Prevention of Shale Instability by Optimizing Drilling Fluid Performance

Abstract

This research work outlines the main results of the shale instability study with respect to shale/ fluid interaction mechanisms, and also analyses the shale strength properties. An approach was undertaken to develop a better understanding of the effects of using polymer drilling fluid on the shale mechanical properties and how to optimize its performance. Wellbore stability in shale is very much influenced by the type of drilling fluid used, here the problems observed were, borehole fluid invasion into the shale, increase in potential pipe sticking, swelling/hydration of shale which results in high level of non-production time (NPT). Different drilling fluids were analyzed for their effects on shale strength. Emphasis here was on the mechanical properties of the shale samples after exposed to different drilling fluids for 24hours. The mud which maintains high compressive strength of the shale is the most preferred (optimum) selection. The shale sample exposed to the polymer drilling fluid shows the lowest strength, from the stress-strain curve. The use of KCl brine in drilling fluids is based on its shale inhibition characteristics as it is thought that the k+ ion provides additional attraction among the clay platelets due to its size and charge. However the shale loses most part of its strength when exposed to the modified polymer drilling muds combined with 5% KCl, 7%, 10% KCl respectively. One of the possible reasons is Kaolinite in the shale. The shale samples will keep more of its original strength when exposed to the modified polymer drilling muds combined with chemical agents with plugging ability to pore throat and micro-fracture. The shale sample exposed to polymer drilling fluid with 10%Nacl, 5% Physical sealing agent and 10% Chemical agent A1 (rich in silica) exhibit the highest strength. From the result of the experiment the drilling fluid density has been reduced immensely, thus the new mud system improves the wellbore stability and keeps the shale formation stable.