FORMATION EVALUATION

The Use of Sidewall Core Analysis in Formation Evaluation

E. H. KOEPF CORE LABORATORIES, INC. R. J. GRANBERRY DALLAS, TEX. MEMBERS AIME Downloaded from http://onepetro.org/jpt/article-pdf/13/05/419/2238154/spe-1635-g-pa.pdf by guest on 01 October 2021

Abstract a well after the formation being sampled has been pene­ trated by the drilling bit. The majority of sidewall samples Analytical techniques and procedures which permit ac­ are obtained with bullet-type or percussion samplers. Other curate measurement of important physical properties and tools permit drilling or punching into the wall of the hole of fluid content of sidewall core samples as received in the at an angle to obtain a core sample. The small size of the laboratory are available. However, hole conditions prior to sample makes special handling techniques necessary, and and during sampling affect the values as measured on the the conditions of sampling affect the interpretation of the core samples. Also, the impact of the percussion sampler analytical data significantly. in the sampling process alters some of the physical char­ acteristics of the sample. Comparisons of data on conven­ One of the first commercial sidewall coring tools was tional and sidewall core samples and experience have described in 1939." The early tools provided only very shown the general direction of these effects. Normally, small samples (~-in. in diameter and up to 1Vz in. in formations along the Gulf Coast have a greater productive length), and their use was limited to fairly soft forma­ capacity than the sidewall core sample data indicate. Water tions. Continuous equipment development has resulted in saturations associated with gas, condensate or oil produc­ facilities which now provide appreciably larger samples of tion are greater in sidewall than in conventional core soft and moderately dense formations. Sidewall sampling samples. Sidewall core data are valuable as exploratory is now the predominant method used by many of the aids, but data from conventional or wireline cores are operators along the Texas-Louisiana Gulf Coast to obtain generally requ:ired for evaluating recoverable reserves, the samples of possible oil or gas producing formations. distribution of reservoir fluids and formation flow char­ Core analysis data obtained from sidewall samples soon acteristics. showed that effective utilization would require changes in Sidewall core data usually establish the presence or ab­ analytical procedures and techniques and would require sence of hydrocarbon content and indicate the probable the development of new criteria for interpreting the data. type of production. Measured permeability and porosity Semi-micro analytical procedures were developed (utilizing values indicate productive capacity. The data show gas-oil scaled-down equipment), and some new, alternate proce­ and water-oil contacts. Sidewall sample data are particu­ dures were developed. Current procedures permit accurate larly valuable as a basis for "calibrating" electrical log measurement of normal core properties on the sidewall data. They are used to check lithology changes indicated core sample as it is received. The interpretation standards by log data, and they permit evaluation of thin and stray have developed, with experience, as essentially empirical sands. Sidewall core samples probably provide the most relationships. reliable data normally obtained on "dirty" or ashy sand The quantity and quality of formation samples provided zones which show low resistivity on the electrical logs, on by conventional coring generally are far superior to side­ sand sections drilled with high-salt-content muds and on wall samples. Conventional-type cores are always desirable shallow bentonitic sands containing fresh water. Considera­ and are necessary for obtaining data used in evaluating tion of both sidewall core analysis data and electrical log the distribution of reservoir fluids, total in-place and re­ data together increases the value of each. In many in­ coverable reserves and fluid-flow characteristics, and in stances, it is necessary to consider both types of data in making detailed reservoir engineering calculations. How­ arriving at a correct interpretation. Greatest value can be ever, sidewall coring is a valuable exploratory tool. It is obtained from the sidewall core data if the analyst has an particularly applicable in cases where many possibly pro­ electrical log of the zone as a guide for general formation ductive zones may be encountered and a normal conven­ characteristics and for zoning the various samples. tional coring program is not practical or where other data indicate that a pay zone may have been drilled through. Introduction The sidewall sample provides qualitative information which can be obtained in no other way after the formation has Sidewall core analysis is the term normally applied to been penetrated. The purpose of this paper is not to com­ the analysis of small core samples taken from the walls of pare data obtained on sidewall and conventional cores but, Original manuscript received in Society of Petroleum Engineers office Oct. 10. 