Geophysical well logging : geological wireline logs and fracture imaging

Abstract

Well logging is a highly advanced technique where complex electronics and sensors are placed inside a logging probe which is lowered on a wireline into a well to carry out measurements continuously or at discrete depth intervals as the probe is moved down or up the well. The objective of the logging can be for example, (1) to study the well, its geometry and completion, (2) to study the rock formation and fractures intersected by the borehole, (3) to determine the reservoir temperature and fluid pressures, and (4) to locate feed points connecting the well to the geothermal reservoir. Well logging has been used systematically in Iceland since 1976 to study and explore geothermal wells, not only the classical logs of temperature and pressure, which the geothermal industry utilizes extensively, but also geological logging tools and logs that are used to study the construction and condition of the well. The present paper gives a brief description of geological wireline logs applied in geothermal exploration in Iceland as well as logs that create an image of the borehole walls. The latter (televiewer log) produces an acoustic “picture” of the wall of the well, where fractures can be easily mapped and their strike and dip determined and other properties of the fractures, whether they are permeable or not, can be studied and their slope and orientations can be determined. The geological wireline logs discussed are the electrical resistivity log of normal configuration, neutron-neutron porosity log and the natural gamma ray log. These logs give valuable information on the lithological section of the wells, the boundaries and thicknesses of the rock units and complement the drill cutting analyses.