Revamping older offshore rigs with modern technology positions them for more efficient and safer drilling to target depth. Weatherford’s James Onifade and Turkish Petroleum Oilfield Services’ Yilmaz Kum and Benhur Sahin explain.
RCD rig up on the BOP stack.
In the current low commodity price climate, avoiding non-productive time (NPT) is increasingly important, especially when drilling offshore to recover hard-to-reach resources. Many scenarios can prompt a decision to shut in or kill a well due to common pressure and safety concerns. However, NPT is avoidable with the introduction of modern, automated-control systems into the equipment configuration and operations of a rig, which enables precise navigation to drill more safely and efficiently.
Case study: Pressure challenges in narrow-margin well
On behalf of operator Turkish Petroleum Corp. (TP), Turkish Petroleum Oilfield Services (TPOFS) needed to drill through narrow margins within the confines of the pore and fracture pressures in the Mediterranean Sea using a jackup drilling rig built in 1988. The operator knew that the well could not be drilled by conventional methodologies. The major challenge was to manage the bottomhole pressure (BHP) within close tolerance without jeopardizing the safety of personnel, environment or equipment.
Weatherford’s managed pressure drilling (MPD) system was chosen as the tool that would enable the drilling operation to “walk the line” between pore and fracture pressures while maintaining the BHP within acceptable limits. The goal of integrating the MPD system into the existing rig system was to more precisely control the annular pressure profile throughout the wellbore and to maintain wellbore stability, especially during connections and other pump-off events.
The MPD components had to be integrated into the layout of the existing rig system in a manner that maintained optimum operations without adding confusion to the drillers’ routine. To solve that challenge, training was provided to ensure that the rig crew was comfortable with the new equipment features and understood how MPD worked in conjunction with their standard equipment makeup.
Rotating control device (RCD) – This device, installed on top of the rig’s annular blowout preventer, creates a closed-loop system with returns directed from the well to the MPD choke manifold through connected flow lines, as opposed to open-to-atmosphere techniques. The RCD contains a bearing assembly, which is locked in place during operations. While the drill pipe rotates, a passive seal is maintained around the pipe via two sealing elements. This enhancement enables accurate measurement of flow-out parameters and provides protection for the rig personnel on the surface.
Automated MPD control manifold – Two fully automated drilling chokes, along with a Coriolis mass flowmeter and a hydraulic power unit, manipulate the choke position. For this operation, the choke manifold was spotted on the starboard side of the cantilever deck and was connected to the RCD with a 6in co-flex main flowline hose. Throughout the drilling operation, the MPD system software recorded critical real-time data, including the flow in and out of the well, fluid density, temperature and volumetric flow rate.
With the bearing assembly installed in the RCD, an additional auxiliary line was put in place to ensure that the trip tank could also be used for flow-checking the well – allowing flow across the well, through the MPD manifold flowmeter, and back to the trip tank.
An automated MPD control system identified a “gas at surface” event. This screenshot shows an increase in flow out of the well (first column) with a corresponding decrease in mud density (second column).
Once tested and calibrated, the MPD system was used to drill and enlarge four distinct hole sections, ranging from 17.5in to 8.5in open hole sizes. Each hole section presented unique challenges, including influx and loss events, the need to maintain constant BHP during connections, and the placement of a heavy mud cap while stripping the drillstring out of hole and background gas from the formation.
During operations, many of the innovative MPD features helped alleviate adverse conditions. For example, the annular pressure control mode of MPD system automatically displaced the heavy mud cap at specific depths, which maintained static overbalance when the drillstring was out of the hole. An influx of water into the wellbore was detected early enough to exercise the necessary control and safe stripping back to the bottom and to circulate the water out of the wellbore.
Also, the automated MPD system clearly identified a “gas at surface” event and eliminated the need to shut in the well altogether. Using conventional drilling methods, an increase in flow out could have been mistakenly interpreted as a kick, leading the driller to shut the well in unnecessarily.
The MPD system was also used in freeing a differentially stuck drillstring. While drilling the 8.5in section with a mud weight of 16.5ppg, the MPD system applied roughly 550psi SBP to maintain an ECD of 18ppg. After an unsuccessful jarring operation, the MPD system reduced the ECD below the formation pore pressure in a matter of seconds. The string was freed and the induced influx was safely circulated out of the wellbore in a controlled manner.
The MPD system also provided early identification of pump washouts. By monitoring flow out and using the loss detection capability of the Coriolis flowmeter, pump washouts were correctly identified before reductions in SPP were even observed.
Overview of MPD value
This case study demonstrates the successful incorporation of an MPD system into an existing, older rig system with minimal disruption to rig operations and equipment. The advanced equipment capabilities of the MPD system included early detection of kick and loss, identification of surface gas events, placement and displacement of heavy mud cap, dynamic pore pressure and leakoff testing, and maintaining constant BHP during drilling and connections.
Throughout the drilling process, the system provided clear trends and crucial information that enhanced operational efficiency and safety. Ultimately, the operator was able to drill effectively to the target depth, thanks to the proven ability of the MPD system to “walk the line” between the pore and fracture pressure limits.
James Onifade is a project engineer responsible for planning, executing and ensuring a safe and efficient application of MPD technology. He holds a bachelor’s degree in mechanical engineering, 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. He is a Chattered Engineer with the Institute of Mechanical Engineers IMechE.
Yılmaz Kum has over 12 years’ deepwater, shallow water and land rig drilling operations experience as a drilling supervisor and engineer. He has bachelor’s degree in petroleum engineering from Istanbul Technical University. Currently, he is an operations manager, responsible for drilling and completions in Iraq.
Benhur Sahin has seven years of experience in the oil and gas industry. He has professional experience in drilling fluids and drilling engineering disciplines. He has a bachelor’s degree in petroleum engineering from the Istanbul Technical University, and is currently studying for his master’s degree in software engineering at Middle East Technical University.