DUAL-DENSITY MUD SYSTEM FOR DEEPWATER DRILLING OPERATIONS

Drilling operations in waters deeper than 3,000 ft are increasing throughout the world, and the industry is considering the challenges imposed by ultradeep waters. Leases have been obtained in water depths up to 10,000 ft, with the requirement that they be drilled within the next decade. Use of current technology to drill these leases will require extremely large floating drilling units and large-diameter marine-riser systems.

This topic presents the results of a feasibility study of the use of an automated gas-lift system for a marine riser that will maintain the hydrostatic pressure in the subsea wellhead equal to the hydrostatic pressure of the seawater at the seafloor.

Hydrostatic control of abnormal formation pressures could still be maintained by a weighted mud system that is not gas cut below the seafloor. Such a dual-density mud system could reduce drilling costs by reducing the number of casing strings required to drill the well and reducing the diameter requirements of the marine riser and subsea blowout preventers (BOP’s). The system would have the advantages of riserless drilling without giving up the well control advantages of a closed, weighted mud system.

Current technology relies on the use of marine risers to extend the well up to the drilling vessel so that the mud and tools can enter and exit the well in much the same way as is done on land. However, when drilling with mud densities that exceed seawater densities, the exposed sediments in the open borehole experience a pressure imposed by the full column of drilling fluid extending all the way up to the drilling vessel. This pressure can cause formation fracture.

The overburden pressure is a result of the hydrostatic pressure of the seawater above the mudline and the sediments below the mudline. This causes the fracture gradient, when expressed as an equivalent mud density, to be much lower than for an equivalent casing depth in an onshore well. The reduction in fracture gradients is more significant in deeper waters and imposes a limit to deepwater drilling operations. More casing-string diameters are required to reach a given depth in a formation-porepressure environment.

One way of overcoming this problem is to start the well at the mudline, with the pressure inside no greater than the pressure outside the wellhead. Another way is to inject gas into the riser at the BOP level to lower annular density in the riser to seawater values. This solution method is called the dual density riser system (DDRS) because weighted mud is still pumped through the drillpipe. The system dynamics are closely related to those of gas-lift operations, and a similar set of valves is used to assist in unloading and aerating the riser annulus.

RESEARCH

The concept of a DDRS is being investigated by use of a computer simulator developed to analyze gas rates, pressure and velocity profiles, and unloading times required for offshore operations. Experiments were conducted in a 6,000-ft test well (Fig. 1) to validate the simulator.

Test well
Fig 1. Test well.

Nitrogen was injected from a cryogenic tank into a 1-1/4-in. pipe inside 3-1/2-in. tubing. Drilling mud was pumped into the annulus between these tubulars and mixed with the nitrogen at the bottom. The mixture returned through a 9-5/8-in. casing annulus. Data were collected by sensors inside an auxiliary 2-3/8-in. perforated tubing, also inside the casing. The sensors were monitored with a conventional logging unit.

OPERATIONS WITH A DDRS

The riser should be filled with seawater before drilling operations with gas injection begins. If it were filled with high-density mud before gas injection begins, the casing would not support the added hydrostatic pressure.

Pipe Connections

If gas injection is stopped when circulation is stopped to make a pipe connection, gas migration will cause an increase in the riser bottom pressure because of the gravitational separation of gas and mud. Before gas injection resumes, the gas can rise 900 ft in 15 minutes because the gas-rise velocity is approximately 1 ft/sec. There will be a 900-ft column of dense liquid above the injection point that must be unloaded, creating pressure spikes at the riser bottom. The solution to this problem is continuation of gas injection but at a much lower rate while making pipe connections.

Pipe Trips

The DDRS should be deactivated when the drillstring is pulled. Seawater should be injected in the riser annulus to maintain the same bottomhole pressure (BHP) and displace the mud/gas mixture.

