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. The dif­ for measurement of permeability, most types of cores. However, the ference in the total water recovered porosity, and other special properties. heated solvent, maintained in a high­ and the water injected during the pressure gas atmosphere, is quite saturation step represents the water Several modifications of the Dean­ hazardous and the method requires content of the core sample as re­ Stark extraction equipment and pro­ extensive safety precautions. This ceived. This procedure permits a cedure have been used. One modifi­ procedure is also for cleaning, only, complete analysis of a large number cation merely involves fabrication of and does not provide data on gas, of samples within a period of 8 to large containers to accommodate a oil, or water content. 12 hours. Further, the total pore volume and the saturation of each fluid are all obtained by direct DEAN-STARK VACUUM RETORT measurements on the same piece of rock. Porosity Accurate determination of the av­ erage formation porosity is one of the most important objectives of core analysis, since this factor limits the available hydrocarbon storage space and is the basis of all pre­ liminary estimates of reserves. In whole-core analysis procedures, as in conventional analysis, porosity may be determined by application of the Boyle's law principle to either

0" WAnR vOLUME VOLUME compression of air or gas into, or I expansion from, the pore spaces of PORE VOLUME extracted and dried samples. How­ I<'ig. I-Whole-core analysis procedures. ever, the method of determining the

12 JOlTRNAL OF PETROLEUM TECHNOLOGY amounts of gas, oil, and water con­ gypsum, contains a definite percent­ the cylindrical shape of the sample, tents of unextracted samples in age of combined water. This water the length of the sample, and the terms of per cent of bulk volume, is removed by heating, and the loss size of the screens through which the and summing to obtain total porosity, of combined water begins at rel­ air is introduced and removed.' is probably the most widely used atively low temp eratures (about procedure. The unextracted samples 90° F). Therefore, a portion of the Interpretation and Application are weighed and fuIly saturated with combined water is removed during of Data water at 2,000 psi (as outlined in the any process for evaporating free discussion of vacuum retorting for water from the pore spaces of the One of the primary uses of core fluid extraction). The fuIly saturated sample. The loss of water is accom­ analysis data is as an aid or criterion samples are weighed immediately panied by an increase in void space, in choosing intervals for drill-stem upon removal from the saturator which is included in a Boyle's law testing and in selecting detailed, in­ chambers. The increase in weight method of measurement. In the re­ dividual weIl completion points and represents the amount of water re­ torting procedure, the water removed procedures. Some of the data ob­ quired to fill the pore spaces oc­ from gypsum is measured and re­ tained by the core analysis pro­ cupied by gas. The fuIly saturated ported as additional water-saturated cedures presented are factual, such core is then subjected to vacuum dis­ pore volume. Therefore, the water as permeability and porosity asso­ tiIlation for the removal and recov­ from the gypsum shows up as in­ ciated with a given depth and the ery of the fluid content. The fluid creased porosity and increased water presence or absence of oil. A study content, when corrected for holdup saturation, but it does not affect the of the core and these data provide in the system, is a measure of the determination of oil content. The the only direct information on the Downloaded from http://onepetro.org/JPT/article-pdf/9/11/11/2239086/spe-817-g.pdf by guest on 30 September 2021 total pore volume. gypsum problem is receiving con­ lithology and characteristics of pore In cases where the porosity is due siderable attention in the industry. development, magnitude and distribu­ tion of permeability, permeability principally to large vugs or where Permeability large fractures are present, it is dif­ profile, position of permeable zones, Whole-core