Petroleum Engineering 411 Well Drilling Lesson 27 Dual Gradient Drilling

DESIGN PROJECT Your PETE 411 Design Project will be assigned on Friday, November 15 The Project Report is Due in Dr. J-W’s Office by 5 p.m. on Monday, Dec. 9

What is Dual Gradient Drilling? (DGD)? In dual-gradient drilling the pressure profile in the annulus appears to have two distinct pressure gradients An example would be a heavy mud below the mudline and a seawater gradient above the mud line

Conventional Riser Drilling - Wellbore Pressures FLOATER SEA WATER HYDROSTATIC DRILLING RISER CHOKE LINE MUD HYDROSTATIC BOP SEAFLOOR DEPTH PRESSURE ATM 16

Static Wellbore Pressures FLOATER MUD HYDROSTATIC PRESSURE DGD MUD HYDROSTATIC PRESSURE Conventional RISER CHOKE LINE BOP DEPTH SEA WATER HYDROSTATIC PRESSURE PRESSURE DGD 10

Dual Gradient Drilling Projects Subsea Mudlift Drilling (SMD) Hollow Glass Spheres Deep Vision Shell Gas Lift H.P. Riser

Subsea Mudlift Drilling ( SMD ) Note Pump and Return Line

Subsea Mudlift Drilling What is Subsea Mudlift Drilling? How does it work? Why do we need it? Pore pressures and fracture pressures Mud weights and casing programs What about connections and trips? What about kicks?

References HW #15 (due 11-15-01) 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 #15 (due 11-15-01)

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 (DGD) Seawater gradient from surface to seafloor Heavier drilling mud inside the wellbore

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

Current Problems Deeper water results in longer, larger diameter 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

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. Weight of 16 lb/gal mud inside riser ~ 2.7 million lbs

What About Subsea Mudlift Drilling? Two 4.5” ID return lines with ~ 400 bbls capacity can do the job Requires much less weight and volume for storage! A smaller vessel can do the job A smaller vessel can easier be upgraded to do the job

What is Subsea Mudlift Drilling? SMD refers to drilling where mud returns DO NOT go through a conventional, large-diameter, drilling riser Instead the returns move from the seafloor to the surface through two small - diameter pipes separate from the drillpipe (outside the main riser pipe) A Mudlift system is used in the Return Line ATM 5

Equivalent Mud Density, ppg Pore Pressure Gradient Fig. 7.21 ADE Pore pressure gradient and fracture gradient data for Jefferson Parish, LA. Fracture Gradient 0.5 ppg 0.5 ppg Pore Pressure Gradient

Conventional Casing Seat Selection SEAFLOOR Frac Pressure Max Mud Wt Min Mud Wt Pore Pressure Equivalent Mud Wt, lb/gal

Typical Overburden Pressure grad.vs. Depth Ref: “Fracture gradient prediction for the new generation,” by B.A. Eaton and T.L. Eaton. World Oil, October 1997. 17.3 ppg 11.5 ppg

SEA WATER HYDROSTATIC PRESSURE Conventional Riser Drilling - Wellbore Pressures FLOATER STATIC PRESSURE CIRCULATING PRESSURE BOP DEPTH SEAFLOOR Drill String SEA WATER HYDROSTATIC PRESSURE PBIT PRESSURE ATM 16

Static Wellbore Pressures MUD HYDROSTATIC PRESSURE SMD MUD HYDROSTATIC PRESSURE Conventional SEAFLOOR DEPTH SEA WATER HYDROSTATIC PRESSURE PRESSURE ATM 10

Example: Static Wellbore Pressures At 30,000 ft, in 10,000 ft of water, the pore pressure is 21,000 psig. For conventional drilling, what is the minimum mud weight that can control this pressure? For SMD, what is the minimum mud weight that can control this pressure? ATM 10

Static Wellbore Pressures P = 0.052 * MW * Depth For conventional drilling, Minimum mud wt. MWmin = 21,000/(0.052 * 30,000) = 13.5 lb/gal Seafloor pressure = 0.052*8.6*10,000 = 4,472 psig For SMD, Minimum mud weight = (21,000 - 4,472)/(0.052 * 20,000) = 15.9 lb/gal ATM 10

SEA WATER HYDROSTATIC PRESSURE Solution: Static Wellbore Pressures SEA WATER HYDROSTATIC PRESSURE 8.6 lb/gal 15.9 lb/gal SMD 13.5 lb/gal Conventional DEPTH 4,472 psi 21,000 psi ATM 10

MUD HYDROSTATIC PRESSURE Conventional SEA WATER HYDROSTATIC PRESSURE Wellbore Pressures MUD HYDROSTATIC PRESSURE Conventional SEAFLOOR FRACTURE PRESSURE DEPTH SEA WATER HYDROSTATIC PRESSURE PORE PRESSURE PRESSURE ATM 11

Wellbore Pressures DEPTH PRESSURE ATM MUD HYDROSTATIC PRESSURE SMD MUD HYDROSTATIC PRESSURE Conventional SEAFLOOR FRACTURE PRESSURE DEPTH SEA WATER HYDROSTATIC PRESSURE PORE PRESSURE PRESSURE ATM 12

