Whilst drilling the 12 1/2"and 8 1/2"sections on a HPHT field in the North Sea,fluid losses were encounteredwhich required the pumping of high concentration Loss Circulation Material(LCM)as well as multiple‘through the bit'cement squeezes.The reservoir had been under production for many years and was now ata point where it had significantly depleted and the formation was very delicate,making it easily subjectedto fracturing.Losses were expected to be an upcoming challenge as the mud weight window was so narrowand due to the HPHT rating of the offshore reservoir and the fractured nature of the geology,this posed asa serious risk that required pre-planned mitigation.The ability to pump ultra-high LCM concentrations was discussed in pre-planning as being one of thekey mitigation strategies to stemming losses.However,it was thought that pumping LCM would blocka typical MWD pulser due to the narrow geometries of the flow paths within.Successful onshore testingwas conducted prior to drilling operations to prove that the MWD pulser selected had the ability to permitthe flow of ultra-high concentrations of LCM.A closed loop system was created onshore and LCM withconcentrations up to 420kg/m~3(147lbs/bbl)was pumped through the MWD tool without blocking it up andwith full functionality throughout.This gave the operator the confidence to pump LCM through the MWDtools,if required in the field.The use of an MPD(Managed Pressure Drilling)system was also required to keep the equivalentcirculating density within the tight mud weight window by maintaining a constant bottom-hole pressurethrough the application of Surface Back-Pressure(SBP).This was critical as the well,when static,wasunderbalanced.The incorporation of MPD also gave the operator the ability to'squeeze'LCM/cement intothe well and formation by applying surface back pressure during'through the bit'LCM/cement jobs.Duringfield operations,the operator did go on to pump a 320 kg/m~3(112 lbs/bbl)LCM pill through the MWD toolwithout plugging during a loss scenario.Pumping LCM through the MWD tool reinforced the formationand enlarged the mud weight window from <0.01sg to 0.03sg allowing operations to continue.In addition,muliple"through the bit"MPD LCM/cement squeezes were conducted on two wells in the same field withno negative effect on any of the MWD tools.The partnership between the MWD and MPD systems formed a key part of the losses management.The MWD system could identify exactly what downhole pressure losses were seen at,which was then transmitted by the pulser to the surface.The MPD system would then use an algorithm based calculation toapply the exact required SBP to maintain a constant bottomhole pressure,that was within the mud window.This provided the operator a safe and efficient solution to continue to the target depth in a very challengingnarrow margin well.
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