2005 Volume 114 Issue 6 Pages 885-900
Characterizing the transport properties of reservoir-forming rocks is one of the most important tasks in reservoir engineering. We review the relationships among permeability, porosity, electrical formation factor, and electrokinetic coupling coefficient under saturated and unsaturated conditions on the basis of the capillary tube model of porous medium, by which one can relate the microscopic physics of the transport properties to the macroscopic behaviors described by Darcy's and Ohm's laws and the cross-coupling effects. These relationships together with the recent models of clay rich sandstones provide a useful guideline for interpreting core, logging, and geophysical survey data. Among various rock properties, permeability in particular needs in situ measurements such as pressure transient tests, because in situi values are usually at least a few orders of magnitude larger than those measured for intact core samples due to the presence of discontinuities such as fractures in reservoirs. Concerning this topic, the concept of fractured rocks, i.e., the double porosity medium and how to characterize fractured reservoirs are described.
Even if the results of extensive field-wide pressure transient tests are available, in addition to drilling and various exploration data, numerical models of reservoirs are never precise, due to the problem of non-uniqueness. However, once exploitation begins in earnest, additional data become available such as temporal trends in downhole flowing pressure and enthalpy (in case of geothermal reservoirs), which may be used in history-matching studies. Because uncertainty in predictions of numerical reservoir models is directly related to the amount of field data available against which the models can be tested, it is clear that the addition of repeat geophysical survey data to the list of pertinent field measurements is likely to improve the reliability of these forecasts. Recently developed computational tools such as the EKP-postprocessor, which can calculate changes in self-potential distribution through electrokinetic coupling caused by changing underground conditions computed by reservoir simulation, enable us to use geophysical monitoring data in history-matching studies.
