Professor Stefan Luthi Faculty of Applied Earth Sciences Technical University Delft The Netherlands The History of Logging (Taken from the book: Geological Well Logs, their use in reservoir modeling, by Stefan Luthi, with permission). Professor Forbes from the Edinburgh Observatory was perhaps the first person to make well log measurements, when, from 1837 to 1842, he lowered temperature sensors into three shafts up to 24 feet deep in order to record temperature variations with depth and time. His results were later analyzed by the physicist Lord Kelvin (Thomson, 1861), who was able to determine secular variations in temperature and particularly the heat flow, which he needed for his calculations of the earth’s age. The separation between data acquisition and interpretation in this largely academic exercise seemed to presage a distinction still present in today’s logging operations. Temperature measurements are still routinely done in modern well logging, but they are rarely used for anything other than correcting other sensors’ responses for temperature effects1. The beginning of commercial well logging is entirely attributed to the initiative of the two French brothers, Conrad Schlumberger (1878-1936), a physicist who graduated from the Ecole Polytechnique France, and Marcel Schlumberger (1884-1953), an engineer from the Ecole Centrale de Paris (Allaud & Martin, 1977). Encouraged and supported by their father Paul, a relatively wealthy businessman, they experimented with electrical measurements at the surface to locate iron and copper deposits. Initial successes in several countries led them to create the Société de Prospection Electrique (SPE). In 1921, Marcel Schlumberger tried for the first time to make resistivity measurements in a borehole in the coal basin of Bessèges (France) with the purpose of validating the surface electrical surveys. The strong resistivity variations they observed convinced the two brothers to pursue this further. In 1927, Conrad outlined the principle of “electrical coring” as well logging was initially called2, in a technical note. Henri Doll was hired to develop the equipment and conduct the first operation in an oil well, which took place on September 5, 1927, in Pechelbronn in Alsace, where at the time a small but thriving oil industry existed. The resulting log, hand-drawn with measuring points spaced one meter apart, represents a turning point in oil exploration. It showed the layered nature of the subsurface and allowed easily identifying the major geological formations such as the top of the Hydrobia Marl, an important regional marker. Subsequent logging in nearby wells made clear that the method could be used for correlation purposes with greater accuracy than drill cuttings and at lower costs than core drilling (Schlumberger et al., 1932). Furthermore, the technique showed potential for detecting hydrocarbon-bearing layers, but the pay zones in Pechelbronn were too thin to demonstrate this. The Schlumberger brothers were able to convince Royal Dutch Shell that the technique was reliable, after a repeat log done by the client in person (Allaud & Martin, 1977), and they were awarded a logging contract in Venezuela. Following a successful campaign, the company expanded to India and Russia, and, after unsuccessful initial trials, established themselves in the United States. They were helped by the almost serendipitous discovery of the spontaneous potential (SP), a voltage observed on two of the resistivity electrodes when no external current was applied to the device. The two measurements combined were shown to locate permeable hydrocarbon-bearing layers, and well logging became an industry on its own, far beyond the supportive role to surface geophysics that it had been designed for. In 1 This is not universally true. Russian specialists are very skilled at interpreting temperature logs, particularly in fractured reservoirs (see chapter 3.3.3.) 2 Carottage électrique in French.

