter the brine until 3 consecutive readings of XX NTU' for example. What I have seen is that once the tail spacer is out and we take periodic readings, the NTU will drop to a point of diminishing returns. If you continue to circulate the brine seems to react with the now water wet casing and pipe and you get some rust discoloration causing the NTU to increase. Working the pipe with brushes and scrapers again can cause this to increase. This can lead to excess time and cost trying to filter discoloured brine.
Since it is fine particles that cause formation damage I always thought it was best to use NTU periodic samples as an indicator the well is cleaning up, and once at the point of diminishing returns to make the final call based on % solids content. If the pill train has been designed and pumped correctly the brine should be in good condition without the need for much continuous filtration.
Further to this you may consider particle size distribution and oil in water content, but that needs some fancier equipment.
Of course if you work for a filtration company, then you are usually happy to filter rusty water all day......
Just my two cents.......
The simple analogy of NTU vs solid content is Microscopic vs. macroscopic.
Water containing 1 milligram of finely divided silica per liter has a turbidity
of 1 NTU, in most cases, the brine used in well testing is filtered to less than 20NTU ( 20miligram of finely divided silica per ltr). It is expected that during well testing, we try as possible as we can to eliminate anything that will further cause formation damage in microscopic scale.
The use of solid content is mostly used while drilling the drain hole, obviously every drilling fluid will have solid particles in various scales. These particles cause formation damage in a larger scale. At this drilling phase, the major concern is to reduce solid content but in macroscopic scale. That is why you run Production Screen Test (PST) on the mud used to drill drain holes.
In summary, NTU during well testing, solid content in drilling phase.