What is Well Logging?
Well logging is the practice of recording and analyzing various kinds of data about the rock formations around a borehole during or after drilling. Also called borehole logging, this activity aims to answer a number of questions pertinent to the commercial viability of the well.
In other words, well logging, or borehole logging, aims to establish whether there is enough—and quality—oil and gas in the well to merit completing it and beginning production.
A Short History of Well Logging
The practice of well logging dates back to the beginning of the 20th century. The first person to do it was Conrad Schlumberger, one of the founders of the services giant of today’s oil and gas industry.
For 11 years until 1920, Conrad Schlumberger was busy with experiments on the electrical resistivity of rocks. His purpose at the time was not oil but iron ore. Schlumberger was looking for a way to establish the presence or absence of iron-bearing ores underground using electricity.
His experiments did more than he had hoped for. Sending electricity underground turned out to be able to also reveal subsurface structures and boundaries between formations. It could, in other words, tell where there were not just ore-bearing rocks but oil and gas-bearing rocks as well. Related: Oil Prices Set For Worst Weekly Plunge In Four Years
The first results of what the industry now calls electric resistivity logging were not particularly reliable as they were done from the surface. So, Schlumberger tried sending the electrical signal into the ground from inside the borehole. This marked the beginnings of not just the company Schlumberger, set up as Societe de Prospection Electrique in 1926, but also of the practice of well logging, which has long since become an essential part of exploratory and production drilling.
What Does Well Logging Record?
Among the properties and characteristics that well logs record are:
Type of formation around the borehole: Oil and gas-bearing rock is sedimentary. It has high porosity and permeability.
Formation thickness: Not all sedimentary rock is equal and not all of it is oil and gas-bearing. Drillers need to know how deep they need to drill to get to the so-called reservoir rock.
Porosity: The more pores there are in a rock formation, the more oil and gas it can hold. Highly porous rock is, therefore, more likely to be reservoir rock.
- Permeability: Linked to porosity, permeability is the capacity of porous rock to transport fluid, water or crude oil. The larger the pores, the higher the permeability.
- Temperature: The temperature in the borehole is important for the casing and cementing phase of drilling. Different temperatures—typically depending on depth—require different types of cement.
- Amount of water in the formation: Water is a common companion of oil and gas underground. Sometimes the water reservoir is close to the oil and gas reservoir and other times they share one reservoir.
- Pressure of the fluids contained in the formation: Well pressure is one of the most important considerations in drilling and the most important one with regard to safety.
- Properties of the oil and gas encountered during drilling: There are different types of gases that are grouped under the “natural gas” label. There are also different types of crude oil. Determining these is another priority of well logging.
Types of Well Logs
The types of well logs that are produced during the drilling of an oil and gas well are classified based on either the property they measure or the way they measure various other properties and the technology they use to do that.
One other major distinction that needs to be made aside from the types of logs used in well logging is the distinction between real-time logging and memory logging.
Real-time logs are made up of data that is recorded at the surface of the well as the logging tool generates it inside the borehole. The data is recorded against the measured depth of the wireline to which the logging tool is attached.
Memory logs compile data and store it during logging, for later retrieval at the surface. These logs are recorded against time, including data about the depth, at which the sensors are recording information about the borehole.
Well logs utilize, among others, visual, electrical, audio, and X-ray technology to analyze rock formation properties and other relevant information. Based on these, they are classified as the following.
Borehole imaging logs: As the name suggests, borehole imaging utilizes technology that generates images of the inside of the well. It is a type of wireline logging and produces detailed images of the rocks in the borehole providing drillers with a literal picture of the formation that lets them know what they can expect in terms of reserves and production rates, and how to maximize the latter.
In terms of technology, borehole imaging can be optical, acoustic, and electrical, as well as combined, which uses both acoustic and electrical technology.
Optical imaging involves cameras that in modern-day well logging provide high-definition color images of the rocks in the well.
Acoustic imaging involves ultrasonic energy that can penetrate the drilling mud and provide images of the formation. Acoustic imaging, however, is “blind” for air-filled holes in the rock.
