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Two kinds of mud cap drilling 1. Pressurized using a RFD to add surface pressure onto the mud cap 2. Floating is a variant in which reservoir pressure is lower than hydrostatic pressure; here, the top of the fluid drops to a balance point down in the well. Uses 1. To control severe loss of circulation during flow drilling operations. 2. Drilling into sour formations 3. For high surface pressures where the well control can’t handle the pressures. (probably rare) Operations Driller loads the annulus with a relatively high density high viscosity mud and closes the choke with surface pressure maintained. (The annular fluid prevents hydrocarbon migration to the surface and subsequent high pressures. The mudcap enables drillers to continue with lower annular pressures. In essence, the MCD technique allows an operator to continue drilling through fractures or faults to total depth with reduced trouble time and cost, minimizing mud losses to the formation.) Drilling is then continued “blind” by pumping a non-damaging fluid (sacrificial fluid) down the drillstring through the bit and into the thief zone. Disadvantages : It does not allow the rig to monitor the downhole pressure directly. If gas breaks through the mud cap, it is difficult to detect and would reach the surface with little warning.

The Basics of PMCD

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Page 1: The Basics of PMCD

Two kinds of mud cap drilling1. Pressurized using a RFD to add surface pressure onto the mud cap2. Floating is a variant in which reservoir pressure is lower than

hydrostatic pressure; here, the top of the fluid drops to a balance point down in the well.

Uses

1. To control severe loss of circulation during flow drilling operations.2. Drilling into sour formations3. For high surface pressures where the well control can’t handle the pressures. (probably rare)

Operations

Driller loads the annulus with a relatively high density high viscosity mud and closes the choke with surface pressure maintained.

(The annular fluid prevents hydrocarbon migration to the surface and subsequent high pressures. The mudcap enables drillers to continue with lower annular pressures. In essence, the MCD technique allows an operator to continue drilling through fractures or faults to total depth with reduced trouble time and cost, minimizing mud losses to the formation.) 

Drilling is then continued “blind” by pumping a non-damaging fluid (sacrificial fluid) down the drillstring through the bit and into the thief zone.

Disadvantages:

It does not allow the rig to monitor the downhole pressure directly. If gas breaks through the mud cap, it is difficult to detect and would reach the surface with little warning. This could result in surface pressures that are above the rated working pressures of the rotating control equipment (only if no back pressure is applied.)

No returns to surface, no samples.

Large amount of fluids needed

Selection of cap mud:

• Non-damaging to the formation.• Not able to form damaging emulsions with either reservoir fluid or drilling fluid.• High rheology downhole to minimize mixing with reservoir fluids.• Mixable in high volume.• Able to be weighted up quickly during drilling operations.• Inexpensive.

Page 2: The Basics of PMCD

• The drilling fluid should be an inexpensive fluid that can be lost into the formation at large volumes and also be compatible with the cap mud.

Schematic of equipment required for mudcap drilling

Page 3: The Basics of PMCD

Fluid Volume Requirements

Annular volumes will depend upon whether the operator desires continuous or periodic injection of annular fluids or whether a floating mudcap is to be used.