This allows the string to be pulled with the annulus open, avoiding wear on the rotating head at the surface. Displacing the mud in the riser annulus with seawater is not uncommon; however, it takes some time to complete. A typical riser size for a deepwater rig is 171/8-in. inner diameter (ID). For a 5,000-ft water depth, the riser volume is 1,420 bbl. In a rig with the DDRS, a higher pump output and smaller-diameter risers should be considered.

Running Casing

Because running casing is a fairly time-consuming operation that does not normally require circulation, the fluid in the riser annulus should be switched over to seawater. As the casing is lowered into the well, a significant mud volume is displaced into the riser annulus. In the case of a large-diameter pipe, such as 3,000 ft of 133/8-in. casing, the mud volume would be approximately 3,000 ft3, which translates into a 1,785-ft height of heavy mud inside the riser annulus after the run is completed.

To avoid injecting nitrogen while running the casing, special float valves could be used. These valves should allow the casing to be open while being lowered, and, after its setting point is reached, a release mechanism should release flappers in the float valves. Similar float valves have been used in the past in deepwater drilling operations.

Kick Detection

Early kick detection is a challenge in a DDRS. Any technique developed for early kick detection probably depends on sensors installed at the BOP below the gas-injection point. For conventional kick detection based on pit-volume increase, the mud volume in the riser needs to be “totalized” with the active pit volume. This can be based on a measurement of hydrostatic pressure at the bottom of the riser.

For example, for a 171/8-in. riser ID with a mud weight of 15.6 lbm/gal, a 10-bbl increase in the riser mud volume would cause a 30-psi increase in the hydrostatic pressure at the BOP level. In addition, if an influx of 1.5 bbl/min occurs, there would be an increase of approximately 5 psi in the first minute at the bottom of the riser. A microprocessor could be assigned to monitor these small changes and provide early warning.

The literature describes three other systems that could be used. One operates by use of the negative pressure pulse generated by a measurement-while-drilling (MWD) tool as a source signal. This signal travels up the annulus and can be monitored by a sensor at the BOP level. Another method is based on the use of a sonic interferometer installed in an MWD tool. Acoustic waves are generated between two parallel walls, and the system is in resonance at certain frequencies.

Different fluids resonate at different frequencies, and the resonance is not disturbed by fluid flow. By varying the wave frequencies sent between the walls and monitoring the signal through a spectrum analyzer, the resonance peaks are detected. If gas flows through the two walls, the resonance disappears because the medium has changed. These two techniques require that a tool be in the hole and will not send any information unless fluid is moving.

One way to monitor the well while tripping or with the pumps off is to use a wellhead sonar. Acoustic waves are generated at the BOP level and directed down the well, while an acoustic sensor installed in the BOP picks up the sonic reflections. If any gas is present in the mud, it will generate a reflection because of the difference in acoustic impedance between the mud and the gas/mud mixture.

Kick Circulation

Kick occurrence is always possible while drilling any well. Once the invading fluids are inside the wellbore, the problem is how to remove them and return to normal drilling operations. Research is under way to develop a technique for the circulation of a kick.

Well-control procedures are more complex with the DDRS because the mud-column hydrostatic pressure inside the drillstring is greater than that in the annulus; therefore, the well cannot be shut in without greatly increasing the pressure at the casing seat.

When the openhole interval is small, bullheading procedures may be the best alternative. Well-control techniques currently used in underbalanced drilling, in which circulation is maintained after taking a kick, also look promising. The BHP can be increased by reducing the rate of gas injection into the bottom of the marine riser.

CONCLUSIONS

The feasibility of using a dual-density mud system has been investigated. The advantages of this system include the following.

  • Lower BHP and thus smaller fluid invasion and higher penetration rates.
  • Greater safety in the event of emergency riser disconnections.
  • Less top-riser tension requirements, extending operations to deeper water.
  • Economically feasible according to preliminary cost estimates.

The development of this system still requires research on the

  • Kick-detection methods.
  • Kick-circulation techniques.
  • Gas-injection automatic controllers.
  • Unloading-operations dynamics.

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