1960. 1References given at end of paper. SPE 1635-G MAY, 1961 419 rather, to discuss the application of sidewall core-analysis TABLE l-COMPARISON OF SIDEWAll (S) AND CONYENTIOI,AL (C) CORE data. ANAL YSIS DATA (MIOCENE, FRIO AND COCKFIELD) Av. Res. Tot. No. Depth Perm. Por. Oil H,O Accuracy of Measurements on Samples as Received GULF COAST Samples .-Jf!L (md) (%) 1'10) (%) Combined Data C 3178 8041 851 28.1 6.4 61.1 S 2160 7255 239 28.5 4.7 69.6 Two generally accepted methods of sidewall core analy­ By Production sis are included in the API RP-40, Recommended Practice Condensate C 967 9131 797 27.7 1.4 53.2 S 526 7536 177 27.7 1.1 63.7 tor Core-Analysis Procednre.' The atmospheric retorting 0;1 C 1111 7035 936 29.2 15.0 53.4 procedure is the more widely used. The techniques are S 601 6815 384 29.8 14.0 57.8 Water C 1101 810() 822 27.5 2.3 75.8 similar to conventional plug-type core analysis, but the S 1033 7367 185 28.1 1.1 79.4 equipment has been scaled down to permit accurate meas­ By formation mements of the small quantities involved. The gas, oil and Miocene C 1044 9149 853 27.3 7.9 58.3 S 557 8160 292 28.4 5.3 69.5 water contents of the sample as recovered at the surface are Frio C 1097 7933 1197 27.7 4.3 61.5 determined directly on one portion of the sample. The S 785 6763 233 28.3 5.4 68.3 Cockfield C 1037 7050 493 29.5 7.2 63.6 summation of these values represents the pore volume of $ 818 7110 208 28.7 3.7 70.9 the sample. The second method of analysis involves ex­ By Formation and Production tracting the oil content of the sample with pentane and de­ Miocene termining the oil remaining after evaporation of the sol­ Condensate C 378 10830 726 27.5 1.9 53.0 vent. Gas content is determined by mercury injection, as S 143 8835 274 28.3 1.9 64.5 on C 393 7605 1109 28.1 17.1 51.1 in the retorting procedure, and the water content is calcu­ S 181 7259 334 29.1 13.1 61.1 Downloaded from http://onepetro.org/jpt/article-pdf/13/05/419/2238154/spe-1635-g-pa.pdf by guest on 01 October 2021 lated from a material-balance relationship. Permeability Woter C 273 9044 659 26.0 2.9 76.0 S 233 8445 271 28.0 1.3 79.0 to air is measured on a separate portion of the sample in Frio both procedures. Condensate C 403 8083 1103 27.4 0.9 51.1 S 220 7071 143 27.3 0.8 63.8 The relative merits of these two methods of analyzing Oil C 224 7471 1246 28.1 15.5 49'.0 sidewall samples, the care which must be exercised in S 208 6389 483 30.2 16.9 54.3 Water C 470 8025 1255 27.6 2.0 76.2 each step of the analysis and the limits of accuracy of the S 357 6791 142 27.7 1.6 79.2 measured data have been studied and reported in consider­ Cockfield Condensate C 186 7947 279 28.6 1.3 58.1 able detail." This study showed that gas, oil and water con­ S 163 7024 137 27.8 0.9 62.9 tents could be measured to within approximately ±2 per Od C 493 6397 658 30.7 13.1 57.2 cent of the actual value, expressed as a percentage of the 5 212 6854 329 29.9 11.9 58.5 Water C 358 7481 377 28.4 2.1 75.2 bulk volume of the sample. Summation of the three values S 443 7265 176 28.5 0.5 79.8 to obtain pore volume yields a porosity value within ±0.5 ---._------porosity per cent of the true porosity of the sample as received. This value may differ from true formation po­ pari sons for this area, as obtained in further study of rou­ rosity, as will be discussed later. tine data: are shown in Table 1. The small size of the available permeability sample and, Reudelhuber and Furen< found that measured sidewall in many cases, the friable texture of the sample make it permeability values tend to be too high for Gulf Coast necessary that it be mounted in plastic or wax before it sands having less than approximately 20 md. For sands can be subjected to air flow for the measurement of per­ with permeability greater than 20 md, the measured side­ meability. It is essential that the mounting material bond wall permeability is too low. It seems probable that the well with the surface but penetrate not more than one tighter sands are shattered to some degree and the cemen­ grain depth. The accuracy of the permeability measure­ tation broken, while the more