52 SUBSURFACE DATA INTEGRATION

Logging history rich with innovation

From bathtub experiments to modern nuclear physics, formation logging has been the backbone of oil and gas development, letting geologists, geophysicists and engineers see more and more of the subsurface.

By Editor in Chief BILL PIKE and Exploration Technology Editor RHONDA DUEY

In 1929 logging operations in the Pechelbronn field involved a specialty fabricated cable, but the winch was still hand-driven. Images courtesy of Schlumberger.

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he advent of well logging in the 1920s and began to develop rapidly. In 1931, the accidental mud (OBM) in the Rangely, Colo., oil fields in its subsequent development into a discovery of spontaneous potential (SP), 1948. OBMs are nonconductive. Normally sophisticated technology revolutionized produced naturally by the borehole mud at the configured electrical surveys require a T 1 the oil and gas exploration and production boundaries of the permeable beds, led to an conductive mud (water-based) system. The industry. The ability to “look and measure” such innovative new logging technique – solution to logging in OBMs was the induction things as formation type, formation dip, simultaneously recording SP and resistivity log, developed in the late 1940s. In induction porosity, fluid type and other important factors curves. This technique enabled producers to logging, “high-frequency AC of constant transformed the drilling and completion of oil differentiate permeable oil-bearing beds from intensity is sent through a transmitter coil. The and gas wells from an ill-defined art into a impermeable, nonproducing ones. alternating magnetic field thus created induces refined science. Logging development By 1936, the industry could augment secondary currents in the formation (that) flow encompasses three major areas: electric resistivity logs with formation sample takers, in circular ground-loop paths coaxial with the logging, sonic or acoustic logging, and nuclear automatic film recorders and multispacing transmitter coil. These ground-loop currents, in logging. An understanding of their development resistivity curves for deep wells. turn, create magnetic fields that induce signals is an understanding of the industry’s technical The 1940s were a period of rapid in the receiver coil. The induced receiver progress. development in logging technology despite the signals are essentially proportional to the intervention of World War II. In 1941, logging conductivity of the formation.” 2 Electric logging took another major step forward with the Throughout the 1950s and 1960s, electric The genesis of electric well logging resides with introduction of the spontaneous-potential logging continued to develop, but it was the Conrad Schlumberger, who while a physics dipmeter, which greatly improved the vertical computerized processing of logs that professor at the Ecole de Mines de Paris, resolution of openhole logs. The tool allowed catapulted the sector ahead in 1962. France, conceived the idea of prospecting for the calculation of a layer’s dip – the deviation of Computerization allowed much faster log metal ore deposits by using their electrical that layer from true horizontal – and the processing, thereby allowing the dramatic conductivity to distinguish them from their less direction of the dip. This measurement was expansion of log data-gathering capability. conductive surroundings. One of the first tests, improved further with the resistivity dipmeter By 1970, the sector was poised on the brink according to Schlumberger historians, was in 1947 and the continuous resistivity dipmeter of significant advances in logging technology, performed in his bathtub, which was filled with in 1952. particularly in the field of nuclear logging. But various rocks for the experiment. Working with During the 1940s, development in other areas advances still were being made in electric his brother Marcel Schlumberger, Conrad forced innovations in logging. One of the most logging, especially in the area of mass and began a series of test surface surveys in Europe, important was the introduction of oil-based speed of data collection. The year 1971 saw the Africa and North America over a 3-year period. Their discoveries included an oil-productive salt dome in Romania, a precursor of things to come. In 1926, the brothers formed Societe de Conrad Schlumberger prospects Prospection Electrique and began to develop for metal ore deposits in the theory that adding resistivity information Normandy, France, in the early from deeper formations would increase the 1920s using technology later effectiveness of their surface prospecting. By employed in well logging. lowering an electric sonde down a 1,600-ft (488- m) well in France’s Pechelbronn field Sept. 5, 1927, the brothers created the first well log. This log was painstakingly recorded point by point, meter by meter, using makeshift equipment and then plotted by stitching together the successive readings (Figure 1). The technology worked simply. Three electrodes – A, M and N – are lowered to the bottom of the wellbore on three insulated wires. Current from electrode A passes through the drilling mud and spreads out into the formation. The potentials measured at M and N are transmitted to the surface where they are measured. By measuring the potential difference between M and N, and the strength of current from A, the apparent formation resistivity is calculated (Figure 2). Following the initial success with the first electric resistivity logs, logging technology

