:582-6 Formation Evaluation Of Oil And Gas Reservoirs By Norman J. Clark and H. M. Shearin, Members AThiE, Core Laboratories,-Inc., Dallas, Tex. Downloaded from http://onepetro.org/SPEFD/proceedings-pdf/55FE/All-55FE/SPE-582-G/2086146/spe-582-g.pdf by guest on 29 September 2021 ABSTRACT The fundamental questions that face the oil maximum economic return? and gas operator in finding, developing, and pro­ ducing oil are set out and the formation evalua­ The reliability of answers to these funda­ tion information that is necessary to answer these mental questions depend upon the availability and questions is outlined. A history of' the develop­ accuracy of data upon which the answers must be ment of' the tools and techniques of formation e­ based. As new oil and gas reservoirs are found valuation is briefly given. The type of informa­ at greater depths and are more costly to find tion that is provided by each of the various me­ and develop, accuracy of the evaluation grows thods is listed and problems enco~1tered in ob­ correspondingly more important. taining this information are discussed. A plan is proposed for the coordinated use of too15 and Formation evaluation encompasses the pro­ techniques of' evaluation by means of' a key well cesses of gathering appropriate, accurate, and program for studying optimum requirements for detailed data on the physical characteristics of formation evaluation data. the formation rock, the occurrence and distribu­ tion of fluids within the formation rOCk, and the rnTRODt£TION processes of interpreting those data for accuracy and reliability so that proper use can be made The petroleum industry today is a product of of the information in developing and operating American business and with its 8,900 producing the oil and gas reservoirs. The need for accu­ companies it is one of the most competitive indus­ rate data is imperative, and the prudent operator tires in .America. Because of this it has emerged attempts to secure proper information on his pro­ from World War II as one of our most highly spe perties through a planned formation evaluation specialized businesses, and never before has program. science been drawn upon and applied more vigo­ rously in shaping a maturing industry. The basic data necessary to evaluate oil and Because we can never physically see the oil gas reservoirs to answer the fundamental questions or gas reservoir in any of its in-place condi­ consist primarily of the following: (a) net reser­ tions, we must indirectly rely upon measurements voir 011 and gas zone thicknesses; (b) productive­ of certain physical parameters to guide us in op­ limits; (c) porosity; (d) permeability; {e} inter­ erating the reservoirs prudently. The develop­ stitial water saturations; (f) residual hydrocar­ ment and operation of an oil or gas accumulation bon saturations; and (g) information on sand un­ has, therefore, become a reservoir engineering iform!ty. These types of data separately or in enterprise; and more arid more operations are logi­ combinations are needed throughout the productive cally being based on factual reservoir informa­ life of any reservoir if employment of sound tion in this manner. production practices is to be assured. During the development of a reservoir, the data are used There are two fundamental questions that to establish spacing and completion intervals of face the oil and gas operator in finding, devel­ wells and to guide drilling practices. Operations oping, and producing oil and gas reservoirs. of wells require the data for such things as stud­ These questions are: ies of rates of production, problem wells, and workover possibilities. Proper reservoir opera­ 1. Is oil or gas present and in paying tions require the data for such things as reser­ quantities? voir engineering studies of pressure maintenance, secondary recovery, efficient rates of produc­ 2. How should the deposit be produced for tion, participation in production, and unitization References and illustrations at end of paper 1 FORMATION EVALUATION OF OIL AND GAS RESERVOms and pooling. In addition, sale or abandonment formations, obtained during drilling, was para­ of properties are greatly facilitated through mount whereas proponents of the rotary rig con­ access to reliable data. sidered that easier and safer drilling, parti­ cularly in the soft rock areas, was paramount. HISTORY OF THE DEVELOaoofr OF TOOLS AND TECHNIQUES FOR OBTAINING EVALUATION DATA As early as Spindletop, it was recognized that some means must be devised to permit the The story of formation evaluation is virtual­ rotary driller to examine specimens of the de­ ly the story of the oil industry itself, one that sired formations penetrated. Crude rotary core was fraught with superstition and intrigue dur­ barrels such as punch, Texas or poor poy, auger, ing its pioneering years; one that was spiced fishtail