permeable, unconsolidated ment depends upon the size and condition of the available sands are adversely affected by a shifting or realignment sample. In some cases, the sample recovered is too small of the sand grains and connecting pores, and possibly to provide a separate portion for permeability measure­ some compaction. ment. In such cases, an order-of-magnitude permeability In all types of coring, the core is subjected to the flush­ can be obtained from empirical charts such as those which ing action of drilling-fluid filtrate. The sidewall sample is have been developed using grain size, degree of sorting, normally flushed to a greater extent than a conventional or saturated sample density and porosity. Reudelhuber and diamond core sample. The well may have been drilled Furen' have pointed out that permeability values meas­ many feet below the sampling point. As a result, the walls ured on sidewall samples normally differ significantly from of the hole were subjected to the scraping action of the those measured on samples of conventional cores from the outer edges of the bit and of the rotating drill pipe. This same formations. may have removed the mud cake several times, and al­ lowed more filtrate to flow into the formation in replacing the cake. Also, Nowak and Krueger" provide evidence of Factors Affecting Formation Representation by Samples mud-solids invasion of porous alundum. Further, Slusser There are a number of factors which influence the de­ and Glenn'" have shown that mud solids may invade gree to which the sidewall sample represents the actual porous media as much as 2 or 3 cm and cause permanent formation and its physical properties, its condition of reduction in the permeability of the porous media. Since fluid saturation and its flow properties. Webster and the sidewall sample represents only 1 to 2 in. of formation, Dawsongrove5 point out that the measured porosity on a including the mud cake, any infiltrated mud particles sidewall sample may be much too high, particularly in would be included in the sample. This flushing process dense or well consolidated formations. This is attributed to naturally results in higher water saturation, less residual the shattering of the sand grains and breaking of cementa­ oil saturation and, usually, less permeability. All of these tion by the percussion-type sampling device. Experience factors must be considered in the interpretation of the has shown, however, that reasonably good porosity checks data. can be obtained on most Gulf Coast productive sands The use of diesel oil in drilling muds has increased which are not so dense and well cemented. Some com- greatly in recent years, particularly in the areas where

420 JOURNAL OF PETROLEUM TECHNOLOGY sidewall samples are most widely used. Contamination comparison of sidewall and conventional core-analysis of the sidewall core sample by diesel-oil mud filtrate must data on a large number of samples (extension of study by be recognized and considered in the interpretation of the Reudelhuber and Furen). These data were compiled from data. approximately 100 Gulf Coast wells in which the same The size and quality of the sidewall sample frequently sand was cored by both methods either in the same well are important factors in interpreting and applying the data. or in the same field where comparable data could be ex­ If the sample is very friable or very badly contaminated pected. The differences in fluid saturations for the different with mud infiltration, it may be better to use the entire groups of gas-, oil- and water-productive sands are con­ sample, after removing the mud coating, for determining sidered typical of the changes brought about by excessive fluid saturations and porosity rather than to divide it into flushing in the sidewall samples. Caution must be exer­ two inadequate portions. cised, however, in reference to this table. Individual sets The frequency of sampling and the total number of of sidewall data may vary considerably due to peculiar samples from a zone are also important factors in inter­ well or sampling conditions, and this table should not be pretation and application of sidewall core-analysis data. used as a basis for direct "adjustment" of individual side­ The limitations of conventional cores where a small cylin­ wall analyses. drical segment is taken as representative of a large reser­ Years of experience have indicated general limiting voir volume have been well recognized. The sidewall core ranges of permeability and total water saturation which sampling procedure, which provides only a fractional por­ are associated with gas- or oil-productive zones in specific tion of each vertical foot of formation, further reduces formations and areas. Comparisons of sidewall and con­ ventional core-analysis data4 and well productivity show

