In hard formations (dolomite - limestone - quartzitic sandstone or even volcanics) the matrix is often of low to very low porosity and of extremely low permeability. Such formations are only considered as potential producers when fractures and fissures drain the hydrocarbons to the well producers when fractures and fissures drain the hydrocarbons to the well bore. It is therefore extremely important to detect the presence of fractures, and several logging tools are able to give a good and reliable information about the location and even the orientation of those fractures. The combination Dual Laterolog-Microspherical log is one of the fracture indicators used nowadays. When analysing the process of mud or mud filtrate invasion in a fractured reservoir and observing the effect on the resistivity logs it was discovered that the Dual Laterolog itself was a fracture indicator and that the difference in conductivity between the two measurements deep and shallow was a function of the volume of hydrocarbon displaced from the fracture system during the invasion process. It allows to compute a lower limit of the fracture porosity-the technique has been successfully applied in various formations and examples are presented.
During the past 10 years numerous papers have been written on fracture detection. They describe how the various logging tools are affected by the presence of fractures and how they detect the fractured zones.
Resistivity tools have a particular behaviour in front of fractureformations-specially the microresistivity devices.
The porosity tools such as the Formation Density and the Compensated Neutron porosity will be affected by the fractures. The correction curve of the Formation Density Log is considered as an indicator as well as the caliper of this tool.
The Sonic Log in highly fractured zones could give cycle skips as a result of very weak signal.
The Variable Density log will show perturbed sonic waves which can be attributed to the presence of fracture planes.
The Dipmeter tool will provide a rather interesting Fracture Identification Log (FIL) an even the strike of the fracture system could be estimated.
The Gamma Ray or the Natural Gamma Ray Spectroscopy tool could show abnormal high radioactivity levels explained by deposition of Uranium salts in the fractures.
The Pe Curve of the Lithodensity tool can be an excellent fracture indicator in wells drilled with baryte mud as a result of the large photoelectric cross-section of the barium contained in the mud weighting material.
Finally Temperature and Flowmeter surveys run during injectibility tests can prove the presence of fractured zones.
Computer programs have been written to analyse the responses of the various fracture indicators and a fracture probability index is computed.
Figure 1 shows an example of the most attractive fracture indicator, the FIL. Fractures are indicated by overlaying the 4 resistivity curves of the dipmeter but are also detected when observing their effect on the rotation of the tool.
Figure 2 is an example of detection of fractures using the Pe curve of the Lithodensity Log.
On Figure 3 a "Geothermal Global" regroups the various fracture indicators and a GLOBAL * evaluation in a Geothermal well.
All these techniques however only allow the detection of fractures and fissures and in fact most of the fracture indicators mentioned above will reflect the abnormal rugosity of the well bore as a result of the presence of weak points in the rock. In addition if only fractures crossing the well bore are "seen" fissures and fractures induced by the drilling will also be detected. Even if a "fracture density" or a "fracture probability index" can be evaluated none of the fracture indicators give a quantitative value of the "Fracture Porosity". An attempt is made to quantify the secondary Porosity Index" (SPI computed at the end of the Coriband processing. It corresponds to the difference between the Coriband final porosity, computed from the Formation Density and the Compensated Neutron Porosity and the Sonic Porosity derived from the Sonic travel time assuming a matrix travel time corresponding to the average matrix density of the Coriband. Such Secondary Porosity Index included the porosity not seen by the Sonic Log, mainly the fractures and the vugs. This Porosity index is subject to borehole effect and could be affected by matrix evaluation problems in very complex lithology. problems in very complex lithology.
Since 1973 one of the most effective techniques to detect open fractures in hard formations was to overlay the recently introduced Dual Laterlog with the Microlaterlog or the Proximity Log. Sharp and erratic variations of the Microresistivity readings were interpreted as fractures. On the other hand when the microdevice pad is passing in front of tight formations, the microresistivity curve pad is passing in front of tight formations, the microresistivity curve overlays the deep and shallow laterlogs. Later on the Microspherically Focused log was combined with the Dual Laterlog providing the more appealing quick-look presentation we use now.
However a closer look at Dual Laterlog - Microspherical log combination run in various hydrocarbon bearing fractured hard rock reservoirs around the world would reveal that, often, large separations are observed between the two laterlog readings in front of the "fractured zones as detected by the behaviour of the micro-device.