Analytical and Interpretive Problems of Production Logging

Abstract

Abstract Analysis of down-hole fluid movement surveys cannot be cataloged. Although there are a few so-called "cook book" methods designed to provide an easy, rapid analysis for some surveys, these should be used sparingly. When standardized interpretations are applied to fluid movement surveys with total disregard to the seemingly infinite number of variables affecting analysis, misleading results can and do occur. Cook book interpretations can cause erroneous results with the flowmeter or spinner surveys, easiest of all the down-hole fluid movement surveys to interpret. The various types of tracer and temperature surveys should never be analyzed with any sort of standardized interpretation. Some of the more common problems resulting from standardized interpretive procedures are presented in this article. Possible results of misinterpretation are presented to provide a background for more effective analysis of down-hole fluid movement surveys. Introduction Production logging, the analysis of down-hole fluid movement, generally requires more interpretation than primary logging before satisfactory results are reached. Some standard interpretation procedures have evolved in an effort to simplify the analysis of fluid movement logs. As long as these cook book procedures are recognized as being simply a basis for initiating the complete interpretation, then valid, usable results will be obtained. If a standardized form is used for the complete interpretation, the analysis of that particular production log is only partially complete, and the results may not indicate what is actually happening. Many things affect analysis of down-hole fluid movement. A few down-hole variables include hole size, laminar flow, counter or crossflow, tool efficiencies, logging speed and even, in some cases, improper running of the survey itself. Although all of these factors do not always affect the analysis of all of the various types of production logs, their effect should not be indiscriminately neglected. This article is not intended to imply that a more thorough analysis of the down-hole fluid movement survey will give the complete picture every time. However, more thorough analysis will minimize unnecessary remedial work to both injection wells and producing wells which often results from cook book methods of interpretation. Flowmeter Surveys This discussion on flowmeters is limited to the packer-type tool since analysis of the continuous flowmeter or spinner survey is generally recognized as being highly interpretive. The packer flowmeter is a very accurate tool for down-hole fluid measurement. Its major limitation is that it can measure fluid in only the pipe or borehole. The flowmeter can provide neither information about the distribution of the fluid after it leaves the borehole in injection wells nor information concerning the location of the flow channels prior to entering the borehole in producing wells. A unique problem involving profiling open-hole injection wells with a flowmeter was discussed by Jones. In this case, natural vertical fractures in the formation precluded an accurate profile by the flowmeter. Fluid flow through the spinner section of the tool created enough friction pressure to cause the fluid to bypass the flowmeter through the vertical fractures. The reduction in measured rate through the flowmeter indicated that a zone above it was taking fluid, when actually the water was reentering the borehole below the flowmeter and going on down the hole. Although the problems of obtaining accurate production profiles are less numerous and severe, one problem warrants consideration. For optimum results, flowmeter production profiles require much more down-hole information on intermittent gas-lift wells than on wells utilizing continuous gas-lift systems. The producing rate during each cycle of intermittent systems is considerably greater than the PI of the well. Consequently, an instantaneous down-hole rate obtained by a flowmeter would be totally inaccurate since the well would be pumping off. Trace A on Fig. 1 is a 15-second flowmeter recording at a point above all producing intervals, indicating a producing rate of 480 B/D. Since this is an intermittent gas-lift well and does not produce at a steady rate, a 15-second recording represents little more than an instantaneous rate. Trace B presents the flowmeter recording during the entire 6 3/4-minute lifting period. The average producing rate for Trace B is 288 B/D, but the picture is not complete until the entire gas-lift cycle is monitored by the flowmeter. Trace C presents the entire 10-minute cycle. JPT P. 993ˆ