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Continuous circulation technology is a form of managed pressure drilling (MPD) where the constant circulation of drilling fluid is maintained by allowing the rig pumps to remain on while adding and removing stands of drillpipe. Using continuous circulation, many drilling challenges are solved through continuous hole cleaning and solids transport, including maintaining constant bottomhole pressure (BHP), reducing the loading of solids and cuttings bed formation resulting from a pumps-off state, mitigating stuck pipe incidents, reducing ballooning effects, providing continuous cooling of bottomhole assembly (BHA) components, and improving formation integrity. Continuous circulation technology was used extensively for its hole-cleaning benefits during a 17-month development, gas drilling, and oil drilling campaign on a deepwater drillship in offshore West Africa from 2015-2016. Operations restarted in 2018 and are ongoing in 2019.
Deepwater drilling using continuous circulation in this field provided a solution for significantly helping improve the rate of penetration (ROP) through continuous hole cleaning when drilling one stand per hour or faster. With constant circulation during the drillpipe connection process, the operator was able to significantly decrease the amount of time spent on hole-cleaning activities after a stand was drilled down in both oil-based mud (OBM) and water-based mud (WBM) applications. The enhanced hole cleaning provided by continuous circulation also helped improve cuttings transport efficiency, mitigate stuck pipe incidents, and enhanced the ability to drill a clean hole.
This continuous circulation system was used on a deepwater drillship in offshore West Africa on 16 wells from 2015-2016, following two wells drilled without this technology, and four wells after the restart in 2018. Overall, more than 535 operating days and more than 1230 successful continuous circulation drillpipe connections were performed during both drilling and tripping operations. Because of the increased ROP provided by continuous hole cleaning with the system, approximately five drilling days were saved per well, totaling to an approximate 85 drilling day reduction over the planned drilling campaign from 2015-2016, resulting in a significant cost savings to the operator and to obtain first production much sooner than anticipated.
Continuous circulation technology has proven to reduce overall nonproductive time (NPT) and total drilling days. During a long-term drilling campaign, additional improvements were recognized with superior prewell planning and front-end engineering design to minimize interference with rig operations and equipment and improve understanding and communication between the service company and operator. A newer generation, dual-activity rig helped prevent downtime by providing a platform for continuous repair and maintenance activities while drilling.
Increasingly, rigs using continuous circulation hold the requirement that all personnel be removed from the rig floor red zone area during single-pressure barrier exposure applications. To address this challenge, a new and enhanced semiautomated connection system was developed that enables an operator to deliver a connection tool to the continuous circulation sub by overhead lift or manipulator arm. Using a remote human machine interface (HMI), the system automatically removes a threaded cap, makes a flowline connection to the side port, automates the flow diversion process while a new stand is connected, and isolates the side port once the connection and flow diversion process is complete--all without the operator's physical interaction. The new system removes personnel from single-barrier pressure environments, helping improve safety to rig floor personnel and, because of the addition of automation, increasing system performance and repeatability to reduce nonproductive time (NPT) and rig drilling days.
Continuous circulation drilling technology has been used significantly in the Mediterranean during both shallow and deepwater offshore applications to address various drilling challenges. Continuous circulation is a variant of managed pressure drilling (MPD) in which mud pumps remain on, thus providing constant circulation of drilling fluid downhole through the wellbore and up the annulus during the process of adding and/or removing stands of drillpipe.
From 2014 to January 2019, the continuous circulation system was successfully deployed in offshore Egypt on 20 wells, totaling more than 1,785 successful continuous circulation drillpipe connections. During the same 5-year period, two offshore wells in Cyprus were drilled, adding more than 305 continuous circulation connections. This followed a separate extended campaign from 2006 to 2012 that used continuous circulation technology to drill 13 wells.
The use of continuous circulation to drill on both fixed platform and floating vessels in the Mediterranean has provided a solution for navigating tight drilling windows. For example, this technology was used during the exploratory phase of a project, which assisted an operator in reaching total depth (TD). This project resulted in the discovery of a significant natural gas field, where operation continues to this day.
Overall, lessons learned from the work in the Mediterranean include continuous circulation providing enhanced hole cleaning and improving cuttings transport efficiency, which helped in mitigating stuck pipe incidents in an extremely long horizontal section. In wells with extremely narrow drilling windows, continuous circulation was used in conjunction with standard backpressure-control MPD to provide an enhanced level of control and automation. In other wells, 8-in. continuous circulation subs were deployed to provide the capability for a 1,200-GPM circulation rate. Between drilling campaigns, onsite repair, maintenance, and recertification activities reduced transport costs and allowed the system to remain prepared for further operation.
