For many years the petroleum engineer who wished to evaluate a new well or work over an old well had three basic sources of data for a reservoir performance study, (1)well logs, using the S.P. and resistivity logs for saturation and permeability indications, (2) micro-devices and core analysis for porosity and permeability values, and (3) performanceand pressure records for reservoir history.
Today's reservoir engineer has at his disposal a great variety of tools to evaluate the quality and capability of a reservoir.
Multiple porosity logs allow a more accurate definition of effective porosity and reservoir lithology.
While new porosity-logging devices allow better reservoir description in the geological sense, it remains for the new family of Production Logging (PLI tools to define in situ formation flow characteristics.
Neutron logs allow gas/fluid differentiation
Temperature logs define gas entry and fluid flow
Flowmeters allow definition of flow direction and capacity
Thermal Neutron Decay tools allow behind-easing fluid-saturation inspection
With these new tools and correct application of older, more time-tested PL devices, the concept of accurate formation evaluation from PL applications has become more acceptable and, because of this, initial completions are efficiently designedand effects of workovers can be accurately observed under downhole fluid-flow conditions.
Examples are given of porosity evaluation by multiple porosity-log analysis compared with open-hole log data. Also flow characteristics of a reservoir are given as detected by repeated PL techniques.
New tools are also described with expectations as to their effectiveness in reservoir performance evaluation.
Today's modern wireline well-logging programs consist of highly sophisticated logging devices that as little as ten years ago were not in existence. As can be seen in the firstdiagram (Figure 11, the number of logs per well over a 20year period has changed from an average of 1 in 1951 to highs of 4 in 1959 and 5 in 1969 1. There have been occasions during these two decades when a certain new type of survey would be the only auxiliary log run on a well for a specific period; eventually, however, as newer interpretation techniques are devised, the log analyst again calls for a full slate of porosity devices, and the number of logs per well increases. The present average is 3.3 logs per well.
In Figure 2 we can see how the number of logs available has grown throughout the years. This diagram shows the relative level of usage and acceptability of the primarylogs that are run on exploration and development wells. Until about 1954 or 1955, only electrical and neutron logs were available. From 1955 on through 1965, Induction, Sonic and Density logs were introduced Then, from 1965 up to the present, we have the introduction of the compensated devices such as the compensated sonic (BHC), compensated density (FOC) and, later, the sidewall or pad-contactneutron linear-neutron devices (SNP).
This means that today's well-log analyst has at his disposal a great number of logs from which he may choose and set up his logging program.