greatly the likelihood of obtaining a truly representative Downloaded from http://onepetro.org/jpt/article-pdf/13/05/419/2238154/spe-1635-g-pa.pdf by guest on 01 October 2021 sample. Since the small sample is selected almost at ran­ that the upper limit of total water saturations associated dom, within a given foot, the number of samples should with gas or oil production is considerably greater for side­ be relatively great. We feel that it is virtually a waste of wall analyses. As an example, maximum water saturation time and effort to obtain one sidewall sample from a as determined on conventional cores to be associated with small zone of possible interest. One sample per foot gas production in certain zones was 50 to 55 per cent. through the zone of interest is recommended as the mini­ Water saturations measured on sidewall samples from the mum number to consider. Minimizing the number of same zones were 60 to 70 per cent. The extent of increase samples may hinder proper interpretation, even if the in water saturation appears to be greater in coarse-grained sample is representative. This applies particularly to the formations with higher permeabilities and porosities. As picking of gas-oil and oil-water contacts. In many in­ stated earlier, permeability measured on sidewall samples stances, sidewall samples are taken only in the center is generally less than true reservoir permeability. Data on portion of the interval which indicates oil or gas on the low-permeability sidewall samples from a zone where log. This procedure essentially eliminates the use of core good over-all permeability is indicated should normally analysis in selecting completion points. There have been receive little emphasis in the interpretation for the zone. many cases where the availability of sidewall core-analysis Experience has shown that sidewall core-analysis data data has been largely responsible for obtaining a satisfac­ and electric or radiation log data are complementary, and tory completion. It is recommended that at least one that both are very desirable for proper evaluation of a sample be taken below an indicated oil-water contact as formation. Many of the shortcomings of the small, scat­ well as through the apparent hydrocarbon-bearing zone in tered samples can be overcome if log data are available to order to determine the relative fluid saturations. give an indication of sand quality and continuity. If the Accurate sampling depths are essential to the proper log indicates a "dirty" or nonhomogeneous zone, samples application of sidewall core-analysis data. These samples which apparently were taken opposite shale lenses (im­ are obtained on wireline equipment identical with or simi­ permeable streaks, etc.) can be discounted, and the inter­ lar to that used to run the various electric or radiation pretation can be made on samples representative of the logs, and they are normally reliable. Occasionally, how­ productive intervals of the zone. In a like manner, the ever, incorrect depth measurements have resulted in sam­ core-analysis data are necessary to evaluate the constants pling outside of the objective zones or in sampling shale used in calculating formation characteristics from the vari­ streaks dispersed in the zone. Errors of this type can be ous down-hole logs. These constants vary appreciably with minimized by careful operation. formation properties and need to be calibrated against measured values rather frequently. Also, certain types of shaly or dirty sands show as low-resistivity sections on Applications of Sidewall Core-Analysis Data logs, but still make good wells. Sidewall data permit proper interpretation of their hydrocarbon content. In cases where Sidewall core-analysis data normally show the actual salt-water-base mud is used or where shallow fresh-water presence or absence of hydrocarbon content and indicate bentonitic sands are encountered, the electric logs may be the type of production and productive capacity. The data distorted and of little value, but the sidewall core data permit accurate location of gaS-Oil and water-oil contacts. permit accurate selection of oil-water contact and perfora­ When used in conjunction with electric or radiation logs, tion points. the sidewall core-analysis data verify lithology, permit evaluation of stray sands and provide a basis for "calibrat­ ing" the electric log data. Examples of the Application of Sidewall Data The consistent and rather wide differences