Mud logging and core analysis are direct methods of formation evaluation. Wireline well logging is the indirect analysis of downhole features by electronic methods. To log a well by wireline (actually conductor line), an instrument called a sonde is put on bottom, and a recorder plots a graph at the surface as the sonde is raised to the surface.
Figure 48: Wireline Logging Company Sondes
The numerous logs offered by wireline companies today gather data in many different ways under many different conditions. Many, but not all, logging devices can be run in a single sonde in one wireline trip.
A specific combination of logs is usually chosen for the types of formation data needed. Correlation between the curves gives a clear picture of lithology, porosity, permeability, and saturation up and down the wellbore. Caliper logs, spontaneous potential logs, resistivity logs, radioactivity logs, or acoustic logs may be included in a typical logging run.
6.1 CALIPER LOGS
A caliper is a tool that measures diameter. A caliper logging sonde measures the inner diameter of the borehole. Arms, springs, or pads are held against the sidewall as the device is pulled out of the hole by wireline. Changes in borehole diameter move the arms in and out and send signals to the surface.
Wellbore diameter may vary widely because of lateral bit movement, caving formations, mud cake, or flexure (rock bowing into the wellbore because of overburden stress). Excessive hole enlargements indicate that caving or washouts are present. Reduced hole diameters indicate filter cake build-up in permeable formations.
It is important to know the hole size / gauge to calculate cement volumes accurately and determine the effect that variations will have on other electronic logs.
6.2 SPONTANEOUS POTENTIAL LOGS
The spontaneous potential, or SP, log is the most common and widely run log. It records the weak, natural electrical currents that flow in formations adjacent to the wellbore. Most minerals are nonconductors when dry. However some, like salt, are excellent conductors when dissolved in water. When a layer of rock or mud cake separates two areas of differing salt content, a weak current flows from the higher salt concentration to the lower.
Usually drilling fluids contain less salt than formation fluids, which may be very salty. As freshwater filtrate invades a permeable formation, spontaneous potential causes weak current to flow from the uninvaded to the invaded zone. More importantly, current flows from the uninvaded rock into the wellbore through any impermeable formation, such as shale, above and below the permeable layer.
The SP curve is recorded in millivolts against depth. This value is useful in calculating the formation water resistivity (Rw). The SP log can be visually interpreted to show formation bed boundaries and thickness, as well as relative permeability of formation rocks. It is also used in well correlation.
Because the SP log is so simple to obtain and provides such basic information, it is included in almost every logging run.
6.3 RESISTIVITY LOGS
Resistivity logs record the resistance of a formation to the flow of electricity through it. Conductivity is the inverse of resistivity. Resistivity is expressed in ohm-meters, conductivity in mhos / meter. Some well log formulas use resistivity while others use conductivity. However, all of these formulas describe the same thing: the flow of electrical current.
Resistance to this flow depends on:
How much water the formation can hold.
How freely the water can move.
How saturated the formation is with water rather than hydrocarbons.
In this way, resistivity is directly related to other formation characteristics. High porosity, high permeability, and high water saturation will present low resistivity. The presence of oil and gas (and dense rock types) will present high resistivity, because hydrocarbons are poor conductors. If well logs show a formation to be very porous and permeable but also to be highly resistive, then it can be inferred that it holds petroleum. Resistivity logs can therefore be used to determine the degree of water saturation (Sw) and hydrocarbon-water contacts.
Resistivity measurements are usually taken at different depths of penetration into a formation, typically 30, 60 and 90 cm. The depth of investigation is controlled by the spacing of electrodes on the sonde. The deeper measurements are likely to be a more true indication of fluid type as they are unlikely to be
affected by mud filtrate invasion. Comparisons of the three measurements can also be an indicator of relative permeability.
Common resistivity logs include the lateral focus log, the induction log, and the microresistivity log.
6.3.1 Lateral Focus Log
The lateral focus log uses a sonde that sends current outward through the rock in a specific pattern. A set of guard electrodes in the sonde focus current sideways into a flat sheet. As the sonde is raised, the sheet of current passes through formation rock. Differences in formation characteristics change the flow of current through the sheet and these changes are logged at the surface.
