Influx management strategy enables automated MPD response

As managed pressure drilling (MPD) gains traction in both land and offshore applications, recent technological advancements for these systems have made this drilling technique more efficient and effective at the time of controlling and circulating influxes out of the wellbore. To this end, one area of advancement has been automation. Today, most MPD systems are able to automatically detect influxes—formation fluids that enter the wellbore while drilling—to prompt the manual application of surface backpressure (SBP) to restore overbalance and enable drilling to continue unimpeded.

Going further, a select group of advanced MPD systems not only automatically detects volumes of influxes as small as 1 bbl, but also automatically restores overbalance in the well by increasing SBP until return flow equals in-flow, and then circulates the influxes out at full rates. These MPD systems control and eliminate influxes on their own, before they become disastrous well-control events. This development is a significant step forward for the industry, as it can prevent costly and time-consuming shut-in scenarios in which the well is shut in at the blowout preventer (BOP), and influxes are circulated using the rig’s choke. MPD is often used in some of the world’s most obstacle-laden wellbores—including slim wellbores, ultra-deep wellbores, HPHT wells, and wells with tight drilling windows and volatile pressure profiles—where avoiding shut-ins provides a major advantage. 

Although MPD systems were not originally intended to be a substitute for, or even a part of, any secondary well control equipment, some MPD systems possess the capability and algorithm to restore required overbalance, when an influx is detected. 

One example of a fully-automated MPD system is the Weatherford Microflux control system (MFC). Comprising a rotating control device (RCD), flowmeter, hydraulic drilling chokes, and an intelligent control unit, the contained, closed-loop system continuously monitors and analyzes return flow. If an influx is detected, the system uses proprietary algorithms to automatically adjust bottomhole pressure by closing the chokes to increase SBP. This quick and early response minimizes the size of influxes, from barrels to gallons. 

MFCs used today have the capability to prevent shut-in scenarios by automating the influx-handling process. The influx management envelope (IME) or MPD matrix traditionally defines the limits within which the MPD system can potentially be used safely to control and circulate these
small influxes.

The system and other fully-automated MPD systems optimize drilling efficiency and safeguard well integrity. By eliminating the need for rig personnel to manually control the well, they also enhance safety and lower costs. Additionally, these MPD systems decrease non-productive time by eliminating the need for flow checks, BOP closures, and other operational delays inherent in conventional well control. 

TRANSITIONING FROM TRADITIONAL MPD TO INFLUX MANAGEMENT ENVELOPE 

The MPD operational matrix was developed primarily to allow the utilization of an MPD system to control and remove small influxes from the wellbore—circulating them out through the MPD system without exceeding the pressure limits.  These systems can handle each scenario highlighted in green, yellow, and orange without exceeding the pressure limits of MPD equipment. For the scenarios highlighted in red, the well would need to be shut in at the BOP and the rig system would take over. This matrix in Fig. 1 considers (and it would be the main limitation) only one influx volume, defined by the “planned limit” of the “influx indicator” column and one “kick intensity,” illustrated by the “backpressure limit” described in the “surface pressure indicator” row. This is operationally restrictive and could lead to less efficient use of
MPD equipment.

Fig. 1. The MPD drilling matrix describes how MPD systems can circulate and remove small influxes from the wellbore, albeit with technical limitations.

But what if a broader operational envelope—an influx management envelope (IME)—could be identified? The IME, which was first illustrated by Weatherford in 2014, and initially called the ISBP-Volume envelope, proposes that the influx volume and pressure limitations are better represented by an “envelope” rather than one single pressure and influx volume limitation that could be circulated out safely, using the MPD system without shutting the BOP.

According to the IME methodology, influx volume and pressure limitations are better represented by a range than any single figure. Because it expands the operational parameters for MPD systems, the IME is poised to replace the traditional matrix going forward.

IME APPROACH FOR MPD ENGINEERING PLANNING, INTEGRATION 

Essentially, an IME is a well-engineering tool that provides an answer to operators’ and drilling contractors’ critical MPD question: Can the MPD system circulate this influx out of the well without jeopardizing the well integrity? It also could be described as a decision-making tool for operators: Once the influx is under control, should we continue to circulate it out using the MPD system, or shut in the well and circulate
it conventionally?

Fig. 2. The influx management envelope is dynamic and can fluctuate, based on certain parameters.

Much like the traditional MPD matrix, an IME defines the limits within which an MPD system can be used safely to control and circulate small influxes. It defines the shape and size of the controlling envelope for various limiting factors, such as shoe strengths lower than the RCD limit, a shut-in scenario while circulating influx, maximum flowrate for circulation, mud compressibility with added backpressure, mud gas separator (MGS) liquid and gas handling capacity, etc. The modified shape of the resulting MPD IME is discussed systematically, in this article. 

This gives operators a more comprehensive, realistic view of the pressure levels that their MPD equipment can handle without requiring a shut-in. As a result, the IME can enhance operators’ understanding of what constitutes safe and efficient MPD operations while removing guesswork from the process.

In Fig. 2, the transition away from a one-pressure, one-volume limit is clear. In this example, the volume of influx—which entered the wellbore during the process of adding SBP to restore the required overbalance, and the ISBP when the influx has stopped—can be read directly inside the light-yellow shaded area of the graph. 

