An Introduction to Dilution-BasedDual Gradient (DBDG) Drilling

Presented at the 2011 IADC Dual Gradient Workshop, 5 May 2011

by Paul Boudreau, Dual Gradient Systems LLC

• Heavier “downhole” mud weight is pumped down drillstring, through bit, and into annulus.

• At injection point (i.e., usually bottom of riser), a lighter density “dilution” mud is injected into annulus at a fixed dilution ratio.

• “Downhole” and “dilution” muds combine at injection point, resulting in column of “diluted” mud weight above injection point.

• “Diluted” mud is returned to rig and, in a continuous process, separated by specialized centrifuges back into “downhole” and “dilution” mud weight components.

How is a Dual Gradient Achieved with the DBDG Process?

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Simplified DBDG Circulating Process

CENTRIFUGE SEPARATION

SHAKERS

Downhole Mud

Dilution Mud

Diluted Mud

Light Weight Discharge

Heavy Weight Discharge

Injection Point

“Downhole”, “Dilution”, and “Diluted” muds are all the same fluid, only with

varying concentrations of barite. DrillstringAnnulus below

Wellhead

Riser

Surface

Boost Line

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How Do Dilution-based and Pump-based Dual Gradient Systems Differ?

Dilution-based Dual Gradient (DBDG)

Pump-based Dual Gradient (PBDG)

Well Design

Wellhead Hydrostatic Pressure

Diluted Riser Mud Hydrostatic

Usually Seawater Hydrostatic

Location of Inflection (Injection) Point

At seafloor, or potentially sub-mudline

At seafloor

Location of Complex Componentry At surface At seafloor

Fewer casing strings compared to single gradient design. Casing points may vary between DBDG

and PBDG based on respective pressure profiles.

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Basic Dual Gradient ConceptPressures and Gradients

To achieve 12.5ppg equivalent mud weight at 20,000ft, the PBDG system would require a 16.5ppg mud below the seabed (at 10,000ft), with seawater hydrostatic at the wellhead.

PBDG: Inflection point at seafloor

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Basic Dual Gradient ConceptPressures and Gradients

To achieve 12.5ppg equivalent mud weight at 20,000ft, the DBDG system could use a 14.5ppg mud below the wellhead (at 10,000ft), with a diluted 10.5ppg mud weight in the riser above the wellhead.

DBDG: Inflection (injection) point

at seafloor

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Dual Gradient Concept with Sub-Mudline InjectionPressures and Gradients

To achieve 12.5ppg equivalent mud weight at 20,000ft, the DBDG system could use a 16.5ppg mud below the sub-mudline injection point (at 13,300ft), with a diluted 10.5ppg mud weight above the injection point.

DBDG w/ SMLI: Inflection (injection) point moved, e.g., to surface casing shoe

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DBDG vs. Single Gradient Well Design

Well Design Considerations• Not all casing seats determined solely

by PP/FG/MW inter-relationship.• Isolation of stray hydrocarbon zones.• Maintaining adequate kick tolerance.• Maintaining multiple barriers between

reservoir and “the world”.• Contingency planning.• Completions design.

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DBDG vs. PBDG Well Design

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DBDG vs. PBDG Well Design

Depleted Sand

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DBDG w/ SMLI vs. DBDG Well Design

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DRILLSTRING

INJECTION STRING

MUD PITS

MUDPUMPS

DILUTION MUD = LIGHT WEIGHT DISCHARGE

FLOW STOP VALVE DOWNHOLE MUD = HEAVY WEIGHT DISCHARGE

DILUTED MUD = MIX MUD = FEED MUD

DEGASSERSHALE

SHAKERS

MUD PITSMUD PITS

CENTRIFUGES

DBDG Circulating System Using Surface BOP Stack (Not Shown)

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DRILLSTRING

BOOST LINE

MARINE RISER

KILLCHOKE LINE

LINE

MUD PITSSUBSEA BOP

MUDPUMPS

DILUTION MUD = LIGHT WEIGHT DISCHARGE

FLOW STOP VALVE DOWNHOLE MUD = HEAVY WEIGHT DISCHARGE

DILUTED MUD = RISER MIX MUD = FEED MUD

CENTRIFUGES

MUD PITSMUD PITS

DEGASSERSHALE

SHAKERS

DBDG Circulating System Using Subsea BOP Stack

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CAPM (Continuous Annular Pressure Management) System

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CAPM Circulating System

DRILLSTRING

CONVENTIONAL FLOWLINE

ROTATING CONTROL DEVICE

INNER RISER MANIFOLD

MARINE RISER

BOOSTER LINE

KILL LINECHOKE LINE

MUD PITS

SUBSEA BOP

MUDPUMPS

DILUTION MUD = LIGHT WEIGHT DISCHARGE

FLOW STOP VALVE DOWNHOLE MUD = HEAVY WEIGHT DISCHARGE

DILUTED MUD = RISER MIX MUD = FEED MUD

SHAKERS

CHOKEDRILLING

CENTRIFUGES

DEGASSERCORIOLIS FLOWMETER

MUD PITSMUD PITS

SHALE

PDP

PDIL

PPWD

PBOP

PRCD

PCHOKE

PRISER

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Customized DBDG Centrifuge

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Customized DBDG Centrifuge

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DGS / TO Centrifuge Testing

SOBM Drilling Fluid

G Force, in/sec2

Feed Rate, gpm

“Diluted” Feed MW,

ppg

“Downhole” Heavy MW,

ppg

“Dilution” Light MW,

ppg

Dilution Rate, QLight / QHeav y

Min - Ave - Max

90/10 OWR 220-1010 490-590 10.5-11.0 11.5-13.7 8.4-8.9 1.1 - 1.5 - 1.9

81/19 OWR, no LCM 500-1010 490-600 10.8-10.9 13.2-15.5 9.0-9.6 1.6 - 2.3 - 2.8

81/19 OWR, w/ LCM 500-780 490-500 10.9-11.1 14.8-15.4 9.3 2.4

Flottweg Centrifuge Testing Results

“Downhole”, “Dilution”, and “Diluted” muds are all the same fluid, only with varying concentrations of barite.

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DGS Flow Stop Valve

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Dual Gradient Drilling FSVU-Tube Arrested with Pumps Off

Upstream - Drillstringwith “Downhole” Mud

Downstream -BHA and Drill Bit

U-tube Pressure Resisted by Spring Forces

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Dual Gradient Drilling FSVBringing on Pumps Through Choke

Pump Pressure + U-Tube Overcome

Spring Force

Flow is Restricted to Provide Main Piston Opening Pressure

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Dual Gradient Drilling FSVMain Valve Opened for Normal Drilling

Choke is By-passed once Main

Valve Opens

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Dual Gradient Drilling FSVValve Closes Once Pumps Off

Upstream - Drillstringwith “Downhole” Mud

Downstream -BHA and Drill Bit

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Thank You