Overcoming Drag While Running Casing in an Extended Reach Well by Varying Drilling Fluids Properties

Abstract Running casing from a floating offshore vessel has increased risk due to having a subsea stack and vessel heave. Floating large casing is not possible to deep depths [MA1]thus requiring additional measures to ensure the casing reaches TD. This paper presents a solution involving using drilling fluids with varying properties to overcome drag to allow the casing to reach the planned depth. Drag modelling combined with hydraulics modelling is used to assess the effects of changing drilling fluid properties in overcoming the drag forces while running the casing. Drilling fluid rheology within the annulus is modified to reduce the hydraulic lift on the casing string while still maintaining wellbore integrity with adequate mud weight. A dual gradient approach is used while running the casing string, with lighter fluid used initially and heavier fluid as the casing is closer to the planned depth. The fluid must also remain within the mud-weight window when circulating out prior to cementing. The presented case study is from an offshore well from a semi-submersible rig in 451 ft. water depth with a subsea wellhead. The hole section in the analysis is a 13½ in. underreamed hole to a measured depth of 10,300 ft. with a TVD of 4,000 ft. and a reach of 8,000 ft. The intermediate casing is a tapered string of 4,800 ft. of 9⅝ in. casing from the shoe with 5,000 ft. of 10¾ in. casing to the wellhead. The open hole section requires 12 ppg mud for wellbore stability and the intermediate section of this well is 1,500 ft. longer than previous wells. The initial mud being planned has low end (6 and 3 rpm) rheology like previous wells of 12 and 11. Switching to a thinner fluid, such as a micronized barite with a rheology of 4 and 3, reduces the hydraulic lift of the casing while maintaining 12 ppg mud weight. Using dual gradient mud inside the casing string of 11 ppg for the initial 5,800 ft. and 13 ppg to surface enables the casing string to overcome additional drag. This dual gradient mud is also able to be circulated out prior to cementing without exceeding the losses limit or dropping below pore pressure. This innovative approach to overcoming drag forces by using drilling fluid weight and rheological properties enables large casing to be run to deeper depths in extended reach wells where fully evacuated floatation and/or rotation are not feasible. Varying the mud weight inside the casing is simultaneously reducing the normal force, and subsequently drag, in the horizontal section while increasing weight in the vertical section thus providing additional pushing force.