permeability measure­ position of barriers to vertical flow ficult to prevent loss of water from ments utilize either a Hassler-type the large surface void spaces dur­ and the magnitude and distribution holder or a unit employing two rub­ of porosity. ing removal from the saturator and ber cradles and a hydraulic ram, or weighing of the saturated sample. The fluid saturation data repre­ "ram-type" compression holder, to Various procedures, such as webbing sents conditions after partial or com­ over the surface voids with a porous seal the core sample. Both procedures plete water flushing and pressure de­ plastic film, filling voids with clean are based upon the same basic prin­ pletion, rather than conditions ex­ sand, etc., have been used to elim­ ciples and yield comparable results. isting in the reservoir. It follows, inate this source of error. None of Horizontal permeability is normaIly then, that some interpretation of these these procedures have been found measured in two directions; one giv­ data is required for their application satisfactory for some types of sam­ ing the maximum value (normally regarding the location of fluid con­ ples which contain large surface vugs along the direction of principal frac­ tacts and the type of production to or fractures. A Boyle's law porosi­ turing), and the other at 90° to the be expected from a given interval. In meter has been ada!lted for measure­ maximum. Vertical permeability can a specific case, interpretation on fac­ ments on this type of sample. The be determined with either unit. tors such as type of fluid production thoroughly extracted and dried sam­ Measurements in the vertical direc­ (gas, oil, water, or a combination of ple of known bulk volume is placed tion require that the ends of the these), position of fluid contacts, defi­ in a chamber of known volume. The samples be cut square. Radial perme­ nition of transition zones, intervals of core-holding chamber is then allowed ability measurements are sometimes possible early water or gas break­ to reach pressure equilibrium with made. Under certain conditions, and through and possibilities of water or an evacuated chamber of known if properly made, these values may gas coning can be developed from volume. The equilibrium pressure be suitable, but this procedure re­ the measured data and empirical re­ value permits calculation of the total duces the available flow path and lations which have been found to pore volume of the sample. thus tends to defeat the purpose of apply to the formation being consid­ Some laboratories use gas com­ whole-core analysis. ered. The development of preliminary estimates of reserves by various meth­ pression rather than gas expansion The core holder, whether the Hass­ ods of reservoir depletion, another to obtain the !lore volume. In this ler unit or rubber cradles for the primary application of core analysis procedure the core chamber and core ram-type unit, must be sized for the data, requires interpretation of some sample, evacuated or at atmospheric particular core sample to assure factors and their use with the factual pressure, are brought into pressure proper sealing and elimination of air data. The following examples present equilibrium with gas under pressure leakage around the sample. Screens whole-core analysis data obtained on in a chamber of known volume. Any of a predetermined size are placed several of the major carbonate form­ reasonable pressure may be used, on opposite sections of the cylin­ ations, together with general inter­ but most laboratories where this drical surface of the sample to pro­ pretation of the data. method is employed utilize pressures vide good distribution of air over of 35 to 75 psig. the inlet section and removal from lIIustrative Examples Any discussion of porosity de­ the opposite side. The rate of air termination on carbonate reservoir flow is measured by means of a Grayburg-San Andres samples should include some com­ calibrated orifice and water or mer­ Fig. 2 presents a coregraph of a ment on the effects of gypsum in­ cury manometers. The calculation of dolomite section in the Grayburg-San clusions. The chemical compound, air permeability takes into account Andres formation. The zone 3,019 to

