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.