The oil and gas industry has made great strides in developing effective techniques to drill longer wells into geologically complex formations, but it also has met challenges in the form of fluid losses and wellbore instability. Efforts to overcome these challenges gave rise to the development of managed-pressure drilling (MPD) techniques in which constant bottomhole pressure (BHP) is maintained by applying backpressure to the annulus of the well to compensate for dynamic pressure losses.

The MPD control system is used to maintain the BHP within a drilling window bordered by the reservoir’s pore pressure and its fracture initiation pressure. Existing automated annular pressure-control systems use a pseudo real-time hydraulics model to determine the appropriate backpressure and then open or close a choke to restrict flow from the well and maintain that backpressure. A dedicated backpressure pump may be used to provide additional fluid volume and an extra degree of precision, but it also adds more equipment, power requirements and transportation costs to the project.

The pressure window maintained by a control system is dictated by timeliness or how quickly the system responds to disturbances; accuracy, or how far the BHP is from the target pressure; and precision, or the amount of variation in BHP. A trade-off often exists between these factors. For example, increasing the speed of the system tends to increase pressure overshoot. Finding the right balance between responsiveness and precision depends on the operator’s skill at tuning the control system.

Advancing choke control
To address the industry need for more precise MPD control from a smaller and less equipment-intensive system, M-I SWACO, a Schlumberger company, embarked on a multiyear project to deliver more streamlined choke control with @balance Control Services. One component of this new system comprises a proportional valve that adjusts flow based on an electrical input signal. This valve replaces multiple directional valves of different sizes to provide speed control, resulting in a simpler and more robust design.

The other design change focused on the control loop, which was modified to output a speed and direction signal. Conventional systems provide single-loop control using a single error term derived from the difference between the measured surface backpressure and the desired set point pressure. To indicate both the direction and duration of the movement of the choke, conventional systems require a system operator to manually set proportional and integral gains.

These parameters interact with each other, presenting choke tuning challenges and introducing the risk of overshooting the set-point pressure.

The @balance Control Services system operates in two modes: e-balance Control System for efficiency-focused wells where simplicity is key and i-balance Control System for difficult wells where intelligence is demanded of the control system. The control loop compensates for the error in the pressure, the speed of the choke and the rate of change in the pressure error.

Being able to rapidly change both the speed and direction of the choke allows a less complex control algorithm. The loop also provides the ability to adjust the speed of the choke in proportion to the rate of change of the pressure correction, which allows a more robust control algorithm and less tendency for overshoot.

Multitiered testing
The new MPD control system underwent a rigorous three-year testing period, beginning with short flowloop testing that helped verify system modifications as these were implemented and established an initial expectation of performance.

Another series of tests analyzed the system’s ability to control step changes in pressure set point at fixed flow rate and its ability to track pressure set-point changes resulting from simulated connections. The new controller demonstrated precise and crisp response to changes in set point, as the graph shows. The controller reached the set point in less than half the time of the existing system with minimal overshoot (less than 5 psi) and quickly stabilized. Field trials were initiated following this round of testing.

Proving field potential
A trial of the new control system was conducted by a North Sea operator drilling an exploration well with a predicted narrow mud-weight window (0.92 parts per gallon [ppg]/0.11 specific gravity [sp gr]) and with large uncertainties regarding pore pressure prediction. The operator decided to drill with a statically underbalanced drilling fluid and use MPD to keep the well overbalanced at all times. This required displacing the well between lighter underbalanced drilling fluids while drilling and a heavier overbalanced drilling fluid to maintain control while tripping.

The new system underwent acceptance testing prior to field deployment. The test well was 2,010 m (6,594 ft) measured depth and 1,535 m (4,947 ft) true vertical depth with a 15.1-ppg (1.81-sp gr) oil-based mud. The system was evaluated under both normal and contingency field operations per the operator’s specifications.

During 10 simulated connections, the system maintained downhole pressure within the operator’s specified upper and lower boundaries (plus or minus 36 psi). The elapsed time from the start of rampdown to the end of ramp-up varied from four minutes to seven minutes. Two simulations tested system performance without the use of a backpressure pump. The results did not differ significantly compared to simulations when the backpressure pump was present. For contingency event simulations, the system maintained BHP within the operator-specified window of plus or minus 72 psi.

Satisfied with these results, the operator deployed the control system in its North Sea well. To verify that the correct mud weight was used to maintain static overbalance, dynamic pore pressure fingerprinting was performed prior to each displacement. The control system provided accurate control of the pressure steps at each stage.

Once the proper mud weight was confirmed, a mud rollover schedule was created using the VIRTUAL HYDRAULICS software suite to model equivalent circulating density and equivalent static density throughout the drilling and tripping process. The control system used this schedule, along with the integrated real-time hydraulics model, to automatically adjust surface backpressure and maintain
constant BHP as the drilling fl uid was displaced down the drillstring and up the annulus. The system successfully completed six displacements between drilling fluids and maintained BHP within a window of plus or minus 0.11 ppg (plus or minus 87 psi).

The control system also was used by an onshore U.S. operator that encountered wellbore instability problems while drilling the intermediate hole sections of wells. While the operator previously used MPD to mitigate wellbore instability problems, the remote nature of this location and limited rig space raised concerns about equipment footprint and system complexity.

The @balance Control Services allowed the removal of the backpressure pump from the MPD system, eliminating approximately 61 m (200 ft) of pipe work, suction lines to the rig’s mud pits, a trip tank and pressure-relief lines. Pump removal also saved more than 31.4 sq m (338 sq ft) of space and reduced electrical power consumption from 480 v per 250 amps to 480 v per 10 amps.

Removing the auxiliary pump called for the MPD crew to trap pressure on connections. This required the control system to quickly close the choke to ensure that the appropriate pressure was obtained without overshooting the desired pressure. The control system maintained BHP to within plus or minus 0.27 ppg (106 psi) during connections, allowing the MPD crew to achieve the desired level of pressure control.

Building on early successes
Since the early fi eld trials, the @balance Control Services system has demonstrated a response time to changing downhole conditions that is two to three times faster and as much as a threefold improvement in accuracy and precision. The system has been deployed on other wells without a backpressure pump, and Schlumberger plans to offer the backpressure pump as only an optional component on future deployments. By reducing the complexity of the field equipment, the system affords further benefits, including the option to reduce crew size.

References
SPE/IADC paper 173126, titled, “Breakthrough advance in MPD automation: A new system manages narrower drilling window with reduced equipment and crew,” was originally presented during the SPE/IADC Drilling Conference and Exhibition in London March 17 to 19, 2015.