by Angelo Calderoni
May 2, 2016

Drilling high-pressure and high-temperature (HP/HT) wells, drilling deep and ultra-deepwater wells, and using extended reach drilling (ERD) may be challenging because of the inherent uncertainties related to downhole pressure limits, pore and fracture pressures, and formation characteristics. Operators may spend an enormous amount of time and money trying to solve problems such as kicks, circulation losses, stuck pipes and low rates of penetration (ROPs). They often come to the erroneous conclusion that the well’s technical limit has been reached and that abandoning the well is the best decision.

Depending on the well, one of the main causes of drilling problems is the interruption of circulation for a connection. In conventional drilling, this stop/start cycle of mud circulation occurs every 30, 60, 90 or 120 feet of hole being drilled. The likelihood of creating downhole problems could be very high.

Continuous Circulation Drilling

In the last few decades, the drilling industry has tried to fight downhole problems related to bottomhole pressure management using managed pressure drilling (MPD) techniques. The MPD techniques may be used to partially compensate for the loss of pressure and equivalent circulating density when circulation is stopped. To maintain the downhole pressure, drilling fluid must be pumped into the annulus against a closed drillstring, and the annulus has to be modeled to calculate how much to pump.

In a continuous circulation system, a downhole steady-state condition is naturally maintained. As a result, the formations do not suffer from pressure oscillations. Hole-cleaning improves, and the ability to pump out of the hole for extended intervals usually means the string can be moved until it is inside the previous casing, reducing the chance of problems in open hole.

One company has created a drilling package that combines continuous circulation, a high-resolution flow rate monitoring system and an anti-friction device. This functionality gives operators the capability to achieve the target in any drilling environment with extremely high health, safety and environmental (HSE) standards and with a drastic reduction of total project time and costs. With this package, it is possible to make connections at full circulating pressure, with drilling fluid continuing to flow through the drillstring after the top drive has been disconnected.

With real-time data acquisition and a model-based software developed using the experience of more than 100 wells, the package’s flow monitoring system can detect kicks and losses precisely, providing a real-time alarm to the driller that can activate well control procedures.

The combination of continuous circulation and flow rate monitoring represents an open MPD loop that can continuously monitor the hydraulic barrier on the well. Unlike conventional MPD, this combination does not require rig modifications because its integration is possible in any rig in operation with low rig-up time.

To reduce friction in the well and the resulting stress on the top drive, the package incorporates an anti-friction device made of special rings integrated with each sub (or in each drill pipe tool joint) that prevent direct contact between the drill pipe tool joints and the casing surface.

Continuous Circulation System Components

The continuous circulation system aspect of the package enables operators to establish a continuous dialogue with the bottom hole, maintaining constant drilling fluid circulation while adding or removing drill pipe stands. This system is composed of a valve, integrated in special subs (see Image 1) or directly integrated in the drill pipe tool joint, positioned on top of the stand. Only the stands that are positioned inside the portion of the well that requires continuous circulation need the subs or special drill pipes.

The clamp allows the valve to open and close in a fully automated way. No-hands operation on pressurized equipment is necessary. Personnel only need to bring the clamp closer to the well center before starting the flow diversion and moving the clamp away when the sequence is completed.

Image 1. Subs used in the continuous circulation systemImage 1. Subs used in the continuous circulation system (Courtesy of Drillmec)

A manifold is installed on the rig floor, close to the stand pipe manifold, diverting flow from the stand pipe manifold directly to the valve. Flow diversion from the standpipe to the valve (and vice versa) is performed without flow interruptions. The manifold design includes the hydraulic power unit and the control system, making the manifold easy to install in any rig thanks to its small footprint.

Changes to the conventional mudflow circuit are introduced only during drill pipe connection and disconnection. This solution guarantees system reliability and efficiency, preventing additional pressure losses in the mud circuit during the drilling phases.

Everything in the continuous circulation system is designed to guarantee high safety standards. A human-machine interface enables the operator to manage the clamp and the manifold from a safe position away from the rig floor. As a backup, the clamp and the manifold can be operated through an electromechanical synoptic panel, which is also placed in a safe position on the drill floor.

Benefits of Continuous Circulation

The benefits of continuous circulation are numerous but vary greatly from well to well, depending on the type of well and the geological conditions. For all wells, however, the main benefits are improved safety, quality of the hole drilled and drilling performance.

The use of a continuous circulation to maintain continuous mud flow while drilling a particular section should have the following safety benefits:

  • elimination of the conditions for connection kicks to occur
  • more accurate detection of kicks in the annulus when penetrating higher pressure zones
  • prevention of a kick up the drillstring during a connection because drill pipe is never open
  • uninterrupted circulation of the drillstring to the bottom if a kick occurs while tripping
  • continuous measuring while drilling/pressure while drilling data transmission without interruption during connections
  • no bottomhole temperature variations
  • significant reduction in temperature-related stresses

In terms of quality, continuous circulation means transportation of the continuous cuttings. This aspect is especially helpful in vertical wells and extended horizontal wells. Continuous circulation provides more effective solids control thanks to the elimination of slugs of solids and solids breakout. It reduces reservoir and hole damage, formation stress and the likelihood of a stuck bit or bottomhole assembly. Systems that use continuous circulation prevent cuttings bed formation and eliminate re-drilling or settled cuttings and debris after making connections. They also improve bit penetration rate.

Continuous cuttings transportation, better hole conditions and well-bore stability allow a new drilling mud rheology designed to significantly increase ROP.

Continuous circulation leads to shorter total connection times and eliminates both mud weight changes while tripping an open hole and circulation time before making connections. Systems using this technology reduce time for testing and handling of connection influx. They also eliminate the need to circulate out accumulated gas after connections in underbalanced drilling wells.

The reduction of total drilling duration, the elimination of mud losses and an increase in equipment lifetime ensure significant drilling cost reductions with an early field production anticipation.