observed in Figs. 1 and 2 present sections of logs and sidewall core­ data obtained on sidewall samples and on conventional analysis data for oil-productive zones which show low­ cores from the same formations made it necessary to de­ resistivity characteristics on the electrical logs due to velop new criteria and interpretative standards for the argillaceous and shaly content, with its resultant high sidewall data. The direction of the differences is fairly well water saturation. In Fig. 1, the SP curve deflection is explained on the basis of the conditions of sampling, but typical of sand intervals in the area. The amplified 16-in. the relative effects ,of the changes on formation produc­ normal and 40-in. induction curves show a slight change, tivity had to be developed by experience. Table 1 shows a but not greater than in other shaly sections of the hole.

)WAY, 196] 421 The changes in the regular l6-in. normal and 40-in. induc­ a wet interval. The four sidewall samples taken in the 27- tion curves are barely detectable. Data from sidewall ft interval are inadequate to give suitable coverage, but cores obtained in the interval indicate good porosity, some the data do indicate limits of the pay interval and show permeability and definite oil saturation. Examination of the presence of hydrocarbons in a permeable zone. The the core sample showed considerable shaly and argillaceous sample at 12.190 ft shows geod porosity but very low material. Experience in the area had shown that com­ permeability, with high water and no oil saturation. The mercial oil production could be obtained from such zones, next two samples, at 12,204 and 12,207 ft, show even even with the high water contents, because much of the better porosity, good permeability, low water and good water is held by the "dirty" components. As would be ex­ residual oil saturation. The fourth sample indicates a return pected, the lower permeability samples show the higher to low capacity and high water saturation. These results, water saturations. The 14-ft interval produced 120 BOPD when considered with the log data, provided the basis for at a GOR of 400 cu ft/bbl on test. testing the interval shown. The interval produced at a rate The interval shown in Fig. 2 shows somewhat greater of 284 BOPD (39.5° API) with a GOR of 933 cu ft/bbl. changes in the electrical logs than did the preceding ex­ It is very doubtful this zone would have been tested with­ ample. However, the low resistivity still may indicate a out the aid of the sidewall core data. wet zone. The sidewall core data shows this interval to Fig. 4 presents an example of a thin oil column which have good porosity, permeability and oil saturation. The was found by sidewall cores and core analysis between high water saturations are normal for the dirty oil-produc­ water-bearing massive sands. The electric logs indicate a tive zones encountered in this area. The l2-ft interval change in SP and in resistivity, but the zone is not thick

produced at a rate of 140 BOPD at a GOR of approxi­ enough to define the reasons for change. The sidewall Downloaded from http://onepetro.org/jpt/article-pdf/13/05/419/2238154/spe-1635-g-pa.pdf by guest on 01 October 2021 mately 400 cu ft /bbl on test. sample shows the presence of high-porosity sand, with Fig. 3 also presents an example of the use of sidewall permeability and with good oil and low water saturations. core-analysis data in a low-resistivity zone to indicate oil A commercial oil producer was completed in this zone. production probabilities. The SP curve indicates a sandy Fig. 5 is an example of a case where sidewall core- interval. The normal and induction logs are indicative of SPONTANEOUS-POTENTIAL '"rn RESISTIV ITY CONDUCTIVITY millivolts ":c-< ohms.m'/m millimhos/m = ohml~:~/m SPONTANEOUS-POTENTIAL rn'" RESISTIVITY CO NDUCTIVITY 0 16' NORMAL 10 2000 40' INDUCTION 0 millivolts "-< ohms.m2/m millimhos/m = Oh~~~~~/m -1lQ.1+ :c 9.. _____ 19~J.~Q.QG!9_~ ____ ..... lQ -1lQ.1+ 0 16' NORMAL 10 2000 40' INDUCTION 0 0 AMP 16' NORMAL 2 Rm~.l8-90°F 9...... __ 1Q:·.J.~9.~n!9.~ __ . --. ..IQ Rmf = .15-85°F 0 AMP IS" NORMAL 2 ( Rm = 09 175°f Rrne = .66 85'F \ 1\ \1 I I ;.'J ~ -+-__ ' i III 12200 I I ~ I- I'" I I I-!-i I u!!I I ~r+~ fJ 8120 ~ I \ --l~ I P \, 'I I . I i t t"\ I -H Li i 12220 I ' !f r-... ]( I) :1 ' I . I I I I 1 I I fffi i H41 8140 I' % POR. SAT II 'J I I) I i I I I I IN. DEPTH PERM. POR PROB. % % TOTAL OIL GAS °API 01. REG. FEET Md. % OIL PROD. IN. DEPTH PERM. POR '% POR SAT PROS. % WATER VOL. VOL TOTAL OIL GAS °API REC FEET Md. 'Yo OIL PROD. 0.5 12190 1.9 23.2 0.0 80.7 (6) 0.0 4.5 WATER VOL VOL 1.0 12204 175. 30.3 16.B 47.8 OIL 5.1 10.7 32 1.3 8119 10.0 25.8 6.6 74.8 OIL 1.7 4.8 41 0.5 12207 lOB. 30.5 19.6 38.2 OIL 6.0 12.5 1.3 8124 82. 29.2 7.2 68.9 OIL 2. I 7.0 41 1.0 12217 4.0 22.9 5.1 69.7 (6) 1.3 5.6 0.8 8127 12. 24.7 7.3 76.9 OIL 1.8 39 1.0 8129 83. 2B.0 7.5 67.4 OIL 2.1 7.0 41 (6) LOW PERMEABILITY 1.3 8133 54. 270 10.0 64.8 OIL 2.7 6.8 41 Fig, 3 Fig. I

