New Method For Quantifying Vuggy Porosity From Digital Optical Borehole Images as Applied to the Karstic Pleistocene Limestone of The Biscayne Aquifer, Southeastern Florida

摘要

Vuggy porosity is gas- or fluid- filled openings in rock matrix that are large enough to bernseen with the unaided eye. Well-connected vugs can form major conduits for flow ofrnground water, especially in carbonate rocks. This paper presents a new method forrnquantification of vuggy porosity calculated from digital borehole images collected fromrn47 test coreholes that penetrate the karstic Pleistocene limestone of the Biscayne aquifer,rnsoutheastern Florida. Basically, the method interprets vugs and background based on therngrayscale color of each in digital borehole images and calculates a percentage of vuggyrnporosity. Development of the method was complicated because environmentalrnconditions created an uneven grayscale contrast in the borehole images that makes itrndifficult to distinguish vugs from background. The irregular contrast was produced byrnunbalanced illumination of the borehole wall, which was a result of eccentering of thernborehole-image logging tool. Experimentation showed that a simple, single grayscalernthreshold would not realistically differentiate between the grayscale contrast of vugs andrnbackground. Therefore, an equation was developed for an effective subtraction of thernchanging grayscale contrast, due to uneven illumination, to produce a grayscale thresholdrnthat successfully identifies vugs. In the equation, a moving average calculated around therncircumference of the borehole and expressed as the background grayscale intensity isrndefined as a baseline from which to identify a grayscale threshold for vugs. A constantrnwas derived empirically by calibration with vuggy porosity values derived from digitalrnimages of slabbed-core samples and used to make the subtraction from the backgroundrnbaseline to derive the vug grayscale threshold as a function of azimuth. The methodrnshould be effective in estimating vuggy porosity in any carbonate aquifer.