NOVEMBER, 1957 13 30 ' ..... Max.o-o Total Water 0-0 ,tenn., Max. 0-0 Total Water 0-0 Millid ...ys % Pore Spac. Millida ..ys % Pon Spott ~ 30 10 10 0 ~~ 40 3,0 10 10 8O~04010~ Porosity x----x Oil Sat. x----x Porosity x----x Oil Sat. x-· - -x PmOtd % Port s,.1 Perco" % Pon Spat. 40 30 10 10 0 104060M 40 30 10 10 10 40 60 80 20

3019 3010 , Il!"~O 1i: ti 10 'j, I 1 ;t ' iii , 11155 ~ I: .. ,I :I 3030 1116 I ~r 00~----~2~~----~~----~----~~----~'00 -r-.

TOTAl WATER SATURArlON, % PORE SPACE ~- 't I', .I ~ i'l . , 3035 11165 ,i Fi~. 3--Poro,ity vs water ,aturation. 7h: 3040 :r. ' ! t ·-f:1~ 'Ir ':i 'r' H-i--t+H++tt+, 4ll'1I170~~~-rit'-!-t1 Perm., Max. 0 - 0 0-0 II 3,025 ft is oolitic dolomite, while Total Water ~i::t t;:~ :!.~ .. '1 the remainder of the cored in­ Millidartys % Pore Space I I x...... lIlt t, ! 40 30 10 10 80 60 40 10 0 :j: ::1 :!: :~! '0 . TI' ~ '1 terval is typically anhydritic with , : " . 11175 .. IF! " :;: ii: fl.::;=; : Itrlr; ,I:: Downloaded from http://onepetro.org/JPT/article-pdf/9/11/11/2239086/spe-817-g.pdf by guest on 30 September 2021 varying amounts of shale. There 1,: ":l.vIr,':'I'~ ,d;, ':1 1,1 , .. rlrl "IS 'til,'" 'It is evidence of some slight hair­ . . ' 11180 ' " , line fracturing in the cored in­ r:; ::: :i;::!fc;::z::; :i!li1fMtnl.; 1 . ::: terval. 11' P' I",rti "§"2 , th" 'r 'I' '" "I " 11185" ,tI ";j The liquid saturations and the ,j. ·1, ,'i :'; I".z;, ~1~.1i , 1 .i permeability are typical of this ;:: ::1 'i! ;:i :;; 'fi'itJ :::d!l. 1:1 1\. ::1 formation, while the porosity is "',' 'I, ,:1,,; :i'rJ: 111 1 . ':' slightly higher than is usually Fig. 5--Whole-eore anal­ encountered. High permeability ysis eoregraph of Devon- in the oolitic zone at the top dis­ ian seetion. tinguishes this zone from the re­ mainder of the interval. The marked intervals above

3,169 ft (Fig. 2) are interpreted Perm., Max. 0-0 Total Wcrttr 0-0 to be oil productive. The basis of Millidarty. % Pore Space 4.0 30 10 10 0 ~O 60 40 10 0 the selection of these intervals is primarily their relatively large decrease in water saturation, as compared to adjacent formation. The remaining formation shows relatively high water saturation and is expected to yield some water production, the amount in­ creasing somewhat with depth. Perforations should be made op­ posite intervals interpreted to be oil productive to minimize water cut. Below 3,165 ft it is noted that the water saturations are ap­ preciably higher than the satura­ tions existing up the hole for comparable porosities. Forma­ tion below 3,172.5 ft is inter­ 4, 5675 preted to he wholly water pro­ l f '... -I- ttl ductive. :1;::;::: '.m'if; Some of these points of inter­ pretation may be illustrated in a different manner by reference to Fig. 3. This shows values of water saturation and porosity for individual samples and the trends of change in the water satura­ tion with porosi ty for th ree groups of samples (A, B, and C) Fig. 4--Whole-('ore anal­ Fig. 6--Whole-core anal­ ysis eoregraph of Ellen­ ysis coregraph of Clear­ at progressively greater depths. burger section. fork section.

