Sodium Silicate was a well-known shale stabiliser and was used in mud systems in the USA in the 1930´s & 50´s. However polymer technology was not as it is today, so parameters such as rheology and fluid loss were difficult to control in the early systems.
Silicate mud has proven over the years to be a very effective mud system for overburden intervals and is still considered by me to be the preferred High Performance Water Based Mud system in Europe. When formulated correctly the system will prevent pore pressure penetration occurring!
I believe that silicate inhibit formation shales decomposition through a pH reduction process due to pore fluid contamination and/or a reaction with free cations (Ca+/Mg+). The silicate oligomers form a precipitate/gel sealing the pore space/surface of the shale. The precipitated silicate forms a physical barrier which ”œpressure isolates” the shale from the water/pressured of the drilling fluid. The mud also maintains pore pressure isolation of the shale.
Some 10 years ago there has been good success in Norway and UK single strings, including wells on Tern , Mallard, Knave,Liverpool Bay, Kittiwake, Cormorant and many single strings wells, as well as the Pelicans development extensively.
Despite extensive testing, we never found a lubricant that worked in a silicate mud.
The system limited to about 230ÂºF as an operational temperature. Can be used in a reservoir but seems a strange choice. No issue with cementing but be careful with spacers.
Other disadvantages include gauge hole, low lubricity and hole cleaning difficulties with deviations above 30Âº.
See attached picture of 17"" BHA - pretty clean!
I remember Gary Wright an Exxon DSV being quite brilliant in running Silicate mud on Penguin wells.
The major continuing concerns are as follows:
1. Elastomer compatibility: Limited increased pump part usage on some rigs. Warn all tool suppliers, and BOP and rig companies beforehand, but this is now not such an active issue.
2. As you can see from comments, MWD needs to be compatible! before hand.
3. Health and Safety: not a problem although very high pH. Mud companies have skin data which shows silicate mud is non-irritating, surprisingly.
4. Hole cleaning: The rheology of silicate muds should be conducive to good hole cleaning, because of good shear thinning characteristics, high low end rheology and low plastic viscosity etc. So, even with high solids build up the mud often looks better than oil mud for hole cleaning.
For high angle wells (see comments on lubricants - not recommended) where cuttings beds have been deposited, back reaming has been a problem. This has also been confirmed from other operators who also have problems with high angle long reach 12Â¼" sections. There are reasons for this; WBM cuttings are stickier, although not necessarily worse for silicates. But the hole is in gauge probably, so worse than for other WBM systems. Also hole cleaning procedures have not been optimised for WBMs yet. Hole tends to stay more in gauge than any other WBM system.
Also accretion or balling is more likely with WBM on the BHA.
This can be engineered around with a bit of planning as wiper trips pumping out to 40 degree angle and then pumping clean. Heavy pills to aid hole cleaning. But it is certainly something to be aware of.
5. Wellbore stability: Not an issue logged a couple of Tertiary sections and observed over gauge hole for the top 1000ft. And oval and in gauge hole for the rest of the section. Much better than we would expect from most WBMs.
We had long open hole times, on Central North Sea well of up to 50 days, and one other to 90 days with no wellbore collapse.
6. ROP performance. With the correct PDC bit the ROPs can be close to OBM, but annular velocities have to be good too.
7. Did a small comparison using field data between OBM used in Central N Sea, proprietary HPWBM water based mud in Norway and low angle silicate 17 "” sections on Pelican. ROP and hours per 1000 feet very similar for OBM compared to silicate. Silicate mud costs higher but lower when cuttings transport and disposal costs taken into account.
Proprietary HPWBM much poorer and more expensive mainly due to not being able to use the full HPWBM system due to environmental regulations.
It is a while since I last had any dealings with silicate fluids but I guess the chemistry won't have changed all that much.
Great for stabilising friable sands. Sodium silicate is used as a grouting agent on big tunnelling projects and so on as it effectively becomes cement in the presence of divalent cations (Calcium & Magnesium in most drilling cases).
Best water-based inhibition of reactive shales due to the same action as above. BP use it a lot in the Caspian in top hole with RMR as it stabilises their wet shales.
Environmentally friendly as the active ingredient becomes inert in the presence of seawater.
The system requires large additions of silicate in wet or reactive formations. The mud man really needs to keep on top of testing & logistics to ensure he has a decent buffer of silicate in the system. A lot of the anecdotes about poor performance of the systems relate to silicate levels being allowed to drop.
Lubrication. The system has no inherent lubricity. Vegetable oil based lubes hydrolyse in the high pH environment so you would be looking at graphite or similar inert substances to get torque values down. It wouldn't be your first choice for adventurous directional work.
Hole cleaning. You might get tight hole at mud weights that work with other WBM. This is down to the rifle barrel silicate wellbore highlighting the real pore pressure of clays that previously sloughed away at lower mud weights. So not really a hole cleaning issue but it certainly feels like it at surface. You get similar issues if you try to drill reactive clays with OBM - suddenly you find out what the pore pressure of the shale really is and have to up the mud weight, which slows down ROP.
HSE. The neat silicate is very alkaline and care needs to be taken especially if you are adding from drums. The complete mud didn't seem to give problems as the active ingredient is neutralized quickly by sweaty palms.
Cleanliness. Shaker hands aren't crazy about thin grey cement all over their workplace. It goes away by itself eventually. Make sure the hands don't fill the mud with untreated seawater as you will deplete the silicate faster than necessary.
Rheology. Not necessarily a con but systems did occasionally exhibit increasing rheology with increasing temperature. Nice & thin in the pits during tripping then getting thicker once the drilling started. Maybe this has been addressed now? Certainly would have an impact on your hydraulics programme inputs.
As far as downhole tools are concerned they should be fine although there were some issues very early on with super-hard elastomers (installed for POBM/EBM sections) crumbling with the silicate fluids but easily cured by going back to softer elastomers.
Someone with more recent experience will be along in a minute I'm sure.
Hope that helps