Influx detection

James Onifade discusses how the first managed pressure drilling operation for an African operator accurately detected and circulated a small influx out of the well, without having to shut it in or threatening the safety of the rig crew.

The MPD rotating control device (RCD) on the BOP.  Images from Weatherford

In their quest to reach hydrocarbon resources found in deeper waters and in reservoirs with narrow and rapidly changing pressure profiles, offshore drillers are continually searching for technologies that offer greater efficiency and safety.

Managed pressure drilling (MPD) is ideally suited to meet these needs. An adaptive drilling process, MPD precisely measures and controls the annular pressure profile throughout the wellbore during the drilling process. The technique possesses a unique ability to carefully “walk the line” between pore and fracture pressures to prevent the uncontrolled influx, or kick, of reservoir fluids into the wellbore and up to the surface of the drilling rig.

Safe and secure control

MPD provides considerable safety advantages over conventional overbalanced drilling techniques, which control the bottomhole pressure (BHP) in a well by varying the drilling mud weight to obtain the necessary hydrostatic pressure that will counter the pore pressure of the formation.

Increasing or decreasing the mud weight is an expensive and time-consuming process that cannot efficiently manage rapidly changing differences between the BHP and pore pressure. MPD techniques use a series of chokes that dynamically control the annular pressure by manipulating the surface back pressure (SBP) to increase the BHP. Choke position can be varied in a matter of seconds, a significant improvement compared to the hours required to circulate and condition the mud weight throughout the wellbore in overbalanced drilling.

MPD also provides considerable safety and environmental benefits. Return flow is diverted away from the rig floor to a dedicated choke manifold by the use of a rotating control device (RCD), thereby reducing accompanying hazards to rig floor personnel.

Flow measurement is critical to the MPD system, and is achieved through a mass flowmeter installed downstream of the chokes on the MPD manifold. The mass flow meter provides accurate flow readings and enables early kick detection. Due to the fact that influxes can be detected and controlled earlier and the annular pressure profile manipulated as required, casing setting depths can be extended. Industry studies suggest that the proper utilization of MPD may lead to the elimination or extension of casing setting points.

Equipped with a mass flow meter, the MPD manifold is used to achieve precise control of a wellbore’s pressure profile while drilling.

Influx control offshore Africa

An operator working off Africa employed the MPD system for the first time, providing early kick detection capabilities and enhancing the ability to manage any influx occurrences into the wellbore during drilling operation. During a coring operation ahead of drilling the 8 ½-in. hole section of a well, the MPD system maintained a 16.2 ppg bottomhole equivalent circulating density by applying approximately 260 psi SBP. After the coring operation, the MPD system maintained 16.2 ppg (645 psi SBP) while the string and core were pulled out of hole, which kept the well in a static flow condition.

Drilling the 8 ½-in. hole section commenced, with the MPD system maintaining 16.1 ppg (approximately 590 psi SBP) while making up connections; this SBP would maintain the BHP above the pore pressure to compensate for any lost annular frictional pressure experienced during connections. A sudden increase in the flow out of the well was observed on the flowmeter while drilling. There was no change in mud density out of the wellbore, which indicated that a uniform mud density was still flowing through the flowmeter. An increase of about 20 psi was also recorded on the standpipe pressure, confirming the possibility of an influx invading the wellbore.

MPD screenshot demonstrates flow into the well and increase in flow out of the wellbore, with the SBP required to control the influx visible in the third column.

Once an influx invasion was confirmed, the control sequence was initiated using Microflux, the MPD system’s influx control sequence, to gradually increase the SBP in order to control the influx of reservoir fluids into the wellbore. The well control sequence was activated within two minutes of the observed increase in flow out. The volume of influx when the well control sequence was activated was measured as 1.42 bbls (60 gals).

The SBP was increased from a system friction pressure loss of 160 psi (16.1ppg ECD) to about 380 psi. The well was observed to be flowing and SBP was further increased to about 560 psi (16.7ppg ECD) in order to control the influx. Once equilibrium between flow in and out of the well was achieved and while circulating at the full drilling rate of 300 gpm; the influx was circulated out of the well following the first circulation of the driller’s method.

The influx was fully controlled in a total period of about 13 minutes. The total volume of gained from the influx was approximately 3.2bbls (140 gals).

Gas was circulated out of the well for more than 30 minutes, until the gas reading resumed the normal pre-influx level. The density trend of the drilling mud and the flow signature also resumed normalcy, indicating that the gas influx had been completely circulated out of the wellbore. A dynamic pore pressure test was then performed to obtain an indication of the pore pressure within the rock formation, and the MPD system was utilized to maintain the BHP above the pore pressure with an additional overbalance of 0.3ppg.

Conclusions

The capability of the MPD system to detect and automatically control influxes is of immense value to the drilling operation. The system utilized the automatic activation of the influx detection and control sequence to provide a faster reaction time and ultimately improve safety and operational efficiency. This faster reaction led to smaller influx volume and reduced surface pressures during the influx circulation operation. The MPD system allowed the influx to be circulated at the prevailing drilling rate, avoiding the need to shut down the pump and circulate at the slow circulation rates. Ultimately, MPD allowed for safe and efficient influx detection and control, with a reduction in nonproductive times recorded during drilling operations.

References

1.) Rohani M.R., “Managed-Pressure Drilling; Techniques And Options For Improving Operational Safety And Efficiency.” Sharif University of Technology, Tehran, Iran. Received September 19, 2011, Accepted January 5, 2011.

James Onifade is a wellsite drilling supervisor with the main responsibility of ensuring a safe and efficient application of the MPD technology. Onifade earned a bachelor’s degree in mechanical engineering in 2002. Onifade also holds a master’s degree in mechanical engineering from the University of Portsmouth and a master’s degree in oil and gas engineering from Robert Gordon University, Aberdeen.