6.3.2 Induction Log
The induction log involves inducing a current in the formation beds. The sonde sets up a doughnut-shaped magnetic field around the wellbore, which generates a current monitored by instruments at the surface. As the sonde is raised through formations, changes in the current are logged. Like the lateral focus log, the induction log is very accurate for investigating thin formation beds.
6.3.3 Microresistivity Log
The microresistivity log is designed to show resistivities very close to the wellbore. It has two curves: one showing resistivity in mud cake, the other showing resistivity less than one-half foot away in the formation.
When the two curves are not identical, an invaded zone is indicated and a possible reservoir is found.
Porosity and permeability can be calculated from the microresistivity curves.
6.4 RADIOACTIVITY LOGS
Just as resistivity logs record natural and induced electrical currents, radioactivity logs record natural and induced radioactivity. Traces of radioactive elements are deposited in formation sediments. Over time, water leaches them out of porous, permeable rock, such as limestone and clean sandstone, but cannot wash them out of impermeable formations, such as shale and clay-filled sands.
6.4.1 Gamma Ray Logs
Gamma ray logs show radiation from these impermeable formations. The gamma ray sonde contains a gamma ray detector, such as a Geiger counter. This tool measures the natural radioactivity of the rocks by detecting elements such as uranium, thorium, and potassium, and is therefore used as a shale indicator.
Shale free sandstones and carbonates, typically, have low gamma readings although certain mineralogies such as K-feldspars, micas and glauconite may increase the values.
The gamma ray log is useful for correlation with the neutron log.
6.4.2 Neutron Logs
The neutron log measures the concentration of hydrogen ions in a formation. A radioactive source is loaded into the sonde and sent downhole. As neutron radiation bombards the rock around the wellbore and the rock gives off gamma rays from the neutrons it has absorbed.
The greatest energy loss occurs on collision with hydrogen atoms, due to the fact that they are of a similar size. Because hydrogen is concentrated within the fluid, whether water or hydrocarbon, the measurement is a function of porosity, although clay lattice-bound water cannot be distinguished from pore water. Where gas is present, the hydrogen concentration decreases and results in the gas effect, a significant drop in the neutron porosity.
Some sondes measure the levels of both gamma rays and unabsorbed neutrons. Other neutron logs can be calibrated to the gamma rays emitted by certain elements such as hydrogen, carbon, oxygen, chlorine, silicon, or calcium. The detected amounts of these elements give information about water and hydrocarbon saturations, salt content, and rock types. All neutron logs give good porosity readings.
6.4.3 Density Logs
The density log, like the neutron log, uses radiation bombardment but uses gamma rays instead of neutrons. These collide with formation electrons and suffer an energy loss. The number of returning particles is a direct function of the bulk density of the formation. Bulk density is the total density of a rock, reflecting rock matrix density, fluid density and pore space volume.
The denser, or less porous, a formation is the more gamma rays it absorbs. On the other hand, the more porous a formation is the less rock there is to stop gamma rays. Mathematical formulas for such figures as porosity, hydrocarbon density, and oil-shale yield can be solved with data taken from density curves. As a direct indicator of compaction, the density log is an excellent tool in overpressure evaluation.
Typically, the density log is only run through zones of interest, rather than the entire length of the borehole.
6.5 ACOUSTIC LOGS
Sound travels through dense rock more quickly than through lighter, more porous rock. The acoustic log, also called the sonic log, shows differences in travel times of sound pulses sent through formation beds.
Shale and clay, as well as porous rock, slow down the pulses. Using information about formation types from other logs, porosity can be determined from acoustic logs. This type of log is also used to verify the integrity (quality and hardness) of the cement bond between casing and the formation.
6.6 TYPICAL LOGGING RUNS
A wide variety of logs can be taken using a single sonde, but a specific combination is usually chosen for the types of formation data needed. Correlation between the curves gives a clear picture of lithology, porosity, permeability, and saturation up and down the wellbore. Figure 49 is a picture of E-log sondes ready to be picked up and made up for a logging job.
Figure 49: E-log Sondes