SPONTANEOUS-POTENTIAL rn'" RESISTIVITY CON DUCTlVI1,:o millivolts SPONTANEOUS-POTENTIAL '"rn RESISTIVITY CONDUCTIVITY :c"-< ohms.m2/m mililmhos/m = ~ millivolts -< ohms.m2/m millimhos/m = Oh~~~Z/m 40· INDUCTION ":c -1lQ..1+ 0 16' NORMAL 102000 0 9_. __ ..3.Q:JNQ~.'T.IQ.~ ____ .. __ .JQ -1lQ.1+ 0 IS' NORMAL 10 2000 40' INDUCTION 0 L ____ 19~J.@.~n!9JL ____ . ..I.Q Q AMP IS' NORMAL 2 Rm = .18- 90°F. Rmf. =.l5-85°F. 0 AMP 16' NORMAL 2 Rm = .09 175°F. Rme.= .66 85°F. ) I S730 I! I~ I \ 1\ \ i : w-- - ~--t i r It. i-rT 8180 t--- i :1; \ 6750 ,I I ~ I',> I I I i I i ! :~ I !\L, iii !y 1.., ! 1_11 1\ LC V- :1 I ' 8200 i I I fl~_ 'd_lkr I 6770 ~i-W- 1 Ii k1-r1 ! ! i (1 ! : ! ! I rrl!t ~i H~ -~--:-- - f-J--i-r-- '% POR SAT I ';-T % % : i : I IN. DEPTH PERM. POR PROB. I I I Ii:J ' i ! I Iii • I I i I I REG FEET TOTAL OIL GAS °API Md. % OIL WATER PROD. VOL VOL IN. DEPTH % POR. SAT % % 1.5 81B3 45. 26.B 9.7 69.0 OIL 2.6 5.7 42 PERM. POR PROB. REC. FEET Md. TOTAL PROD. OIL GAS °API 1.5 818B 265. 27.6 10.5 64.9 OIL 2.9 6.8 % OIL WATER VOL. VOL. 1.3 BI91 60. 29.5 6.4 75.9 OIL 1.9 5.2 42 6760 2B.1 16.0 44.1 OIL 4.5 11.2 O.B BI93 2B.1 6761 139 36. 8.5 67.5 OIL 2.4 6.7 42 27.5 IB.5 51.0 OIL 5.1 8.4 f--- 6763 29.3 IB.8 55.0 OIL 5.5 7.7 Fig. 2 Fig. 4

422 JOURNAL OF PETROLEUM TECHXOLOCY SPONTANEOUS-POTENTIAL '"rn RESISTIV ITY CONDUCTIVITY SPONTANEOUS-POTENTIAL '"rn RESISTIVITY CONDUCTIVITY -I millivolts ""-I ohms m2/m millimhos/m = o~~~~Z/'" millivolts :J:"" ohms.m'/m millimhos/m = Dh~~O~Z/'" x 0 16' NORMAL 10 2000 40' INOUCTION 0 -1lQ..I+ 0 16' NORMAL 10 2QOO 40' INOUCTION 0 -1l.9..1 + 19.".J!!~Yg!9.L._ _LQ 9______i9:L@_~£T!9H. ___ ... _IQ Rm =! 52-88"F 9... _____ ..... Rmf =O.80-152°F. 0 AMP 16' NORMAL 2 Q AMP 16' NORMAL 2 Rm~0.88-152'f Rmc =0.82 152'f. I ~ 'I> II I 1 1\ ~ 1\1 I 4900 JJ 9480 I ~ ~ ". 1'- /Jr- 11 j- "; C: r-I- r-... ~ 1< p I I i '~I- ! I I ( I ,! k--f> i 1\ i 1( I I i I H- II 9500 4920 , I J ~ i (I ~ [ I l

66. 30.8 7.5 OIL 2.3 6.8 Downloaded from http://onepetro.org/jpt/article-pdf/13/05/419/2238154/spe-1635-g-pa.pdf by guest on 01 October 2021 Fig. 5 I 5 9487 91. 35.1 9.4 64.4 OIL 3.3 9.2 37 1.5 9488 71. 30.7 4.2 76.2 OIL 1.3 6.0 analysis data were needed to substantiate electrical log 1.5 9489 77. 34.9 9.2 67.8 OIL 3.2 8.0 interpretation in a situation where two formation tests (6)-LOW PERMEABILITY indicated essentially no productivity. The electrical logs indicate all of the thin sand setcion at about 4,906 ft to Fig. 6 be hydrocarbon-bearing. One formation test was dry with the entire zone without fear of bottom water, thus provid­ zero flowing pressure, and a second test showed only 100- ing greater productive capacity. psi bottom-hole flowing pressure, indicating a very low­ Table 2 presents results of core-analysis data on two permeability zone. Sidewall core data showed good perme­ sets of sidewall samples taken over the same section of a ability over the entire interval. Saturation data indicated well. One set of samples indicates zero or very low perme­ gas production in the top of the zone with water in the ability and relatively high water saturations. These data bottom, more permeable portion. In view of the log char­ did not appear to be consistent with other information on acteristics, it was concluded that the more permeable sec­ the well. A second set of sidewall samples was obtained, tion had been completely flushed with drilling-fluid filtrate and special care was exercised in obtaining these samples and that its high permeability should be favorable for gas from the desired depths. Data obtained on the second set production. The well was completed and showed an initial of samples indicated appreciable permeability, better po­ potential of 2,240 Mcf/D of dry gas with a tubing pres­ rosity and sufficient productive capacity for an economical sure above 1,700 psi on a 16/64-in. choke. well. These data emphasize the importance of taking a It is doubtful that an operator would set pipe on a 4 or sufficient density of samples in an interval and of being 5-ft section after two essentially dry tests. Sidewall core sure that samples are taken at the depth selected. analysis showed good permeability and productive ca­ pacity, but indicated water in the bottom portion. The The data shown in Table 3 present a comparison of induction log