Small-Diameter Inertial-Grade Sensors Significantly Enhance Wellbore Survey Accuracy

Abstract

Tests with a new directional-survey instrument show a significant enhancement in the accuracy of defining a wellbore trajectory. Several prototype systems developed over the previous two years by Gyrodata Inc, prototype systems developed over the previous two years by Gyrodata Inc, in conjunction with Incosym Inc., a developer of guidance systems and components for the aerospace industry, have successfully undergone tests in the laboratory and in the field. The testing period, which included both land and offshore operations, indicates that the original design objective for borehole position uncertainty less than 1.7 feet per 1,000 feet of hole has been met. Although prototype development has been completed to the extent that criteria for manufacturing have been established, an ongoing testing programme is in effect to verify field accuracies and system application in a wider range.

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 systems measuring techniques and operational procedures, and additional benefits 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.

Introduction

“In a paper presented in 1980, Wolff and de Wardt- 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 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.” This was the beginning of the predicting borehole position uncertainty.” This was the beginning of the referenced paper by Uttecht and de Wardt, of which this paper is a follow-up and extension.

As in the previous paper the central topic to be discussed is the development of a new gyroscopic-based technology which, the authors believe, has the potential to meet the requirement for -wellbore survey accuracy in most situations. While some of the information will be similar in order to view the technology in Us proper perspective, this paper departs from its predecessor in several ways. More test data has been gathered since the last writing, including; field experience on offshore platforms, and some surprising things have been learned concerning the platforms, and some surprising things have been learned concerning the procedures for running survey instruments through the wellbore. procedures for running survey instruments through the wellbore.