Automatic Coordinated Control of Pump Rates and Choke Valve for Compensating Pressure Fluctuations during Surge and Swab Operations

Abstract

In this paper we present new methods for coordinated control of pump rates and choke valve for compensating pressure fluctuations during surge and swab operations. In hydrocarbon well drilling, automatic control of the choke valve improves the control of the bottom-hole pressure. This has made it possible to drill wells where it previously was not possible to reach total depth, due to narrow pressure margins in the reservoir. This concept is developed further in this paper, to also include automatic control of pump rates, coordinated with the choke valve control. In configurations where the choke valve is fitted with a flow rate controllable annulus back-pressure pump, the situation calls for structured methods for automatic and coordinated control of the main mud pump, the back-pressure pump and the annulus return choke valve. In this paper, well known methods for multivariable control are applied to the drilling process. These methods are based on identification of coupling and interaction between system input (controllable variables), e.g. pump rates and choke valve settings, and output (measured or estimated variables), e.g. pressure at given depths. It is shown how the well pressure profile can be kept within tight margins, subject to disturbances such as drillstring movement in surge and swab operations. Automatic coordinated control is shown through numerical simulations on a detailed, space-discretized well model. Comparisons are made with the case of complete manual control and the intermediate case where only the annulus return choke line pump is automated, using a simple linear controller. The results with automatic coordinated control of pump rates and choke valve are very promising - the performance of such a solution, measured by variance in pressure fluctuations, is improved - compared to both the case of manual control and the case of only automated choke line pump control. Introduction During surge and swab operations the downhole pressure will be affected by volume changes in the well caused by insertion or extraction of the drillstring, in addition to friction effects between the drillstring and the fluid in the annulus. Multiple control variables are adjusted in order to maintain underbalanced conditions along the well section exposed to the reservoir. The most important control variables are drillstring fluid pump volume rate, qp, drilling fluid density, rho, the annulus back pressure pump, qc, and choke valve opening area, Ac. Multiple measurements are available for evaluating whether the required downhole pressure conditions are maintained. Typical measurements available are flow rates, pump pressure, choke valve differential pressure and downhole pressure. To avoid pressure fluctuations during the drilling operations, multivariable process control can be required. In Balchen 1 there are descriptions of several approaches for controlling such a multivariable process. Control methods such as decoupling, feed-forward, modal control, LQG-control and robust control could be used. In addition the available models of the drilling process should be utilized to improve the control system.

Nonlinear Estimation and Control during Pipe Connection in a Drilling Operation

“…Automatic control of the bottom hole pressure in a drilling operation has been studied by [1,6,7]. However, in these works, the backpressure pump is assumed to be absent in the drilling rig.…”

Design and Comparison of Constrained MPC With PID Controller for Heave Disturbance Attenuation in Offshore Managed Pressure Drilling Systems

“…Rasmussen and Sangesland (2007) compared and evaluated different MPD methods for compensation of surge and swab pressure. In Nygaard et al (2007a), it is shown that surge and swab pressure fluctuation in the bottom hole pressure during pipe connection can be suppressed by controlling the choke and main pump. Nygaard et al (2007b) used a nonlinear model predictive control algorithm to obtain optimal choke pressure for controlling the bottom-hole pressure during pipe connection in a gas dominant well.…”

Adaptive observer design for the bottomhole pressure of a managed pressure drilling system

“…In many cases the bottomhole pressure (the pressure at the bit) is used as the variable to control [4], [5], [6], [7]. The bottomhole pressure is measured, but the signal is usually transmitted by mud-pulse telemetry which is powered by a mud flow turbine.…”

“…Previous attempts at using low order models for control and estimation of the bottomhole pressure can be found in [14] and [5]. In [14] nonlinear model predictive control (NMPC) was used together with an unscented Kalman filter to control the bottomhole pressure.…”

“…The bottomhole pressure was kept fairly stable. In [5] it is shown that pressure variations in the bottomhole pressure during surge and swab can be suppressed by controlling the choke and main pump. The control is based on a fourth order model and assumes that all parameters and the bottomhole pressure is known, hence there is no estimation scheme involved.…”