Downhole Measurements of Drill String Forces and Motions

Abstract

A self-contained downhole recording instrument was developed and used to measure and record drilling string forces and motions. The eight signals recorded by pulse-width modulation on magnetic tape were: axial, torsional, and bending loads; axial, angular, and lateral accelerations; and internal (pipe) and external (annular) pressure. The device was used over a two-year period to collect data in fifteen wells under a wide range of drilling conditions. After about nine minutes of cumulative recording time, the tool was retrieved and brought to the surface. Data were converted from the magnetic tape to analog type oscillograph display and, in some cases, were digitized for analysis purposes. Normal variations in measured downhole bit load usually ranged between 25 and 50 percent of the mean value. Maximum bit loads reached over 3.5 times mean loads in some instances. Frequencies of weight, torque and bending traces showed evidences of rock bit tooth action, of cone action, of rotation, and also of pump pulsations. Large annular pressure variations accompanied large load variations.