Use of Dielectric Constant Logs
Published:January 01, 1985
Standard resistivity tools, such as the deep laterolog and the deep induction log, generate an electromagnetic wave which operates in a frequency range of 35 to 20,000 hertz (where hertz = cycles/second). At these frequencies, the predominant influence on the wave is the conductivity of the substance it is traveling through. In a reservoir, conductivity is strongly influenced (of course) by the salinity of formation water.
At higher frequencies in the 30 megahertz (million hertz) to 1.1 gigahertz (billion hertz) range, the dielectric properties of a substance become very important to wave propagation (Hilchie, 1982). High dielectric constant values are associated with polar compounds like water. Since water is a polar compound, it requires energy to orient all its dipoles. Thus, an electromagnetic field moving through water is weakened. Rock matrix and hydrocarbons are both non-polar compounds with very low dielectric constants, and weaken an electromagnetic field less than water does.
The Gearhart and Dresser Atlas logs measure dielectric constant and operate at frequencies of 30 MHZ and 47 MHZ, respectively. Schlumberger's log is called an electromagnetic propagation tool (EPT)† and operates at 1.1 gigahertz. It measures the propagation time of the electromagnetic wave by reduction in wave amplitude and by shifts in the phase of the wave (Serra, 1984).
Table 7 lists dielectric constants and propagation times for various materials. It is apparent that water has a much greater wave travel time and dielectric constant than any other material on the list. Because of this, the dielectric log can be used to detect water-versus hydrocarbon-bearing zones (for a detailed discussion of the dielectric and EPT† logs, see Serra, 1984; Dewan, 1983; and Hilchie, 1982).
Figures & Tables
Handbook of Log Evaluation Techniques for Carbonate Reservoirs
Will a reservoir produce hydrocarbons? This is a particularly troublesome question in carbonates because, frequently, the answer is anything but straightforward.
Despite the best geology put together from carefully crafted depositional and seismic models, only after a well is drilled into a carbonate reservoir, can a geologist decide whether or not the well will give up commercial quantities of hydrocarbons or, indeed, any hydrocarbons at all. Besides information from surrounding wells, data from drill stem tests, cores, cuttings, and open-hole logs ensure the best basis for making a decision about a well's productivity; unfortunately, drill stem tests or core data are not always available so the geologist is forced to fall back on open-hole logs for most of his or her information.
Because of unique pore characteristics in carbonate rocks and their affect on resistivity logs, geologists can easily make some incorrect judgements. They sometimes decide a well is productive when it's not, or they sometimes overlook a good well.
Problems occur because carbonate reservoirs can have several types of porosity which include intergranular, intercrystalline, vuggy, moldic, and fracture. In addition to these different types of porosity, the pore size may be large (megaporosity) or very small (microporosity). The different pore types and sizes result from both depositional and diagenetic processes.