Ultra Deep Water Cementing, Challenges and Solutions

Abstract

Abstract Ultra Deep water drilling operation under Sagar Samriddhi Project started with two charted hire rigs Belford dolphin and Discoverer Seven Seas from Nov. 03. Since the commissioning of two rigs, total no. of 24 wells have been completed. This paper discusses real challenges faced during cementation of critical surface casing as well as liner and production casing. The maximum operating water depth was 3008 mt. Shallow water hazards like shallow water and gas flow also encountered during drilling of top hole sections. The lowest sea bed temp. Of 2 ºC encountered in no. of wells. The narrow margin of pore and fracture. Pressure window necessitated the requirement of cement slurry of wt. not more than 11 ppg in many wells. The requirement of 5oo psi compressive strength of light wt. slurry within 24 hrs. time span at 15/20 º C was one of the main challenges. All the above challenges were resolved using Schlumberger's PSD system combined with low temp. Gas Migration control additive, used for the first time in India. Proper cementing of liner more than 1500mts. Length with slurry wt. of 12.2 ppg was the biggest challenge faced during planning. Also the big temp. difference between the liner shoe and hanger top further aggravated the problem. However, the problems were solved using PSD system with proper Gas Migration control additive packages. Reduction of ECD during 7″ liner cementation of well having high temp., was dealt effectively using high temp. Gas Migration control additive in well K1 X, the deepest well drilled in Indian Offshore. Production casing in four wells, having very narrow window of pore and fracture. pressure, perfect isolation was achieved using PSD system and proper Gas Migration control additive system. 1. Cementing Challenges for surface Casing: Ultra Deep water drilling poses several challenges for Cementing surface casing. Large diameter surface casings are set in unconsolidated formations, with a narrow pore-fracture pressure window and high potential for shallow flow hazards. This problem is further aggravated due to very low temperature found at the seabed and first few hundred meters below mud line. Also, the young soft formation below the seabed does not allow the use of normal 15.8 ppg cement slurry and conventional low-density extended cements are not considered suitable due to the time required to set and achieve the compressive strength specified for this application. Recommendation: Slurry system requirement for deep water Environment:High performance light wt. slurry.Low slurry wt. in the range of 11–12 ppg.Controlled fluid loss less than 50 c.c.Low transition time. (time to gain 100- 500 lbf/100 ft2)Low permeability.Ability to set and attain good compressive strength at very low temperature.Zero shrinkage.Ductile cement system which can withstand cyclic loading. Operational requirement:Pump slurry vol. not less than 100 % excess.Pump weighted spacer at least 100 bbls.Centralize casing at least 50 mt. around shoe and 30 mt. inside previous shoe.Use effective Gas Migration control additive system suitable at low temp.Use inner string for pumping slurry and keep the end 5 mt. above the float collar to reduce contamination.Extended LOT should be taken for realistic value.