Performance of Limestone Reservoirs

Abstract

Abstract During the past 20 years, research and development in the study of reservoirbehavior have dealt principally with flow of oil through sandstones. Manyreservoir studies of sand fields have proved valuable in promoting recoveryefficiency. This paper discusses fundamental principles governing oil and gas productionfrom sandstone and limestone alike and presents the results of investigationsrelating to the application of analytical techniques used for sandstonereservoir studies to the study of limestone reservoir performance. Thecharacteristics of limestone porous systems, porosity-permeabilityrelationships, distribution and occurrence of oil, and characteristics of flowthrough such systems are discussed. Recognition is made of the similarities ordifferences which these factors exhibit in limestone and sandstone systems. Comparisons between operating data for typical limestone and sand reservoirsare presented. It is indicated that the distribution and movement of fluids inand through porous limestones follow the same fundamental principles underlyingsuch processes in sandstones. This fundamental similarity may readily bediscernible in the performance of many limestone reservoirs. The volumetric balance and unsteady state radial flow equation, the fluiddisplacement equation, use of electrical analogue devices, and other analyticaltechniques to study the behavior of limestone fields appear fundamentallyapplicable, but do require thorough understanding of the properties of theformation, of the fluids, their behavior during flow, and adequate productionoperating data. Need for more complete coring and comprehensive examination ofcore properties is stressed. The results of "active oil" studies, andof flow and interference tests are presented. Well spacing, well completion, and efficient rates of production in limestone reservoirs are brieflydiscussed. Introduction Limestone and dolomite reservoirs constitute the largest source of supply ofcrude oil in the world, an estimated 60 per cent of present production comingfrom carbonate reservoirs. In many large geologic provinces such as Mexico, theMiddle East, and more recently Canada, almost all the oil is found in this typeof rock. In the United States, all of the major oil-producing areas exceptCalifornia and Pennsylvania contain oil-bearing carbonate formations. Thediscovery in recent years of large oil reserves in the Silurian, Devonian, andOrdovician formations in West Texas, in addition to the large reserves in thePermian, has accentuated the interest of operators, geologists, and engineersin limestone formations and in the many problems associated with understandingthe performance of these reservoirs. The rapid increase in discovery of oil inlimestone formations and the present-day position of prominence held by thesefields in the production and reserve picture in all parts of the worldemphasize the horizons opened to the reservoir technologist in the field ofgeological and production research. T.P. 2935