Page 43 - Basic Well Log Analysis for Geologist

CHAPTER II

Oe

THE SPONTANEOUS POTENTIAL LOG

General

This chapter and succeeding chapters (II through V) Bed thickness—As a formation thins (i.e. < 10 feet
introduce the reader to specific log types such as SP, thick) the SP measured in the borehole will record an SP
resistivity, porosity, and gamma ray logs. The text discusses value less than SSP (Fig. 10b). However, the SP curve can
how different log types meusure various properties in the be corrected by chart for the effects of bed thickness. As a
well bore and surrounding formations, what factors affect general rule whenever the SP curve is narrow and pointed in
these measurements, where a particular curve is recorded, shape, the SP should be corrected for bed thickness.
and how data are obtained from the log using both charts Bed resistivitv—Higher resistivities reduce the deflection
and mathematical formulas. of the SP curves.
The spontaneous potential (SP) log was one of the earliest Borehole and invasion—Hilchie (1978) indicates that the
electric logs used in the petroleum industry, and has eftects of borehole diameter and invasion on the SP log are
continued to play a significant role in well log very small and, in general. can be ignored.
interpretation. By far the largest number of wells today have Shale content—The presence of shale in a permeable
this type of log included in their log suites. Primarily the formation reduces the SP deflection (Fig. 10b). In
spontaneous potential log is used to identify inipermeable water-bearing zones the amount of SP reduction is
zones such as shale, and perincauble zones such as sand. proportional to the amount of shale in the formation. In
However, as will be discussed later, the SP log has several hydrocarbon-bearing zones the amount of SP reduction is
other uses perhaps equally important. greater than the volume of shale and is called “hydrocarbon
The spontaneous potential log ts a record of direct current suppression” (Hilchie, 1978).
(DC) voltage differences between the naturally occurring The SP response of shales is relatively constant and
potential of a moveable electrode in the well bore, and the follows a straight line called a shale baseline. SP curve
potential of a fixed electrode located at the surface (Doll. deflections are measured from this shale baseline.
1948). It is measured in millivolts. Permeable zones are indicated where there is SP deflection
Electric currents arising primarily from electrochemical from the shale baseline. For example, if the SP curve moves
factors within the borehole create the SP log response. either to the left (negative deflection: Ry > Ry) or to the
These electrochemical factors are brought about by right (positive deflection: Ry < Ry) of the shale baseline,
differences in salinities between mud filtrate (R,,~) and permeable zones are present. Permeable bed boundaries
formation water resistivity (R,,) within permeable beds. are detected by the point of inflection from the shale
Because a conductive fluid is needed in the borehole for the baseline.
SP log to operate, tt cannot be used in non-conductive (i.e. But. take note, when recording non-permeable zones or
otl-based) drilling muds. permeable zones where Ry, is equal to Ry, the SP curve
The SP log is reeorded on the left hand track of the log in will not deflect from the shale baseline. The magnitude of
track #1 and is used to: (1) detect permeable beds, (2) SP deflection is due to the difference in resistivity between
detect boundaries of permeable beds. (3) determine mud filtrate (Ry) and formation water (R,,) and not to the
formation water resistivity (R,,), and (4) determine the amount of permeability.
thickness, bed resistivity, invasion, borehole diameter. shale County. Oklahoma. In this example, the SP curve is used to
volume of shale in permeable beds. An auxiliary use of the »
SP curve ts in the detection of hydrocarbons by the
Calculated from the SP Curve
\_ suppression of the SP response. Resistivity of Formation Water (R,,)
_ The concept of static spontancous potential (SSP) is Figure i] is an electric induction log with an SP curve
“important because SSP represents the mavinion SP that a from a Pennsylvanian upper Morrow sandstone in Beaver
thick, shale-free, porous and permeable formation can have
fora given ratio between R,,,/R,,. SSP is determined by
find a value for Ry by the following procedure: After you
determine the formation temperature, you correct the
formula or chart and is a necessary clement for determining
resistivities (obtained from the log heading) of the mud
accurate values of R, and volume of shale. The SP value
filtrate (Ry) and drilling mud (R,,) to formation
that 1s measured in the borehole Is influenced by bed
Next, to minimize for the effect of bed thickness, the SP
content, and most important—the ratio of Ryy/Ry, (Fig. 10a). temperature (see Chapter I).

28