Recently, completion technologies using downhole flow-control valves allowing production control over multiple zones have become available. The central idea is that the downhole control may be used to adjust flow distributions along the wellbore to compensate undesired fluid-front movement. In this paper, we address several technical issues related to downhole controls. We consider the completion, the reservoir, the measurement, and the feedback algorithm that adjusts flow-control devices as a single system with quantitative models for each of the components. Both flow-rate and pressure control systems are discussed.
Downhole control is modeled for electrical, reversible hydraulic and irreversible hydraulic valves.
System design requires a quantitative description of both the orifices in the control valves, and the feedback control algorithm. The design methodology for different valve systems is described and the disadvantages of hydraulic systems are discussed. In particular, it is shown that the hydraulic valves are susceptible to instabilities. Computations also show that all other factors remaining equal, instabilities of the feedback system occur most easily in low permeability zones. For layered systems that communicate, a front movement equation is derived using perturbation techniques.