Analysis of Naturally Fractured Reservoirs From Conventional Well Logs(includes associated papers 6420 and 6421 )

Abstract

Methods are presented for detecting and evaluating naturally fracturedreservoirs from porosity (sonic, neutron, and density)and resistivitylogs. It is shown that the porosity exponent of a naturally fracturedreservoir is smaller than the porosity exponent of the matrix. Charts havebeen generated for estimating the porosity exponent for these reservoirsas a function of total porosity, matrix porosity, and matrix-porosity exponent. Introduction The principles for the techniques presented here weredescribed previously using sonic and resistivity logs.The approach followed in this study was to use empiricalequations that had been derived for granular mediain the hope that they could be useful in the analysis offractured reservoirs. It was anticipated that thisapproach would result in a distinctive means of detectingand evaluating fractured media. The purpose of thispaper is to extend the method to other porosity logs, and to present ways to estimate fracture and totalporosity from logs. A theoretical model composed of cubes" indicated that the double-porosity exponent, m, shouldbe relatively small(ranging from about 1.1 to 1.3) fornaturally fractured systems. Towle was apparently thefirst investigator to indicate the similarity of a synthetic pore system (represented by cubes with spaces inbetween) to a fracture-type system. However, this modelconsidered only fractured porosity (matrix porosity waszero). This paper analyzes the behavior of the porosityexponent, m, in a naturally fractured reservoir by meansof a double-porosity model. It is found that the value ofm is certainly small and may range somewhere betweenabout 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 thedouble-porosity exponent, m, (obtained from logs) with the matrix-porosity exponent, mb, (determined in thelaboratory) gives a reliable way to detect naturallyfractured systems. Values of water saturation are determined using aparameter, P, derived originally for the analysis ofintergranular media, and extended in this study to analyzefractured media. This parameter is a function of formation resistivity and porosity tool response. It has been foundempirically that P has a square-root-normal distributionfor zones 100-percent saturated with water. Hydrocarbonzones deviate from this distribution. By determining themean value of P at a water saturation of 100 percent, it ispossible to evaluate the resistivity index, I, for hydrocarbonzones and, hence, the values of water saturation. Log Properties for Detecting FracturesState of the Art Sonic amplitude logs have been used extensively inattempts to detect fractures. When the acoustic velocitygenerated by a logging tool is recorded, four wave typescan 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 beattenuated more by vertical and high-angle fractures, while the shear wave seems to be more sensitive tohorizontal and low-angle fractures. However, experience has indicated that this measurement is not universally applicable because changes in amplitude as largeas those caused by fractures can be produced byvariations in lithology and tool centralization; and because, in practice, there might be solid contact across thefractures, so that the degree of acoustic discontinuity isdiminished. JPT P. 764^