Abstract

Wellbore instability encountered while drilling troublesome shale formations is a problem common to most major oilfields in the world. Indonesia is no exception. Despite concentrated efforts to control this problem, drilling and completion operations continue to be plagued by hole enlargements, poor cement jobs, tight hole, stuck drill pipe, fill on bottom after trips, solids build-up in the mud-all resulting in higher costs and possible loss of the well. This paper discusses the various techniques used to minimize and possibly eliminate shale instability through proper selection and control of the drilling fluid.

The greatest cause of shale instability is an unfavorable reaction of the drilling fluid with sensitive formations. The various techniques to minimize and possibly eliminate this reaction, through the proper selection of drilling fluids, require a knowledge of the type shales causing the instability. Thus, the first section of this paper will classify shales. The second section of the paper will cover the types of fluids that provide maximum shale stability. The third section will discuss laboratory tests required prior to making a fluid selection. The concluding section illustrates specific shale stability problems encountered in Indonesia.