Quick detection allows kicks to be circulated out without closing subsea BOP
By Brian Grayson, Weatherford
Mounting deepwater experience suggests riser gas may be a problem that has seen its day—at least from a technology perspective. By precisely detecting and controlling gas influxes, managed pressure drilling (MPD) can effectively mitigate the risk of gas in the marine riser, where it is beyond the reach of conventional well control.
The solution is both proactive and reactive. Within a closed-loop drilling system (CLD), MPD severely limits the occurrence and volume of an influx as a matter of standard operation. The potential for riser gas is reduced accordingly. MPD control also prevents the gas from dissolving into the mud system only to break out of solution higher in the hole or riser.
If gas enters the riser, MPD provides a controlled means of circulating it out of the well. A growing deepwater experience base supports the effectiveness of the methodology. However, use of the emerging deepwater application is constrained by the availability of specialized MPD equipment and of rigs able to provide the prerequisite CLD system.
The dividing line
From an MPD perspective, a hole in the water is the same as a hole in the ground. Gas is detected and managed the same wherever it occurs in the fluid column, from wellbore to marine riser.
This is not the case for conventional circulating systems that are open to the atmosphere. The dividing line for these systems is the BOP on the sea floor. Once in the riser, gas is beyond the reach of this primary well control element. Response is further compromised by the difficulty of effectively detecting gas coming out of solution in the riser.
Conventional riser gas handling provides limited control of an impending pressure event: When mud gains are noted, the BOP is shut, the diverter is opened, and returns are routed away from the rig.
Starting at the beginning
MPD ideally prevents riser gas from occurring in the first place and, in fact, this is the case with many wells where the technology has detected and mitigated kicks that could have otherwise resulted in riser gas incidents.
With the greater precision of MPD, an influx is detected immediately and control measures are initiated before the gas dissolves into the mud.
Because the influx size is so small, MPD also provides the ability to safely circulate it out of the hole at high rates. This option is available when surface pressure and gas breakout are within the limits of the MPD and rig riser systems. Provided the kick size and intensity levels are within pre-calculated and engineered levels, an influx or kick can be circulated out of the system without closing the subsea BOP.
For this reason, a key step in deepwater MPD engineering is the application of kick modeling software to determine the size and intensity limits of the system. The advance modeling enables real-time mitigation that allows a micro-influx to be circulated quickly.
Entrained gas problems
While any influx or kick presents the potential for gas entering the riser, entrained gas is the most problematic. Due to many factors, including kick intensity, size and type (water, oil, dry gas, methane), gas can be entrained in the fluid system downhole only to break out of solution somewhere above the subsea BOP and below the rotary table.
Laboratory tests have demonstrated that gas solubility and migration in oil-based mud is a significant phenomenon from a well control perspective. Riser gas is a common result.
Gas migration related to natural gas influxes in synthetic-based mud was examined in a 5,884-ft deep vertical test well at Louisiana State University’s Petroleum Engineering Research & Technology Transfer Laboratory. The tests used Weatherford’s Microflux control system and GC Tracer gas chromatograph.
Natural gas was injected at the bottom of the test well that approximated bottomhole conditions. The well was shut in, and pressure readings were monitored. The observations confirmed the solubility of natural gas in oil-based fluids. Although it is generally believed that migration does not occur after gas goes into solution, the tests noted significant migration of dissolved gas.
Gas solubility was confirmed through a rapid drop in shut-in casing and drill pipe pressures. Gas migration was also observed. In a 44-hr shut-in test, gas was immediately observed at the surface when the well was opened. A second 5-hr test found gas had migrated more than halfway up the well.
The gas chromatograph, used to evaluate gas volume and composition at the well site, provided useful surface data when the multiphase flow reduced the reliability of the flow meter readings. A significant increase in lighter, methane hydrocarbons was observed when gas was being circulated out.
Riser gas solution
Based on Weatherford modeling of worst-case methane gas behavior, breakout typically occurs at 2,000 ft to 3,000 ft below the rotary table. This becomes an issue in deepwater because the gas will break out above the subsea BOP, which is typically 5,000 ft to 6,000 ft below the rotary table.
Whether gas comes out of solution 100 ft below the BOP or 100 ft above the BOP, the MPD solution is the same. Safety dictates that the potential of having gas occur above the BOP requires BOP closure, whether drilling conventionally or with a closed-loop system. However, MPD methods enable a more informed and precise response.