indicated gas or oil over the interval with results obtained by the analysis of conventional cores from no bottom water. Having both sets of data permitted a zone in a well (Oligocene sand-South Louisiana) and proper evaluation of the zone. results obtained on sidewall samples which were taken at Fig. 6 presents an example where both the sidewall core­ 6-in. intervals through the same zone. The data indicate a analysis data and the electrical logs indicated an oil­ productive zone. However, the operator felt that the elec­ TABLE 3-CONVENTIONAL AND SIDEWALL CORE ANALYSIS trical log indicated water in the lower portion of the zone. [OLIGOCENE SAND-SOUTH LOUISIANA) The sidewall core data indicate oil production for the en­ Residual Total Perm. Por. Oil Water tire zone. The interval 9,480 to 9,488 ft was perforated. Type Core (md) ("/0). Sat. ("/0) Sat. ["/0) Remarks Initial potential flow was 121 BOPD (35.4° API) at a Conventional 569. 32.3 13.3 58.7 9971-79 It GOR of 708 cu ft/bbl, with only 0.1 per cent BS&W. Sidewall 262. 32.0 11.7 69.5 9971-78.5 ft Sidewall 168. 30.6 10.4 68.5 Avg. data for 8 The sidewall core data allowed the operator to perforate additional wells

TABLE 2-RESULTS OF CORE-ANALYSIS DATA ON TWO SETS OF SIDEWALL SAMPLES TAKEN OVER SAME SECTION OF A WELL

% Porosity Sat. Per Cent Per Cent In. Depth Permability Porosity Total Prob. Oil Gas Ree. ~ ~- - ("/0) Oil Water Prod. Vol. Vol. °API 0.5 10300 8.9 22.1 2.7 72.5 (6) 0.6 5.5 1.3 10306'/2 6.2 12.3 i1.2 59.6 (6) 2.5 6.5 1.3 10342 4.3 19.4 2.1 74.3 (6) 0.4 4.6 2.3 10346 0.0 21.3 0.0 80.2 0.0 4.2 0.5 10348 3.4 21.7 5.0 73.6 (6) 1.1 4.6 0.5 10353 0.0 21.2 3.3 81.0 0.7 4.3 1.8 10372'/2 0.0 22.2 1.8 75.7 0.4 5.0 0.5 10293 508. 29.2 2.0 48.6 Condo 0.6 14.3 0.8 10299 53. 25.4 2.7 63.0 Condo 0.7 8.7 1.0 10305 0.0 22.0 9.6 69.1 2.1 4.7 1.3 10309 .0.0 19.1 0.0 78.3 0.0 4.1 0.8 10346 159. 26.1 2.3 62.8 Condo 0.6 9.1 0.8 10349 189. 27.8 1.8 58.3 Condo 0.5 11.2 0.8 10351 16B 27.1 1.8 60.2 Ccnd 0.5 10.3 0.8 10358 49. 23.8 1.7 65.8 (8) (1.4 7.7 (6) Low permeability. (8) Possibly condensate productive.

MAY, 1961 'l23 TABLE 4-SIDEWALL CORE-ANALYSIS DATA 0/0 Por. Sat. % % In. Depth Perm. Por. Total Prob. Oil Gas Rec. (md) ("!o) Oil Water Prod. Vol. Vol. o API Description __ (ftL -(i4j- 0.8 11011 ----n- 24.2 5.3 -73.2 1.3 5.2 -33 SDYF 6i SHY SILTY V LIMY GOOD ODOR FAiNi-F-Cu­ 1.3 11251 5.1 17_8 0.0 71.5 (6) 0.0 5.1 SO F GR V SHY SILTY V LIMY NO ODOR NO FLU 0.5 11253 268. 26.9 21.1 49.1 OIL 5.7 8.0 30 SO V F GR SLI SHY SILTY GOOD ODOR BRT BLUE WH FLU 1.0 11255 40. 24.5 10.2 56.8 OIL 2.5 8.1 SO V F GR Sll SHY GOOD ODOR BLUE WH FLU 0.8 11267 85. 29.1 9.3 57.1 Oil 2.7 9.8 30 SO V F GR SLI SHY SILTY GOOD ODOR BLUE WH FLU 1.3 11283 SHALE-NO ANALYSIS 0.5 11291 331. 32.1 0.6 66.5 COND 0.2 10.6 SO V F GR SLI SHY SILTY V FAINT FLU V FAINT ODOR 0.5 11294 515. 28.4 0.0 59.2 COND 0.0 11.6 SO V F GR SLI SHY SILTY SLI LIMY V FAINT ODOR V FAINT FLU 0.5 11297 715. 32.9 0.0 62.3 COND 0.0 12.4 SO F GR SLI SHY SILTY V FAINT ODOR V FAINT FLU 0.5 11300 620_ 32.5 0.6 62_2 COND 0.2 ,12.1 SO F GR SLI SHY SILTY LIMY FAINT ODOR FAINT FLU 1.5 11312 SHALE-NO ANALYSIS (6) Low permeability. (8) Possibly condensate productive. (14) Additional samples needed to clarify marginal fluid saturations. ------marked reduction in permeability, a slight reduction in oil required for data to be used in accurate evaluations and saturation, an appreciable increase in water saturation and in reservoir engineering calculations. a rather good check on porosity. These results exemplify Sidewall core data and electrical log data are mutually the general descriptive characteristics presented earlier in complementary. General formation characteristics shown this paper. They are consistent with results reported by by the electrical logs partially compensate for the small Reudelhuber and Furen4 from a large number of samples representation of a specific zone by the sidewall sample and with the results of further study of their data as and frequently permit positive interpretation of lithology Downloaded from http://onepetro.org/jpt/article-pdf/13/05/419/2238154/spe-1635-g-pa.pdf by guest on 01 October 2021 presented in Table 1. The formation is moderately soft and fluid content of the zone which may be contradictory with good permeability. Reduced permeability would re­ to the indications of the log. Sidewall core data are prob­ sult from both mud-solids infiltration and sample altera­ ably most useful in "dirty" low-resistivity zones. The side­ tion by the bullet sampler. Low oil and high water satura­ wall sample will show whether the changes indicated by tions are characteristic of the formation