With the combination of a proper drilling fluid design and continuous circulation during drillpipe connections, the operator can maintain a constant bottomhole pressure (CBHP) necessary to navigate wells with a narrow pore pressure and fracture pressure margin. By eliminating the large pressure swings caused during conventional drilling when cycling the pumps on and off, influx or fluid loss situations can be reduced or eliminated.
With a long and positive history, continuous circulation technology has helped eliminate a variety of drilling challenges by increasing operational efficiencies and reducing overall nonproductive time (NPT), all leading to a reduction in drilling days, and therefore, operator costs.
Continuous circulation technology is used to maintain constant circulation of drilling fluid in a well by enabling the rig pumps to remain on during all steps of the drillpipe connection process. Continuous circulation is a managed pressure drilling (MPD) technique; it improves drilling success for difficult high-pressure high-temperature (HPHT), narrow pore pressure/fracture gradient, and extended reach horizontal wells.
Traditionally, a continuous circulation system relies on a manifold connected into the rig standpipe, which diverts flow to and from the topdrive to a side port on a sub that is threaded into the top of each drilling stand. Historically, this side port flowline is connected manually by an operator, within the rig floor red zone, in a single barrier pressure environment. To enhance safety by removing exposure to any single barrier pressure applications, a new system was developed that automates and enhances the current manual process.
The automated continuous circulation system includes a connection tool that is mounted on a manipulator arm; after it is delivered to the drillstring and clamped, it will use a human machine interface (HMI) to automatically and remotely remove a threaded side port safety cap, connect the side port flowline, and control the manifold flow diversion process. The system is controlled at the HMI by an advanced software system that is capable of functioning autonomously, with operator verification steps. Internal robotic mechanisms drive the system to perform the exact steps as a human without requiring modifications to the proven continuous ciruclation sub design, all while providing instantaneous feedback to the operator located at the remote HMI.
A prototype tool was assembled and successfully tested in November 2016 in the Val D'Agri oilfield region in southern Italy. With a rapid technical development cycle of less than one year in a down market, a commercial tool was developed and deployed, including the implementation of all lessons learned. This system is the first in the industry to provide threaded engagement of the side port flowline, and a successful undermount delivery arm. This paper presents results from more than 1,000 diversion connections in both laboratory and field environments.
With a 10-year track history of the manual system, the automated system enables the operator to improve upon proven technology to safely deploy continuous circulation capabilities in offshoreapplications, from fixed platforms to floaters, in areas with strict industry certification regulations, personnel in red zone limitations, and double pressure barrier requirements. The system reduces overall added connection time from typical manual systems, increases safety, and maintains the benefits of continuous circulation to reduce nonproductive time (NPT) and total drilling days.
The Perla Gas Field is a naturally-fractured carbonate formation in offshore Venezuela that often requires advanced technology to complete and optimize drilling operations while also minimizing nonproductive time (NPT). To overcome the challenges and reduce NPT demonstrated on a well drilled in 2010 in the Perla field, continuous circulation and managed pressure drilling (MPD) technologies were used to drill two subsequent wells in 2016.
Drilling in the Perla field is a challenge because of its narrow pore pressure and fracture pressure window, which increases the potential for issues such as lost circulation, kicks, wellbore instability, and connection gas. To overcome challenges encountered in 2010 while also decreasing NPT, a combination of MPD and continuous circulation technologies were selected to drill the wells. To navigate the tight drilling windows, MPD was selected for this campaign, with continuous circulation implemented to supplement the MPD process by removing any transient time or pump cycling effects during connections.
Continuous circulation allows the operator to circulate fluid downhole during the drill pipe connections. The system includes a manifold connected into the standpipe that performs the diversion, and subs installed onto the top of each stand of drill pipe, creating two independent flow paths for drilling an open hole section. MPD creates a closed loop system and allows the operator to safely add or remove backpressure at the surface in order to control bottom hole pressure (BHP).
In Perla, a well was drilled in 2010 with significant losses as a result of an unexpectedly narrow drilling window of 0.5 ppg equivalent mud weight between pore and fracture pressures (9.8 to 10.3 ppg, respectively). In 2016, the new combined solution enabled drilling of the lateral in both wells by maintaining a constant BHP during all phases, mitigating influxes or losses, removing static mud pressure spikes (breaking gels), reducing connection gas, and improving hole cleaning. A statically underbalanced 9 ppg fluid was used while targeting a 10.2 ppg equivalent mud weight throughout the length of the 8½ in lateral on more than 150 drill pipe connections to decrease tripping time, get liner to depth, and improve the estimated time to total depth, decreasing overall risk, NPT, and cost.
In exploratory and development wells, risks increase when different than expected well conditions are encountered during drilling. This case study provides an example of the viability of combining MPD and continuous circulation, a solution which is highly applicable in offshore Latin America, including but not limited to Venezuela.