Analysis of Naturally Fractured Reservoirs From Sonic and Resistivity Logs
- Roberto Aguilera (Colorado School of Mines)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- November 1974
- Document Type
- Journal Paper
- 1,233 - 1,238
- 1974. Society of Petroleum Engineers
- 5.8.7 Carbonate Reservoir, 3.3.1 Production Logging, 1.14 Casing and Cementing, 5.8.6 Naturally Fractured Reservoir, 5.6.4 Drillstem/Well Testing, 5.6.1 Open hole/cased hole log analysis
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This study was an effort to find a reliable means of detecting and analyzing fractured systems from logs. It appears from the cases considered that the porosity exponent m is an adequate criterion to detect such a system and that the parameter P, a function of formation resistivity and sonic velocity, provides a reliable means of distinguishing the hydrocarbon-bearing zones.
Detection and evaluation of naturally fractured reservoirs is becoming more important every day. This investigation was undertaken to try to find a reliable means of detecting and evaluating fractured systems from sonic and resistivity logs. A theoretical model developed for detection purposes indicates that for such systems the porosity exponent m (commonly referred to as cementation factor) should be relatively small, ranging between 1.1 and 1.3. This criterion has been successfully used in recognizing natural fractures in different wells of the Altamont trend. (It must be pointed out that the method is limited to gross sections where a statistically significant number of zones is available, some of which are 100 percent water saturated and fractured.) For evaluation purposes a previously defined parameter, P, has been used. This parameter is a parameter, P, has been used. This parameter is a function of formation resistivity and sonic velocity. By determining the mean value of P at a water saturation of 100 percent, it is possible to evaluate the resistivity index, I, for hydrocarbon zones, and hence values of water saturation. This technique has also been used in wells of the Altamont trend. For illustration, analyses of a commercial well and of a noncommercial well are presented.
The idealized model considered in this study is presented in Fig. 1. It is similar to the one presented by Warren and Root to analyze pressure behavior in fractured reservoirs and to the one presented by Towle to study the relationship between formation resistivity factor and porosity. In this model it was assumed that the spacing between parallelpipeds represented the fractures. This assumption does not appear to be unreasonable. Since porosity is defined as the ratio of the void space in a porosity is defined as the ratio of the void space in a rock to the bulk volume of that rock, porosity could be expressed for the case of this model as
= 1 - X 3,...............................(1)
= porosity, fraction, X = length of the block, fraction.
The resistance of each block is given by the equation
r = RL/A,...................................(2)
R = resistivity L = length of current path A = cross-sectional area of the current path.
As defined by Archiel the formation factor is given by
F = Ro/Rw,..................................(3)
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