Vol. No. 11

November 1999

Frontiers of Technology

Formation Evaluation

Logging and Testing

In ancient China, wells were drilled to depths of as much as 3,000 ft (914 m) to locate and tap sources of salt brine. Their primitive drilling technique resembled cable-tool drilling methods used centuries later to explore for oil. To obtain knowledge of the formation below, cuttings and fluids brought to the surface by bailing operations were dumped on the ground and examined by Chinese drillers. Cursory examination of the cuttings provided information on the formations penetrated and enabled the drillers to determine how close they were to finding the needed brine.

When early oil pioneers drilled their wells 2,000 years later, they knew little about the formations they were drilling and showed practically no interest in the stratigraphy their bits penetrated. Instead, their interest was focused on making holes and looking for the presence of oil. Later, realizing it was very helpful to know something about the formation, they examined and recorded the characteristics of cuttings brought to the surface by bailing operations. Eventually mineralogists applied a microscope to the cuttings and advanced formation-evaluation efforts further by measuring the density, hardness, and electrical properties of the rocks and by making chemical analyses of them.¹

Core Sampling and Mud Logging

For almost 50 years, recorded descriptions of drill cuttings were the sole source of formation knowledge. Around 1920, the first core-barrel sampling tools were put to work in California, west Texas, and Colorado. These tools cut core samplings of the formations from the bottom of the borehole. After collection, the cores were analyzed and underwent experimentation in the laboratory to garner valuable reservoir data on the formations being drilled. Over the years, diamond-coring tools became the preferred method of collecting core samples.

While mechanical coring was an improvement on simple cuttings records, it was expensive because it had to be done continuously during the drilling of the well. In an attempt to be more cost-effective, efforts were made to obtain as much data as possible from cuttings samples.

The advent of rotary rigs and the use of drilling mud to circulate cuttings from the bottom of the hole to the surface produced an interest in comparisons of cuttings obtained from different drilling depths. The cuttings were treated with acetone or ether and were exposed to ultraviolet light to detect the presence of small amounts of oil. This test was repeated again and again during drilling, and the results, or lack thereof, were documented.

During the late 1930s, John T. Hayward developed the continuous mud-analysis log, which shows the combined results of mud analysis for both gas and oil and relates these results to factors such as drilling rate and depth.

Actually, Hayward was more interested in the gases and liquids in the mud than the cuttings. From appropriate measurements at the surface, he succeeded in determining the content of oil and gas in the various formations traversed. He then correlated these observations with depth to create a continuous diagram of the oil and gas content of the formations penetrated while drilling was in progress.

As a result of his work, continuous mud-analysis logging furnishes a variety of useful formation-evaluation data, including the amount of methane, liquid hydrocarbons, and oil in the cuttings. “This log will eliminate frequent mechanical coring. Cores will now be taken only when a show of oil or gas make it advisable,” proclaimed Hayward when asked about the long-term implications of his technique.³

Electric Logging

In March 1921, Marcel Schlumberger and several associates used a 2,500-ft (820-m) borehole and conducted downhole resistivity measurements to see if they could enhance the interpretation of surface seismic data. They found that their measurements did indeed reflect the variation in the nature of subsurface formations penetrated by the wellbore. Six years later, in a 1,640-ft (500-m) well in France’s Pechelbronn field, experimental physicist Henri Doll successfully produced the world’s first “electric log” using successive resistivity readings to create a resistivity curve.

The technique actually was invented by Conrad Schlumberger with the help of his brother, Marcel. The Schlumberger brothers believed that, among the physical properties of metal ores, their electrical conductivity could be used to distinguish them from their surroundings.

Very basic equipment was used to record their first map of equipotential curves in a field near Caen, France, in 1911. Plots of curves derived from their surveys confirmed both an ability to detect metal ores and a capability to reveal features of the subsurface structure. Subsequently, this information led to the location of subsurface structures that could form traps for minerals.

To understand the measurements made at the surface better, the Schlumbergers knew they had to incorporate resistivity information from deeper formations. The result was Henri Doll’s 1927 Pechelbronn field log.⁴

The procedure used at Pechelbronn was crude and makeshift in nature.

But, it didn’t take long to realize that the resulting resistivity log could be a valuable formation-evaluation tool. Clays have a low resistivity. Porous sands are conductive if saturated with salt water, are moderately resistive if the water is fresh, and are very resistive if the impregnating fluid is oil. Thus, important clues could be deduced from the log as to the formation’s character. As for oil sands, a relationship was determined to exist between resistivity and oil potential—the higher the resistivity, the better the production.

