Riser margins were abandoned when fluid columns in risers became too long with increasing water depth accessed when floating drilling units with subsea blowout preventers (BOPs) moved to deep water. Removing risers in disconnect events means wells go underbalanced and rely on BOPs to contain wellbore pressure. With the advent of a simple retrofit dual-gradient system based on partially evacuating the riser by pumping mud returns back to surface from an outlet in the riser, riser margin in the form of an overbalanced well can be reintroduced in many cases.
Setting the Scene
Deepwater drilling has a multitude of unique challenges.
- Water depth: Cold temperatures combine with long fluid columns with high pressures at the seabed most often sitting squarely in the hydrate-formation zone. Ocean forces put mechanical stresses on the riser system. Perhaps most challenging, a highly complex hydraulic system, the subsea BOP, must be operated remotely in a very hostile environment.
- Deepwater depositional environment: This leads in most cases to a very tight margin between formation strength and pore pressure.
- Existing emergency procedures: In the case of a loss of position of the rig, a complex sequence of disconnecting from the BOP must be initiated. The BOP also needs to be closed before the lower marine riser package is disconnected and must reliably seal the wellbore because, by removing the riser, the well becomes underbalanced because primary well control by mud weight is lost.
Status Quo
The solutions that the industry has embarked on to address these challenges have often been overly complex evolutionary developments instead of clean-sheet designs, leading to high costs and an unavoidable increase of mechanical failure modes. The best example for this approach is the current generation of subsea BOPs. In their functionality, these devices remain close to the original patent in 1929; but, today, they are very large, complex, and heavy (350 tons) devices that require an enormous amount of maintenance and testing.
Even if a BOP is closed on an inflow in time so the flow is entirely contained below the BOP, circulating out this kick by an established “driller’s method” very often proves to be difficult if not impossible. This is mainly related to two factors. One is the risk of hydrate formation. The moment gas and free water are mixed in the BOP area, hydrates will form. The second factor is the narrow margin between fracture gradient and pore pressure. Very often, the formation is not able to support the backpressure created by the choke line friction, and circulation without losses can never be established.
However, the narrow margin is not detrimental during well control events only. It also leads to drawbacks for normal drilling and cementing. In the worst case in deepwater wells, the moment the preceding shoe is drilled out, equivalent circulating density (ECD) is simply too high to drill a single foot of formation. When circulating, losses are immediately induced, and the overpressure exerted by the mud statically is too small to hold back formation contents, leading to a kick.
Dual-Gradient Drilling
Dual-gradient drilling’s effect in deep water in essence is a technique to equalize or significantly reduce the pressure differential inside and outside the riser at the mudline. It is a managed-pressure-drilling technique that makes designing a deepwater well almost as simple as designing a land well because you can start at zero and follow with the fluid gradient the gap between pore and fracture gradient, paralleling them by adjusting the gradient of the fluid in the wellbore only, not the riser, establishing overbalance without danger of fracturing the formation. This is generally achieved by lightening the fluid in the riser. This then allows the use of a heavier mud in the wellbore. The heavier gradient allows mud pressure to increase more rapidly over depth than it could in single-gradient systems. The mud exerts less pressure at the preceding casing shoe, where it should be lower, and more pressure deeper down, where the mud is needed to control pore pressure.
Riser Margin
A dual-gradient system that has seawater pressure inside the riser at the mudline while maintaining overbalance over pore pressure in the wellbore has achieved, by definition, riser margin. The removal of the riser in the case of a disconnect event, contrary to current practice, would not lead to loss of primary well control, with the well becoming underbalanced and a blowout prevented only by a fully functioning disconnect sequence and a fully sealing BOP. Therefore, what is done when drilling riserless tophole to control shallow water and gas (i.e., having kill mud in the actual wellbore) now becomes technically possible when we are risered up in deep water.
A Simple Low-Complexity Retrofit Dual-Gradient System
A simple retrofit dual-gradient drilling system would be very rewarding. The removal of the drilling-window constraint because of a mud gradient that is too steep in a single-gradient system leads to a breakthrough simplification of deepwater-well design. It not only has the potential to improve the economics of deepwater developments but also can enable drilling wells previously considered undrillable. For this reason, after a successful field trial of the deepwater riserless mud recovery (RMR) system (Fig. 1) in Malaysia, ideas were evaluated to extend this technology to risered operations. Fig. 2 shows a conceptual drawing of the full retrofit system.
The following modifications are necessary:
- Two riser joints have to be modified to accommodate the pumps.
- A winch has to be sited with access to the moonpool. This winch would be similar to the pod line winches for the BOP but would need to carry only some 1,500 ft of power and control umbilical to power the upper pump.
- In case a mud return hose is used instead of the integrated mud return line in the riser, a hose landing platform similar to what is currently used in shallow-water RMR systems needs to be included.