CALCULATING THE IME

Various calculation and simulation tools—including any software or set of equations that are run to understand the driller’s method—can be used to produce an IME for different operational scenarios. These calculations can incorporate important well parameters; simulations for scenarios that are likely to occur as a result of specific well conditions; and safety factors. Calculations should always be based on realistic assumptions, with individual discretion given as to how conservative to make them.

The IME is developed by estimating the initial surface backpressure (ISBP) and the corresponding maximum surface backpressures (MSBP) for various relevant influx volumes and kick intensities, using specific software or programs that can simulate kicks. The ISBP is the initial SBP at which the MPD system restores the overbalance. Since the MPD system reacts to influx by systematically increasing SBP while maintaining drilling flowrates, the ISBP is similar to the concept of ICP in conventional well control. The MSBP is the maximum surface backpressure that the MPD system will apply to maintain constant bottomhole pressure, when the influx is circulated to surface and is at the MPD chokes. Curves representing ISBP versus volume and MSBP versus volume are constructed for specific kick intensities. From these curves, the cases that resulted in MSBP equal to the SBP limit are picked to develop the IME. For the case shown, the SBP limit was 800 psi. 

Since this case exemplifies an SBP limit lower than the surface equipment pressure limit, the proposed IME is then revised for the accurate volumes that could be circulated at an SBP limit of 800 psi, for which case the MSBP would be limited below the surface equipment pressure limit of 1,000 psi. The revised IME calculation is further corrected for shut-in-scenarios, which prevents SBP from exceeding the 1,000-psi equipment limits. This correction is necessary to manage SBP at, or below, the pressure limits of the MPD surface equipment, if the rig pumps had to be stopped and the well had to be shut-in on the MPD chokes before engaging BOPs. Additionally, the IME is revised for available SPP and for mud compressibility with added SBP. Finally, a check for the mud gas separator (MGS) is made to ascertain peak liquid and to make sure gas flowrates are maintained below the MGS throughput capacity. 

PUTTING IT ALL TOGETHER 

The IME can be used alongside any MPD system to enable more informed, efficient and safe operations. The following case study demonstrates why the IME can be an especially powerful tool for handling influxes using fully automated MPD systems. 

While drilling an onshore well in the Montney formation of Alberta, Canada, the Microflux control system successfully detected and controlled a gas influx of 1.88 bbl—with an intensity of 0.57 ppg—in oil-based mud (OBM). Known as one of the largest tight gas reservoirs in the Western Canada Sedimentary basin, the Montney formation is a complex, heterogeneous formation with abnormal pore-pressure gradients ranging from 0.55 to 0.70 psi/ft.

As the operator drilled an 8,500-ft horizontal section using the constant bottomhole pressure (CBHP) method of MPD, an increase in return flow occurred at 8,063 ft. At the time of its detection, the influx volume was 17 gal. In response, the chokes in the system instantly applied an SBP of 510 psi to restore overbalance while continuing to monitor the return flow. Applying SBP and re-establishing a balance between the in-flow and out-flow increased standpipe pressure to 235 psi. After verifying this condition for 20 sec, the system applied an additional 100 psi of SBP as a safety precaution. Then, the influx was circulated at the drilling rate and dissolved into the OBM. The entire process took less than 4 min.

In comparison to conventional well control methods, which would have involved shutting in the well, the Microflux system enabled recapturing well control faster, minimizing the size of the influx and lowering pressure. Various simulations showed that if the choke response time was delayed—if it took between 5 min. and 10 min., on average, to shut in the well—total influx volume could have exceeded 50 bbl by the time the BHP was balanced. This would cause an extremely challenging well control situation.

This larger influx would demand higher pressures to create overbalance. As opposed to the 510 psi applied by the system’s chokes, the shut-in casing pressure required to control the influx with conventional well control methods would have reached 700 psi. This high casing pressure and peak gas flowrates would have been too much for the mud-gas separator to handle. Additionally, the resulting pit gain would surpass the maximum allowable annular surface pressure (MAASP). During circulation, surface pressures could approach 1,500 psi and increase the risk of exceeding the fracture gradient at the casing shoe, which is the weakest point in the open hole.

Clearly, the control system performed effectively in the automatic detection, control and circulation of the influx. How could the addition of the IME further improve the operations of this system and other fully-automated MPD systems? With the IME, operators can better understand the actual limits within which the MPD systems can operate without exceeding MAASP or MSG capacities. As a result, the IME facilitates faster decision-making while being engaged in effective influx response and management. For operators, the decision to either circulate an influx using the MPD system or to shut in the well at the BOP to preserve well integrity can be made even faster and with more confidence. 

In a scenario where quick reaction time means minimal influxes, and therefore lower pressures, better well control, and enhanced safety, the IME provides a critical benefit. 

CONCLUSION

With the evolution of MPD technology toward automatic influx detection and control, and growing consensus around the limitations of the traditional MPD matrix, the IME provides a reliable means of determining the feasibility of using MPD systems to safely control and circulate small influxes. The IME can assist in making advanced decisions with greater precision and in less time, so that MPD systems can be used to peak effectiveness. The use of IME in combination with an automated MPD system can prevent shut-in scenarios, especially in narrow drilling margins. This can result in much safer and cost-effective drilling operations, reducing the overall risks associated with conventional well control.