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introduction of combination logging systems. Sonic logging introduced in 1962, the borehole wall is Combos allowed loggers to acquire different In 1946, working in large part off technology irradiated with a gamma ray source. A gamma sets of data simultaneously rather than in developed during World War II, the logging ray counter then records the reflected rays. The sequential runs. Thus were born tools that sector produced the first sonic log, the casing number of gamma rays returned vs. those provided gamma ray, SP, resistivity, sonic and collar locator. This technology allowed more diffused relates to the density of the formation. caliper measurements in a single run. accurate depth measurement inside casing and The industry took nuclear logging one step more exact placement of perforations and further with the introduction of neutron logs in completion equipment. Sonic logs generally the late 1960s. Neutron logs also measure work by generating signals in the 20-kHz to returned gamma rays, but in this instance, 30-kHz range, although some tools operate at those generated by fast- or slow-moving higher ranges. Most acoustic logging tools have neutrons. Neutrons are emitted by mixed centralizers at one end and three caliper arms radioactive sources. “Most of the (neutron’s) at the other end to keep the tools centered in energy lost is done so during collisions with the hole. An acoustic wave transducer atop the hydrogen nuclei … After having traveled a tool generates the acoustic signal. Two certain distance, a neutron becomes ‘thermal’ receivers at the lower end of the tool receive or ‘slow’ and is captured by an atom, which the signal. “After a signal from a transducer emits a capture gamma ray.”6 Since the distance travels through the fluid and strikes the a neutron can travel without hitting a hydrogen borehole wall at the critical angle, a nuclei varies with the amount of hydrogen compressional wave is transmitted down the present, both porosity and formation contents borewall in the direction of the receivers. As it can be determined. The original neutron logs propagates along the borewall toward the were augmented later by pulsed neutron logs receivers, fluid waves are continually refracted and neutron spectroscopy logs. back into the fluid. However, only two refracted One of the biggest breakthroughs in recent ray paths are of interest: the ray paths taken logging history has been the advent of nuclear back toward the two receiving transducers ... If magnetic resonance (NMR) logging. The we measure the difference in time of arrival of technology has proven more potentially the compressional wave at the two receivers, beneficial and more confounding than its early we can determine the compressional wave developers could have imagined. travel time through the rock formation opposite NMR tools function by creating a magnetic the two receivers.”3 field in the borehole and then sending out During the next three decades, sonic logging pulses that polarize the hydrogen in water, oil moved into several measurement fields, and gas in the formation. As these hydrogen including: nuclei realign themselves to the original • porosity measurement; magnetic field, they induce signals in the tool’s • cement bond evaluation; receiver, which are recorded by electronics.

• fracture detection; Recording • lithology determination; equipment Pulley • mechanical rock properties measurement; • borewall and casing inspection; • seismic calibration; • abnormal formation pressure detection; and • gas-bearing formations identification.4 Although nuclear logging has supplanted some of its functions, acoustic logging remains a vital part of the logging suite and regularly is run in some form in combination logging tools.

Nuclear logging Logging using radiation of nuclear origin got its start in 1940. The initial nuclear logging tools recorded “the natural gamma radiation emitted by the formations crossed through by boreholes. Of the three identified nuclear radiations – alpha, beta and gamma – only N M gamma radiation, which is of the electromechanical type, can be used in well

logging because it alone has sufficient A penetrating power to go through the formation and the steel casing.”5 Figure 2. Resistivity logging employs an electrode Figure 1.The first electric log was obtained Sept. From passive radioactive monitoring in the (A) to generate currents M and N. From 27, 1927, on the Diefenbach 2905 well, Rig. No. 7, gamma ray tool, the logging industry moved measurements of the potential difference between at Pechelbronn,Alsace, France.The resistivity curve rapidly to active nuclear bombardment and M and N, and the strength of current A, the was created by plotting successive readings. measurement. In a formation density log, first apparent formation resistivity is calculated. www.EandPnet.com | Hart’s E&P | September 2002 SUBSURFACE DATA INTEGRATION 55

The amplitude of the signal relates directly to goal. That goal was within range of the porosity, and the signal relaxation time relates sophisticated logging tools by the advent of the to the size of the pore spaces containing fluids, combination tools in the early 1960s. providing an indication of permeability. NMR The barrier to earlier implementation of is a fluids-only measurement; however, due to MWD and LWD was not the logging tools but the interactions of the pore fluids with rock the method by which to send the information surfaces, the rock matrix can significantly to the surface while drilling with jointed pipe. influence the fluid response. As it turns out, a key advance in logging tools The technology has existed since the early was not another logging technology but rather 1960s, but it has taken several decades to mud-pulse technology, which allows near- refine the process, with Numar – now a continual transmittal of logging information Halliburton subsidiary – the first to bring a from tools on the bottom of the drillstring to continuous NMR logging tool to the market. processors at the surface through The result has been an offering of tools and measurement of short, varying variances in associated products that provide better depth mud pressure created by a component of the of investigation and more information than logging suite downhole. It is possible to ever before while traveling at the same pace as employ almost any logging suite combination a traditional triple combo. on the bottom of drillpipe and log the hole as it is drilled. While some operators remain Pipe-conveyed logging reluctant to allow decisions on a well to be For at least two decades, highly deviated holes made solely on MWD/LWD logs, reliability and This formation density tool was a major innovation in have required loggers to run their suites of correlation have improved dramatically . early nuclear logging technology. tools on pipe. Initially those efforts took the form of traditional logging tools run on coiled References 49-14, 49-15, 1992. tubing with electric line run inside the tubing. 1. Etnyre, Lee M., Finding Oil and Gas from 3. Etnyre, pp. 94-95. Almost instantaneous information received Well Logs, Van Nostrand Reinhold, New York, 4. Ibid. continually at the surface – now known as p. 161, 1989. 5. Debrandes, Robert, Encyclopedia of Well logging-while-drilling (LWD) and 2. Bradley, Howard B., Petroleum Logging, Editions Technip, Paris, France, pp. measurement-while-drilling (MWD) – while Engineering Handbook, Society of Petroleum 150-151, 1985. the well was being drilled had always been a Engineers, Richardson, Texas, 3rd edition, pp. 6. Debrandes, p. 9

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