Analysis of Naturally Fractured Reservoirs From Conventional Well Logs(includes associated papers 6420 and 6421 )
- R. Aguilera (Argentina-Cities Service Development Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1976
- Document Type
- Journal Paper
- 764 - 772
- 1976. Society of Petroleum Engineers
- 2.2.2 Perforating, 5.8.6 Naturally Fractured Reservoir, 5.6.4 Drillstem/Well Testing, 1.2.3 Rock properties, 5.2 Reservoir Fluid Dynamics, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.5.2 Core Analysis, 5.6.1 Open hole/cased hole log analysis, 4.3.4 Scale
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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.
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.
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