Several challenges can await when drilling deepwater wells. One common denominator is narrow operating windows. Combined with the height of the hydrostatic column from RKB down to the Seabed, where the well and the operating window starts, the well design becomes complex with frequent casings and liners.
There are different options for improving drilling conditions in deepwater. MPD is a natural tool for addressing narrow operating windows providing near Constant Bottom Hole Pressure (CBHP) – especially for the reservoir section when the Equivalent Circulating Density (ECD) is high.
In the upper sections of the well, Controlled Mud Level (CML) MPD can create a Dual Gradient Effect enabling two or more sections to be drilled as one. Besides significant cost savings, eliminating a casing/liner also simplifies the plan for the rest of the well. The pressure profile generated by CML is ideal to use in the upper parts of the well, and high ROPs in large hole sizes does not case any issues with this method.
Conventional well control becomes challenging in (ultra) deepwater wells because of the friction generated in the long choke line. Again, MPD is a natural tool providing influx management by using a riser closure device.
Check out: 11 Things All Drilling and Completion Engineers Need to Know About MPD
20K rigs have entered the market to drill previously inaccessible reservoirs. 20K rigs are rigs capable of drilling in environments requiring 20,000psi pressure control.
These environments, typically deeper wells with high pressure and temperature, encounter the same tight margins as deepwater wells because of high ECD.
There is an upcoming campaign where a single MPD method is not sufficient, but a combination of Surface Back Pressure (SBP) and CML is required to stay within the operating window. In this case, CML reduces ECD by lowering the fluid level, while back-pressure is applied during connections to maintain the desired Bottom Hole Pressure (BHP). This combination of MPD methods also enables influx management with two different approaches.
Areas previously considered impossible to drill are now being explored. New fields bring increased uncertainty such as estimating the wide range of pore pressures and fracture gradients that come with exploration wells. Planning contingencies, evaluating the next steps based on real-time data and adjusting while drilling ahead is necessary.
For these wells CML MPD, or a Dual MPD system combining CML with SBP, provides the flexibility to adjust according to actual well conditions. In addition to operating with a CBHP, CML MPD can adjust the setpoint value both ways by increasing or reducing the fluid level in the riser.
The Early Kick and Loss Detection System, equipped with pressure sensors positioned subsea inside the well, provides near instant feedback on gains or losses in the well. Keeping the fluid level below the telescopic joint eliminates the uncertainty from rig motion, allowing the riser to function as a Trip Tank for direct well monitoring. Combined with the ability to instantly adjust the BHP, the system is ideally suited for managing uncertainty.
Discover: EC-Drill® Dual MPD
Addressing Depletion and Loss Zones
Drilling new phases in mature fields pose challenges not faced in previous campaigns. Depletion levels can be uncertain, with some pockets still retaining virgin pressure. A common approach is to start drilling with a BHP over high-case pore pressure and then try to manage the losses caused by depletion. There are also fields where a hydrostatic column of unweighted drilling fluid leads to losses because of the high degree of depletion, or when entering a total loss zone.
Operating in a CBHP mode will keep the BHP above expected pore pressure, reduce the likelihood of going on losses, and enable longer reservoir sections. The ability to reduce the BHP according to actual well conditions is critical to enable drilling and completion in depleted zones. CML MPD is ideal for these conditions with its ability to remedy losses by reducing the BHP.
In highly depleted fields or when drilling into total loss zones like Karstified formations, an unweighted mud system with a reduced riser fluid level is planned to create hydrostatic pressure lower than water. Additionally, there is the planned contingency for addressing total losses by operating in Controlled Mud Cap Drilling (CMCD) mode. In CMCD mode, sacrificial fluid (Seawater) is pumped down the drillstring while the annulus is controlled, monitored and kept overbalanced with the Subsea Pump Module (SPM) in the CML. The annulus volume is maintained by using a topfill pump and the boostline, while the SPM keeps the desired level in the riser.
Read more: How Does CML MPD Work
Drilling the previously deemed impossible wells are certainly exciting from a technical point of view and a testament to the technological innovation within the drilling industry. It is also a necessity to unlock and develop new and mature fields.