Flow and displacement of viscoplastic fluids in eccentric annuli.

Abstract

In the construction of oil and gas well, improper displacement work (primary cementing job) may cause poor zonal isolation, cement channeling, remedial squeeze cementing, and thus lead to a severe problem. The leakage of the wellbore fluids and contamination of sensitive zones resulting from poor primary cementing job such as the Gulf of Mexico's oil spill in April 2010 has been the biggest disaster accidents for mankind history since then. The experimental flow system was developed for systematic study of both single-fluid flow and two-fluid displacement in eccentric annuli. This flow is relevant to the drilling operation and cementing operation in oil well completions, where drilling fluids are displaced from the annulus between the casing and the well bore by a series of spacer/wash fluids and cement slurries. The design of the system is based on the helical flow geometry, which is a combination of annular axial flow and tangential rotating flow in an annulus. The annular flow apparatus can be operated at various degrees of eccentricity and different angles of inclination to simulate the type of flows in oilfield drilling and cementing operations. A special feature of the flow system is that the inner pipe can be rotated during displacement, allowing the effect of casing rotation on the performance of the annular displacement process to be studied. Single fluid flow with various models of rheological fluids and displacement tests with various models of rheological fluids, especially viscoplastic fluids representing drilling and cementing fluids was conducted at different eccentricities, pipe inclinations, and over a range of flow rates and cylinder rotational speeds. Regarding the entire results of the displacement experiments, to our knowledge, the images of the moving fluid-fluid interface in annular displacement flow, especially with a rotating cylinder, generated in this work may be considered to be the world leading, since such visual information is not available in the open literature. In addition to flow visualization, the velocity of moving fluidvi-fluid interface in annular displacement flow was studied by measuring the conductivity of tested fluids. The result reveals the dynamics of displacement flow in annuli. A method was developed for simultaneously determining the displacement efficiency by measuring the conductivity of the mixed fluid phase exiting the annulus. The results obtained are of sufficient accuracy to be useful for assessing the effects of key variables such as pipe standoff, pump rate, pipe rotation and fluid rheology, on the effectiveness of annular displacement process. The relationship between the pressure gradient and the displacement efficiency has been established for the displacement process. The basic general mixing rule of a binary system occupied in term of pressure gradient may be illustrated. The pressure gradient of the mixture at any specific time of the displacement process may be illustrated by the equation comprising of the pressure gradient of each pure component in the mixture (in mass, mole, or volume fraction) derived from the results of the single-fluid flow experiments. Finally, the graph plots of the model prediction for the friction pressure gradient of the displacement process were shown, illustrating that the results from model equation collapses properly on the experimental result. To summarize, the results obtained in this work is useful for assessing the effects of fundamental variables such as eccentricity, pipe inclination, flow rate, pipe rotation and fluid rheology, that impact on the effectiveness of the annular displacement process. In particular, the flow visualization data, the velocity of moving fluid-fluid interface, the displacement efficiency, and friction pressure modelling results obtained from this project can be used to compare with, and validate, Halliburton’s 3-D simulation CFD, implemented.