Interactive Digital Wellsite Formation Evaluation - The Prolog System
Authors: Frost Elton , Fertl Walter 
Published: December 2007

Summary
The advent of digital well logging equipment and associated interpretive software allows sophisticated handling of sonde calibrations, trouble-free logging operations, and accurate wellsite log interpretation analysis. Prolog, an interpretive computerized wellsite log analysis package incorporated into our computerized logging service (CLS) units, facilitates reliable, on-the-spot decisions in a minimum of time. Prolog is based on two basic analysis packages, Wellsite CRA for complex reservoir analysis and Wellsite SAND for clastic sediment sequences. Both CRA and SAND present detailed formation evaluation in a four-track format supported by depth shift routines, a true vertical depth option, data-edit routines including borehole (washout) and environmental (invasion) log response corrections, and crossplot routines (frequency, Pickett, Hingle, etc.). This paper outlines the basic philosophy of the interactive Prolog system, presents flow charts and pertinent equations for both the CRA and SAND programs, illustrates the edit options based on comparative log response studies, and discusses the data format of wellsite Prolog analyses based on field examples from Texas.

Introduction
Prolog is a computerized wellsite log analysis program package. The programs in Prolog are designed to enhance the information obtained from CLS units. Prolog is designed to eliminate tedious, time-consuming and error-prone wellsite hand calculations by offering several unique advancements for fast, reliable, computerized interactive formation evaluation at the wellsite. Prolog programs include the following. DSHFT. CLS log suites may be put on depth to a base log by depth shifts, depth compression, or depth expansion. TVD. CLS log data may be corrected to true vertical depth based on directional survey or Diplog information CROSS. Sophisticated cross plotting capability including normal porosity crossplots, x-y and z plots. histograms, and Hingle and Pickett plots. EDIT. Environmental and borehole rugosity corrections. SAND. Detailed wellsite analysis in clastic formations, such as sand/shale sequences. CRA. Detailed wellsite analysis in complex lithologies, such as carbonates. All Prolog programs automatically handle both customary and metric logging units.

Prolog DSHFT
Program DSHFT allows CLS log data to be shifted, compressed. or expanded in depth. The program will shift any curve or combination of logging curves. Under normal circumstances there is little need to use DSHFT, since the CLS data normally are real-time merged. Extensive experience has shown that real-time merged logs normally exhibit less than a 0.5-ft depth discrepancy between log runs. This is accomplished by a sophisticated merging technique (Fig. 1) that allows the user to monitor a correlation curve from the previous log run for depth correlation along with data from the logging run in progress. This ensures that the logs are on depth with respect to each other.

Prolog TVD
Program TVD creates a tape correlated to true vertical depth with the radius of constant curvature technique. The program requires input of data pairs comprised of indicated depths and borehole angles normally obtained from a Diplog or directional survey.

Prolog CROSS
Program CROSS is a multipurpose program designed to provide the wellsite log analyst with a wide variety of crossplot capabilities using CLS log data. Program options includefrequency crossplots,z-axis crossplots,histograms,special function Hingle and Pickett crossplots, andspecial porosity and water saturation listings.
JPT
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