Directional Survey Operations Management
Authors: Thorogood John L. 
Published: December 2007

"‘A dialog must take place between drilling engineers and the end user because survey programs cannot be constructed in isolation without reference to cost, instrument capabilities, and operational practicalities."practicalities."

Introduction

Directional surveying usually is discussed in terms of distinct topics, such as specific instrument types, survey-data-reduction procedures, or instrument error analysis. procedures, or instrument error analysis. Rarely have the three themes been combined into a single framework within which the reasons for, and costs of, any particular survey program can be clearly balanced against each other. The survey program for any well can be based on three general rules:tools intended for use in a well must be qualified for the application;accuracy objectives must be specified for each well; andthe proposed program must be shown to satisfy proposed program must be shown to satisfy the well's objectives. The purpose of this paper is to review some of the issues underlying the management of survey programs and to encourage the end users of well survey data to adopt a critical and d mating attitude toward, the information on which they base important reservoir engineering and petrophysical decisions. petrophysical decisions. Instrument Qualification

The accuracy specification of a survey tool is an essential consideration. To manage survey operations effectively, however, it is much more important to be able to predict the behavior of an instrument reliably in any given operational situation. The precise method used to model performance is not critical, but given the pivotal Tole in the process of survey operations management, process of survey operations management, outlined in Ref. 1 and shown in Fig. 1, the performance specification for each tool must performance specification for each tool must be thoroughly validated. For a survey tool to be qualified properly for use, two essential issues have to be addressed: performance specification and quality assurance. performance specification and quality assurance. Performance Specification. The behavior Performance Specification. The behavior of the tool must be fully characterized in relation to time, temperature, latitude, inclination, azimuth, and other relevant aspects of the survey environment. The validity of tills on must be rigorously demonstrated with evidence from test runs and independent analysis.

Quality Assurance.

A detailed quality-assurance procedure must exist to demonstrate to the client that the instrument has performed to specification during the survey. A set of procedures with associated tolerances must provide necessary and sufficient information on which to base an acceptance or rejection decision. As yet, little consensus exists within the industry as to how these two aspects of survey instrument performance should be approached. Together they represent the single most important area for further development in directional surveying.

General Instrument Capabilities

A frequently asked question concerns the capabilities of modem survey tools. In general, current "high-accuracy" gyroscopic tools are capable of determining wellbore position and true vertical depth to an position and true vertical depth to an accuracy that satisfies the requirements of the majority of wells being drilled today. The performance of certain gyrocompassing performance of certain gyrocompassing devices may degrade m high-angle east/west wells at high latitudes, but these limitations can be overcome by use of attitude reference systems. Extreme temperatures create difficulties for all tools, particularly where rating of the electronic components may impose severe restrictions on the available combinations of depth and survey duration. One of the main problems with gyroscopic survey systems is that their use interrupts the drilling process. Because the full cost of a gyroscopic survey includes rig time, an economic incentive often exists to acquire the necessary information as a byproduct of the drilling process, either directly from measurement-while-drilling surveys or from drop-type multishots. The new generation of solid-state magnetic multishots is irelatively cheap to run, requires minimal incremental rig time, and dispenses with the need for third-party running gear. While the sensors themselves may be accurate to 0.05 degrees inclination and 0.15 degrees azimuth, their overall performance is limited by the effect of the following external factors.

P. 1250