1934, the Schlumberger Well Services company was founded in Houston, Texas, and it immediately expanded at a rapid pace. Logging proved successful in other parts of the world, notably Mexico, Ecuador, Argentina, Germany, Austria, and Borneo. The number of crews grew rapidly as illustrated by table 1.2.1, with a particular acceleration after the spontaneous log was introduced in 1931. In the early 1930s the first tools for sidewall coring and perforating were developed. Well geometry surveys using first the teleclinometry, then the photoclinometry system, were introduced by Sperry Sun and Schlumberger, and were crucial in accurately locating the positions of deviated wells. In 1938 the Wells Survey Company performed the first gamma ray logging in Oklahoma and commercialized it one year later by promoting it for accurate perforating. In 1942, Schlumberger introduced the first dipmeter tool, initially as an “anisotropy tool”, then based on a spontaneous potential (SP) principle with three arms. It was replaced ten years later by passively focussed micro-electrical devices. The growing number of tools required a significant parallel development in cable technology and surface acquisition systems, which was mostly done by service companies. Research labs from oil companies, among others Mobil, Shell, and Chevron, significantly contributed to the development of many logging tools and in many cases built prototype tools themselves. In 1949, after intensive research, Doll was able to prove the feasibility of induction logging. This opened the market to the large number of wells drilled with freshwater or oil-based muds. In the early 1950, Schlumberger developed several new electrical measurements such as the microlog, and brought an early version of the neutron porosity log onto the market. Efforts by Mobil and Esso (then Magnolia and Humble) led to the introduction of the acoustic log in the mid-50s, initially without and then with borehole compensation. It provided the first accurate and relatively direct porosity measurement, a quantity sorely needed for the proper assessment of hydrocarbon reservoirs. Already in 1942, Gus Archie from Shell had proposed empirical relationships between porosity, fluid resistivity, rock resistivity, and water saturation, which seemed to hold true in a large variety of rock types. Now, with a porosity log, a shallow, and a deep resistivity measurement, it became possible to determine fluid saturations from logs, as was advocated by early proponents of quantitative log interpretation (Wyllie & Rose, 1950). The original use of logs - geological correlation and location of hydrocarbon-bearing zones - was thus slowly but steadily supplanted by “formation evaluation”, or petrophysics. The 1950s also saw the first formation fluid sampling tool, the first shaped charges for perforating, and nuclear magnetic resonance logging saw the light. In the 1960s, density logging and sophisticated focussed resistivity devices were put in commercial use. Mobil’s borehole televiewer, an ultrasonic borehole-scanning tool, was the first downhole imaging measurement that could be used in oil wells. With this, most of the basic logging techniques as known today were in place. Much of the logging development in the 1970 was related to the growing use of electronics. Tool modeling and design was increasingly done with the aid of computer simulations. Downhole sensors, transmission, and surface acquisition were increasingly controlled by microprocessors. The analog-digital conversion moved steadily towards the sensors, to a point where today’s logging systems are essentially all digital in nature. Data processing became entirely a task performed by computers, and data interpretation was increasingly done on computers using specialized software. With drilling going to greater depths, logging cables, data transmission rates, tool specifications and tool reliability all had to be adapted to the changing environments and the surface acquisition systems had to be able to handle rapidly increasing data volumes. The 1980s saw the emergence of logging-while-drilling (LWD), mostly fuelled by the increased use of directional drilling after steerable downhole motors had been introduced by Smith International and Eastman-Christensen. This development had started off in the 1960s with downhole measurements of the well deviation, torque and weight on the bit and simple formation parameters. Initially it was done in stationary mode whenever drilling was stopped, and then in measurement-while-drilling (MWD) mode, whereby the data was sent to the surface by mud pulses, or, in the case of turbodrilling, by a wireline cable. Teleco, Sperry Sun, and Anadrill, a Schlumberger company, introduced single-resistivity downhole measurements in the early 1980s, where the data was stored in a downhole memory and later retrieved. The urgent need to have information from the drill bit as soon as possible caused development to proceed at a rapid pace, and in the mid-1980s Sperry-Sun introduced several tools that allowed formation evaluation from LWD measurements. In 1988, real-time LWD saw the light with Schlumberger’s compensated dual-

