18. Dual-Gradient Drilling - Petroleum Engineering 661...

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1 Petroleum Engineering 661 Drilling Engineering Lesson 18 Dual Gradient Drilling
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2 What is Dual Gradient Drilling? What is Dual Gradient Drilling? (DGD)? (DGD)? In dual-gradient drilling the pressure profile in the annulus has two distinct pressure gradients An example would be a heavy mud below the mudline and a seawater gradient above the mud line
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3 A T M Conventional Riser Drilling - Wellbore Pressures SEAFLOOR SEA WATER HYDROSTATIC PRESSURE DEPTH MUD HYDROSTATIC BOP FLOATER DRILLING RISER CHOKE LINE
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4 Static Wellbore Pressures SEA WATER HYDROSTATIC PRESSURE PRESSURE D E P T H MUD HYDROSTATIC PRESSURE DGD MUD HYDROSTATIC PRESSURE Conventional BOP FLOATER RISER CHOKE LINE DGD
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5 Dual Gradient Drilling Projects Dual Gradient Drilling Projects Subsea Mudlift Drilling (SMD) Hollow Glass Spheres Deep Vision Shell
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6 Subsea Mudlift Drilling ( SMD ) Note Pump and Separate Return Line
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7 Subsea Mudlift Drilling What is Subsea Mudlift Drilling? How does it work? Why do we need it? Pore pressures and fracture gradients Mud weights and casing programs What about connections and trips? What about kicks?
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8 1. Riserless Drilling: Circumventing the Size/cost Cycle in Deepwater,” by Allen D. Gault. May 1996, Offshore , p.49 2. Subsea Mudlift Drilling JIP: Achieving dual-gradient technology,” by K.L. Smith et al., World Oil - Deepwater Technology, August 1999, pp 21-28. HW #10 (due 04-08-02) References
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9 Current SMD Concepts A water-filled drilling riser One or more separate small- diameter mud return line(s) from seafloor to surface (e.g., two 4.5-in ID lines) A “dual mud density” system Seawater gradient from surface to seafloor Heavier drilling mud inside the wellbore
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10 Current SMD Concepts - cont’d A seafloor mud pump to lift mud to surface Pressure inside wellbore at seafloor is ~ the same as the pressure in the ocean at seafloor Theoretically the well is always dead Important in case of drive-off Retains a Riser Margin
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11 Static Wellbore Pressures SEA WATER HYDROSTATIC PRESSURE PRESSURE D E P T H MUD HYDROSTATIC PRESSURE DGD MUD HYDROSTATIC PRESSURE Conventional BOP FLOATER RISER CHOKE LINE DGD
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12 Current Problems Deeper water results in longer and heavier drilling risers High pore pressures and low fracture pressures lead to more casing strings This leads to larger wellheads, even larger and heavier risers, and finally to bigger and more expensive rigs Well control is more difficult - because of the pore pressure / fracture pressure proximity, and long choke lines with high friction pressure drops
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13 Effect of Increasing Water Depth Weight of drilling riser increases with depth. In 10,000 ft of water: 21-inch riser has an internal capacity of ~ 4,000 bbls! (value ~ $1 million) Weight of riser ~ 2 million lbs.
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18. Dual-Gradient Drilling - Petroleum Engineering 661...

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