Following its first use in France in 1927, electric logging was introduced in Venezuela, the U.S.S.R., and the Dutch East Indies in 1929. In 1932, after a series of demonstrations, electric logging came to the U.S. when Shell Oil issued Schlumberger contracts for work in California. By the end of 1933, the initiation period was over, and 12 crews were applying the technique worldwide.

SP Curve

Four years later, in 1931, the discovery of a “spontaneous potential” (SP) phenomenon produced naturally between the borehole mud and formation water in permeable beds introduced a new basic measurement—the SP curve. Attempts by researchers to explain how the phenomenon works resulted in agreement that it was due to electrocapillarity (filtration of liquid from the borehole into the permeable formations).

However, this explanation did not prove itself in subsequent logging, and another cause was added to explain the SP curve—the electrochemical effect. Laboratory and field work confirmed the importance of the chemical effect, but everyone involved agreed that knowledge of the phenomenon needed further clarification. During the ensuing decade, the work of several researchers (Mounce, Rust, and Tixier) indicated that the SP effect was mainly a chemical potential, with only a small and sometimes negligible filtration potential, but it was M.R.J. Wyllie, a Gulf Oil Co. researcher, who provided a comprehensive explanation of the SP curve. In a 1948 technical paper titled “A Quantitative Analysis of the Electrochemical Component of the S.P. Curve,” Wyllie suggested that SP consists of two different effects.

The explanation he offered greatly enhanced the science of SP logging. When recorded simultaneously with the resistivity curve, permeable oil-bearing beds could be differentiated from impermeable, nonproducing beds. The combination of the resistivity and SP curves considerably increased the chances of probable conclusions as to the characteristics of the formation material.

In early uses, the SP curve was used exclusively as a tool for locating permeable beds and defining their boundaries. Later, with the introduction of quantitative analysis methods, the SP log was used to derive information on formation-water resistivity, an essential element for computing water saturation from log data.

In the late 1940s and early 1950s, Schlumberger introduced the microlog, laterolog, and microlaterolog. The microlog provided a more accurate determination of permeable beds and their boundaries in limestone, sand, and shale, where the SP log wasn’t satisfactory.

Laterologging began with a device called the “guarded electrode,” which was invented by Conrad Schlumberger in the 1920s. From this, the laterolog was developed for use in wells drilled with highly conductive mud because it more sharply defined bed sequences in hard formations. The microlaterolog soon followed. It provided a more reasonable estimate of the resistivity of an invaded zone (Rxo) and of residual oil saturation in practically all formation types.

In the early 1970s, Schlumberger introduced its Dual Laterolog-Rxo tool. The dual laterolog answered the need for a tool capable of producing useful resistivity measurements even when true formation resistivity and mud resistivity are very high, as in the case of carbonates and evaporites drilled with salty mud. It also provided greatly improved thin-bed resolution.⁶

Induction Logging

Resistivity logging is a valuable tool, but it does not operate well under all conditions. This is especially true in cases in which there is no liquid filling the borehole to allow contact to be established between the electrodes and the formation or in cases in which oil-based mud is used. For these situations, induction logging is much more suitable.

Applied in shallow ore exploration for over 25 years, the induction process had not been used in oil exploration before 1942. Its oil industry application is credited to Henri Doll. While seeking a way of using the induction process to create a military vehicle that could detect enemy mines in its path during World War II, Doll realized the possibilities that might be gained by applying the induction process to oil-exploration logging. His colleagues at Schlumberger strongly opposed his use of the induction process in logging, citing problems posed by very small signal strength, high direct mutual-coupling interference, and the lack of adequate supporting technology. Nevertheless, Doll persisted in this complicated task, leading a team that was determined to develop an order-of-magnitude improvement in logging technology.

In induction logging, which was introduced in the mid-1940s, a sonde that employs alternating current of constant magnitude and a coil (transmitter) is lowered into the well to create an alternating magnetic field from which eddy currents are introduced into the formation. The eddy currents follow circular paths centered on the axis of the sonde. The eddy currents, in turn, create a secondary magnetic field that induces an electromotive force, or “signal,” in a second coil (receiver) also located in the sonde. The signal is amplified, rectified to direct current, then transmitted to the surface, where it registers in the form of a continuous log.

History validated Doll’s vision, perseverance, and faith with the eventual success of his induction-logging tool. Since its first commercial use in 1946, induction logging has become one of the most widely used logging methods in the world and has overtaken electric logging because it is regarded as superior in many applications.⁷

In 1963, the dual induction-laterolog tool was introduced. This tool provided the simultaneous recording of three resistivity measurements and the SP curve. All measurements are focused to give true formation resistivity in a variety of conditions for wells drilled with freshwater muds.

