Application of Small-Diameter Inertial Grade Gyroscopes Significantly Reduces Borehole Position Uncertainty
Initial tests with a new directional survey tool show a significant enhancement in attainable accuracy over conventional instrumentation. Two prototype systems, developed over the last two prototype systems, developed over the last two years by Syrodata, Inc., have recently been tested in a well in West Texas. Although many more tests are required, preliminary results indicate that the original design objective for borehole position uncertainty less than 1.7 feet per 1,000 feet of hole has been met. The Gyrodata Wellbore Surveyor employs an inertial grade rate gyro adapted from the aerospace industry. In combination with its other sensors and electronics, the device can sense the orientation of the earth’s spin vector at each independent survey station. As a result, the major systematic errors associated with conventional gyros — geographical reference and unaccountable drift — are eliminated. Other sources of inaccuracy are minimized by the system’s measuring techniques and operational procedures, and additional benefits should arise procedures, and additional benefits should arise from faster survey speed and increased reliability. A true north reference device can also employ a small outside diameter since it requires only one gyro and one accelerometer, rather than the two or three of each needed in an inertial navigation system.
In a paper presented in 1980 , Wolff and deWardt concluded that uncertainty in directional surveying was worse than had been assumed, and the magnitude was cause for concern in conducting efficient drilling and production operations. These conclusions were based on an analysis of surveys in several North Sea wells and the development of a new model for predicting borehole position uncertainty. Discrepancies between results, when more than one survey had been conducted on the same well, had led to the initial concern with the problem. The differences were large and in no way justified by the model used at that time to generate uncertainty ellipses. This model assumed that the major measuring errors consisted of mostly random components, which had a tendency to compensate one another during the course of the well. Conversely, the Wolff/deWardt model showed that the errors were largely systematic in nature and could not be negated from one survey station to the next. As a result, uncertainties with the systematic model were often an order of magnitude larger.