Once the well is closed, the MPD/CLD system contains and controls the gas breakout so it can be released from the riser and routed through a high-rate gas-handling system. With the BOP closed, the MPD system quickly indicates whether the gas is above or below the subsea BOP. If the kick is below the BOP, then the MPD system will no longer see it at the surface. If the gas is above the BOPs, gas expansion will still be seen at the surface.
While the use of MPD in deepwater is a recent development, applications in West Africa and Indonesia show how these systems can prevent riser gas.
Weatherford’s SeaShield Model 7875 Below-Tension-Ring (BTR) rotating control device (RCD) is made up below the tension ring and integral to the riser to support operations from dynamically positioned vessels. It was first deployed in 2010 in Indonesia. In the eight rank wildcats drilled so far with the system, there has been no instance of formation gas breaking out of solution above the BOP and inside the riser. In eliminating riser gas, MPD control algorithms have quickly detected flow anomalies, minimized them and circulated them out.
A riser-degassing event has yet to be performed, and there has been only one instance akin to riser gas handling when an influx was detected and circulated out through the riser. In this instance, the subsea BOP and MPD annular BOP were closed, and the fluid system above the BOP was circulated out through the automated MPD choke manifold and the high-rate mud gas separator. The mud in the riser was then weighted up, the BOP was opened, and the rest of the well circulated to a higher mud weight while managing pressure with MPD.
In the deep waters offshore Ghana, MPD control allowed the identification and handling of influxes before they developed into riser gas. The lack of riser gas problems in this difficult well further demonstrates the effectiveness of the methodology.
While conventional drilling had failed in two attempts due to pressure-related wellbore problems, MPD enabled the well to be drilled without borehole instability issues, underreaming or contingency liners.
Riser gas was routinely mitigated. The MPD system identified small influxes and losses while drilling and applied annular backpressure to control and circulate them out of the hole. Backpressure was also applied when pulling out of the hole to prevent swabbing and maintain wellbore stability.
In addition to the BTR-RCD, the system included equipment below the RCD. An annular preventer and flow spool were required to facilitate changing the RCD sealing elements while continuing to circulate and hold pressure on the well.
An MPD engineering study examined the circulation system to identify optimum mud weights in each well section and establish system limits in handing kicks. The company’s kick simulation and analysis software was used to examine pressure loadings on the casing shoe, choke line, RCD, riser and mud-gas separator to ensure the pressure capacity needed to safely circulate an influx out through the MPD choke line.
The pace at which the industry can adopt this level of riser gas prevention and handling, as well as other CLD-enabled capabilities, depends on overcoming deployment constraints aboard floating drilling structures. This process is constrained in part by the ability of semisubmersibles and drillships that were originally built for conventional, open-to-the-atmosphere circulating systems to readily accommodate a CLD system.
It requires more than a drill string isolation tool and flow spool to make a rig CLD-capable. Additional considerations in upgrading existing rig systems may include higher volume mud-gas separator, higher circulating rate capabilities and riser pressure capabilities. Any existing architecture constraints of the telescopic joints, ball joints and tension rings also must be addressed.
This lack of CLD readiness is also seen in physical parameters, such as riser component configurations and IDs, rotary table IDs, deck requirements to accommodate MPD equipment, as well as the availability of experienced personnel and basic cost efficiency. Ultimately, this creates a bottleneck in the deployment of MPD equipment.
In newbuilds, these considerations will become part of the design parameters and requirements and provide part of the solution to MPD deepwater capacity. Increased availability of MPD capabilities may also be driven by governmental mandates and by efforts between E&P companies, drilling contractors and service companies to create a CLD infrastructure. The standardized integration of a riser degassing system with the ability to incorporate an RCD would provide a “plug and play” MPD capability.
This process also involves transferring ownership of the CLD-enabling equipment – a riser degassing system with RCD body – to the drilling contractor. This would require a new set of competencies for the drilling crew but would enhance their efficiency. Bridging these competencies would allow service companies to focus on the specific subsurface-related problems defined by the drilling engineering team and enable the application of CLD methodologies.
With the solution to riser gas confirmed in many wells, it is increasingly incumbent on the industry to incorporate CLD systems and MPD methodologies as deepwater best practices. These changes to technology, infrastructure, training and even business practices are key steps to significantly reduce the risk of riser gas, as well as improving the safety, efficiency and capabilities of deepwater drilling.