Bottomhole Pressure Control During Drilling Operations in Gas-Dominant Wells
- Gerhard Haukenes Nygaard (Intl Research Inst of Stavanger) | Erlend H. Vefring (Intl Research Inst of Stavanger) | Kjell Kåre Fjelde (Intl Research Inst of Stavanger) | Geir Naevdal (Intl Research Inst of Stavanger) | Rolf Johan Lorentzen (Intl Research Inst of Stavanger) | Saba Mylvaganam (Telemark U. College)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2007
- Document Type
- Journal Paper
- 49 - 61
- 2007. Society of Petroleum Engineers
- 5.1 Reservoir Characterisation, 5.2 Reservoir Fluid Dynamics, 5.3.2 Multiphase Flow, 1.10 Drilling Equipment, 4.3.4 Scale, 1.7.5 Well Control, 1.6 Drilling Operations, 5.1.1 Exploration, Development, Structural Geology, 1.7.1 Underbalanced Drilling, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.4.2 Gas Injection Methods
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To obtain an underbalanced pressure condition, nitrogen gas can be injected into the drillstring. Simultaneous injection of liquids and gases leads to a highly dynamic flow system. During pipe connections, pressure transients can cause the bottomhole pressure to rise above the pore pressure of the reservoir or fall below the reservoir collapse pressure. Migration of gas during pipe connection and inflow from the reservoir will also cause bottomhole pressure changes.
This paper presents a methodology for controlling the bottomhole pressure during drilling operations in gas-dominant wells. The methodology incorporates a dynamic model of the well fluid flow and the well-reservoir interaction. Available control actions during the drilling process are the gas injection rate prior to the pipe connection and choke valve settings during the pipe connection. Measurement of the pump rates, pump pressures, choke pressure and the bottomhole pressure are also available to support the control actions. However, during pipe connections and in the event of transient signal failures, the bottomhole pressure measurements will be suppressed.
The control methodology used is based on a nonlinear model predictive control system, which predicts the near-future behavior of the well, and uses these predictions to obtain the optimal choke settings. The model parameters are calibrated using measurements from the well to ensure that the model is suitable for the predictions.
A field-based case with gas injection has been examined using this control methodology. The results indicate that model based control can be utilized in developing an automated and integrated pump rate and choke-control system for underbalanced drilling operations.
Injection of nitrogen gas into the drillstring while drilling is often used to obtain underbalanced pressure conditions in the reservoir section of the well. When drilling into a low-pressure reservoir, where the reservoir pore pressure is substantially lower than 1 SG, a large amount of nitrogen gas is needed to achieve underbalanced conditions. This causes the gas properties of the fluid mixture to be dominant in the well.
During pipe connection, where the fluid velocity is reduced, gas and liquid segregate because of gravitational forces. In addition, loss of friction pressure causes the pressure difference between the reservoir pressure and the bottomhole well pressure to increase. This results in a larger influx of reservoir fluids into the well.
To achieve more stable pressure conditions in the well, the choke setting and the circulation pump rates can be adjusted (Perez-Tellez et al. 2004; Nygaard et al. 2004). This paper evaluates a method for controlling the bottomhole pressure during the whole drilling operation, including operations related to pipe connections.
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