When drilling shale formations with water-based drilling fluids, wellbore instability problems arise from the interaction of water with clay minerals. As a result, there will be higher solids loading into the wellbore and the borehole diameter will not be constant along the depth of the well. Hence, the chance to get pipe stuck increases and the hole cleaning efficiency of drilling mud decreases significantly. To combat shale drilling problems, different types of inhibitive water-based muds were developed. The performance and effectiveness to inhibit shale formations vary between one type of shale inhibitors to the other. Oil and gas industry has standard procedures to test the inhibition performance of drilling fluids such as dispersion test and swelling test. This paper builds on that and introduces a method to test the long-term shale inhibition capability of shale inhibitors. Due to the presence of some unique geological conditions, sometimes there is a need to replace a high cost inhibitive mud by a low cost non-inhibitive mud to drill the next formation. If the original inhibition provided by the inhibitive drilling fluid does not last long enough to drill the second zone, the previously drilled inhibitive shale formation starts reacting again with the non-inhibitive drilling fluid leading to various drilling problems. Hence, there is a need for isolation of the reactive shale zone by a casing string before drilling ahead. Inhibitive muds with long-term inhibition potential such as 24 hours or more may eliminate the need of a protective casing for the previously drilled shale formation.