The S.P. Log: Theoretical Analysis and Principles of Interpretation

Abstract

Abstract The S.P. log is shown to be a measurement of the potential drop along thedrill hole, caused by ohmic effect in the mud. The notion of static S.P. isbrought forward, and its relation to the S.P. log is discussed. Other factorsinfluencing the shape and amplitude of the log are considered; attention isgiven to conditions encountered in practice. Numerous figures are givenillustrating graphically the results; these figures are of particular interestfor comparison with field examples. The S.P. log, although indicating permeability, is not an absolutemeasurement of permeability, nor of porosity, of the formations traversed by adrill hole. It is affected by several parameters, such as resistivity offormations and mud, thickness of formations, and others, which should beappraised carefully. Simple rules have been established for a betterdistinction of the boundaries of permeable sections, particularly in difficultcases, such as those encountered in highly resistive formations. A systematicapplication of the established principles will assist in obtaining moreinformation from the S.P. log than was possible thus far; for instance, underfavorable conditions, presence of oil may be detected, or amount of shale insands may be estimated. Introduction The S.P. log, or spontaneous potential log, has been known and widely usedduring the last 15 years for the location of permeable beds traversed by drillholes. In electrical logging practice, the S.P. log is shown on the left hand sidetrack of the record (as may be seen in later examples) where it can be easilycorrelated and interpreted with the resistivity curves located to the right.Usually, the S.P. log consists of a base line, more or less straight, havingexcursions or "peaks" to the left. The base line frequently has beenfound to correspond to impervious beds, while the peaks are usually foundopposite permeable strata. Measurements which will indicate positively the presence of permeability inthe formations, and which will give accurately the boundaries of the permeablezones, are of great importance in oil-field practice. Thus far, the S.P. log isthe best approach to such determinations; unfortunately, its interpretation isnot always evident. With respect to the base line of the S.P. log, it may be noticed that thisline is not always at a definite location on the chart. Sometimes it may shiftabruptly, while other times a gradual drift is apparent. As far as the peaks are concerned, their shape is not uniform; some arerounded while others are sharp. Also, from other data, it may be found thatoccasionally the peaks extend appreciably beyond the boundaries of permeablezones into zones which are not everywhere permeable. A comparison with permeability measurements made on cores has oftenconfirmed that there was no definite correspondence between the magnitudes ofthe peaks and the permeability values. T.P. 2463