High Accuracy Directional Surveying Of Wells Employing Inertial Techniques

ABSTRACT Drilling of multiple wells from offshore platforms has greatly increased the accuracy requirements for directional surveying. To meet this need a 3-dimensional method of surveying deviated wells to within one foot to the 13 3/8" Casing Shoe has been developed. The basis of the equipment is an inertial system which consists of a gyro stabilised instrument cluster with three accelerometers measuring the displacements. The advent of Inertial techniques offers the opportunity to complement, monitor and improve conventional survey techniques. INTRODUCTION The drilling of multiple wells from offshore platforms has greatly increased the accuracy requirements for directional surveying. Conductors are commonly drilled with seven or eight foot spacing and the accuracy required in this situation is essentially beyond the capability of conventional directional surveying equipment. With such spacings between wells the possibility of well collision cannot be excluded particularly since the density of wells may increase in future. One may anticipate an increasing loss of production during concurrent drilling and production operations due to the uncertainty of hole position. The need to define the bottom hole position in cases of loss of well control has increased. The source of concern arises from the doubts about the accuracy of conventional surveys and the doubt that the position of a deep well could be known within the detection range of homing devices. TECHNOLOGY Taking the original premise - that there is concern and doubt about the accuracy of conventional surveys - a premise which will not be rigorously evaluated in this paper but which surveys will endeavour to illustrate - the first question which arises is to define alternate systems. Reliability and accuracy of gyro systems has existed for aircraft navigation and a natural progression arose to examine the more sophisticated navigation systems of military and commercial aircraft. Sophisticated inertial navigation systems were developed in the 1960's for missiles and all weather combat aircraft. More recently they were applied as survey equipment in the artillery and topographical surveys. The navigation performance for aircraft is typically 1 nautical mile per hour - accuracy sufficient to assist transatlantic crossing the system is, however, capable of increased accuracy under different operating conditions. THE SYSTEM An Inertial Navigating system can be defined as one which is independent of information from outside the system except for gravitational effects. The System consists of five main parts:Inertial platform containing accelerometers and gyroscopes mounted on three gimbals.Electronics control and computing system which processes the output signals from the inertial platform and calculates displacements and monitors the servos which stabilise the inertial platform.CDU (Control and Display Unit) which performs two-way communication between the system and the operator when initiating the system.Random Access Memory store for data storage.