Formation evaluation (well logging) is the method to evaluate reservoir formations. In this article, I will describe about the basic formation evaluation and its equipment.
The basic logging tools are as follows:
Caliper Log: One or more arms attach on wellbore surface in order to record hole diameter.
Behavior of Caliper Log response with sand in open hole (See the image below)
Gamma Ray Log: It measures the natural gamma ray level in formation. Basically, Gamma ray emission is produced by three radioactive series found in the Earth’s crust which are Potassium (K40) series, Uranium series and Thorium series. Shale and Coal exhibit high gamma ray radiation, but Sand and Carbonate exhibits low gamma ray radiation.
In clean sand, gamma ray reading should be around 40 API. If reading value is between 40-75 API, it can be classified as sand, not clean sand. In shale formation, reading is about 120-180 API which is quite different from gamma ray reading in sand. For Coal, the reading is very high about 200+ API, depending on formation.
Applications of Gamma Ray are listed below:
– Depth Control
– Bed Boundaries
– Bed Thickness
– Volume of Shale
(Image from http://www.kgs.ku.edu)
Resistivity Log: It measures formation resistivity. The basic concept of electric is applied. Current can only pass through the water in the formation, hence the resistivity depends on: 1) Resistivity of the formation water, 2) Amount of water and hydrocarbon present in formation and 3) Pore structure.
Resistivity Measurement Concept
High resistivity reading reflects high hydrocarbon content in formation because hydrocarbon is non-conductive fluid. In contrast, low resistivity reading demonstrates high water in formation, called “WET sand”, because water is conductive fluid. Resistivity is the key to hydrocarbon determination.
There are 2 types of resistivity log as follows:
1) Induction Log: Formation resistivity is measured by inducing current flow which produces Electromagnetic Field (EMF) as per Faraday’s Law. EMF produces the ground loop and the ground loop produces EMF back which cuts the receiver coil. Transmitter and receiver coils measure the resistivity of the formation by inducing current flow. Induction log is suitable for non-conductive drilling fluid as SDF. The resistivity in shale is around 1.5-4 ohm-m and the reading in wet sand is around 4-10 ohm-m. Unocal Thailand has cut-off criteria for pay sand, which must have the deep resistivity more than 10 ohm-m. In Shale formation, there is no separation between deep and medium resistivity because Shale is non-permeable zone that means no mud filtration in to formation. However, the separation between deep and medium resistivity can be seen because Sand is permeable zone.
2) Laterologs log: Laterolog log has a basic circuitry of emitting and measuring electrodes across which a potential drop in measured to give rock resistivity. It is suitable for conductive drilling fluid as water-base mud. Unocal Thailand does not use the Laterolog because SDF is always used in 6 1/8” open hole section.
Density Log: The density log measures the bulk density of the formation using Compton Scattering of Gamma ray. The bulk density can be related to porosity when the lithology is known. The equation relating porosity and density is below:
Ø = density derived porosity
ρma = matrix density ( for sandstone pma ~ 2.65 – 2.7 gm/cc)
ρb = formation bulk density
ρf = fluid density (1.1 salt mud, and 1.0 fresh mud)
Matrix density commonly used is shown in the following table.
The density log uses a pad-type sonde with a spring-load back up arm that also provides a caliper measurement. Density instruments generally consist of a gamma ray source, usually cesium-137 and two detonators. The source and detectors are located on a pad which is forced against the side of open hole. The long spacer detector reads mostly the formation. The short spaced detonator measures a great deal of both formation and the materials that occur between the pad and the formation (see the figure below)
Density Log showing the configuration of the source and detectors of a compensated density logging tool. Courtesy of Schlumbeger
The gamma rays leaving the source are scattered by the orbital electrons of the atoms in the materials. If the material is very dense (contain many electrons), the gamma rays are scattered more resulting in lowering the level of energy of gamma ray. Therefore, there are fewer gamma rays reaching the detonators. In other hands, low density formation (contains few electrons), the emitted gamma rays are not slow down much so there will be more gamma rays reaching the detonators.
Moreover, the density tool can identify lithology of formation by “Photo-electric Value” (PE). The common value of PE for each lithology is listed below:
|Lithology||Photo-electric Value (PE)|
In shale, bulk density will be read about 2.55-2.6 gm/cc. In sand, bulk density can be around 2.00-2.5 gm/cc, which demonstrates low density because it is porous formation. For Coal, the bulk density reading is very low comparing with sand or shale.
Neutron Log: Neutron log measures the formation ability to attenuate the passage of neutron through the formation. This is a measure of the hydrogen content of the formation. Hydrogen in clean reservoir rocks can represent presence of water or oil.
Neutrons are electrically neutral particles of about the same mass as hydrogen atom. The sources used in neutron log are combinations mineral like Am (americium) and Be (beryllium). The neutrons leave the source with high energy (high velocity) and collide with formation materials in an elastic manner. Neutrons rebound from heavy nuclei with high energy but lose energy when they hit with hydrogen nuclei. Detectors designed to detect low energy neutrons detect neutrons after colliding with mineral atom in formation in term of count rate. High count rate demonstrates low hydrogen atom that means low porosity formation. In contrast, low count rate reflect to high hydrogen atom that means high porosity formation. The richness in hydrogen or quantity per unit volume is converted directly to “neutron porosity units”.
Neutron porosity is “real porosity” in clean limestone, but the other lithologies as sand and dolomite requires conversion factors.
In shale, the reading of neutron porosity will be high due to the content of water trapped inside. Normally in oil or wet sand, neutron porosity is about 15-30 %, but in gas sand, porosity reading is around 10-15%. The porosity in gas zone is tool low because hydrogen atom in gas is lower than water or oil comparing in the same volume. In Coal, neutron reading is around 40-50 % porosity due to larger amount of trapped water.
Sonic Log: Sonic log measures the shortest time required for a compression wave to travel vertically through one foot of formation adjacent to the wellbore. The sonic travel time can be related to porosity when the lithology is known. The Wyllie equation is used to relate travel time and porosity as below:
Δt log = the formation travel time reading from log
Δt ma = the matrix travel time at zero porosity
Δt f = the fluid travel time ( in Gulf of Thailand ~ 180 usec/ft)
All travel times are in microseconds per foot.
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