Mechanisms of Differential Sticking and a Simple Well Site Test for Monitoring and Optimizing Drilling Mud Properties

Abstract

Abstract Differential sticking has a major impact on drilling efficiency and well costs. Although some problems can be related to the characteristics of rock formations, or to drilling parameters not related to drilling mud properties, it is widely accepted that the mud formulation and condition strongly influences the risk and severity of differential sticking incidents. In this paper we use our own laboratory data and information from industry publications to formulate the mechanisms of differential sticking and the key drilling mud parameters that can influence the process. Using this knowledge, we have built a simple and robust device for measuring the sticking properties of muds in the laboratory or at the well-site. This device comprises a half area HTHP fluid loss cell which is modified to allow a metal sphere to be placed in contact with a growing mud filtercake. The torque required to rotate the sphere free from the mud cake is determined using an electronic torque gauge. We find that a plot of torque against the 3/4 power of time (this power term derives from the build-up of cake around a spherical object) gives a linear fit to the data, and that the slope of this line is indicative of the sticking potential of the mud. An extensive analysis of laboratory-mixed and field muds indicates that the test gives a good indication of mud condition and allows the user to investigate various options for reducing the sticking potential of the fluid. This information is useful both in the initial selection of mud type and in making technical and economic analyses of various mud treatment options. In addition to the laboratory evaluation described above, the sticking device has been used in several wellsite tests with encouraging results. Some of these are described.