Tax changes made by the Chancellor in the 2014 budget introduced a new allowance for uHPHT (ultra-high pressure high temperature) oil and gas field clusters, to help address the challenges and opportunities faced by the mature North Sea basin.
Following a webcast organised by Oil & Gas UK, Philippe Guys, Managing Director of Total E&P UK outlined where the industry must focus to overcome potential challenges drawing upon the experiences of their base in the North Sea, Elgin-Franklin fields.
Elgin G4 incident
Following two gas leaks in 2012 at their Elgin platform, Total had to stop production until almost 12 months later in 2013. Causes of the leaks were chalk activity above reservoir and A annulus pressurised, and the failure of 10 3/4" production casing which had stress corrosion cracking due to the joint dope.
After this development, a new well integrity criteria and associated operation philosophy were adopted, and overburden studies and chalk data acquisition were collected. Mr Guys also highlighted 5 main challenges the Elgin case study had raised for the oil and gas industry as a whole.
5 uHPHT challenges
1 - Annulus management
Systems are required on all the wellhead platforms to maintain the well A, B & C annulii within operating pressure ranges. The original design intent was to handle thermal cycling of the wells but the degraded wells put additional demands on these systems.
2 - Chloride Stress Corrosion Cracking (internal & external)
The risk of internal CSCC can be managed by dilution with washawter and material substitution. This a requirement as concentration will effect massively as pressure drops in the system. This raises the issue of also finding a material that permanently resolves the problem.
An extensive piece of research is needed to determine operational limits of such 'substitute materials'.
For external CSCC, risk needs to be managed by a high integrity coating and maintenance too. Specific inspections and remedy programme must be completed to control risk on uncoated services.
3 - Downhole scaling
The occurrence of dry scales (NaCl & CaCO3) need to be effectively treated by periodic acid washes. Wet scale (ZnS & PbS) also needs to be treated and progress has already been made on both phenomenons.
Mitigation is being developed and a scale inhibitor has been qualified at 180 Deg C. Working is ongoing to develop a potential scale squeeze chemical effective at 225 Deg C.
4 - Extended reach drilling
Generally speaking, Philippe Guys mentioned that extended reach drilling is an ongoing machinery issue. Total bases at Glenelg and West Franklin phase 1 were drilled with highly deviated wells from Elgin and Franklin - creating a step out of 4km.
5 - Drilling infill wells
The use of 'stress caging mud' was also raised as an issue for drilling infill wells in depleted reservoirs. Linear drilling and Managed Pressure Drilling has enabled infill drilling much later in field life than currently anticipated.
A good but challenging sign for the rest of the UKCS.
Opportunities are available
Despite raising their concerns, Total E&P UK pressed the issue that this development from the UK government is a huge step forward. Now technology has advanced and mature basins can be exploited further, Total estimated that the newly labeled uHPHT-ready fields represent 54% of their operated reserves.
And with only 2% of their worldwide portfolio achieved through uHPHT, the company are rubbing their hands at the prospect of more global legislation changes.
Photo Credit: Total E&P UK