Computerized Log Analysis for Efficient Evaluation of Gas Wells and Gas Storage Reservoirs
Authors: Schmidt A.W. , Tinch D.H. , Carpenter B.N. , Hoyle W.R. 
Published: December 2007

SCHMIDT, A.W., SCHLUMBERGER-DOLL RESEARCH CENTER, RIDGEFIELD, CONN. TINCH, D.H., SCHLUMBERGER-DOLL RESEARCH CENTER, RIDGEFIELD, CONN.MEMBERS AIME CARPENTER, B.N., SCHLUMBERGER-DOLL RESEARCH CENTER, RIDGEFIELD, CONN. HOYLE, W.R., SCHLUMBERGER-DOLL RESEARCH CENTER, RIDGEFIELD, CONN.

Abstract
Open-hole log interpretation methods are employed to provide effective porosity data that are then used with cased-hole neutron logs to compute gas saturations in storage reservoirs. A method of using density and neutron logs to solve the problem of porosity determination in shallowly invaded gas reservoirs is developed. A system of digital magnetic tape and a computer was used to process the large amounts of log data.
Line printer frequency plots and customized "Computed Logs" are graphical techniques developed to display the computer output in analyzable form. The computed logs were used in constructing cross-sections as well as porosity and hydrocarbon fraction maps. The computer was used to integrate these maps into total reservoir and gas storage volumes.

Introduction
In the evaluation of gas-producing or gas-storage reservoirs a primary necessity is the accurate determination of porosity. When the lithology is known and the formation is fully liquid saturated, porosity usually may be determined with a single log-density, neutron, or sonic. However, porosity determination from logs is frequently complicated by unknown lithological characteristics. The presence of gas further complicates accurate porosity determination. These difficulties are overcome by combining several porosity-sensitive devices; for example, the interpretation of porosity in the presence of gas is done with the combination of open-hole density and neutron logs. Such multiple-log interpretations can be handled by interpretation charts and cross-plot techniques. If made over many levels, the work becomes long and tedious.
For gas storage, logs are used to monitor gas in place. This is done by combining open-hole porosity data with cased-hole neutron logs.' The neutron logs are run at periodic intervals or after periods of large gas injection or withdrawal. Evaluation of these monitoring logs on a fieldwide basis has tended to be qualitative rather than quantitative because of the manpower required for manually processing the large amount of data.
The labor of these techniques and the necessity for handling large amounts of log data invited the application of digital computer techniques to these methods. This requires that the data be in computer-compatible digitized form, preferably on magnetic tape. Work on large-scale projects has been greatly expedited by the introduction of equipment for recording logging signals on digital magnetic tape at the well site. This equipment, introduced experimentally in the Illinois basin in 1965, is now in general use in the U.S.
Along with appropriate computer programs, graphic techniques were developed to display as logs the computed results from these tapes.' In computed log form it is easy to scan quickly the zones within a well and to compare or correlate between wells.
In this paper we will discuss in particular the application of computer processing to:
The use of open-hole logs for determining porosity in gas-bearing zones. Attention also will be given to determining the amount of gas originally present in new wells. The evaluation of neutron logs through casing in order to monitor changes in amounts of gas in place. The application of these results from individual wells to field-wide studies.
A few comments will be made on the data processing equipment and techniques.

Reservoir Evaluation from Open-Hole Logs
Two log combinations are used to illustrate computerized techniques in gas reservoir evaluation.
For clean formations: density and neutron logs for porosity, plus an induction log for water saturation. This combination is used to handle the case of shallow invasion in which gas is present within the range of investigation of density and neutron logs. For shaly sands: sonic, density, and gamma ray logs. This combination yields values of effective porosity and shaliness.

Shallow Invasion of Gas-Bearing Formations
It is worthwhile at this point to review how the interpretation of the different logs may be complicated by the presence of gas close to the borehole.
JPT
P. 959ˆ