Computer Evaluation of Logs
Authors: Breitenbach E.A. 
Published: December 2007

A computer program has been developed to afford rapid and complete quantitative log analysis for exploration and production decisions. The computation consists of automatic selection of tops and bottoms of porous intervals from the digitized data, and then pint-by-point calculations within each selected interval. Nearly all log types can be analyzed. This paper presents the calculation techniques found to be applicable to machine evaluation and gives examples of their use.

Introduction

As the quantitative interpretation of well logs entails repetitive use of charts and equations, it is natural that digital computer programs would he written. A Universal Log Interpretation Computer Program (ULICP) has been developed to afford rapid and complete analysis for exploration and production decisions, Application of this program enables the log analyst to:

apply rapid quantitative log analysis for exploration and production decisions on either a field, a well or on a single zone basis; apply concepts of interpretation requiring detailed numerical analysis; analyze all porous intervals on each log, rather than a few selected zones, and complete the analysis in less time than previously required. Computations are reported for every digitized point in each zone; use empirical techniques applicable to a given area as an integral part of the computation; experiment with the empirical coefficients and exponents in the interpretation equations to find the best possible solution; and automatically plot both the original and computed data to scales analogous to the field prints.

The primary utility of a log interpretation program stems from its ability to do an overwhelming amount of work with very little man-power. If a calculation procedure or thought process can be formalized to the extent that step-by-step logic can be written, a computer program can be developed that follows this logic. The major question is one of economics. A feasibility study of the costs for such a program indicates that digital processing is economical primarily as a means of increasing the productivity of the log analyst. In effect, the probability of mission productive intervals in any well, because of lack of time to do detailed calculations, is greatly reduced.

The nucleus of ULICP is programmed to compute large sections of a suite of logs by selecting zones automatically and then performing all pertinent computations on a data point by data point basis within each zone. An entire suite of logs can be processed in this manner with very little manual intervention. Sufficient programming logic is available so that each log analyst can request computations pertinent to his area. These requests are made by simple additions or deletions of information on the input header cards. Hence, the log analyst is always in complete control of the computation process. The evaluation of a suite of logs requires pre-editing. digitization, computation, presentation of results and interpretation. Work by the log analyst is required only in pre-editing and interpretation. Thus, he is allowed more time for comprehensive interpretation, rather than calculation. For continuity, the discussion of ULICP is organized sequentially: pre-editing, digitization, computation and presentation of results.

Discussion

PRE-EDITING

The extent of pre-editing prior to computation is dependent on the format of the original data. For analog prints, it requires inspection, correlation and editing of the logs, plus the entering of required data on special forms. For digitized data such as magnetic tape field recordings. only the special forms are necessary. The process for analog prints will be given here. The inspection and correlation process involves the selection of sections for digitization and the correlation of the traces to a common depth. To decrease the cost of digitization, traces that exhibit large variations in shale formations can be redrawn to a non-zero baseline. The next step is to enter all pertinent data on the input header cards. The header cards presently used for ULICP are presented as Figs. 1 through 6. Cards 1, 2, 3 and 4 (Fig. 1) are defined as the Main Header Cards. They describe the particular well for output identification and give basic information. Cards 5 through 15 (Figs. 2 through 6) are defined as the Block Header Cards. As such. they define the log types and the interpretation parameters for the block of data immediately following. Cards 1 through 10 are required for every computer run. Cards 11 through 14 pertain only to nuclear logs. Card 11 is required if any nuclear log is Supplied, and cards 12 through 14 are required only when gamma-ray. density or neutron log data, respectively, are supplied. Card 15 (Fig. 6) must be supplied when cross-plot calculations are required.

JPT

P. 493ˆ