Managed pressure drilling episode 4
3- Pressurized Mud Cap Drilling:
The pressurized mud cap drilling technique (PMCD) is used when dealing with reservoirs that could result in a severe loss of circulation. Depleted reservoirs, which have lower reservoir pressures because of the production from other wells, often cause lost of circulation problems. As the mud is lost into the depleted zone, the hydrostatic pressure of the wellbore decreases to balance the reservoir pressure at the depleted zone.
At this point, the wellbore pressure is below the reservoir pressure of a zone that is not as deep as the loss zone. This causes gas to begin to flow into the wellbore. One way to keep such a well under control is to fill up the well at a rate that exceeds the gas percolation rate.
The PMCD (Pressurized Mud Cap Drilling) method uses a heavier mud pumped down the annulus to keep the gas influx from reaching the rig floor.
Fig. 1 Pressure-gradient profile for pressurized mud cap drilling method (From Hannegan).
Fig. 1, where BP: Back Pressure, BHP: Bottom Hole Pressure, shows the pressure profile of the pressurized mud cap method. A lighter mud is used to drill the depleted section and the heavier mud forces the fluid into the loss zone. Drilling continues and all the lighter mud and any influx are forced into the depleted zone. This method keeps the well under control even though all returns go to the depleted zone.
The important aspects of PMCD are the RCD (Rotating Control Device), cap mud, and drilling fluid. The RCD enables the operator to pump the cap mud into the annulus and to also keep pressure at the surface to compensate for the lower mud weight of the drilling fluid used to control the reservoir pressure during PMCD.
Requirements for the selection of the cap mud:
- Nondamaging 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.
- Able to be weighted up quickly during drilling operations.
- 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.
The advantages of the PMCD method:
- It can keep the well under control even while suffering severe losses to the formation.
- The rig is still protected by two barriers, the BOP and the mud cap.
- Using a lighter drilling fluid increases the rate of penetration (ROP)
- The lighter mud costs less than the mud that would be lost in conventional drilling.
- Lighter drilling fluid causes less damage to the reservoir.