Prevent Poor Cementing Results By The Use of Bottom Cementing Plugs

The objective of primary cementing is to support the pipe and achieve a seal so that the desired fluids can be produced from the well. Although a lot of effort is spent in designing for effective fluid displacement in the annulus, displacement in the pipe is frequently overlooked. When fluids are being circulated down the pipe, the balance of forces is not correct for efficient displacement. The heavier fluid is on top, displacing the lighter fluid below it, with buoyant forces causing the heavier fluid to tend to fall through the lighter fluid. Wiper plugs are available to prevent this, but in practice, bottom plugs frequently are not used. When bottom plugs are not used, the cement may bypass the spacer. The cement may then mix with or bypass mud, resulting in poor displacement, leading to poor isolation of various well intervals. Other problems include inter-facial mixing resulting in high permeability cement and exposure of pipe to corrosive fluids, or high displacement pressures due to high viscosity of mixes of incompatible fluids. An investigation of the physics of fluid displacement in pipes was undertaken. The study combines the effects of density and rheology and defines the condition which exists when wiper plugs are not used to separate fluids flowing down the pipe. A computer model provides a qualitative evaluation of the efficiency of the displacement process and indicates the possible instability of the displacement front. Case histories are given which demonstrate the effects of contamination or of one fluid bypassing another and the results of the use of bottom plugs.