Nuclear Logging

In 1896, H. Becquerel discovered radioactivity when his photographic plate was affected by a preparation of uranium. By the early 1900s, it became evident that all terrestrial materials contain measurable quantities of radioactive elements in extremely minute quantities. Over time, these radioactive elements disintegrate and transform into other elements. As they disintegrate, they emit energy in the form of alpha, beta, and gamma rays. In rock formations, this radioactivity can be measured and logged to determine the types and nature of rock formations being drilled.

Gamma Ray Logging

During the late 1930s, electric logging had been accepted as a viable method of determining formation materials as the borehole was drilled. However, it could not be used if the hole was lined with steel casing. Therefore, the development of a logging method for use in these applications was crucial. The result was gamma ray logging, which measures and records natural gamma ray activity in formations contacted by the borehole.

A Tulsa, Oklahoma, group made the first gamma ray log in a well near Oklahoma City. The results clearly demonstrated that the technology could reveal the lithology of the borehole. A company was founded soon, and the first commercial gamma ray survey was done for the Stanolind Oil and Gas Co. in May 1940 in Texas’ Spindletop field. Subsequent use of the technology determined that it was particularly good for defining oil beds, spelling out the geology, and as a substitute for the SP curve in hard formations or with salty muds.

The gamma ray spectrometry log, first used in 1970, is a refinement of the gamma ray log. Like the gamma ray log, it detects naturally occurring gamma rays, but it also defines the energy spectrum of the radiation. Because potassium, thorium, and uranium are responsible for the energy spectrum observed by the tool, their respective elemental concentrations can be calculated. Calculated-concentration curves show a correlation to depositional environment, diagenetic processes, clay type, and volume. Also, it is useful in estimating shale content.

Neutron Logging

While experimentation and development was occurring in gamma ray logging, R.E. Fearon advanced, in 1938, his idea for a different type of nuclear-logging service. Bruno Pontecorvo subsequently perfected it in 1941. The process is called neutron logging, and it involves the bombardment of the formations along the borehole with neutrons. Then, the secondary gamma ray activity generated by the bombardment is measured.

Since the variations in gamma ray activity observed on neutron logs are a result of the hydrogen content of the formations, they offer a measurement of porosity. Therefore, porosity determination has become one of the most important applications for the neutron log.

Early data obtained in gamma ray and neutron logging were of a qualitative, not quantitative, nature, and there was no zero line in the diagrams. However, improvements in the late 1940s incorporated zero lines into the logs, and numerical scales of gamma ray and neutron intensities were added.

The continued rapid and intensive development of both resistivity- and nuclear-logging techniques resulted in a movement from qualitative interpretation of logs to quantitative interpretation with the 1942 publication of a paper by G.E. Archie. The paper detailed his discovery of a relationship between electrical resistivity and formation-water saturation. Archie’s work inspired an intensive investigation of data that were obtained from logs of subsurface surveys and their relation to fundamental reservoir properties, such as porosity, permeability, water salinity, and reservoir limits.⁸

Formation-Density Logging

Another nuclear-logging technique introduced in the 1960s was the formation-density log. The device, which uses a gamma ray source and detector, is placed in contact with the borehole wall to measure the bulk densities of formations in situ.

Field applications have demonstrated that measurement of formation density is a useful and revealing technique for determining the porosity, lithology, and fluid content of formations in conditions that hamper other logging methods, such as the logging of empty or gas-filled holes.

Sonic Logging

Unlike nuclear logging, sonic or acoustic logging operates on the principle that sound waves (elastic waves) travel through dense rock more quickly than through lighter, more porous rock. The technique, which resembles electric logging, uses a transmitter and receivers combined in one downhole tool to measure, in microseconds, the time differences required for sound pulses to traverse formation beds.

Some of the first experimental sonic logs were made in the early 1930s, but the first commercial logs weren’t made until 1954. Sonic logs provided a more accurate porosity interpretation of formation fractures and water table.

Pressure-Transient Testing

Instruments for measuring pressures in oil and gas wells were developed during the 1920s, and a study by Pierce and Rawlins in 1929 reported a relationship between bottomhole pressure and potential production rate. The study encouraged the development of improved instruments and recording devices. By 1933, there were more than 10 different kinds of pressure-measuring/-recording instruments in use.

Early measurements were “static” and were acquired by lowering a pressure-measuring device to the bottom of a well that had been shut in for between 24 and 72 hours. However, engineers soon recognized that, in most formations, the static pressure reading was a function of shut-in time and mainly reflected the permeability of the reservoir rock around the well. What engineers wanted was another basic measurement—the pressure-transient reading, in which the pressure variation with time is recorded after the flow rate of the well is changed.

