Abstract
Formation damage caused by kill fluids invasion and filter cake deposition in the perforation tunnel has rarely been addressed in the scientific literature. In this paper, a systematic experimental program using CaCO3-HEC slurry to kill perforated wells is presented. The perforation target is simulated using a cylindrical core with a drilled perforation, and borescope has been used to observe the distribution of filter cake in the perforation tunnel. Important parameters affecting perforation plugging and cleanup have been identified. The effectiveness of perforation cleanup methods, such as backflush and backsurge have been studied and the mechanism of filter cake removal investigated.
Experimental results show that among the various damage parameters investigated, only core permeability and fluid viscosity have big effects on Perforation Flow Efficiency (PFE), which is the comparative flow capacity of a damaged perforation to that of a clean perforation. Perforation Flow Efficiency decreases with increasing permeability.
The mechanism of filter cake removal strongly depends on the perforation cleanup techniques. When backflush is used to clean the plugged perforations, a threshold differential pressure has to be reached before flow can be initiated and a minimum volume of fluid must be flowed to achieve optimum PFE. Depending on the damage and cleanup conditions, plugged perforations may be partially or completely cleaned by backflush. Sequential backflush is advantageous over single constant rate backflush for high permeability core, where matrix damage is severe. Increasing backflush rate increases PFE. When backsurge is used, all the filter cake deposited can be removed instantaneously, and the PFE obtain is very high. Backsurge pressure is more important than backsurge volume.
The results obtained in this study will be very helpful in the design of kill pills and the design of perforation cleanup methods in the field.
