Discovering the Reservoir

Contributed by Mark Mau and Henry Edmundson

The early pioneers didn’t devote much thought to the nature and behavior of oil reservoirs. Their idea of developing a field was to drill as many wells as possible and then produce them at maximum capacity, placing the wells on pump when the natural flow dried up. However, as more oil was discovered and production took off, a few curious souls started to wonder how the oil seeped through the rock to the well, indeed how large was the reservoir so recently discovered. The age of reservoir engineering was dawning.

Darcy’s Law of Permeability

Fortuitously, the key scientific foundations had been established by French engineer Henry Darcy in the 1850s. Darcy spent his entire career at the Corps des Ponts et Chaussées (Office of Bridges and Roads), a government agency in his hometown Dijon, France. He suffered ill health and died prematurely at the age of 54 from pneumonia, but his ailments did not blunt his sense of humor. Corresponding to a young colleague while preparing a publication, he wrote, “It was written to me that you had toasted recently to my health. I fear that this obligation will be imposed on you for a long time, but the wine of Burgundy is good and I feel a little less sorry for this devotion than the one you will need to read these proofs.”

Darcy’s decisive innovation was published in an appendix to his work “Les Fontaines Publiques de la Ville de Dijon” in 1856. In this work, which summarized the result of experiments he had made by flowing water through a sand-filled cylindrical tube, he postulated that flow through the sand was linearly proportional to the pressure drop across the sand. Darcy called the constant of proportionality “permeability.”

The oil field began to wake up to Darcy’s ideas when the US Bureau of Mines was established in 1910, and four years later added a Petroleum Division to study and understand the physical processes involved in oil production. The key breakthrough, however, would come from the USGS, where Perley Gilman Nutting, who previously worked for the US Bureau of Standards and Eastman Kodak and had joined the Survey in 1924, started looking afresh at Darcy’s permeability definition. Crucially, Nutting worked out how to introduce viscosity, a measure of the resistance of a fluid to being moved. The viscosity idea had come from French physicist Jean Léonard Marie Poiseuille in 1842, and the unit of viscosity “poise” was named after him. In his 1930 AAPG paper “Physical Analysis of Oil Sands,” Nutting introduced a viscosity term in Darcy’s equation and in the process suggested a simple method of measuring the permeability of oil sands using small rock samples.

Nutting’s work paved the way for the first standard method for measuring permeability. In their seminal 1933 paper “The Measurement of the Permeability of Porous Media for Homogeneous Fluids,” Ralph Wyckoff, Morris Muskat, Holbrook Botset and Donald Reed of Gulf Research and Development Company in Pittsburgh, Pennsylvania, described a detailed technique for the measurement of the permeability of porous media. They named the permeability measurement unit a “darcy.”

Mud Logging

As the industry started coming to grips with picturing what was going on in the reservoir, their attention focused on the rock being drilled through. The first clues were gained by inspecting the cuttings extracted from the well during drilling. With the advent of rotary drilling, the cuttings were collected from the circulating mud exiting the well and then analyzed by a geologist on the wellsite, a practice called mud logging. Early mud logging focused on looking for telltale signs of oil and gas, including watching for oil sheen in the mud returns and looking for gas coming out of the mud as it depressured. But attention was also paid to the rock type. At first, mud loggers relied on the naked eye or, at best, a hand lens. The cuttings were identified by main lithological type—sandstone, limestone and shale, for example—and described in terms of color and texture.

The next step for mud loggers was harnessing paleontology. The importance of examining fossils and organic remains had been recognized as early as 1790 by Polish geologists carrying out surface investigations of petroliferous areas in the Carpathian mountains. Throughout the 19th century, Polish geology continued to be at the forefront of paleontological research, culminating in the efforts of Jόzef Grzybowski, a professor of geology at the University of Krakow. In the 1890s, Grzybowski was investigating mud returns from oil wells in the Carpathian mountains and reported that one group of marine microfossils called foraminifera was especially suited to providing a record of the sediments age.

Two companies, Humble Oil and Rio Bravo Oil Company, a subsidiary of the Southern Pacific Railroad, were quick to embrace paleontology; Humble Oil’s first paleontologist was Alva Ellisor, based at its Fort Worth headquarters. Humble’s embrace of paleontology was not without its detractors. The traditionalists believed that the chronologic range of microfossils was too wide to make them useful in age determinations, but a new generation of micropaleontologists, many women among them, fought to overturn this belief and, for that matter, the concept that the oil field was exclusively a man’s world.

