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Planning requirements for Managed Pressure Drilling (Mud Cap)

The Mud Cap Drilling (MPD, Managed Pressure Drilling) is a technique that can be used even in case of total circulation losses.

Circulation losses are traditionally cured with Loss Circulation Materials (LCMs) pills, gunk pills and cement plugs, but these methods have disadvantages:

  • uncertain about results
  • time consuming and expensive,
  • potential damage to reservoirs.

Severe circulation losses are often encountered drilling fractured formations, particularly carbonate (limestone, dolomite, etc.), specially when long vertical section are concerned, because of the differential pressure gradient of the drilling fluid vs. the formation fluids.

 

A sacrificial fluid is pumped through the drillstring and lost to the formation while the annuls is full of the so called “dual gradient” fluid system, i.e. the sacrificial fluid itself on the lower portion of the hole, and the (heavy) Mud Cap Drilling in the upper portion.

Although the Mud Cap Drilling (MPD, Managed Pressure Drilling),  is not underbalance drilling in the sense that formation fluids are not allowed to flow to surface,  the hydrostatic head of the dual system is such that it equals (or it is slightly lower than) the formation gradient.

Requirements for Mud Cap Drilling

For optimum use of Mud Cap Drilling (MPD, Managed Pressure Drilling),  the following is essential:

  • Heavy (total) circulation losses in fractured formations
  • Fractures that can take the drilling cuttings
  • Availability of low cost sacrificial fluid
  • Capability to build mud volume fast and effectively
  • Adequate additional surface equipment (Rotating BOP, high pressure pumping system).

For most effective results, the following options are preferred:

  • Last casing set as close as possible to the top of fractured zone
  • Small (say 8 1/2”) hole size to help hole cleaning and minimize injection rate.

Main equipment and rig upgrade for Mud Cap Drilling

 

  • Rotating Control Device (RCD).
  • Large amount of water (sacrificial fluid) that will be pumped through the drilling string, into the loss zone and carrying the drilling cuttings.
  • High pressure mud circulating system (pumps, standpipe, rotary hose, top-drive, etc.) may need to be upgraded in order to allow for the implementation of the Mud Cap Drilling (MPD, Managed Pressure Drilling) (7500 psi WP system is generally required).

Drilling Technique

The Mud Cap Drilling (MPD, Managed Pressure Drilling) technique applies when conventional circulation is no longer possible. It involves “blind” drilling with the choke completely closed while using an RCD (Rotating Control Device, Rotating Head, Rotating BOP) to seal the annulus.

While the sacrificial fluid is continuously pumped through the drillstring in order to refrigerate the bit and remove the drilled cuttings, mud is periodically injected into the annulus to prevent hydrocarbons from migrating to the surface.

The Mud Cap Drilling (MPD, Managed Pressure Drilling) is a cpmplicated and expensive system. For these reasons it is used only when other system for controlling losses were proven ineffective. Large quantities of water and mud are required. Additional equipment and specialized personnel is also required.

An accurate evaluation of all factors concerned with the drilling through hydrocarbon reservoirs, including rig time, material availability and consumption, reservoir damage, drilling hazards, etc. shall be performed prior the decision to switch from conventional drilling to Mud Cap Drilling (MPD, Managed Pressure Drilling) drilling is taken.

Sour gas considerations

The presence of H2S means that no formation fluids are allowed at surface unless in strictly controlled manner (e.g. while well testing or during production operations).

Drilling with Mud Cap Drilling (MPD, Managed Pressure Drilling), means that the annulus will be closed and no returns circulated back to the surface.

All down-hole equipment and blowout preventer equipment (BOPE) for the Mud Cap Drilling (MPD, Managed Pressure Drilling) operation will be rated for H2S service.

Since H2S corrosion on steel is in reality a form a Hydrogen embrittlement, if the hydrogen ions are in solution downhole rather than tied to sulfur ions, they may be more likely to attack the steel. Therefore, all fluids pumped downhole, including the sacrificial fluid, shall be treated to inhibit (or at least mitigate) the corrosion.

Along with these preventive measures, the mud shall be adjusted to maximize the pH. The drill string shall be made up of steel suitable for the H2S environment.

Circulation losses and the Mud Cap Drilling (MPD, Managed Pressure Drilling)

Fractures and/or vugs are characteristic of carbonate formation, so that circulation losses are often experienced here. In fractured reservoir, it could happen that a drilling fluid properly balancing the reservoir pressure at top, greatly overbalance the reservoir pressure with increasing depth, resulting in circulation losses. This is because of the differential specific gravity between the drilling fluid and the formation fluid. In practical terms it may happen that circulation losses are experienced drilling the bottom of the reservoir section and that balancing the pressure at depth of losses results in underbalance conditions higher in the reservoir.

Traditional methods to cure losses are often unsuccessful if multiple leak off zones may be encountered in sequence or because massively fractured zones cannot be cured. In extreme cases, running of technical (or expandable) liner is the most practical solution.

The Mud Cap Drilling (MPD, Managed Pressure Drilling) system, on the contrary, allows drilling ahead in circulation losses zones by controlling the annulus pressure and injecting a sacrificial fluid into the thief formations.

Converting from conventional to Mud Cap Drilling

The Mud Cap Drilling (Mud Cap Drilling (MPD, Managed Pressure Drilling), drilling procedure can be used to continue drilling safely after approximately severe (total or un-manageable) circulation losses occur. The nature of mud losses shall be carefully analyzed to determine the applicability of the Mud Cap Drilling (MPD, Managed Pressure Drilling) system, with particular regard to past experiences, nature of fracture and their extend, knowledge of reservoir petrophysical characteristics, nature of reservoir fluids, hole size, nature of the sacrificial fluid required. In particular, to ensure sufficient hole cleaning, the presence of large fractures or Karsts, capable of receiving the drilling cuttings, is required.

Mud Cap Drilling (MPD, Managed Pressure Drilling) equipment rig up and drilling rig features

Rig Features. If necessary, rig equipment will be upgraded to bring the fluid circulating system pressure and volume ratings up to the requirements for the Mud Cap Drilling (MPD, Managed Pressure Drilling) procedures. Performance test shall prove the pumping capability (either rig pumps or auxiliary HP pumps) to cope with the Mud Cap Drilling (MPD, Managed Pressure Drilling) requirements.

Mud System. The rig mud mixing system must be capable to add corrosion inhibitors and other treatment chemicals to the Sacrificial Fluid and build the required mud for the annular mud system at the same time.

Rotating Control Device (RCD). The approved Rotating BOP or Rotating Head (Rotating Control Device or RCD hereinafter) shall be installed onto the rig BOP stack prior to drill out the last casing shoe for possible Mud Cap Drilling (MPD, Managed Pressure Drilling) operations. The RCD (Rotating Control Device, Rotating Head, Rotating BOP) shall be commissioned and tested according with the Supplier procedures. The drilling adapter shall be installed and the mandrel connected to the bell-nipple/flow-line to allow circulation and control mud level when drilling in normal circulating mode. Ensure that there is room enough below the rig floor and above the rig BOP stack to accommodate the required equipment

High-Pressure Pumps. If the rig pumps are not sufficient for the scope of the Mud Cap Drilling (MPD, Managed Pressure Drilling), auxiliary high pressure pumps are required in addition to or in place of the rig pumps. Auxiliary high pressure pumps, needed for the injection if the sacrificial fluid through the drillstring, shall feature an intrinsically safe control panel installed close to the driller’s consolle to facilitate communication with the pump operator.