Computerized image analysis provides direct, rapid, and highly accurate measurement of pore size in thin or plane sections. The method can be applied to a wide range of rock types and requires only conventional sample-preparation techniques. Back-scattered electron or fluorescence microscopy images of impregnated samples are scanned, converted to digital form, and stored and processed on a microcomputer. Individual picture points from a matrix of up to 600,000 elements per scan are classified as rock or pore based on gray level. Pore size is obtained by area measurements of individuals pores and by Feret's diameter, the maximum spacing between parallel tangents to a pore in up to 56 directions. The measurements are readily summarized as pore size distributions.

Cumulative porosity vs. pore diameter crossplots indicate how porosity is distributed and can be used to aid reservoir evaluation and production assessment. The crossplots can also be used to determine the amount of porosity contributed by different pore types. Individual pores, or groups of pores classified by size or shape, can be interactively identified on the image analyzer monitor, enabling the user to make a visual association of pore type with size. Rocks having a wide range of pore size can be analyzed at more than one level of magnification, and the data can be merged to form a composite pore size distribution for the sample.

Pore size distributions of carbonate rocks containing complex pore systems show changes in slope that are indicative of pore type. Other carbonates, including fine crystalline dolomites and microcrystalline limestones, show relatively uniform pore size distributions, which reflect a single pore type.

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