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
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.
© 2007. Society of Petroleum Engineers
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- Original manuscript received:
14 September 2005
- Meeting paper published:
11 October 2004
- Revised manuscript received:
6 December 2006
- Manuscript approved:
19 December 2006
- Version of record:
20 March 2007