resistivity tool, which was combinable with a gamma ray, density, and neutron tool. Their measurements were transmitted with pulses in the borehole mud which propagated in the borehole from the downhole sensor to the surface. This paved the way to evaluate the formation penetrated by directional drilling in real-time and, therefore, to adjust the well trajectory immediately when needed. Other measurements were developed at a rapid pace and today most wireline techniques are also available in LWD mode. Together with the MWD measurements, LWD has created a positive feedback loop on well design, since these measurements allowed for sophisticated wellbore positioning of sidetracks, multiple targets, horizontal or extended reach wells. The 1980s also saw a strong acceleration of borehole imaging technology, a field of particular importance for geologists. It was spurred by micro-electrical imaging, a method of recording an image of the borehole wall with numerous small electrical sensors. The term “imaging” was soon applied to any measurement with two independent variables, such as depth/radius, depth/azimuth, or depth/time, and it expanded to other electrical as well as acoustic and nuclear measurements. This move away from the one-dimensional logs towards visualizing the subsurface helped the geoscientists unravel the complexities of reservoirs better than ever. In the early 1990s, Schlumberger’s RAB* tool performed the first LWD electrical imaging using electrodes on the rotating bottom hole assembly. Also in the 1990s, nuclear magnetic logging, a technique somewhat dormant since its inception by Chevron in the 1950s, became commercially available as a robust wireline measurement. By measuring the pore size distribution, it provides the volume of capillary-bound fluid - a quantity long sought after by petrophysicists - and an estimate of permeability. Today there are numerous service companies active in well logging, with three major players: Schlumberger, Baker-Atlas and Halliburton. Desbrandes (1985), Serra (1984), and the logging company catalogs contain compilations of the currently available well logging services. A list of all major aspects of well log applications is compiled by the Society of Professional Well Log Analysts (SPWLA) and published in their trade journal “The Log Analyst” (starting in 2000: “Petrophysics”) in annual updates (Prensky (1997). More recently, most of the larger oil service companies have developed into general contractors, or integrated service companies, providing a wide array of services from seismic surveys, drilling, mud chemicals, cementing, completions, wireline and LWD logging, production and other downstream facilities, data processing and interpretation. This concentration of services allows these companies to tender for entire reservoir development and management contracts, whereby the integration of services under one company provides a synergistic and cost-effective operation - an aspect of particular importance in today’s complex field developments and economic environment. Well logging is an integral part in these as well as in conventional, operator-controlled projects. It is one of the principal means of collecting relevant downhole data upon which many decisions in field development are based. The history of well logging depicts how the technique has developed from its humble beginnings to a wide range of services and recently into a powerful combination of real-time measurements with directional drilling. While the first measurements were simple tools for geological correlation, they soon developed into petrophysical tools permitting reliable and repeatable formation evaluation. Today’s tools allow drilling progress to be monitored from the surface, with the direction adjusted according to the geological situation encountered in the wellbore. Formation analysis can be done in real time with advanced analytical techniques. Rock layering can be visualized on surface monitors, and fluid as well as rock samples can be taken accurately at locations of interest. Logging in all its modern forms thus contributes greatly to improving the success rate of oil and gas wells, and to produce the reserves more efficiently and more effectively. In conclusion, the history of well logging can be divided, perhaps somewhat arbitrarily, into four distinct phases:

* Mark of Schlumberger

• The conceptual phase (1921-1927): Well logs were intended to support and complement surface geophysical surveys.

• The acceptance phase (1927-1949): Well logging became increasingly useful for layer correlation, identification of hydrocarbon-bearing zones, well surveying and perforating.

• The maturity phase (1949-1985): The advent of several porosity and resistivity logs rings in the golden age of petrophysics. Well logging is primarily used for quantifying oil and water saturations. Additionally, numerous new non-petrophysical logging methods are developed.

• The reinvention phase (since 1985): Numerous technological breakthroughs such as logging-while-drilling, borehole imaging, and nuclear magnetic resonance redefine well logging as an important aid for efficient reservoir development and management.

References Allaud LA, Martin M (1977) Schlumberger, history of a technique. John Wiley Sons, New York. Archie GE (1942) Electrical resistivity log as an aid in determining some reservoir characteristics. Trans Am Inst Min Eng 146: 54-62. Desbrandes R (1985) Encyclopedia of well logging. Gulf, Houston. Doll HG (1949) Introduction to induction logging and application to logging of wells drilled with oil-base muds. Petr Trans Am Inst Min Eng 186, 148. Hallenburg JK (1984) Geophysical logging for mineral and engineering applications. PennWell, Tulsa. Prensky SE (1997) Bibliography of well-log applications: 1996 Update. Log Analyst 38: 17-88. Schlumberger C, Schlumberger M, Doll HG (1932) Electrical coring: A method of determining bottom-hole data by electrical measurements. Amer Inst Min Eng Techn. Paper 462. Serra O (1984) Fundamentals of well-log interpretation. 1. The acquisition of logging data. Elsevier, Amsterdam. Thomson W (1861) On the reduction of observations from underground temperature. Trans Roy Soc Edinburgh 22: 405-427. Tittman J (1986) Geophysical well logging. Academic Press, Orlando. Wyllie MRJ, Rose WD (1950) Some theoretical considerations related to the quantitative evaluation of the physical characteristics of reservoir rocks. J Petr Tech 189. Table Table 1.2.1. The number of Schlumberger logging crews in the early years and in the year 2000.

Year No. of Crews

1928 1 1929 3 1930 7 1931 10 1932 14 1933 24 1934 45 1935 65 2000 1100a

a The estimated worldwide number of all logging units is 4100 (Russia 600, China 700, Rest of World 2800)