An Introductory Discussion of the Reservoir Performance of Limestone Formations

Abstract

Field experience with limestone and sandstone production indicates the existence of wide difierences between the reservoir behavior of these two types of formation. Little attention appears to have been given to the separate study of the flow of fluids and the retention of fluids in limestones. This paper presents data and arguments to demonstrate the existence of a difierence· between the two types of formation, and urges the separate intensive investigation of limestone reservoirs. All experimental data presented pertain to dolomitic limestone formations in West Texas and New Mexico. Two kinds of porous media are recognizedintergranular and intermediate. Intergranuiar rocks are those in which the porosity and permeability are determined by the geometrical properties and the sorting of the sedimentary units; intermediate rocks, those in which there is no direct relationship between grain properties and the porosity and permeability. Limestones in general are intermediate media. The partial relationship between porosity and permeability of any class of porous media is represented by an area of finite extent and definite shape on the permeability-porosity plane. The horizontal and vertical variations of porosity and of permeability in limestone and sandstone formations are discussed and compared. Comparisons are made of connate-water content, the relative permeability-saturation relationship, and capillary phenomena in the two kinds of rocks. A number of tentative conclusions are drawn relative to the reservoir properties of lime- Manuscript received at the office of the Institute Sept. 25. 1944. Issued as T.P. 1791 in PETROLEUM TECHNOLOGV. January 1945. • Shel1 Oil Co.. Midland. Texas. t Consulting Petroleum Engineer and Geol· ogist. Midland. Texas. stones; in particular, (1) dolomitic limestones are oil bearing and apparently are oil productive in zones of permeability less than 0.1 millidarcy, and (2) primary depletion (by gas expansion) oil saturations may be lower in dolomitic limestones than in sandstones.