FEATURE ARTICLE Whole-Core Analysis Methods and Interpretation of Data from Carbonate Reservoirs R. S. BYNUM, JR. CORE LABORATORIES, INC. MEMBER A/ME MIDLAND, TEX. E. H. KOEPF DALLAS, TEX. MEMBER AIME Downloaded from http://onepetro.org/JPT/article-pdf/9/11/11/2239086/spe-817-g.pdf by guest on 30 September 2021 Introduction rienced man, properly preserved by truck bed) has been found to be a quick-freezing in dry ice or by seal­ suitable technique for preserving the During recent years reservoirs ing in plastic or other suitable means, saturation condition of the core while where the hydrocarbon storage ca­ and delivered immediately to a well­ being sent to the laboratory. In pacity and the permeability char­ equipped laboratory where the tests some instances where the distance of acteristics are due chiefly to fractures are made on maximum-sized samples. delivery is short the use of plastic and vugs, and tight formations with Current coring equipment and prac­ bags or tubes, or other methods of non-homogeneous porosity develop­ tices permit a close approach to these preventing gain or loss of moisture, ment, have become of major import­ optimum conditions. may be used. ance in West Texas oil production. At the laboratory the core is laid Diamond coring equipment and Coring and Sampling of Core out in rows for further examination methods were devised which per­ and marking for samples. Detailed mitted excellent recovery of cores Although the successful applica­ information is frequently desired re­ from these formations. Examination tion of the diamond bit was a great lating to the number, size and major of these cores with erratic porosity step forward in the coring of dense linear direction of fractures, size and development frequently indicated in­ carbonate reservoirs, there still is no distribution of vugs and abrupt dividual fracture or vug volume means available to recover a sample changes in lithology. Some lab­ greater than the "perm-plug" sample of the reservoir in an undisturbed oratories are equipped to photograph normally used in conventional anal­ condition. Normally, the drilling mud a 50-ft section of core, where such ysis. It is evident that actual reser­ filtrate flushes the COFe to an irre­ information is desired, and enlarge­ voir properties are more closely ap­ ducible oil saturation. As the core ments can be made of sections of proached in the sample as the sample is brought to the surface, the gas in particular interest for detailed study size is increased and that the entire solution in this residual oil is evolved, of the fracture or vug system. core from this type of formation expands and expels oil and water A recent addition to test equip­ should be analyzed. until liquid saturations are reduced ment for use at this stage of analysis Although the literature on general to the point where only gas will flow. involves a gamma-ray unit for ob­ whole-core analysis procedures has The core, as recovered at the sur­ taining a log of the core on the been rather limited"",3 many innova­ face, then, contains residual oil after laboratoIY layout table. A compar­ tions have been introduced, both in water flushing and solution gas drive, ison of this detailed gamma-ray log methods of analysis and in the in­ some original water and some mud and the corresponding analysis and terpretation and application of the filtrate, and solution gas from the visual examination of the core with data. The experienced core analyst oil. There appears to be a relation­ a "down-hole" log permits precise has determined a set of optimum ship between the formation volume and accurate selection of the com­ conditions which permit a confident factor of the oil and the efficiency of pletion depth or depths in zones of approach to his specialized work. liquid expulsion by the gas as it ex­ alternating pay and non-pay intervals. These optimum conditions include a pands with pressure reduction. Sample sections of 12 to 20 in. completely cored interval, beginning The analysis of oil well cores ac­ are marked off along the entire core several feet above the pay zone, cut tually begins on the derrick floor, in order to take advantage of the with large diameter coring equip­ immediately after removal from the long pieces recovered. Some samples ment at maximum speed in a prop­ barrel. The core should be wiped, may consist of a single 12- to 20-in. erly selected drilling fluid, pulled to not washed, free of drilling mud to piece; whereas, other samples may the derrick floor in minimum time, prevent further absorIJtion of water. be composed of several 6- to 8-in. described and logged by an expe- The core is measured and a log is pieces. The selected samples are prepared, including a description of marked for easy identification