Developing an Influx-Management Envelope for a Deepwater MPD Operation
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The influx-management-envelope (IME) concept is an advanced, innovative way to assess influx conditions in managed-pressure-drilling (MPD) operations, offering an improved tool for the decision-making process. The complete paper presents the process of developing the IME concept for a deepwater MPD operation for the first time.
The IME Concept
The IME concept defines the operational envelope in case of incidental influxes during MPD operations. It is based on kick-tolerance concepts, in which a relationship between volume and intensity of the influx is established and plotted as a graph. A resulting combination of regions within the volume-and-intensity graph depicts the conditions in which an influx can be removed safely from the wellbore by use of the elements of the primary wellbore barrier; otherwise, the secondary wellbore barrier should be engaged and the well shut in conventionally. The regions are color coded for ease of identification during the process of managing an influx. Three regions have been described:
- The Green region identifies normal MPD operations, where drilling and other operations continue as planned. No influx is detected in the wellbore, and surface pressures are within planned parameters.
- The Yellow region defines the operational region where an influx has been detected in the wellbore and the influx can be circulated safely to surface within the boundaries of the primary wellbore barrier.
- The Red region defines the conditions in which it is deemed that some limits of the primary barrier would be exceeded during the process, indicating that the well should be secured with a secondary-barrier envelope.
The Orange Region. In this work, an additional (optional) Orange region is added as a subset of the Yellow region (Fig. 1 above). In the Orange region, an influx can be removed safely within the primary wellbore barrier but one or more parameters will need to be modified to avoid exceeding some limits in the process.
IME Considerations for Implementation. There are certain considerations regarding the IME concept that the designer and the operational team must understand fully to safely implement this valuable decision-making tool. These include the following:
- A specific IME should be prepared for each well section planned to be drilled using the MPD system. The IME should be adjusted after changes in mud weight, drilling rates, or surface backpressure (SBP).
- The measured (or calculated) parameters to use on the IME must be taken as approximately as possible from the moment when the flow from the formation into the wellbore has been stopped. This key assumption is referred to as influx cessation.
- Although the volume limit between the Green and the Yellow regions is typically established at an arbitrary value, close to the estimated minimum influx-detection capability of the MPD system, any detected influx volume should trigger the actions considered for the Yellow region, regardless of the detected volume being lower than the detection limit.
- Once the decision is made to circulate an influx within the primary barrier, because the volume and intensity parameters are deemed to be within the Yellow region, the operation must follow established procedures, but the IME should not be referenced further during the circulation process.
- Consideration should be given to establish a procedure to follow up the circulation process, to ensure that actual conditions remain safe at all times.
- A procedure for handing over to the secondary wellbore barrier must be in place in the event that the IME control parameters at influx cessation exceed the Yellow-region limits.
- The project team must keep in mind that IME control parameters at influx cessation exceeding the Yellow-region limits do not necessarily imply that there is a severe, imminent risk to the operations or safety.
The IME Limits
In the process of preparing IMEs for an MPD operation, a key step is to correctly define the circulation limits that will dictate the limits between the Yellow, Red, and (potentially) Orange regions in the IME. There will be essentially two types of limits. The most important one, which should guide the process, is the group of pressure limits. The second type is given by expected liquid- and gas-flow rates through the surface equipment.
From the IME concept, parameter combinations (influx volumes and SBP) not expected to exceed the established pressure limits are considered to be safe to circulate through the MPD equipment.
Preliminary Conditions. Ability To Shut In. It must be possible to stop circulation and the well at any time during the influx-removal process without exceeding downhole or surface pressure limits.
Adjusted Post-Influx SBP. This is calculated to provide better insight about the intensity of the influx once the event has been detected and the inflow stopped by the addition of SBP.
SBP Safety Margin. The project team decided to establish a safety margin for the SBP after influx-cessation confirmation (i.e., flow out is equal to or less than flow in) to avoid secondary influxes caused by bottomhole-pressure variations incidental to the MPD operation.
