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Pressure holding capability of Barite plug from degraded mud
22 June 2016

I am looking for any experience, field or lab, demonstrating whether or not old OBM or WBM can degrade and create pressure holding barite plugs.

 

There are many examples of the pressure holding capability of barite plugs when pumped as specific pills designed to settle (low viscosity carrier fluids), however I can find very little data related to degraded mud.  We have some recent experience re-entering historic wells with OBM up to 30years old and have found the mud to be in surprisingly good shape and no evidence of having fully degraded with large Barite drop out.  We also have experience on a number of historic wells where we have seen annular communication indicating lack of isolation from either cement or degraded mud solids.  However conversely I am unaware of any examples where it has been demonstrated that annular isolation was achieved through degraded mud solids but this may just be due to the fact that we don’t go looking closer when we don’t have isolation issues.

 

One theory is that the settling process is self‐limiting as increased sag increases the particle concentration of the lower region that in turn increases viscosity, reduces the density difference between a particle and the surrounding fluid and increases particle collisions. So the process will eventually come to equilibrium when all the particles are in contact with each other and the suspension becomes self‐supporting but still able to transmit pressure.

 

I would welcome any comments or experience people have to share on this topic.

Thanks

Colin


2 answer(s)
ianpetitt
(retired) Well Fluids Team leader
SPREADAssociates
Total Posts: 37
Join Date: 14/06/06
Colin,

I had been looking at this for a number of years. The Aberdeen laboratory work for OBM long term stability is documented in SAR 8080. Before I retired we have done samples from six months to annual samples over five year period  which had been stored at downhole temperatues.  
I have an excel sheet where I have documented the various abandonments  and re-entries where we have recovered mud samples after a number of years; include SW-07,  Brent South abandonments, DA-27 re-entry etc. I also have data from re-entered wells in Norway and the Netherlands.   
This work was done prior to the Commander re-entry - the actual mud recovered from Commander well fitted with the graphs which had been submitted to the well design engineer. 
The summary of the laboratory work and offset samples analysis was discussed with Dave Reid and has been included as a section in the new updated global casing design manual.
You should be able to get a copy of the graphs from Aled who plotted the OBM and WBM weights  for me. 
If you can't my data and information internally.   
I will have a look if I can find it for you and send it.  

Regards Ian

 
Carigali
Freelance Fluids Engineer
SPREADAssociates
Total Posts: 9
Join Date: 09/12/09

Hi Colin.

This is an interesting topic. I have also seen OBM that has hardly degraded in many years (decades); e.g. a North Holland (land) gas well after around 30 years.

I don’t think that it is theory that  “One theory is that the settling process is self‐limiting as increased sag increases the particle concentration of the lower region that in turn increases viscosity, reduces the density difference between a particle and the surrounding fluid and increases particle collisions.”

What happens follows Stokes Law and the viscosity part of Stokes Equation follows the behaviour of the viscosity of suspensions as relates to the Einstein Equation.

I have experienced this as a fact when (pilot testing on the rig) required preparing barite plugs (as in anger at the well situation). I could get 18.5 ppg to settle but 20.0 ppg would not.

As you mention, you are not so much interested in “barite plugs when pumped as specific pills designed to settle (low viscosity carrier fluids)” but more about suspension fluids and their sustainability, particularly in respect of density.

I may be wrong about the principles of suspension fluids but I have been involved in template drilling with OBM in the East Shetlands Basin on locations where a platform will later be put down. (Existing subsea template wells can be completed quickly with the platform allowing quickly, a fuel source for the platforms turbines.)  I have been out on the drilling of the template wells and the re-entry of those wells. The fluid returns (of OBM) as the cement plugs showed the mud in good shape. Washing down on to the cement plug showed only one or two feet of approach before taking weight. The fluid for suspension had a minimum 40 lbs per 100 sq. ft. YP. and a high E.S.

I am not entirely sure what is the reason as to why you are looking at this and what you might want to do in the future. Bare in mind, this is about cased hole suspensions. Open hole brings in other factors such as diffusion. There are some very good papers written on diffusion in wells.

I may be wrong but I don’t know of being able to get 30 years out of a WBM with included insoluble solids suspension. The development of brine technology and corrosion control has superseded what was available 30 years or earlier ago.

Regards,

Stu.

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