Pressure holding capability of Barite plug from degraded mud

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)

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