Cement Displacement and Pressure Control Using Magneto-Rheological Fluids

Abstract

Abstract Competent cementing to achieve lasting zonal isolation is a crucial requirement during well construction. Key to achieving a good cementation is proper displacement of drilling fluid out of annular spaces and placement of the cement in them. The viscosity contrast between drilling fluid, spacer(s) and cement as well as centralization of the casing in the wellbore are important variables affecting displacement efficiency. For instance, when the casing is not well-centralized in the wellbore, fluids flow more easily and faster through the wider annular gap, while displacement lags behind and may be incomplete in the narrower gap. Such non-uniform displacement and/or incomplete cement placement can lead to unreliable zonal isolation, which in turn may compromise the productive life of the well. In this paper, a new cementing technique is presented wherein the rheological properties and the flow direction of the cement slurry can be controlled in real-time to achieve more uniform displacements, even when casing is not optimally centralized. This new technique is based on the principles of magneto-rheology, where an external magnetic field is applied to a cement slurry carrying magnetic particles. In the presence of the magnetic field, the viscoelastic properties of such a modified slurry change instantaneously to cause a stiffening behavior characterized by yield point values that are increased by several orders of magnitude. By varying the intensity and the direction of the magnetic field as well as the dosage of magnetic particles, the flow properties of the slurry can be altered and the flow can be beneficially directed as desired. The magneto-rheological behavior presents other benefits as well. The instantaneous stiffening effect when a magnetic field is switched on can be exploited to generate temporary top-, bottom- or straddle packers for pressure control, and the specific sensitivity of the magnetic slurry to an external field may be exploited for sensing the quality and integrity of cement sheath over the life of the well. Voids, gaps or cracks in the cemented annulus cause disturbances of magnetic field lines that may be picked up by suitable detectors.