A Quantitative Investigation of the Laminar-to-Turbulent Transition: Application to Efficient Mud Cleaning

Abstract

Abstract Spacer and wash fluids play an essential role in the cementing of oil and gas wells. They are pumped ahead of the cement to optimize the mud displacement, enable a proper hole cleaning, and obtain a good cement bond at the formation and the casing surfaces. The design of spacer fluids relies on an ensemble of physical and chemical characteristics of the spacer including flow characteristics (rheological properties), the stability of the weighted spacer suspension, and the compatibility with other wellbore fluids. The flow characteristics of spacer fluids are the key to successful cementing. The wellbore fluids pumped in the field usually behave as viscoplastic fluids. The non-Newtonian nature of these fluids must be accurately modeled when determining the flow behavior around a centered or eccentric annulus to accurately determine flow regime and dynamic pressures in the annulus along the entire column of fluid. Turbulent flow is the preferred flow regime for efficient mud removal in the annulus, provided the cleaning fluid is effectively in turbulent flow all around the eccentered annulus. Therefore, a reliable criterion for the critical Reynolds number is needed to properly design the well-cleaning process and determine whether it is feasible to be in turbulent flow. In an effort to better understand the fluid dynamics involved with mud cleaning, we have conducted an experimental study on a series of spacer and model fluids. Axial velocity and relative fluctuation intensity measurements were conducted using three different techniques:ultrasonic Doppler technique,hot film anemometry, andlaser Doppler velocimetry. On the basis of these results, we have been able to fully characterize the laminar-to-turbulent transition and evaluate the pressure drop caused by highly non-Newtonian fluid. The results of this study help us define more reliable estimates of the minimum pump rate that will lead to substantial well-cleaning efficiency, taking into account the rheological behavior of non-Newtonian fluids. Introduction The main objective of a cement job is to provide complete and permanent isolation of the permeable zones located behind the casing. To meet this objective, the drilling fluid must be removed from the annulus and the annular space must be completely filled with a cement slurry that can adhere to the formation and casing surfaces. Therefore, thorough mud removal and proper slurry placement are essential to obtaining zonal isolation. The mud displacement is achieved with preflushes; i.e., washes (nonweighted fluids) or weighted spacer fluids, or combinations of washes and spacers separating mud and cement. The most efficient displacement is achieved when the preflush is flowing in the turbulent regime, a practice that has been proved by field experience1–3 and laboratory studies.4 In addition to being in turbulent flow, it has also been observed that a sufficient contact time is required for good mud removal. Whenever possible, low-viscosity and low-density water-based or oil-based fluids are preferred for their low cost and the improved cleaning observed when there is a large density difference between the mud and the displacing fluid.5 However, often the need to maintain a minimum amount of hydrostatic pressure in the well for stability purposes requires the use of weighted fluids. Low-viscosity weighted fluids, called turbulent spacers, are used for this purpose. These fluids are loaded with solids and polymers to prevent any sedimentation of solids; they are thus non-Newtonian suspensions, and a good criterion is required to predict whether they will effectively flow in turbulent regime.