- A handling frame to run and retrieve the docked pump modules, best located in a BOP trolley and combined with the hose landing platform, needs to be built.
- An electrical tie-in to the rig system for the mud return pumps needs to be provided. Most sixth-generation rigs will have enough spare electrical capacity to accommodate this.
- The mud return line needs to be tied into the low-pressure mud system, with provisions for a precision flowmeter.
All of these modifications can be achieved while the rig is operating.
Well-Control and Process-Safety Considerations
The pumped riser dual-gradient system provides opportunities for novel well-control practices. Some intrinsic system advantages will allow much earlier detection of any flow anomaly while drilling than would be possible with conventional operations. The fluid level in the riser is not affected by rig motion, such as heave, pitch, and roll, which makes volumetric kick detection notoriously difficult in floating drilling. The fluid level is kept constant by a set of pressure sensors similar to those used in the RMR system, where it has been proved that the fluid level can be maintained within just an inch or two. Any change in flow is immediately detected by a change in the power consumption of the mud return pumps. Studies performed previously on occasion of the deepwater RMR field trial have shown kick detection capability at less than a barrel. Flowmeters are also part of the system and provide additional calibration.
One of the biggest advantages of the pumped riser dual-gradient system is that the empty part of the riser becomes a giant expansion chamber and, therefore, a perfect mud/gas separator. With the fluid level at the maximum of 3,000 ft, the void volume of the riser is approximately 1,000 bbl. This feature can be used to develop some new well-control procedures.
The overarching objective of these proposed well-control procedures is to maintain or immediately regain primary well control through overbalance exerted by the mud column without ever having to resort to secondary well-control methods unless set limits are exceeded.
If a positive (inflow) flow anomaly is detected, the following measures should be taken:
- Stop mud return pumps and maximize riser boost to increase the fluid level in the riser quickly.
- Leave the drilling pump rate as it is. Stop drilling and space out.
- Monitor both the flow intensity indicator and the volume totalizer to ensure that a set maximum inflow intensity is not exceeded, the inflow is actually decreasing, and a set maximum allowable gain is not exceeded.
- The method of raising the fluid level in the riser with full boost pump rate and normal pump rate on the drilling pumps allows a gain in overpressure of close to 100 psi/min (depending on mud weight in the riser) above ECD because the rig pumps are not shut down. So, any typical deepwater inflow should stop in a minute or two.
Now, the decision must be made about how to treat the inflow in the wellbore. In a series of full-scale experiments conducted in 1986, defined quantities of air were injected below the closed subsea BOP on a floating rig in 3,000 ft of water. The BOP then was opened, and the behavior of the riser filled with 13.2-lbm/gal water-based mud was observed. Fig. 3 shows the setup used for these experiments. The effect of smaller quantities of air (10 or 20 bbl at depths yielding the equivalent of up to 5,000-bbl surface gas volume) was a stringing out in the riser and only foaming and bubbles at the riser top, with no unloading of the full riser or any slugs.
The observed limited effect of small quantities of gas at depth is applicable only for long fluid columns such as those in deep water. On a shallow-water floating rig, gas below the BOP would have an effect similar to that of the gas at the shallow kickoff valves in a gas lift system and could very well flow spontaneously.
These considerations lead to the proposal of the following treatment of the inflow volume after the flow anomaly has been stopped:
- When the material balance of flow in again equals flow out, increase the riser level by another 150-psi overbalance (or whatever company procedures require).
- Commence circulating the well through the drillstring at normal pump rates. Monitor the pump strokes until the tail of the inflow is well past the BOP while the tip is still at least 3,000 ft below the fluid level in the riser.
- Stop pumping through the drillstring, space out, and close the BOP to isolate the wellbore from the riser.
- Switch on the mud return pumps, and reduce the fluid level to the lowest technically possible level, therefore allowing the maximum void in the riser, serving as expansion chamber and mud/gas separator.
- Boost the riser at a slow rate of 200–300 gal/min to give the inflow maximum time to string out, while operating the mud return pumps to keep the fluid level constant. Do so until 1.5 “bottoms up” in the riser has been achieved. Because the suction point will be very close to the fluid level, gas will break out well below and there is no danger that gas in solution will enter the mud return pumps.
- Alert the mudlogger that he may be able to recover some valuable gas/formation-fluid samples.
The key to success of this proposed well-control practice is very early kick detection from the superior detection abilities of this system, immediate and dynamic control of the flow anomaly, and then a rapid clearing of the wellbore and the riser of any inflow so the risk of loss of wellbore is minimized.
This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 22889, "A Step Change in Safety: Drilling Deepwater Wells With Riser Margin," by Robert Ziegler, SPE, Petronas, prepared for the 2012 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. Copyright 2012 Offshore Technology Conference, Reproduced by permission.