Analysis of Fractured Limestone Cores

A method is outlined for the analysis of large cores, developed primarily forthe purpose of obtaining reliable data on fractured or vugularlimestones. Porosity and fluid saturations are determined by a modified Dean-Starkextraction after initially bringing the samples to 100 per cent liquidsaturation by a vacuum-pressure method. Horizontal permeabilities on the wholesamples are determined in two directions, parallel and perpendicular to thedirection of principal fracturing. Results are presented for various types of formations. A comparison is madebetween data obtained by this special method and the method of conventionalanalysis, and discussion is given of the relative advantages and limitations ofeach. In view of this comparison a modified method for the analysis offractured and vugular formations is proposed, which method retains theadvantages of the previously outlined techniques but gives promise of speedingup the analysis. Introduction For many years the greater part of the production in the Permian Basin was fromformations of Permian and Pennsylvanian age. While production from theseformations is still the predominant production, deeper prospecting has broughtto light older formations with oil productive characteristics. Attempts to corethese deeper horizons were at first usually characterized by poor recovery. Theold rule of thumb, "good core recovery - poor well" was originated andremained in effect until recently. With the advent of diamond coring and theincreased efficiency in drilling and coring operations, the old adage has lostits significance. Formations of Devonian, Silurian and Ordovician ages weresuccessfully cored with varying degrees of recovery. The cores thus obtainedwere surprising and in some cases disappointing. Good recovery from theseformations led to speculation as to the feasibility of analysis and the meaningof data obtained from conventional analysis. Commercial laboratories wereengaged to analyze these cores, and their results indicated that data obtainedby conventional methods would be of little value. From visual inspection it wasapparent that in many cases the effective porosity for storage of hydrocarbons, and permeability, were contained largely in fractures and solution cavitiesrather than in the primary crystalline structure of the formation. In othercases the solution cavities augmented the effective porosity of the primarycrystalline structure, or fractures enhanced the permeability of the latter sothat the formation might respond favorably to acidization and becomecommercially productive. Since the spacing of the cavities and fractures wasoften comparable to the core diameter, it was evident that the entire coreshould be analyzed instead of small fragments. T.P. 2913