Cleanup of Wall-Building Filter Cakes

Abstract

Abstract Building up external filter cakes during drilling and completion operations has long been one of the most practical ways to minimize fluid loss to the formation. The effectiveness of the filter cake is critical especially for long openhole completions with no perforations. It is usually assumed that this wall-building material will be removed during oil production, due to pressure drawdown. However, it is quite commonly observed that only a small fraction of the cake is removed, leaving large portions of it remaining on the formation face. This condition results in a small wellbore area open to flow and a large skin. To increase the flow area and minimize skin, the filter cake needs to be completely removed. Solids from filter cakes can also cause plugging of screens and gravel packs. This is particularly important in horizontal wells completed openhole. Experimental results are reported for mud cake lift-off tests. The flow initiation pressures required to flow through or remove drilling and completion fluids filter cakes are measured together with the return permeability of the rock. In this study, sized CaCO3 and sized salt drill-in mud were tested on rock samples with permeability ranges between 3 to 2000 md. The effects of mud filtration pressure, rock permeability, and flow back rate were also investigated. The results show that sized CaCO3 fluids require lower flow initiation pressures and are less damaging than sized salt fluids. Despite having low mud filtrate volume and less internal damage, low permeability samples need relatively high flow initiation pressures compared to high permeability samples. Total mud cake removal is observed for experiments with limestone cores while only cracks and small holes were seen for experiments with sandstones. This observation suggests that cake removal is not just a function of permeability but depends on mineralogy as well. High overbalance pressures were also found to increase the magnitude of the flow initiation pressure.