Dip-Log Computer Chart
- Gilbert Swift (Well Surveys Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- September 1959
- Document Type
- Journal Paper
- 23 - 28
- 1959. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 5.6.1 Open hole/cased hole log analysis
- 1 in the last 30 days
- 174 since 2007
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A conventional Dip-Log consists of a set of three formation logging curves which register the depths of formation changes at three points around the face of the borehole, together with records of the orientation of the logging tool and the diameter of the well. The three curves may be electrical (resistivity) or mechanical (caliper) recordings.
The three curves which define the bedding planes are presented in essentially the same manner on all commercial dip-surveys. The orientation data, however, is presented in a variety of ways, one of which is illustrated by the Dip-Log shown in Fig. 1. In this form of presentation the direction of the well deviation with respect to the north-seeking end of the compass and the compass direction with respect to the direction of the No. 1 Arm are each recorded, by a separately identified trace, together with the three resistivity traces, in the right-hand column of the log. The compass trace is identified by being recorded as a series of dots while the deviation direction trace consists of dashes.
The diameter of the borehole and the angle of deviation from vertical are recorded by separate traces operating in the left-hand column of the Dip-Log.
While there are numerous other ways of presenting the orientation data the task of computing the amount and direction of dip is substantially the same regardless of the way in which the basic data is read from the log.
The problem facing the user of the Dip-Log is that of computing from the observed depth differences and the recorded tool orientation data, the amount of dip relative to a horizontal plane (the true dip) and its direction with respect to north.
A number of different methods have been used in the past to solve this problem. Some of these require complicated computations involving the use of trigonometric tables. Others require several separate graph sheets together with numerical computations; while still others employ costly mechanical devices which model the subsurface situation. In contrast, the present computer chart is an 8 1/2 X 11 in. printed sheet on which a simple sequence of graphical steps enables the user to obtain the solution. This chart is used with a transluscent tracing paper overlay which becomes a permanent and reproducible record of each solution. The simplification is obtained primarily through the use of a grid based on the gnomonic projection instead of the stereographic net employed in the previous graphical methods.
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