Mud Displacement In Primary Cementation

Abstract Displacement experiments were carried out in a pilot-plant model, to arrive at means of improving zonal isolation. Scaled and full-sized annuli, with and without hole enlargements, were simulated. Various displacement fluids were investigated. The amount of mud left behind was derived indirectly by measuring the remnant radio-activity caused by the mud, containing a radio active tracer. Special attention was given to the influence of hole enlargements, fluid velocity, density and rheological properties of mud and displacing fluids on the mud displacement. The most significant parameters were found to be the displacement velocity and the flow properties of the fluids. The study showed that depending on the conditions and the properties of mud and displacing fluids, a good displacement efficiency can be achieved both for low and high displacement rates. Introduction Lack of zonal isolation believed to be caused mainly by inadequate mud removal, the presence of an excessive mud filter cake and contamination of mud and cement. A considerable amount of work has been carried out and is published, in this field. However, opinions on how to solve the various problems encountered are still divided and there is no clear insight into the importance of parameters such as fluid velocity, density and rheological properties of mud or cement and the use of preflushes. In addition many studies on this subject were based upon small test series with scaled models. In experimental studies carried out at the Koninklijke/Shell Exploratie en Produktie Laboratorium, Rijswijk (The Netherlands), a different set-up was used which permitted large series of experiments to be carried out in which one parameter could be varied at a time. Based on the results of some 270 tests, conclusions could be drawn on the relative importance of the various parameters that influence mud/cement displacement and parameters that influence mud/cement displacement and thus zonal isolation. THE MUD/CEMENT DISPLACEMENT PROCESS After drilling the borehole and running the casing the following situation will prevail in the borehole; -the space between casing and borehole is filled with mud, -a mud cake is present on the borehole wall; its thickness varies from significant in places opposite permeable zones to virtually nil at locations opposite impermeable zones, e.g. shale layers, -a gauge hole with washed-out sections. Ideally, the mud cake and all the mud should be fully displaced by the cement slurry to obtain a good solution between different zones.