MUD HYDROSTATIC PRESSURE Conventional SEA WATER HYDROSTATIC PRESSURE Casing Requirements - Conventional MUD HYDROSTATIC PRESSURE Conventional SEAFLOOR DEPTH FRACTURE PRESSURE SEA WATER HYDROSTATIC PRESSURE PORE PRESSURE PRESSURE ATM 13

MUD HYDROSTATIC PRESSURE SMD SEA WATER HYDROSTATIC PRESSURE Casing Requirements - SMD MUD HYDROSTATIC PRESSURE SMD SEAFLOOR FRACTURE PRESSURE DEPTH SEA WATER HYDROSTATIC PRESSURE PORE PRESSURE PRESSURE ATM 14

Pressure Considerations MUD HYDROSTATIC PRESSURE SMD MUD HYDROSTATIC PRESSURE Conventional SMD SEAFLOOR FRACTURE PRESSURE DEPTH SEA WATER HYDROSTATIC PRESSURE PORE PRESSURE PRESSURE ATM 15

SEA WATER HYDROSTATIC PRESSURE Wellbore Pressures - Conventional FLOATER STATIC PRESSURE CIRCULATING PRESSURE SEAFLOOR BOP DEPTH SEA WATER HYDROSTATIC PRESSURE PBIT PRESSURE ATM 16

ANNULUS AND RETURN LINE SEA WATER HYDROSTATIC PRESSURE Static Pressures - SMD FLOATER ANNULUS AND RETURN LINE SEAFLOOR BOP DEPTH SEA WATER HYDROSTATIC PRESSURE PRESSURE ATM 17

SEA WATER HYDROSTATIC PRESSURE Drillstring Circulating Pressures CONVENTIONAL SMD SEAFLOOR DEPTH SEA WATER HYDROSTATIC PRESSURE PBIT PRESSURE ATM 19

CONVENTIONAL (13.5 lb/gal) SEA WATER HYDROSTATIC PRESSURE Annulus Circulating Pressures CONVENTIONAL (13.5 lb/gal) SMD (15.9 lb/gal) SEAFLOOR PPUMP DEPTH SEA WATER HYDROSTATIC PRESSURE PRESSURE ATM 20

ANNULUS AND RETURN LINE SEA WATER HYDROSTATIC PRESSURE Circulating Pressures - SMD DRILLSTRING PRESSURE ANNULUS AND RETURN LINE SEAFLOOR PPUMP DEPTH SEA WATER HYDROSTATIC PRESSURE PBIT PRESSURE ATM 21

Circulating Pressures - Summary CONVENTIONAL PBIT PRESSURE PML_PUMP SMD DISTANCE FROM STANDPIPE ATM 21

Transients Transient Behavior when Stopping Rig Pump (U-tubing or Free-fall) Why does the drillpipe fluid level fall? How fast does the Fluid Level in the drillpipe drop? How far does the Fluid Level drop? ATM 22

~SEAWATER HYDROSTATIC PRESSURE U-Tubing in SMD FLOATER ~SEAWATER HYDROSTATIC PRESSURE Drillstring STATIC FLUID LEVEL Annulus BOP MUDLIFT ATM 23

ANNULUS AND RETURN LINE SEA WATER HYDROSTATIC PRESSURE Static Pressures - SMD Static Fluid Level in DP ANNULUS AND RETURN LINE DRILLSTRING PRESSURE SEAFLOOR DEPTH SEA WATER HYDROSTATIC PRESSURE PRESSURE ATM 18

10,000 ft Water Depth ATM 24

10,000 ft Water Depth ATM 25

Well Control Considerations How do you shut a well in after taking a kick? With a DSV this is almost routine Better still, it is not necessary to shut the well in. The wellbore pressures can be increased by temporarily slowing down the mudlift pump Friction in the choke line is handled by the Mudlift Pump and is not seen by the weak formations ATM 22

General Summary Dual Gradient Drilling is a method that offers potential for lowering drilling costs in very deep waters: Fewer casing strings Smaller rigs Less time on location The method utilizes one or more small-diameter return lines from the seafloor to the surface. The drillpipe is separate from the return lines ATM 36

Summary - cont’d A mudlift system (pump) is used to feed the return lines, thereby making a “dual-density” mud system possible Wellhead pressure is maintained at seawater hydrostatic, so well is “dead” at all times Well control is quite similar to that in conventional drilling with a riser, but offers a number of significant advantages ATM 37

THE END ATM 38

Dual Gradient Alternatives Gas Lift? Glass Beads? Pumps? Gas Lift? Glass Beads? Dual Gradient Alternatives How to Handle Connections? Trips? Rotating BOP

~SEAWATER HYDROSTATIC PRESSURE Advanced SMD System FLOATER ~SEAWATER HYDROSTATIC PRESSURE 10,000’ SEAFLOOR BOP MUDLIFT Circulation Rate 650 gpm Drillpipe OD 6 5/8 in Return Line ID 6 in Hole Size 12 1/4 in 30,000’ ATM 6