Electrical imaging utilizes the property of electrical resistivity of the rock to generate data by sending alternating current into the formation and recording data about the properties of the rock and its structural features.
Electrical resistivity logs: The very first well logs recorded by Conrad Schlumberger were electrical resistivity logs. Also called resistivity logs, these measure the electrical resistivity—the opposite of conductivity—in rock formations. Oil and gas are not conductors of electricity but water is. Therefore, if a rock formation of an oil and gas-bearing type manifests high electrical resistivity, the chances of its actually containing oil and gas become higher.
Spontaneous potential logs: These logs also use electricity but in a different way, with a focus on the spontaneous potential of various types of matter to generate an electrical current. They detect and record the difference in the spontaneous potential of the different layers in the rock formation by comparing the SP of an electrode placed at a certain depth in the borehole with a reference electrode on the surface. These SP differences can tell the logger more about the permeability and therefore oil and gas potential of the rock layers.
Induction logs: These were used when electrical resistivity technology had not yet progressed enough to be able to measure and record resistivity in an environment of oil-rich mud in the borehole because oil is non-conductive. Induction logs worked by creating magnetic fields from alternating current and these magnetic fields in their turn induced a current in the rock, which created another magnetic field, and that magnetic field induced a new current. This process made it possible to measure the resistivity of the different layers in the rock formation.
Acoustic/Sonic logs: Acoustic or sonic logs measure the rock formation’s capacity to transmit seismic waves, meaning the time that it takes a seismic wave to travel through the formation. This technology, very similar to seismic imaging, call tell drillers a lot about the different layers of rock in the formation, their porosity, and the borders between them. This is important data for drillers that helps to identify the most promising parts of a formation in terms of oil and gas resources.
Radioactivity (Gamma ray) logs: Just as there are differences in electrical resistivity and spontaneous potential between different kinds of rock, so are there differences in radioactivity. Shale, for example, contains more naturally occurring radioactive elements than other rocks. Shale can also contain a lot of oil and gas as evidenced by the leading industry developments in the United States. Gamma-ray logs that measure and record the gamma-ray radiation from the rock formation in the well are among the cheaper ways to detect oil and gas-bearing rock or confirm its presence in the area of exploration.
Nuclear magnetic resonance logs: Using the same imaging principle as medical MRI, nuclear magnetic resonance logs target the hydrogen atoms in fluids contained in rock formations and produce data about the volume, contents, and viscosity of these fluids—oil, gas, or water—and how they are distributed within the rock.
Most of these logs are recorded during the well-drilling stage but the industry also distinguishes between open-hole and cased-hole logging. The latter refers to logging that is done after the well has been cased and completed. This means that steel tubing has been inserted into the borehole and the space between the rock and the tubing—the annulus—has been filled with cement.
After completion, corrosion well logging is performed regularly throughout the lifetime of the well as a means of monitoring the integrity of the casing and the tubing. In corrosion well logging, loggers use non-invasive technology including ultrasonic waves and magnetic and electromagnetic transducers: devices that convert physical properties such as pressure into electrical signals.
Well Logging Methods
Wireline logging: The first method of well logging that Schlumberger used and one of the dominant well logging methods to this day, wireline logging consists of lowering a logging tool on an electrical cable—a wireline—into the borehole and recording the data it generates. Today, this tool features a number of sensors that detect and send back to the surface the data engineers need to establish the viability of a well.
Wireline logging involves a number of different logs, therefore, each measuring a certain property or characteristic of the rock formations and the conditions in the well that are then brought together for a complete picture. Because wireline logging uses an electrical cable to operate the logging tool, wireline logs are often also called electrical logs, to be distinguished from electrical resistivity logs that record the resistivity of rock formations and the fluids they contain.
Logging-while-drilling: As the name suggests, this method of logging is applied during the process of drilling, with the logging tool with the sensors attached to the drillstring.
