Who is still using NTU as a cleanliness indicator (and why)?
25 February 2019
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
Turbidity readings cannot determine the solids content but the concentration of particles hence monitor filter performance and wellbore clean up.
API RP 13J provides a lab procedure for correlating turbidity readings to TSS.
In the past we've used both combined.
For critical application we've used on site particle counters.
Hope this helps.
In your particular case Dave I would say solids content <0.5%; I read TWOP on an exploration well that is not going to be gravel packed so assume DST and no completion! Having run many DST's in my life we always aimed for solids content.
From my experience, use of NTU is a marvelous tool for service companies to sell expensive cartridges and DE set-up, like most of the others above, I am usually happy with solids < 0.5%.
In the same spirit, choice of Absolute/Nominal values for cartridges is too often not driven by technical considerations.
When I started out at the well site, NTUs were the usual criteria for brine cleanliness. We attempted to achieve it with the usual cleanup assembly, then spacer train, over-displacement of brine and filtering returns through cartridge filters "rigged up in series" with the return flow i.e attempting to filter all the returns before they went back in the well.
What I usually observed: the cartridge filters could not handle high flow rates (especially if there was a little residual oil in the brine), so the mass transport of solids out of the well was low because the brine flow rate had to be relatively low due to filtration equipment limits. If the pumps were stopped to change out a load of plugged filters, then started after 15 - 30 min, it was noticeable that you got short flow of cleaner brine from the top of the well and then a slug of very dirty brine. I infer the slug of dirty brine was likely due to solids settling while circulation was stopped. Once the "NTU spec" was apparently met, if the brine flow rate was raised with the filters out of the circulation path, the brine rapidly went out of NTU spec - I infer because the higher flow rate picked up more fines in the well.
This led us to think about what we are trying to achieve, and the best way to achieve it.
For wells with high porosity/permeability, where some minor production impairment was sustainable, and/or a "perforate past the damage" philosophy we concluded that the main driver was removal of solids that would adversely affect completion installation and operation. For these, we would specify use of Wellbore Patroller type tools to remove large solids, a minimum of 2 full brine circulations on filters rigged up in series, and brine spec based on solids content (typically 1.0 % or 0.5% based on mud engineer's centrifuge).
Where there was more impairment concern (e.g. gas wells with low permeabilities very sensitive to formation damage) we still stuck with the NTU approach "in the absence of anything better", but I still have doubts on its validity. (I am not an expert on Stokesian settling, but i believe it takes several days for ultra fines / colloids to settle through say a 3000m brine column. If these fines are also disturbed by running downhole tooling, is it more likely they are in suspension when the well is perforated and then produced out as the well cleans up?)
We tried a high flow circulating system (i.e drill string in the well pumping at highest possible flow rate), using multiple pits as a combined active to increase residence time for solids settling, and the filter circuit running in parallel on the active pits to remove fines. In theory this would be the fastest way to get the total well solids load removed, as it is the most effective way to transport all the fines out the well, the nature of the active pit fluids to be filtered should be more homogeneous than stop/start flow and any filter problems can be addressed without stopping the well flow.
We found that this approach was more successful in agitating a lot of fines in the well and suspending it in the fluid flow. But we found the filter units we had did not have enough capacity to remove the fines quickly, so it took longer to clean up the well to the NTU spec. The approach was stopped on rig time / cost basis, we had to revert to getting an "apparently clean spec" with filters in series at low flow rate, and we were not allowed to progress it on the next well by using a higher capacity filter unit. C'est la vie...
To cut a long story short, I'd go with 0.5% solids, and use high capacity filters rigged up in parallel on a large active, with the highest possible flow rate in the well.
Senior Completions Engineer / Well Examiner
As a Completion Engineer of many years standing I have not used NTUs as a cleanliness criteria for may years. I concur 100% with the responder who states that milk would probably fail an NTU test.
For the last 20 years at least, I have programmed any completion brine cleanliness criteria at 0.5% solids using an electric centrifuge, with no issues emanating that I recall.
Experience with NTU is that it takes far too long to reach some of the proposed NTU values in the past with absolutely no added value. It is entirely possible to have 5% solids with any fluid above the solids passing the NTU criteria, Clean brine? I think not.
NTUs are basically used as they are so easy to measure IMO.
Measuring Solids Content realistically takes a bit more effort. In the past mud companies have been sending out the 15 ml hand cranked centrifuges for measuring TSS. These just wont work as the tube needs to be 100 ml to try at read gradients at 0.05%.
Ideally an electric 100 ml tube centriguge will be in the mud lab to measure prepared TSS in brines prior to pumping.
The returns to be measured by a pneumatic (intrinsically safe) 100 ml centrifuge with protocols in place to allow proper collection and testing time.
Come on you rotten lot!
To say that I am disappointed that only two (2) members out of almost 3000 have BOTHERED to respond to this pressing (and common) question would be an under-statement.
I set this site up almost 20 years ago, to give back to an industry that provided for my father (and four children) and myself (and four). I've spent over $200,000 (yes!) in this site.
95% of you are NOT contributing, that's called PASSENGERS!
Whether you are: too lazy, too busy, don't know how to start a contribution, don't want to share (only take!), have nothing to add, don't care.
We cannot differentiate which.
We have a tracker on how long people have been members and how much (or little) they have contributed.
Starting 15th April 2019, we will be working our way from oldest members and deleting accounts that have not contributed, since this was always designed to be an active forum. You will receive notification to your registered email that you have been deleted and can re-register ONLY if you are prepared to be active.
Before that happens, I would urge you to come online and make a comment on SPREAD (perhaps saying what value you see and/or why you don't contribute) which will flag you as interested/active/alive(!)
I hope you don't find this too harsh, but I would rather 150 active members in total, collaborating (on our FREE site) than 2850 hangers-on.
There is a facility to de-activate your account and we have a team ready to do so. All you need to do is say so.
Otherwise, please get ACTIVE!
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.