Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 145

Fig. 11.7 Effects of logging speed and time constant on gamma ray log
Perturbing Effects on Gamma Ray Logs
Gamma ray logs are subject to a number of perturbing effects including:
Sonde position in the hole (centering/eccentering)
Hole size
Mud weight
Casing size and weight
Cement thickness
Since there are innumerable combinations of hole size, mud weights, and tool
positions, an arbitrary standard set of conditions is defined as a 3 5/8″ OD tool
eccentered in an 8″ hole illed with 10-lb mud. Service company chart topics pro-
vide analog systems for manually applying corrections but modern logs are almost
universally subjected to environmental corrections in real time as the logs are being
run that take into account the disturbing effects of temperature, hydrostatic pres-
sure, mud weight, hole and casing size, etc.
Estimating Shale Content from Gamma Ray Logs
Since it is common to find radioactive materials associated with the clay minerals
that constitute shales, it is a commonly accepted practice to use the relative gamma
ray deflection as a shale-volume indicator. The simplest procedure is to rescale the
gamma ray log between its minimum and maximum values from 0 to 100 % shale.
A number of studies have shown that this is not necessarily the best method, and
alternative relationships have been proposed. To further explain these methods, the