In December 1921, a 26-year-old Esther Applin, who had joined Rio Bravo Oil Company that year as a micropaleontologist, presented ongoing studies at a Paleontological Society meeting in Amherst, Massachusetts, suggesting that microfossils could be used to date oil-bearing Gulf Coast formations. Professor J.J. Galloway of The University of Texas at Austin stood up and objected: “Gentlemen, here is this chit of a girl, right out of college, telling us that we can use foraminifera to determine the age of a formation.” Four years later Applin was vindicated when she coauthored a paper with Alva Ellisor and Hedwig Kniker of The University of Texas’s Bureau of Economic Geology, demonstrating conclusively that the chronological sequence of oil-bearing zones in the Gulf Coast could be established using microfossils. Applin, Ellisor and Kniker’s work helped advance the scientific study of micropaleontology and provided a badly needed index of fossils for oil drilling operations.

Taking Core Samples

Paleontologists working in the 1920s not only had drilling cuttings at their disposal but also drilling cores, large-size samples of the reservoir rock obtained using a special drilling tool. The first coring tool was invented by French engineer and tunnel designer Rodolphe Leschot, around 1863. He conceived the idea of a hollow tubular tool set with diamonds cutters at one end and with circulating fluid passing through. Leschot used it for drilling faster blast holes while tunneling through Mount Cenis on the France-Italy border for a railway. In the 1870s, this simple device was replaced by a double-barreled tool with inner and outer barrels separated by ball bearings. This allowed the inner barrel to remain stationary to receive a core while the outer barrel was rotated by the drillstring to cut the core. These double-barreled tools were not equipped with diamond cutters until Milan Bullock from Chicago added them in 1892.

The Bullock coring tool worked best in hard formations, so in soft-rock regions, such as the US Gulf Coast and California where many companies were active, the search for an effective coring tool was still on. In 1919, Shell trialed a double-barreled core drill developed expressly for loosely consolidated formations; this was an invention of Jan Koster of the Holland Geological Survey. But the core drill wasn’t strong enough and soft formations easily balled up inside it. However, one of Shell’s Californian geologists, a mining engineer called John “Brick” Elliott—nicknamed for his shock of red hair—did not give up. He quit Shell in 1920, set up his own consulting company in Los Angeles and within a relatively short period developed an improved double-barreled coring tool that featured reaming teeth that prevented the tool from balling up. Elliott’s core drill was capable of recovering core two inches in diameter and several feet long. At first, oil company executives were skeptical, but in August 1921 Elliott successfully recovered cores from a well at the Huntington Beach oil field. His new coring tool became the basis for all rotary core barrels used today.

Learning to extract information from cores paralleled these advances. At first, inspection by the field geologist was qualitative. But soon, the physical properties of the cores became of interest. As early as 1880, pioneering petroleum geologist and engineer John Franklin Carll took cores from the Pennsylvania Venango Sands—the formation where Colonel Drake had struck oil at a very shallow depth in 1859—and began visually estimating their porosity. In 1885, Frederick Haynes Newell, a mining engineering student at the Massachusetts Institute of Technology (MIT), forced water, kerosene and crude oil through small discs cut from cores, in an effort to understand the fluid-flow properties of the rock, but failed to link this to permeability.

By the early 1920s, determining the amount of pore space in a rock, or porosity, had turned quantitative in the hands of Arles Melcher, a geologist at the USGS’s Physical Laboratory. In 1924, Melcher measured porosity from cores across a complete section of the Bradford Sand, near Custer City, Pennsylvania. And in 1925, Melcher measured the flowrate of crude oil as a function of pressure in cores from a variety of Oklahoma oil sands. A year later, Melcher’s work was further refined by Charles Fettke of the Pennsylvania Geological Survey, who developed many of the classic laboratory methods for core analysis. The same year, William Russell of the South Dakota Geological Survey introduced a new method for the determination of porosity that became standard. The method compares the weight of a core sample filled with air with the weight when filled with a saturating liquid.

Commercial core laboratories were now starting to emerge. The first was founded in 1928 in Bradford, Pennsylvania, by petroleum engineer-geologist Paul Torrey. Torrey’s lab determined the porosity of the reservoir cores, and also oil and water saturation. However, these early saturation measurements were palpably unreliable because they failed to take into account changes caused by the lifting of the core out of the well.

