The versatility of closed-loop drilling methodologies is providing a wide scope for successfully meeting a varying range of operational and economic objectives.
Closed-loop circulating systems are a platform for deploying a broad scope of fit-for-purpose solutions in any drilling environment. Just the addition of a rotating control device (RCD) enhances personnel safety and reduces asset risk. An RCD in place atop the surface BOP or as an integral part of a riser system enables pressure and flow monitoring within the closed-loop circulating system.
The highly sensitive kick-loss detection achieved with a closed-loop system provides a unique and almost immediate window for detecting very small pressure changes due to downhole losses or influxes.
The next step in scalability is managed pressure drilling (MPD), which is achieved by manipulating surface back-pressure via an annular automated choke manifold. Changing choke settings results in a rapid modification of equivalent circulating density (ECD) throughout the well-bore without the cost and effort of changing mud weight.
This control, either manual or automated, opens a toolbox of methodologies for proactively managing wellbore and riser pressures ahead of conventional BOP and mud responses.
Closed-loop methods cut across operational and economic applications to enable the drilling of previously undrillable assets both on land and offshore.
This success is built on years of closed-loop technology application offshore and onshore the Middle East and North Africa – with each application having its own set of operational and economic requirements. For example, land drilling requires a cost-effective MPD solution. Jackup operations present space constraints, and in deeper water, floating structures have to meet unique dynamic positioning, heave, and pressure conditions.
In these varied applications, scalable closed-loop methods provide almost instantaneous detection of influxes and losses as well as the means to control them. The resulting ability to mitigate and minimize pressure-related events effectively and economically is a significant factor in the viability of many wells.
Deep Mediterranean drilling
Drilling in the Mediterranean Sea is challenging. Initially, many in the industry doubted drilling was possible for some of the region’s more difficult reservoirs. Notable success has been achieved with advances in MPD among other innovative technologies.
One of the more recent examples occurred in the Mediteranean Sea on a vertical exploratory well using Weatherford MPD services. During the planning phase, the operator identified drilling problems associated with an extremely narrow drilling window combined with the possibility for lost circulation and kicks due to instantaneous pressure ramps and formation uncertainty.
Weatherford’s MPD system was deployed to eliminate these challenges. The system provided early detection of hydrocarbon influxes and drilling fluid losses and precisely managed wellbore pressure throughout the operation.
The HP/HT well where the technology was applied is the deepest in the area. It was drilled in about 1,000 m (3,281 ft) water depth and reached a total depth of more than 6,000 m (19,685 ft) without any well control or major safety issues in relation to drilling hazards. MPD began at about 4,000 m (13,123 ft) and was used in four hole sections including a sidetrack section.
Within the closed-loop system, MPD methods were used to maintain constant bottomhole pressure (BHP) with a lower mud density and surface backpressure. This improved drilling efficiency, allowed precise control within the narrow pressure margins, and minimized kick and loss incidents.
Closely managing these parameters while drilling and tripping eliminated nonproductive time (NPT) associated with formation drilling problems, resulting in all four hole sections being drilled in record times.
In addition to being the deepest vertical well in the region, it also is the first to reach its reservoir objective. The operator drilled the well deeper than originally anticipated with no downtime related to hole problems.
The MPD system on this well was an advanced closed-loop application that used a Microflux control system. The technology uses specialized software that applies proprietary algorithms to flow and pressure data. The results can be used to inform conventional well control procedures or can be integrated with MPD methodologies to provide a higher level of pressure management.
Coriolis flowmeters provided comparative data on mud flow in and out of the hole to determine any influx or loss of mud. The meters also measured weight density and temperature of mud returns.
In addition to the annular choke manifold used to control surface backpressure, the system included a marine Model 7875 RCD, which contains and redirects annular flow away from the rig floor. In doing so, the RCD closes the circulating loop to provide marine MPD service on fixed, jackup, or floating structures. The RCD is one of a series of Weatherford marine environment models that includes the industry’s first below-the-tension ring RCD.
Using MPD to drill the difficult well achieved many advantages. Drilling efficiency was improved through dynamic equivalent mud weight (EMW) management, which in the closed-loop environment was managed almost instantaneously by adjusting surface backpressure. The traditional cost of time and materials required to change mud weights was eliminated.
Controlling EMW with surface backpressure also reduced environmental risks associated with the oil-based mud. Drilling efficiency was improved with more accurate pore pressure and fracture gradient determination that closely defined the drilling window margins.
Drilling costs were significantly reduced by minimizing NPT related to well control issues. And there were fewer uncertainties due to faster, more accurate downhole pressure information.
Better informed decisions and the ability to quickly identify and manage downhole pressure fluctuations avoided the need to shut in the well. Formation influxes were easily circulated out of the well at drilling rates.
The ability to trap well pressure in order to control EMW during connections resulted in faster connection times of six to 13 minutes. For all drillpipe connections, pressure was captured at about 150 psi to 300 psi by stopping the pumps while closing the annular MPD choke manifold. If pressure dropped to the minimum limit, fluid was flowed across the annulus using a booster pump.
Overall health, safety, and environmental factors were enhanced by the closed-loop MPD system as well. Maintaining constant BHP to prevent formation inflow eliminated unexpected gas influxes.
This deepwater Mediterranean well demonstrated the ability of MPD to drill multiple abnormal pressure zones in HP/HT wells. Drilling performance was enhanced by providing a mud weight closer to pore pressure, which enabled a constant BHP during connections and trips and provided better pressure control while weighting up the mud system.
These achievements are based on implementing a closed-loop circulating system. The change sets the stage for a scalable system of methodologies that reduce risk and enhance performance across a broad scope of drilling applications from land development to the most challenging offshore wells.