This has a couple of advantages over wireline logging. First, it makes it possible to measure and record the properties of formations and the conditions in the borehole in directional drilling. Wireline logging is impossible in directional boreholes because they are not vertical. Second, it measures the properties of the surrounding formation before mud enters it and affects them.
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Mud logging: Mud logging involves the analysis of drilling mud and rock cuttings in order to determine what minerals are contained in the formation and whether there are oil and gas resources in it.
Often performed by third parties at the drilling site, mud logging uses samples of drilling mud as it returns from the bottom of the borehole during drilling and before it goes back down to lubricate the drillbit. Samples of rock cuttings are also taken during this process as the drilling mud brings them up to the surface.
Using these samples, mud loggers establish if there are traces of crude oil or gas in the drilling mud, released by the formation where drilling is performed. From the rock cuttings, the mud loggers learn the mineralogical properties of the formation. This is not just a part of the evaluation process for the oil and gas-bearing potential of the rock, in which the well is being drilled but also a way of determining what behavior the drillers can expect from this rock in terms of porosity and permeability, among others.
Mud logging also includes monitoring the quality and quantity of the drilling mud as well as the rate of penetration of the drillbit into the formation, which provides important information about the speed of drilling and allows for making more accurate calculations of drilling times.
Gas logging: This involves isolating gases from the drilling mud and analyzing them to determine their nature and concentration in different rock layers in the well and the drilling mud.
To extract the gas from the drilling mud, loggers use so-called gas traps, where it is separated from the liquid and transported to other devices for analysis that determine its concentration in a given volume and also its constituent elements.
Gas logging is an important part of well logging because gas flowing from the formation into the drilling mud is not the only kind of gas that can be found in a borehole. Related: China’s Emissions Are Plunging, But It Might Not Last
Experts distinguish between liberated gas, which is released from the formation during drilling, and recycled gas, which goes up with the drilling mud to the surface of the borehole and then travels back into the borehole with the mud again.
There is also contaminant gas, which is introduced into the borehole as part of the drilling process by the drillers. This usually happens when the drillers are using oil-based mud or a diesel additive. Contaminant gas can also be the result of introducing a tracer chemical into the drilling mud to track lag times for the drilling mud’s cycles of going down and up the borehole and for rock cuttings. In this case, the contaminant is acetylene gas, formed when calcium carbide—a tracer chemical—reacts with water.
It is important to distinguish between the different types of gas that can be located in a borehole in order to get an accurate picture of the hydrocarbon reserves in the reservoir rock. The continual monitoring and analysis of the gas in gas logging helps loggers to distinguish between the spontaneous release of liberated gas as part of the drilling process—gas influx—and the tapping of a gas reservoir.
Core logging: Core logging, or coring, involves drilling out a cylindrical sample of rock from the borehole and studying it to provide drillers with direct knowledge of the type of rock and all relevant properties, including porosity, permeability, fluid saturation (how much fluid is in the rock), and density (how many grains of rock there are in a unit of volume), among others.
These properties will tell the drillers how much oil and gas is likely to be contained in the reservoir rock and how they could be expected to flow. This, in turn, helps drillers anticipate production rates and optimize them.
Coring is the only way to directly study the physical properties of the rock formation that the well is being drilled into rather than relying on data generated by sensors in logging tools used in wireline logging and logging-while-drilling. This latter kind of data has its limitations: the sensors on a wireline logging tool or an LWD tool cannot extract all the relevant information about a rock’s properties. It must be done in laboratories using physical samples of the rock.
There are two types of coring in the oil and gas industry. One is taking a sample from the bottom of the borehole, called a conventional core. The other is taking a sample from the wall of the well, called a sidewall core. The difference between the two is in the cost and time involved. Obtaining conventional cores is costlier than taking sidewall cores and it takes more time. An added benefit of sidewall cores is that one can obtain several rock samples during a single sampling operation.
Technological advancements made since the first well log recorded by Conrad Schlumberger have made well logging an extremely precise tool for measuring all important properties of the rock and the fluids in that rock at a drill site. This has helped both improve exploration results and optimize oil and gas production.
By Michael Kern for Oilprice.com
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