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Well Bore Strengthening - Coal Seam Drilling
11 May 2019


I’m looking for worldwide experience with regards to stabilization of coal seams in high angle hole sections. The seams are deep in the well so can’t limit the thickness of coal exposed. We also need to drill with mud weights >11.0 ppg for wellbore stabilization purposes, so drilling with a lighter system to limit cleat invasion is a non-starter.

Lo-Energy drilling practises have been utilised to limit the induced damage whilst drilling through the seams and the mud has contained a broad range of WBS strengthening material in an attempt to support the coals.  Unfortunately, the mud rheology is seriously impacted with the volume of WBS material in the system– LGS & 6 RPM a lot higher than desired.

Looking for information on specialised material utilised by companies when dealing with coal seams. Am aware that some operators in SE Asia have been utilising a Well Bore Strengthening product from Impact Fluid Solutions. Are there any other products out there that people have used successfully? The end aim is to limit the impact on fluid rheology, whilst successfully stabilising the coals with mud weights in the 11.5 – 12.0ppg range.

I am aware of the papers that have previously published SPE papers

SPE 125246: Practices Implemented to Achieve Record Performance in Narrow Margin Drilling in Bass Strait Extended Reach 

SPE 128712: Utilizing an Engineered Particle Drilling Fluid to Overcome Coal Drilling Challenge

I'm looking to see if there has been any advancement in product deveopment to deal with a well known problem.

Thanks in advance.

2 answer(s)
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 502
Join Date: 10/01/05

Experiences of deep coals seams in higher angle development wells, offshore Malaysia were that these were geopressured seam problems and not geo-mechanical. 

First and foremost I would be seeking consultation with a geomechanics guru to assure we are labelling the problem correctly?

Proactive strengthening while drilling (SWD) approach to achieve wellbore strengthening (WBS) via pre treating drilling fluids is usually applied when drilling window narrows (fracturing formations is likely) and/or loss circulation is imminent. 

Coal from my experience generally does not fall into these categories? So what works in other SWD/WBS situations circumstances may not work in coal.

For our geopressured coals seams we did not uncover any lesson learned from region that was related to WBS materials that could be used/applied w.r.t geopressured.

We did assure lo-energy drilling practices while drilling/tripping.  
e.g. Drill through, make connection with minimal mechanical hydraulic torsional effects and then drill ahead vs ream the section. Main Problem however was not while drilling or on the few connections made as BHA passed through seam.

Main problem noted was on trip out (as wellbore diameter at seam was smaller) due to geopressured  situation that existed. 

In these cases we generally had to backream 'stabilisers and bit through the seam' and then continue POH with elevators thereafter. note: We never saw that much instability at the shakers though! Just some coal remnants as we drilled our way back through the seam.

Another problem that followed was running casing through the seam. Regional experiences LL telling us we needed a special guide or reamer shoe to again navigate and be able to get through the seam. 

On an ERD well added time will sure increase risk e.g. prior to BHA being required to be pulled out through zone. 

Perhaps new technology tool at back(top) of BHA available to assist during trip out may be prudent to look to make trip out easier.  

Hazard/Risk assessment versus extent of problems times/costs/delays being experienced could warrant a trip to run and isolate the zone with an expandable. Maybe? 

If this is not a geopressured problem. 
Proprietary graphite blend, marble, nut husks, ground petroleum coke, graphite and proprietary cellulosic blends are what is stated as works work best when adopting a proactive strengthening while drilling approach (SWD)  to act as a wellbore strengthening material. How this applies to a coal seam. I'm not sure.

As before collaborating and discussing with geomechanics and other specialists in these subject fields should be resourced.      

WCF Accredited Instructor / Assessor
SPREAD Associates
Total Posts: 9
Join Date: 14/11/15
Iain, we had very good success in limiting fluid invasion into highly permeable (~2 Darcy) sands with Resinex from Australian Mud Co. They have a soak solution to remove it too. I don’t recall any adverse rheology.
Posted by

Iain McCourt

Lead ERD Advisor / Engineer / Instructor

Merlin ERD Limited

Total Posts: 17
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