Shale/Mud Inhibition Defined With Rig-Site Methods

Summary The successful drilling of troublesome shale requires evaluation of the shales and muds during the drilling process so that the proper methods and muds can be selected. With rig-site testing in mind, six tests were studied for their usefulness in defining the severity of the shale/mud chemical reaction. All tests showed some value in the analysis of wellbore problems and solutions; however, the swelling, rolling, and cation-exchange-capacity (CEC) tests proved most useful. The swelling test uses a newly developed device to describe the early-time swelling properties of native or recompacted shale and provides an analytical method for classifying shales according to their reaction rates. In general, soft shales have a short reaction time, which increases with its hardness. This test also shows the inhibition state of a mud and can therefore be used in mud monitoring and mud-treatment design. The rolling test provides useful information on the dispersion tendencies of a shale-cuttings/mud mixture, and the CEC test provides basic information on the effective clay content of a shale. This paper discusses techniques used to evaluate shales and muds at the rig site and presents the procedures, results, and parameters that affect each technique. Information concerning a new device for measuring shale swelling and the procedure for reconstituting shale cuttings is given.