2 PAPERS REQUEST:
4398-PA
DOI What's this?
10.2118/4398-PA
Title Analysis of Naturally Fractured Reservoirs From Sonic and Resistivity Logs
Authors Aguilera, Roberto, Colorado School of Mines
Journal Journal of Petroleum Technology
Volume Volume 26, Number 11
Date November 1974
Pages 1233-1238
Copyright 1974
Language English
Preview 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.
Introduction
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.
Theory
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)
where
= porosity, fraction, X = length of the block, fraction.
The resistance of each block is given by the equation
r = RL/A,...................................(2)
where
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)
JPT
P. 1233
Number of Pages 6
Paper Number 5342-PA
DOI What's this?
10.2118/5342-PA
Title Analysis of Naturally Fractured Reservoirs From Conventional Well Logs(includes associated papers 6420 and 6421 )
Authors Aguilera, R., Argentina-Cities Service Development Co.
Journal Journal of Petroleum Technology
Volume Volume 28, Number 7
Date July 1976
Pages 764-772
Copyright 1976
Language English
Preview Methods are presented for detecting and evaluating naturally fractured reservoirs from porosity (sonic, neutron, and density)and resistivity logs. It is shown that the porosity exponent of a naturally fractured reservoir is smaller than the porosity exponent of the matrix. Charts have been generated for estimating the porosity exponent for these reservoirs as a function of total porosity, matrix porosity, and matrix-porosity exponent.
Introduction
The principles for the techniques presented here were described previously using sonic and resistivity logs. The approach followed in this study was to use empirical equations that had been derived for granular media in the hope that they could be useful in the analysis of fractured reservoirs. It was anticipated that this approach would result in a distinctive means of detecting and evaluating fractured media. The purpose of this paper is to extend the method to other porosity logs, and to present ways to estimate fracture and total porosity from logs. A theoretical model composed of cubes" indicated that the double-porosity exponent, m, should be relatively small(ranging from about 1.1 to 1.3) for naturally fractured systems. Towle was apparently the first investigator to indicate the similarity of a synthetic pore system (represented by cubes with spaces in between) to a fracture-type system. However, this model considered only fractured porosity (matrix porosity was zero). This paper analyzes the behavior of the porosity exponent, m, in a naturally fractured reservoir by means of a double-porosity model. It is found that the value of m is certainly small and may range somewhere between about 1.1 and the porosity exponent value of the matrix, depending on the degree of fracturing of the formation. Consequently, it appears that a comparison of the double-porosity exponent, m, (obtained from logs) with the matrix-porosity exponent, mb, (determined in the laboratory) gives a reliable way to detect naturally fractured systems.
Values of water saturation are determined using a parameter, P, derived originally for the analysis of intergranular media, and extended in this study to analyze fractured media. This parameter is a function of formation resistivity and porosity tool response. It has been found empirically that P has a square-root-normal distribution for zones 100-percent saturated with water. Hydrocarbon zones deviate from this distribution. 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, the values of water saturation.
Log Properties for Detecting Fractures State of the Art
Sonic amplitude logs have been used extensively in attempts to detect fractures. When the acoustic velocity generated by a logging tool is recorded, four wave types can be identified: a compressional wave, a shear wave, a fluid or water wave, and a low-velocity wave. Generally, the compressional wave has been found to be attenuated more by vertical and high-angle fractures, while the shear wave seems to be more sensitive to horizontal and low-angle fractures. However, experience has indicated that this measurement is not universally applicable because changes in amplitude as large as those caused by fractures can be produced by variations in lithology and tool centralization; and because, in practice, there might be solid contact across the fractures, so that the degree of acoustic discontinuity is diminished.
JPT
P. 764^
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