Assessment Depth. The IMEs were prepared for total depth (TD) of each section and were considered as the worst-case scenario for an influx.
Mud-Pump Rates. The mud-circulating rates planned for all three hole sections exceed the liquid-flow-rate limit of the mud/gas separator in the presence of gas. Therefore, the project team established that, in the event of dynamic circulation of an influx, the flow rate would need to be reduced when the front of the influx reaches approximately 1000 m from the surface.
Minimum Influx-Detection Limit. For the graphical representation of IMEs in this project, a minimum influx-detection limit of 2 bbl has been arbitrarily selected, defining the limit between normal MPD operations (Green region) and contingency influx response (Yellow, Orange, or Red region). However, for this project, any detected influx would trigger influx response, placing operations automatically outside of the Green region, even if the detected volume is less than 2 bbl. Normal MPD operations can be conducted only in the absence of influx in the wellbore.
Anchor-Point (AP) Location. This project has planned to maintain the AP at the bit location for all three hole sections. Different AP locations would have an effect on the process for calculating the section maximum and, thus, the maximum allowable influx volume.
Kick-circulation conditions were evaluated for combinations of influx volume and SBP required to stop the kick under MPD conditions. Two methods were used to develop the limits (post-influx SBP and influx volume) for the IMEs: the single-bubble approach and hydraulic modeling with a transient multiphase-flow-simulation commercial software package.
The single-bubble approach disregards gas dissolution in the mud system as well as influx dispersion during circulation, resulting in smaller allowable influx volumes for MPD circulation than those calculated by transient simulation by approximately an order of magnitude for all hole sections analyzed.
When high limits were calculated by the IME process, more-conservative limits for influx circulation were proposed on the basis of an arbitrary maximum influx size of 20 bbl, regardless of intensity. The logic was that, if an influx is not identified and controlled by 20 bbl, there should be concerns with either the equipment or personnel competency, because the MPD system should be capable of detecting kicks before 2 bbl and controlling kicks before 10 bbl.
The single-bubble approach, detailed in the complete paper, calculates the maximum allowable influx at the bottom of the well at the time of influx cessation.
Transient Simulation. Extensive transient modeling was performed with commercial software capable of two-phase-flow simulation as well as reservoir-inflow performance. The information available from the MPD program for the project was used to simulate influxes at TD of each section, for various assumptions of kick intensity, and then circulate them dynamically through the primary barrier observing the resulting control parameters. This iterative process was used to find combinations of kick intensity and influx volumes that reach (but do not exceed) the established circulation limits (pressures and flow rates).
It is important to note here that the commercial software used for this work is not necessarily intended for simulation of influxes in full dynamic MPD conditions. Some adjustments were required during the simulation work to represent the circulation conditions as close to reality as possible.
The results from transient simulation (the process selected by the team for developing the IMEs for the project) are discussed in detail in the complete paper.
The IME is a valuable tool for decision making on management of influxes in MPD operations. Full considerations for implementation include the following:
- In this particular project, the IME pressure limits are dictated by the downhole pressure limits.
- The Orange region is defined and implemented for situations in which circulation of an influx through the primary barrier is possible, but additional steps are needed to remain within its limits.
- The capacity to shut the well in at any moment during influx circulation, without exceeding surface or subsurface pressure limits, is required.
- Two methods used for calculating the maximum influx sizes are the single-bubble approach and dynamic transient simulation. The latter allows better establishment of the system’s capability to circulate influxes through the primary barrier.
- The maximum allowable influxes in the final IMEs for each section are truncated to a maximum arbitrary value of 20 bbl, established by the project team.
- The concept of the IME and the approach taken have been integrated into the project through procedures included in the well-control bridging document, as well as in crew-training sessions.
- A procedure was established to update the IME for each section after MPD fingerprinting and performing of formation-integrity tests.
Developing an Influx-Management Envelope for a Deepwater MPD Operation
01 May 2018
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