JOURNAL OF PETROLEUM TECHNOLOGY Each of the solid points used to de­ water and residual oil saturation and Porosity ranges from 3.5 to 17 fine the curves is an average of the an increase in measured gas content per cent while permeability ranges measun~d data from approximately occurs. Permeable samples exhibiting from <0.1 to 33 md. The higher 20 individual samples. The flatness no residual oil and high water satura­ permeability measurements are gen­ of the curves, i.e., the large decrease tion are not normally water produc­ erally due to slight fracturing. Water in water saturation accompanying tive. The permeability is measured saturations tend to increase with only a small increase in porosity, re­ through fractures which have been depth and show a very sharp in­ flects the "amplified" appearance of voided of oil content by the flushing crease, indicating an oil-water con­ the water-saturation profile as com­ action of the mud filtrate. The high tact, at a depth of 5,683 ft. The in­ pared to the porosity profile of the water content is contained in the im­ crease in porosity with increase in coregraph. The general consistency permeable, low-porosity matrix and water saturation is an additional in­ and essentially parallel trend of the is not producible. A visual examina­ dication that the interval below 5,683 curves indicate that the choice of the tion of the fracture system frequently ft is water productive. Residual oil water saturation profile as one of the indicates probability of oil produc­ saturation decreases with depth, main criteria for picking oil pay in­ tion from the fracture porosity. This reaching a very low value at and tervals is good and should provide entire interval is in the oil column below the oil-water contact. fairly sharp definition of these inter­ and is interpreted to be oil produc­ This formation is oil productive vals. On the average, in this well, the tive. The high gas content is typical where permeable between 5,646 and water saturation changes by 10 per of data from formations containing 5,683 ft. Water-productive forma­ cent (of pore space) for a porosity vugs and solution channels. tion occurs below 5,683 ft. Comple­ change of 2 per cent (of bulk vol­ A plot of total water saturation vs tion can best be accomplished be­ Downloaded from http://onepetro.org/JPT/article-pdf/9/11/11/2239086/spe-817-g.pdf by guest on 30 September 2021 ume). porosity for this section is included tween 5,646 and 5,665 ft in the more The slope and placement of the in Fig. 3. The average data points permeable and porous zone. This also porosity-saturation trends of Fig. 3 define the trend as shown. Although takes advantage of the underlying low are an indication of the water-pro­ the water saturations are relatively permeability interval to minimize the ductive characteristics of this forma­ high and the curve shows a large possibility of water coning from the tion. The high values of water satura­ change in water saturation with water-productive formation below. tion associated with porosities only change in porosity, the total water very slightly lower than in the oil­ present as per cent of the rock vol­ Conclusions productive stringers lend substantia­ ume is so low that no water flow is 1. Erratic porosity development in tion to an interpretation of significant expected. Completion should be carbonate reservoirs where a major water production in the intervals of made sufficiently high above the portion of the hydrocarbon storage lower porosity throughout the sec­ known water level to preclude water capacity is in large vugs or fractures tion. coning through fractures and vugs. makes it necessary to analyze large The shifting of the porosity-satura­ Devonian samples of the full diameter core to tion trends of Fig. 3 to the right for Fig. 5 is an example of a Devon­ obtain values approaching average the successive groups of samples, ian dolomite showing a wide range reservoir properties and conditions. each approximately 75 ft lower, is in of porosity and permeability and a 2. Whole-core analysis procedures line with what would be expected high development of vug porosity. have been developed which provide from capillary pressure considera­ The fluid saturations are typical useful and generally representative tions. This confirms an approach to for this type of formation. Water factual data on the presence or ab­ water which is not quite so obvious content decreases markedly where sence of oil, porosity and permea­ in a study of the coregraph profile. porosity increases in the vuggy zones. bility, and their distribution. The cross-plotting of various A decrease is also observed in re­ 3. The whole-core analysis data parameters of the core-analysis data sidual oil saturation in the vuggy provide a suitable basis for interpre­ is not standard procedure for indi­ zones due to more efficient flushing tation of type of fluid production, vidual welI interpretation. The pres­ action of the drilling mud filtrate. fluid contacts, definition of transi­ ent example relating porosity, water The high gas content is typical of tion zones, probable water or gas saturation and depth shows that there cores from formations containing breakthrough and preliminary esti­ do exist certain trends and correla­ large vugs or open fractures. mates of reserves and recoveries. tions in whole-core analysis data that This Devonian zone is interpreted might not be apparent on first sight, to be oil productive. The intervals of References but which may be noticed by the ex­ higher water saturation should not 1. Atkinson, Burton, and Johnston, Dav­ perienced analyst. Similar cross-plots produce water because of their much id: "Core Analysis of Fractured Dolo­ of other parameters, such as per­ lower porosity and essentially zero mite in the Permian Basin," Trans. meability, gas content, oil content, matrix permeability. Perforation can AIME (1949) 179, 128. may also be used to assist in the in­ 2. Kelton, F. c.: "Analysis of Fractured include the entire interval analyzed, Limestone Cores," Trans. AIME terpretation. providing the section is sufficiently (1949) 189, 225. 3. Stewar!, Charles B., and Spurlock, J. Ellenberger above the known water level to pre­ clude the possibility of water coning W.: "How to Analyze Large Core The coregraph of an Ellenburger Samples;' Oil alld Gas JOllr. (Sept. 15, through the large channels and vugs. dolomite zone showing the typically 1952), 51, 89. Clearfork 4. Collins, R. E.: "Determination of the low porosity and erratic permeability Traverse Permeabilities of Large Core caused by fractures and vugs, is pre­ A coregraph of a typical Clearfork Samples from Petroleum Reservoirs," sented in Fig. 4. The zone 13,120 to section consisting of dolomite with JOllr. of Appl. Phys. (1952), 23, 681. 13,130 ft illustrates high flushing ef­ small amounts of shale and anhy­ *** ficiency in a vuggy zone causing a drite is shown in Fig. 6. The forma­ EDITOR'S NOTE: A PICTURE AND reduced residual oil. With an in­ tion is stylolitic and evidences slight BIOGRAPHICAL SKETCH OF R. S. By­ crease in vug porosity, a decrease in fracture and vug development. NUM, JR., APPEAR ON PAGE 31.

NOVEMBER, 1957 15