and Original manuscript received in Soeiety of Petroleum Engineers office on April 3, ID;)7. the lithology and visual character­ weighed. Sample weights and vol­ Revised manuscript received Aug. 12, 1957. Paper presented at Permian Basin Oil RI:'­ istics such as fractures, vugs, or stain­ umes will normally be of the order covery Conference in Midland. Tex., April ing. The use of large freeze chests of 4,000 to 8,000 gm and 1,500 to 18-19, 1957. lReferences given at end of paper. (either free or mounted on a pickUp 3,000 cc. SPE 817-G NOVEM"ER, 1957 11 Whole-Core Analysis Procedures long segment of whole core and to The other generally accepted, and provide a large volume of solvent. probably the most widely used, Whole-core analysis procedures Extraction times of two to three method of extracting fluids from have been developed for determin­ weeks are required for removal of whole-core samples is by vacuum ing porosity and permeability of ex­ normal crude oils. This procedure retorting. The core as received at tracted core samples and saturation permits measurement of water con­ the laboratory is divided into prop­ of gas, oil and water in the core as tent of the sample. The more im­ erly marked and identified samples received. Measurements made on portant oil content is calculated as of 12 to 20 in. Each sample may be large pieces of relatively homogene­ a difference value, and any errors in comprised of one or several pieces. ous formations have shown excellent weighing or due to grain loss are re­ The samples are weighed and placed agreement of permeability and por­ flected in this calculated oil content in high-pressure saturator chambers. osity with values obtained in conven­ value. Gases and air are removed by evacu­ tional plug-type analysis. These data A second modification involves ation, and de-aerated water is charged have shown that the procedures evaporating a solvent from a large to the saturators. The core samples yield comparable results where the vat, allowing the vapors to condense are kept in the saturators at 2,000 porosity development is suitably rep­ on the cooled lid and on the core psi for one to four hours, depending resented by the plug-type sample. samples suspended in the upper sec­ upon the type of sample. The fully Water saturations as determined in tion above the boiling solvent. In saturated samples are weighed and the whole-core procedures are usually both cases, the solvent is repeatedly placed in vacuum retorts. Distillation somewhat greater than by plug anal­ vaporized and condensed or dripped is started at a relatively low tem­ ysis because the entire core, includ­ Downloaded from http://onepetro.org/JPT/article-pdf/9/11/11/2239086/spe-817-g.pdf by guest on 30 September 2021 onto the sample to be cleaned. This perature and only a slight reduction ing shale streaks and high filtrate procedure is quite slow, and two to in pressure. After the major por­ content near the surface, are in­ four weeks of leaching are required tion of the water content has been cluded in the material retorted. The for satisfactory removal of normal distilled over at a temperature of ap­ high water saturation values of the crude residuals. This process pro­ proximately 250° F, the temperature whole-core procedures are normally vides a clean sample but does not is increased and the pressure is fur­ accompained by correspondingly permit determination of fluid content. ther reduced. The distilIation is con­ lower oil saturation values. A second extraction method3 in­ tinued for approximately four hours, 0 Fluid Extraction and Fluid volves pressure injection of a hot sol­ to a final temperature of 450 F and Saturation vent saturated with carbon dioxide a pressure of 5 to 10 mm of mer­ Three general methods of extract­ into the sample, heating to a mod­ cury. ing fluids from whole-core samples erately high temperature, and allow­ The distilled fluids are collected (shown schematically in Fig. 1) are ing the gas to expand with subse­ in receiving tubes maintained at ap­ used in the industry. Two procedures, quent reduction of pressure. The ex­ proximately - 60° F in a dry ice­ vacuum retorting and one modifica­ panding gases flush the solvent alcohol bath. The total liquid re­ tion of the Dean-Stark extraction, through the pore spaces to remove covery (plus a correction for hold-up permit measurement of the fluid ex­ the oil. Five to ten repetitions of this in the system) represents the total tracted and determination of fluid procedure are normally required for pore volume. The corrected oil re­ saturations. The other procedures cleaning. This procedure results in covery represents total residual oil provide clean or extracted samples proper cleaning of live crudes from content of the pore spaces.