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Managed pressure drilling

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IADC definition

Managed Pressure Drilling (MPD) is an adaptive drilling process used to precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly. The intention of MPD is to avoid continuous influx of formation fluids to the surface. Any influx incidental to the operation will be safely contained using an appropriate process. Any influx incidental to the operation will be safely contained using an appropriate process.[1]

Advantages of Managed Pressure Drilling:

  • Managed Pressure Drilling allows more accurate bottom hole pressure control, resulting in fewer pressure fluctuations and it allows better control of the well.
  • The use of Managed Pressure Drilling techniques can enable wells to be drilled whilst minimizing losses and kicks and possibly avoiding setting additional casing strings.

Application of Managed Pressure Drilling

The field of wellbore pressure management has broad applications in the drilling industry and provides solutions to problems in[2]:

  • Extending casing points to limit the total number of casing strings and subsequent hole size reduction
  • Limiting the NPT (Non-Productive Time) associated with differentially stuck pipe
  • Avoiding the lost circulation - well kick sequence
  • Limiting lost circulation
  • Drilling with total lost returns
  • Increasing the rate of penetration (ROP)
  • Deepwater drilling with lost circulation and water flows.

Managed Pressure Drilling Techniques

Constant Bottom-Hole Pressure (CBHP) is the methodology within MPD, whereby bottomhole pressure is kept constant at a specific depth, with the rig mud pumps on or off. [3] The CBHP method is commonly used in wells with highly uncertain and/or narrow drilling window, unstable formations where starting and stopping the pumps creates formation instability; HPHT wells.

In conventional drilling when the pumps are stopped in a very narrow drilling window, the bottom-hole pressure drops to a value below the pore pressure and an influx may be taken. Once the pumps are brought up back to drilling speed, the friction brings the bottom-hole pressure up to a level where the fracture pressure of the rock is exceeded and losses are encountered. The only option during conventional drilling is to drill with a reduced pump rate. Reduced pump speed, though temporarily controls the problem, slows down the drilling process since ROP needs to be controlled to reduce the risk of loading up the well with cuttings, delaying the completion of the well and increasing the risk of stuck pipe.

In order to stay within the drilling window, it is required to reduce the bottom-hole pressure variations between the circulating and non circulating modes. Using the MPD choke pressure to make adjustments on surface allows for better control of the bottom hole pressure.  Some surface backpressure is held as the pumps are stopped to keep the bottom hole pressure above the pore pressure and to avoid a potential influx.  As the pumps are brought up to speed, the choke pressure is lowered, thus allowing the friction to control the bottom hole pressure. Before the next connection is to be made, the choke pressure is once again increased to compensate for the loss of the friction and maintain the bottom hole pressure above the pore pressure. This procedure is illustrated in figure below and allows to maintain an almost constant bottom hole pressure at all times.

 
Constant Bottom-Hole Pressure

Precise flow modeling and hydraulic analysis are essential to evaluate alternative mud weight scenarios with MPD to reduce the static mud weight and the ECD at the modeled flow rates, rotary speed, and penetration rates. The use of well monitoring tools/equipment, specifically flow meters allows determining the actual required mud properties and backpressure on site.

Pressurized Mud-Cap Drilling (PMCD) refers to drilling with no returns to surface where an annulus fluid column, assisted by surface pressure, is maintained above a formation that is capable of accepting fluid and cuttings. The well is controlled by using a Light Annular Mud (LAM) that has a slightly lower density than is required to balance the formation pressure and is maintained above an open-hole formation that is taking all injected sacrificial (SAC) fluid and drilled cuttings assisted by surface pressure. The LAM density is chosen based on ability to make LAM and the desired surface pressure that can be maintained and observed. Periodically injecting more of the same fluid into the annulus provides a means to control the surface backpressure within the operating limits of the Rotating Control Device (RCD) and/or riser system. The annular fluid is injected at a rate high enough to ensure that gas is not migrating up the annulus. The injection rate and associated annular velocity are designed to stop gas migration to surface and to force any formation gas back into the well – effectively bullheading the gas back into the formation. [4].

Pressurized Mud-Cap Drilling is a time-tested technique to safely penetrate the formations difficult or impractical to drill with other methods. PMCD is widely used in fractured or carbonate reservoirs that experience total fluid losses. Large volumes of sacrificial fluid are required and specialized rig modifications are minimal for PMCD operations. PMCD allows to keep dangerous gasses like H2S downhole, thus considerably enhancing the safety of the project.

Dual Gradient (DG) is the drilling technology that uses or simulates the effect of two fluids of different gradients in the annulus to create dual hydrostatic gradients to better manage the annular pressure profile[5].It refers to offshore drilling operations, where mud returns do not travel through a conventional, large-diameter drilling riser. The returns are either dumped at the seafloor (pump and dump) or returned back to the rig, from the seafloor, through one or more small-diameter return lines. A seafloor or mud-lift pump takes returns from the well annulus at the seafloor and pumps it back to the surface. By adjusting the inlet pressure of the seafloor pump to near seawater hydrostatic pressure, a dual-pressure gradient is imposed on the wellbore annulus, much the same way riserless drilling imposes the seawater hydrostatic pressure in the annulus of the well.

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