During that same year core analysis spread to academic research. In 1928, the Pennsylvania State College, today’s Pennsylvania State University, established training programs in oil production with help from local oil producers. George Fancher, James Lewis and Kenneth Barnes ran the first courses and embarked on a unique three-year core collection and analysis project. Fancher, Lewis and Barnes collected and analyzed more than 400 sandstone cores, storing them in a college building that became known as the “Pennsylvania Core Depository.” This project was the first large-scale, systematic determination of the properties of porous media from cores.

In 1933, the research trio published their results in the now classic article “Some Physical Characteristics of Oil Sands” in the college’s Mineral Industries Experiment Station bulletin, providing extensive references to the various core analysis methods and techniques evolved up to that time. With relation to permeability, they demonstrated a difference between permeability to air and to water, suggesting that discrepancies were largely due to hydration of clays present in the cores. And they presented many original porosity determinations, including the determination of so-called effective porosity, which is the volume of pore space that is interconnected and contributes to flow, as opposed to total porosity that also includes deadends and isolated noncontributing porosity. Core analysis now had a firm scientific basis and was key for understanding the reservoir.

The Birth of Well Logging

But coring and the subsequent analysis of the cores remained a time-consuming and cumbersome business. What was needed was a shortcut, a quick alternative that could yield at least some of the results coring provided. In the late 1920s, just such a miracle occurred, and given its provenance in a picturesque wine region of Alsace, France, this miracle became known as “carottage électrique”, or electrical coring.

Near the small village of Pechelbronn in Alsace, oil had been excavated by hand since the 1740s. By the 1920s, the Pechelbronn oil fields counted more than 3,000 wells, and the local oil company was drilling more every day. Following the slump after World War I, demand was on the rise again, and in June 1926, the company opened a new refinery that could handle 80,000 metric tons per year. The question facing the company directors was whether all these new wells would produce enough to feed the refinery. In early 1927, the Pechelbronn company, which had already contracted Schlumberger for surface electrical surveys, discussed with Conrad Schlumberger the idea of making resistivity measurements in the borehole to see if this could help the company geologists obtain a better understanding of the oil-bearing formations.

The opportunity came at the right time for Conrad and his brother Marcel. They had just lost a lucrative contract with Roxana Petroleum, a Shell subsidiary, after delivering disappointing results along the US Gulf Coast using surface electrical prospecting. At the same time, they were facing competition from other geophysical methods such as magnetics, gravity and especially seismics.

In fact, Marcel Schlumberger had already tried resistivity measurements in the borehole in 1921. In March that year, he made resistivity measurements over a few feet at the bottom of a 760-meter hole in Molières-sur-Cèze in southern France. The results were inconclusive, but the feasibility of a downhole resistivity measurement had been proved. The geophysical community was skeptical. The German geophysicist Richard Ambronn had been doing similar experiments in Germany and maintained that below a certain depth, all geologic formations were so compact as to become infinitely resistive. Nothing of the kind had been observed with Schlumberger’s surface measurements, but then they did not penetrate very deeply.

On September 5, 1927, Henri-Georges Doll, Conrad’s son-in-law and two colleagues, Roger Jost and Charles Scheibli, proceeded to a Pechelbronn well called Diefenbach 2905 and conducted the first electrical logging operation in an oil well. The well was 500 meters deep, and they logged an interval of 140 meters, starting from a depth of 279 meters. They rigged up a hand-operated winch that lowered into the hole three insulated wires—cables of the type used for lighting fixtures—tied together here and there by friction tape. The longest of the wires was used to inject current into the well and formation, with a return at the surface. The other two wires, shorter and of slightly different lengths, measured the resulting potential field and provided the resistivity measurement. Measurements were made point by point at intervals of one meter; the entire operation took five hours. The result was a resistivity log that distinguished between the many layers of sand and shale pierced by the borehole. Doll, Jost and Scheibli repaired to the local tavern for a celebratory dinner.

Continuing through 1928, resistivity logging was conducted throughout the Pechelbronn oil field, and the resulting correlations of resistivity from one well to the next revolutionized the stratigraphic understanding of the field. Soon the Pechelbronn Oil Company was raising the capacity of its new refinery to 100,000 metric tons per year. In 1929, the new logging technique went global. Schlumberger logging crews were engaged by Shell for their explorations in Venezuela, the US and the Dutch East Indies, and by the Soviet Union for the oilfields of Grozny, Chechnya and Baku, Azerbaijan.