and shot types were used to bring core with fortunes and bankruptcy; and finally, one samples to the surface for examination. It was that has developed an economic stability through not until 1920, however, that a double barrel the support of sound engineering practices. core bit was developed and became available to Downloaded from http://onepetro.org/SPEFD/proceedings-pdf/55FE/All-55FE/SPE-582-G/2086146/spe-582-g.pdf by guest on 29 September 2021 the industry. Although the employment of geo­ CABLE TOOL PERIOD (BEFORE 1900) logic methods camnenced in about 1914 in the drilling industry,2 core logs made during this The period of the industry prior to the period of time were based principally upon sur­ turn of the 20th century is variously termed the face indications recorded by the driller or cable tool period, or the lamp and lubricating sampler.3 These included cuttings caught at the oil period. During this time cable tool rigs surface, penetration rates, pipe vibrations, were used exclusively for drilling and little dif rainbows on mud pits, and blowouts. ficulty was experienced in evaluating formations penetrated by the cable tool drilL The presence PERIOD OF INDtETRY EXPANSION (1920-1930) of oil in cuttings that were brought to the sur­ face in the bail~r was easily detected, and, if During the period from 1920 to 1930, the oil the rock had sufficient permeability to produce, industry was greatly influenced by a post-war the presence of oil often resulted in gushers. inflationary trend. More oil was both dis­ covered and produced during this decade than As early as 1880 Carlll pointed out the ele­ bad been discovered and produced during the pre­ mentary differences in recovery efficiency be­ vious history subsequent to the Drake discovery. 2 tween the different reservoir drive mechanisms Except for a general slowdown during the great and predicted the need for gathering data and depression of the early 1930's, this period information leading to the development of ap­ marked the beginning of an industrial expansion propriate reservoir study methods. Little was within the industry which bas continued until done during Carll's day, however, toward de­ the present time. With unregulated production velopment of this line of endeavor. By contrast the order of the day, the industry was on a to periods that followed, this early period of ''boom and doom" basis. Expansion in operations the industry remains quire insignificant in its touched off a wave of equip:nent research because influence on modern practices. of the incentive to better evaluate properties, through better test methods. Ten years of core TRANSITION PERIOD - CABLE TOOL TO ROTARY bit development followed the advent of the first (1900-1920) double barrel core bits before conventional bits that could be pulled without pulling the drill The advent of the rotary drill in Corsicana stem were in widespread use. Notable develop­ and shortly thereafter in Spindletop in 1899 ment included the Elliot bit4 and the Robishaw brought in a new era in drilling. Mud in the bit in 1927. Important work was done by the rotary-drilled hole permitted continuous pressure Bureau of Mines5,6 during this period of devel­ control over formations penetrated and thereby opment of laboratory procedures for core analy­ a great advancement in the art of drilling wells sis and by 1930 the industry was analyzing cores fast and safely. While it was a wonderful ad­ on a fairly widespread basis. Field an~sis vancement in the art of making hole, mud in c.he bad been developed to the extent that a com­ hole presented a great problem to the driller by mercial boil-out kit was available for use by preventing him from being able directly to eval­ field geologists in determining core saturations uate for oil within the formations penetrated. on location by the solvent extraction method. Millions of barrels of 011 have since been proven to have been passed undetected by the :Beginning in the 1920's a number of drill drill during the years that followed the advent stem test patents were filed or issued on drill of the rotary rig before adequate formation e­ stem test tools which were developed to permit valuation techniques were developed to assist testing of sand penetrated without setting pipe. the driller in locating productive horizons. For The first test was performed in a well near the 20 years after the rotary rig made its first Humble field early in 1920 by Edwards,7 the in­ appearance therefore, a controversy ensued with­ ventor of the tooL Difficulty in tool opera­ in the industry. Propoents of the cable tool tion and water recovery on test discouraged the rig considered that better information on the use of the drill stem tester until about 1926 2 NORMAN J. CLARK AND H. M. SHEARIN when a number of patents were filed on various tools were available, excellent for many pur­ tools.