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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.
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.
Bottomhole pressure management is essential to address challenging issues in hostile drilling environmentsbecause of wellbore pressure disparity during pump cycles while in circulating or noncirculating modes. This disparity can cause substantial instability on equivalent circulating density (ECD). While drilling in deepwater, HPHT, or any other hostile environment, these conditions may result in costly nonproductive time, may jeopardize operational safety, or may disrupt the well drillability.
One of the most advanced methods to enhance ECD management during drilling and connection is use of a sub-based uninterrupted circulating system. This technology has been proven to improve drilling performance by maintaining continuous circulation during drilling and connection to retain wellbore pressure stability without significant changes to the rig operation system. This system enables operators to improve drilling performance in wells with a slim pressure window, challenging geomechanical instability, extreme temperature (geothermal), high and long tangent section where hole cleaning is very essential, as well as in extended reach. This system also can be easily integrated with managed pressure drilling technology that exists in the present market.
This system uses a small-footprint, sub-based system with an automated flow switching mechanism to eliminate personnel exposure to the high-pressure system. By maintaining steady state circulation throughout the drilling process, the CFS remedies wellbore pressure disparity to retain continuous circulation and ECD management along the entire wellbore. This condition can effectively assist the operator to reduce nonproductive time, to enhance operational safety, and to enable drilling the well to target depth in a timely and safe manner. This paper will describe the equipment design of this uninterrupted circulating system as well as the best practice and constraints during operation. Generic factory testing will also be discussed to provide comprehensive understanding the reliability of the system.
There is an increasing need to drill difficult reservoirs in a cost effective way. Over the past few years Managed Pressure Drilling (MPD) has made it possible to drill reservoirs which have a narrow window between pore and fracture pressure gradients. Before the introduction of MPD techniques, safely drilling and completing these formations was very costly and not always successful. Cycling the mud pumps off and on for connections affects the pressure and is a major problem for MPD. Techniques and equipment have been developed to make a connection while continuing to circulate the drilling mud to maintain constant pressure. Since 2005, the Continuous Circulation System (CCS) has allowed continuous circulation during connections with traditional jointed drill pipe, by using a chamber around the connection. Several companies have developed continuous circulation subs which are threaded between tool joints to achieve continuous circulation without a pressure chamber. These have had varying degrees of success from both an operational and safety standpoint. This paper describes the current MPD market and describes the philosophy adopted for a new side-entry sub.