immediately sur­ the electrical logs when high-resistivity zones are en­ rounding the wellbore. Average data obtained on sidewall countered are due to reduction in porosity or to the pres­ samples from eight other wells in the field, which were ence of hydrocarbons. The determination of the mere drilled over a period of 12 months prior to the well cored presence or absence of hydrocarbons, as shown by the conventionally, are shown in Line 3. All of the sidewall sidewall sample, is frequently of great importance. The data show a consistent trend from the data obtained on sidewall core will provide an order of magnitude of per­ the conventional core. The availability of data on one meability, which is important in evaluating the probable set of large cores or knowledge of characteristics of spe­ productivity of a zone. The sidewall sample is an im­ cific formations usually permits reliable interpretation of portant aid in the proper "calibration" and interpretation sidewall core data. of electrical and radioactive well logs. Table 4 presents data obtained on sidewall samples from Although hole conditions and sidewall sampling pro­ another Oligocene section. Samples from the interval cedures markedly affect the properties and saturation con­ 11,253 ft to 11,267 ft show permeability and porosity ditions of sidewall core samples, these samples do provide which indicate productive capacity, and the saturation data essential, although qualitative, information on lithology, indicate water-free oil production. Samples from the in­ fluid content and productive capacity which cannot be terval 11,291 ft to 11 ,300 ft indicate good permeability obtained by methods other than direct measurement on a and porosity. Saturation data indicate gas or condensate, sample of the formation. The combined use of sidewall but no oil productivity. The small residual oil found in core data and electrical log data will enhance the value two of the four samples and the lack of dry gas produc­ of each and usually will result in the proper interpretation tion from such great depths in this area lead to interpre­ of the type of production and relative productive capacity tation as condensate productive. The absence of sand in of a zone. the shale sample recovered at 11,283 ft indicates the pos­ References sibility of a separation barrier which would permit produc­ 1. Leonardon, E. G. and McCann, D. c.: "Exploring Drill Holes tion of oil from a zone only slightly above a gas or by Sample-taking Bullets", Trans., AIME (1939) 132, 85. condensate zone. The availability of electrical or radio­ 2. API Recommended Practice for Core-Analysis Procedure, API­ active logs to provide general formation characteristics RP-40 (Aug., 1960). over this interval would permit much more positive inter­ 3. Jenkins, R. E. and Koepf, E. H.: "An Evaluation of Side­ wall Core Analysis Techniques", Paper 788-G presented at pretation. The oil completion from 11,250 to 11,254 ft pro­ 87th AIME Annual Meeting in New Orleans, La. (Feb. 27, duced 192 BOPD through a VB-in. choke. The condensate 1957) . completion from 11,282 to 11,286 ft made 1.5 MMcf/D 4. Reudelhuber, F. O. and Furen, J. E.: "Interpretation and of gas plus 12 bbl of condensate through a lis-in. choke. Application of Sidewall Core Analysis Data", Trans., Gulf Coast Assn. of Geol. Soc. (1957) VII. 5. Webster, G. M. and Dawsongrove, G. E.: "The Alteration of Rock Properties by Percussion Sidewall Coring", Trans., AIME Summary and Conclusions (1959) 216, 385. 6. Nowak, T. J. and Krueger, R. F.: "The 'Effect of Mud Filtrate Laboratory techniques permit reliable determination of and Mud Particles Upon the Permeabilities of Cores", Drill. porosity, permeability and fluid saturations on sidewall and Prod. Prac., API (19'51) 164. 7. Slusser, M. L., Glenn, E. E. and Huitt, J. 1.: "Factors Affect­ samples as received in the laboratory. Comparisons of ing Well Productivity: I. Drilling Fluid Filtration", Trans., data from sidewall core samples with data from conven­ AI ME (1957) 210, 126. tional core samples have shown that different limiting 8. Glenn, E. E. and Slusser, M. 1.: "Factors Affecting Well values and standards of interpretation are necessary. Suf­ Productivity: II. Drilling Fluid Particle Invasion into Porous ficient experience has been gained to permit reliable quali­ Media", Trans., AIME (1957) 210, 132. *** tative interpretation in most areas where sidewall coring is EDITOR'S NOTE: PICTURES AND BIOGRAPHICAL SKETCHES widely used. Conventional, wireIine or diamond cores are C>P E. H. KOEPP AND R. J. GRANBERRY APPEAR ON PAGE 459.

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