Meanwhile, Conrad was playing with new ideas for his well logging. Realizing that oil was infinitely resistive to electricity, he postulated that if a zone was oil-bearing and reasonably thick, logging with a longer spacing between the wires measuring potential would see deeper into the formation and record a higher resistivity; the phenomenon couldn’t be observed in Pechelbronn because the oil zones were too thin. So he asked his handful of field engineers to try the idea and report back. Marcel Jabiol was the solitary Schlumberger engineer in northern Sumatra at the time and within a month had tried Conrad’s proposal and observed exactly what had been predicted. His telegram in June 1930 with the good news triggered celebrations in the small Paris office. Electrical logging could now locate oil from the borehole.

The company’s fortunes, like everyone else’s, were rocked by the world depression that had hit in November 1929. But by the end of 1932, drilling revived and with it the acceptance of Schlumberger’s electrical logging. Also helping was the introduction of a new logging measurement called the spontaneous potential (SP). Natural electrical potentials in the subsurface had been discovered by the British geologist, natural philosopher and inventor Robert Were Fox in 1830 in ore deposits in Cornwall, England. In the borehole environment, naturally occurring potentials are caused by electrochemical interactions between the borehole fluid and adjacent sand and shale formations.

Conrad Schlumberger had received a French patent on the SP in 1929, claiming it could be used to locate permeable strata, but found no practical application. A year later and quite by chance, Doll observed natural potentials while logging in the Seminole oil field in Oklahoma. With the battery disconnected, he noticed the needle of the potentiometer vibrating back and forth as the electrodes were being lowered into the well. Experiments on the SP phenomenon followed at Pechelbronn, and by 1930 it was concluded that the SP could differentiate permeable beds, such as sand and limestone, from impermeable formations, such as shale. The combination of SP and resistivity curves turned out to be of much greater value than the resistivity log alone in locating and scoping out production possibilities.

SP measurements proved to have other benefits as well. In 1935, Doll had the idea of adding SP electrodes to the arms of a three-arm caliper tool and attaching it to the so-called teleinclinometer tool that Conrad and Marcel Schlumberger had developed in 1932 for measuring borehole deviation and direction. The three SP measurements distributed around the borehole combined with teleinclinometer data promised the first downhole measurement of the rock strata dip and direction. The combination was tested in Long Beach, California, and successfully commercialized in Louisiana in 1941.

Sidewall Sampling

Even though electrical well logging showed promise for evaluating downhole formations, the technique did not eclipse coring. Logging may have facilitated stratigraphic correlation and given early pioneers a way to distinguish between shale and porous rock and between hydrocarbon- and water-bearing rock, but it wasn’t quite the same as having a piece of rock in front of you. Coring was the answer, but it was still expensive.

An alternative and cheaper solution to coring was sidewall coring, in which a device is lowered into the hole that can sample the formation sideways from the borehole at any desired depth. The earliest attempts in the mid-1920s used the drillstring to convey such a coring tool. At the desired depth, a boring device or knife arrangement would be pushed obliquely into the borehole wall to cut a sample out of the formation. In the early 1930s, the first sidewall-coring tools that could be lowered on a cable came on the market. An early version, made by the Sperry Sun Well Surveying Company, based out of Philadelphia, was a miniature drilling tool that could be projected laterally, penetrate a few inches into the formation to take a sample. However, these devices were mechanically complex and unreliable. Then in the early 1930s, Marcel Schlumberger had the idea of using an explosive charge to shoot a cup into the formation, the cup being attached with strong cables to a logging tool and retrieved by simply pulling on the tool.

After trying numerous configurations of cup, explosive charge and cable, Marcel came up with a workable solution that was tested in Pechelbronn in 1935 and patented in 1936. Most of the early experiments were conducted in a test well accessible within the basement of the fledgling Schlumberger headquarters in the seventh arrondissement of Paris. His tool could fire any number of cups, or bullets, into the formation at different depths and, after each firing, pull the sample back into the borehole. The samples were then pulled to the surface and presented for inspection and analysis. Marcel Schlumberger took his prototype sample-taker to the US Gulf Coast in April 1936 for trials and performed the first commercial job in southwest Texas in September 1936. Since then, sidewall coring has become standard in the oil field, and the original design has hardly been improved on.

This entry is based on Groundbreakers: The story of oilfield technology and the people who made it happen, by Mark Mau and Henry Edmundson. You can find the book at