Abstract

Vuggy carbonates, such as the reef facies of the Arun limestone, require modification of standard core analysis procedures. The best procedures for vuggy carbonates may not be the methods of choice for other types of rocks. When large, well-connected vugs are present, virtually any material can and does enter the pore spaces. Core analysis problems arise because whole drilling mud enters the pore spaces during coring. Jacketing materials and mercury also enter the pore spaces. For example, whole drilling mud has invaded the Arun limestone so deeply that, in some cases, vugs in the center of 3.5-inch diameter cores are filled with mud solids. Standard core cleaning techniques are designed to remove hydrocarbons and brine from the core, but not solids. Mud invasion and efficiency of cleaning techniques can be monitored with X-ray CAT scanning. Bulk volume of these core samples cannot be determined with normal procedures because mercury enters large vugs. Bulk volume should, therefore, be determined by direct measurement of the sample dimension with calipers instead of techniques that require immersion. To calculate porosity, pore volume can be obtained from the difference between the bulk volume and grain volume. To measure pore compressibility, change in pore volume with pressure must be determined. Measured changes in pore volume are erroneously large, however, due to jacket intrusion, and may be misinterpreted as sample crushing. New techniques have consequently been developed at Mobil to measure pore compressibility. Capillary pressure measurements on vuggy core samples are also difficult. Mercury injection measurements on full-diameter samples are preferred. All core analysis procedures should be carefully evaluated before application to vuggy formation. Many assumptions fail when vugs are present.