Morris Muskat was first to offer an extrapolation theory. Muskat’s theory related the change in pressure with time to the parameters of the reservoir. In 1937, he presented a method for extrapolating the measured well pressure to a true static pressure, and he stated at that time that his method was only a qualitative application because it did not take into account the important aspect of fluid compressibility.

The first comprehensive treatment of pressure behavior in oil wells that included the effects of compressibility was that of C.C. Miller, A.B. Dyes, and C.A. Hutchinson in 1950. The following year, D.R. Horner presented a somewhat different treatment. The two papers still furnish the fundamental basis for modern theory and analysis of oilwell pressure behavior.⁹

One of the greatest contributors to the field of pressure-transient analysis was the late Henry J. Ramey Jr. His 1970 paper investigating wellbore storage and skin effect in transient liquid flow ushered in the modern era of pressure-transient analysis.

And, despite the development of other tests, most engineers agree that the standard for pressure-transient testing will always be the pressure-buildup test. “It is the most direct method of obtaining an average pressure for reservoir analysis, it is operationally simple, and the theory is well developed,” stated a reservoir engineer when asked about the importance of the pressure-buildup test.

Drillstem and Formation Testing

It has long been realized that the sampling of fluids and pressure in the porous strata of the formation being drilled can provide valuable information on the formation and its ability to yield oil and/or gas. However, early methods of obtaining these data required the setting of casing. This was expensive and, therefore, made testing expensive.

Working in El Dorado, Arkansas, in the late 1920s, E.C. Johnston and his brother M.O. Johnston developed the first drillstem tester in 1927 and subsequently refined it in the early 1930s. The test is a measurement of pressure behavior at the drill stem and is a valuable way for an engineer to obtain important sampling information on the formation fluid and to establish the probability of commercial production.

In the 1950s, Schlumberger Co. introduced a more advanced method for testing formations. The Schlumberger formation- testing tool, placed in operation in 1953, fires a shaped charge through a rubber pad that has been expanded in the hole until it is securely fixed in the hole at the depth required. Formation fluids flow through the perforation and connecting tubing into a container housed inside the tool. When filled, the container is closed, sealing the fluid sample at the formation pressure. The tool then is brought to the surface, where the sample can be examined. The information obtained by this method is often better than results obtained from drillstem
testing.¹⁰

The Future

The future of logging and testing is bright indeed. Most of the basic logging and testing tools and techniques developed during the past 70 years have been refined and, in some cases, reinvented to perform the logging and testing function of formation evaluation. However, even newer and more ingenious technology is being introduced thanks to advances in electronics miniaturization and in computer hardware/software that can go downhole rather than remain at the surface. These tools have made real-time logging while drilling possible.

This is a very fundamental change from the past, when it was necessary to halt drilling while logs were run periodically to obtain measurements. In a sense, the continuous-logging version of the original “realtime” continuous mud-analysis technique sought by John Hayward in the 1930s has been attained.

Integration is revolutionizing logging tools as they are packaged into logging-tool strings that weigh half as much, perform multiple functions in fewer trips, and are easier to use. Interpretation-software advances are making the information that logging tools collect more useful to operators who are looking for tools that provide more powerful solutions in difficult-to-interpret, more-problematic zones. Higher resolution in the tools is making it possible to identify pay zones that previously were overlooked due to complex lithology.

References

Leonardon, E.G.: “Chapter 8: Logging, Sampling and Testing,” History of Petroleum Engineering, API, New York City (1961) 495.
Hayward, J.T.: “Continuous Logging at Rotary Drilling Wells,” API Drilling and Production Practice, API, New York City (1940).
Leonardon, E.G.: “Chapter 8: Logging, Sampling and Testing,” History of Petroleum Engineering, API, New York City (1961) 516–520.
http://www.slb.com/.
Wyllie, M.R.J.: “A Quantitative Analysis of the Electrochemical Component of the S.P. Curve,” paper presented at the 1948 AIME Petroleum Division Fall Meeting, Dallas 4–6 Oct.
Suau, J. et al.: “The Dual Laterolog-Rxo-Tool,” paper SPE 4018 presented at the 1972 SPE Annual Meeting, San Antonio, TX, 8–11 Oct.
“Henri-Georges Doll: 1902–1991,” Schlumberger Oilfield Review (Jan. 1992) 4.
Leonardon, E.G.: “Chapter 8: Logging, Sampling and Testing,” History of Petroleum Engineering, API, New York City (1961) 542–549, 828.
Matthews, C.S. and Russell, D.G.: Pressure Buildup and Flow Tests in Wells, Monograph Series, SPE, Richardson, TX (1967) 1, 1–3.
Leonardon, E.G.: “Chapter 8: Logging, Sampling and Testing,” History of Petroleum Engineering, API, New York City (1961).