Axisymmetric Displacements of Miscible Fluids in Annulars with Washouts for Applications in Primary Cementing Operations

Abstract

Abstract Primary cementing is "the process of installing cement in the annulus between the casing and the formation exposed to the well", NELSON (1990). The cement provides permanent zonal isolation to prevent contamination or undesirable fluid migration toward the annulus. It protects the casing from corrosion and provides hydraulic and mechanical stability throughout the oil well productive life. During the drilling stage, irregularities in the cross-section, defined as washouts, can occur caused by partial collapses of the open hole section due to the presence of poorly consolidated rocks in the formation. A successful primary cementing operation will depend on whether the spacer fluid system and the cement slurry adequately and completely displace the drilling mud from the annulus and washouts. Motivated by this industrial problem, the present work performs a Direct Numerical Simulation of the vertical displacement between two miscible Newtonian fluids confined in eccentric annular spaces containing an expansion followed by a contraction. We investigate how different viscosities and densities of the injected and displaced fluids, the miscibility between them, the injection rate, and the dimension of the rectangular washout affect the two-phase flow, and calculate displacement efficiencies. Our numerical algorithm was developed in C language using finite differences and spectral methods and solves the Navier-Stokes equations with variable viscosity in cylindrical coordinates coupled to an advection-diffusion equation for a scalar field that measures the concentration of the more viscous fluid. Our results predict very high displacement efficiencies, close to 100%. Although the fluids used in industrial cementing processes are non-Newtonian, the high displacement efficiencies found in the current results motivate the use of spacer fluids as a strategy to control the interface properties. Thus, approximating the field conditions to the ones encountered in the displacement between miscible Newtonian fluids can lead to increases in displacement efficiency. Needless to say, this might have important consequences for the safety and integrity of oil wells. We emphasize that the axisymmetric displacement between miscible Newtonian fluids leads to very high displacement efficiencies, close to 100%. The calculation of such efficiencies for miscible fluids considers the miscibility between them and is easily obtained from averaged values of the concentration parameter.