A New Nuclear Log for the Determination of Reservoir Salinity

The salinity log was developed as a means for determining the presence and degree of salinity in formations surrounding the borehole. Being a radiation- or nuclear-type log permits its application in wells that have been cased and in wells where air or other gases may have been the drilling fluid. Two curves ("chlorine" and "hydrogen") are recorded simultaneously with a single detector, and these curves are compared in the salinity determination. The two curves have been independently calibrated to show their response to changes in salinity and borehole size. The salinity log has been run successfully in various oil-producing areas of the United States and Canada. Because of changes in the invaded zone with time, the log is most useful when run several days after cementing casing in the well. Introduction Everyone familiar with the interpretation of electric logs recognizes that in many cases these logs do not provide positive answers. This is particularly true of the logs that were recorded five or more years ago. To at least partially satisfy the needs created by these limitations of the electrical jog approach, the salinity log was developed. In addition to not having some of the open-hole limitations of the electric log, the radioactive-type salinity log can be recorded through casing. It is believed that the invaded zone undergoes changes subsequent to the setting of casing in a well. This change may be effected by fluid migration, diffusion of formation fluid and drilling filtrate, gravity segregation, etc., and the rate of change will be influenced by the types of fluids involved, formation permeabilities and pressure differentials. Therefore, the maximum value from the salinity log probably will be realized by running it after sufficient time has elapsed subsequent to setting casing to allow the invaded zone to have effectively dissipated.