Petrophysics
Encyclopedia
Petrophysics is the study of the physical and chemical properties that describe the occurrence and behavior of rocks, soils and fluids http://www.seed.slb.com/qa2/FAQView.cfm?ID=914. Petrophysics mainly studies reservoirs of resources, including ore
deposits and oil
or natural gas
reservoirs. Petrophysicists in the oil and gas industry typically are employed in helping the engineers and other geoscientists understand the rock properties of the reservoir. Petrophysicists evaluate the reservoir rock properties by employing well log measurements, in which a string of measurement tools are inserted in the borehole, core
measurements, in which rock samples are retrieved from subsurface, and sometimes seismic measurements, and combining them with geology and geophysics.
Petrophysical studies are used by petroleum engineering
, geology
, mineralogy
, exploration geophysics
and other related studies.
Some of the key properties studied in petrophysics are lithology
, porosity
, water saturation, permeability, density
, solid mechanics
, magnetization
, electrical conductivity, thermal conductivity
and radioactivity.
While most petrophysicists work in the oil and gas industry, some also work in the mining and water resource industries. The properties measured or computed fall into three broad categories: conventional petrophysical properties, rock mechanical properties, and ore quality.
Lithology
: What type of rock is it. When combined with local geology and core study, geoscientists can use log measurements such as natural gamma
, neutron
, density
, Photoelectric, resistivity
or their combination to determine the lithology downhole.
Porosity
:
The amount of pore (or fluid occupied) space in the rock. This is typically measured using an instrument that measures the reaction of the rock to bombardment by neutrons
or by gamma rays
. Sonic wave speed
and NMR
logs are also measured to derive rock porosity.
Water saturation:
The fraction of the pore space occupied by water. This is typically measured using an instrument that measures the resistivity
of the rock.
Permeability
: The quantity of fluid (usually hydrocarbon) that can flow from the rock as a function of time and pressure. Formation testing
is so far the only direct downhole permeability measurement. In case of its absence, which is common in most cases, permeability estimation is derived from other measurements, such as porosity
, NMR
, sonic
, by empirical correlations.
Thickness of rock with enough permeability to deliver fluids to a well bore. This property is often called “Net reservoir rock.” In the oil and gas industry, another quantity “Net Pay” is computed which is the thickness of rock that can deliver hydrocarbons to the well bore at a profitable rate.
Reservoir models are built upon their measured and derived properties to estimate the amount of hydrocarbon present in the reservoir, the rate at which that hydrocarbon can be produced to the Earth’s surface through wellbores and the fluid flow in rocks. In the water resource industry, similar models are used to compute how much water can be produced to the surface over long periods of time, without depleting the aquifer.
The rocks compressive strength which is the compressive stress that causes a rock to fail.
The rocks flexibility,
the relationship between stress and deformation for a rock. Converted-wave analysis
is also used to determine subsurface lithology and porosity.
These measurements are useful to design programs to drill wells that produce oil and gas. The measurements are also used to design dams, roads, foundations for buildings and many other large construction projects. They can also be used to help interpret seismic signals from the Earth, either man-made seismic signals or those from earthquakes.
and core analysis is a direct measurement of petrophysical properties. In the petroleum industry rock samples are retrieved from subsurface and measured by core labs of oil company or some commercial core measurement service companies. This process is time consuming and expensive, thus can not be applied to all the wells drilled in a field.
Well Logging
is used as a relatively inexpensive method to obtain petrophysical properties downhole. Measurement tools are conveyed downhole using either wireline or LWD
method.
An example of wireline logs is shown in Figure 1. The first “track”, shows the natural gamma radiation level of the rock. The gamma radiation level “log” shows increasing radiation to the right and decreasing radiation to the left. The rocks emitting less radiation have more yellow shading. The detector is very sensitive and the amount of radiation is very low. In clastic rock formations, rocks that have smaller amounts of radiation are more likely to be coarser grained and have more pore space, rocks with higher amounts of radiation are more likely to have finer grains and less pore space (Poupon, Clavier, Dumanoir, Gaymard, and Misk, Journal of Petroleum Technology, July, 1970, page 868).
The second track over in the plot records the depth below the reference point which is usually the Kelly bush or rotary table in feet, so these rocks are 11,900 feet below the surface of earth.
In the third track, the electrical resistivity of the rock is presented. The water in this rock is salty and the salt in the water causes the water to be electrically conductive such that lower resistivity is caused by increasing water saturation and decreasing hydrocarbon saturation (Brown, A Mathematical Comparison of Common Saturation Equations, SPWLA twenty-seventh annual logging symposium, June, 1986, paper T, appendix 1).
The fourth track, shows the computed water saturation, both as “total” water (including the water bound to the rock) in magenta and the “effective water” or water that is free to flow in black. Both quantities are given as a fraction of the total pore space.
The fifth track shows the fraction of the total rock that is pore space, filled with fluids. The display of the pore space is divided into green for oil and blue for movable water. The black line shows the fraction of the pore space which contains either water or oil that can move, or be “produced.” In addition to the what is included in blackline, the magenta line includes the water that is permanently bound to the rock.
The last track is a representation of the solid portion of the rock. The yellow pattern represents the fraction of the rock (excluding fluids) that is composed of coarser grained sandstone. The gray pattern represents the fraction of rock that is composed of finer grained “shale.” The sandstone is the part of the rock that contains the producible hydrocarbons and water.
The following definition and petrophysics model are a typical shaly sand formation model which assumes:
1. Shale is composed of silt, clay and their bounded water which will not flow.
2. Hydrocarbon are stored only in pore space in sand matrix.
ΦT - Total porosity (PHIT), which includes the pore space in sand and shale.
Sw - Total water saturation, the fraction of the pore space occupied by water.
Φe - Effective shale corrected porosity which includes only the pore space in sand. The pore space in shale which is filled with bounded water is excluded.
Swe - Effective shale corrected water saturation. The volumetric fraction of Φe which is occupied by water.
Vsh - Volumetric fraction of shale. This includes medium to very fine silt plus clay and the shale bound water.
Φsh - Shale porosity. Volumetric fraction of pore space in shale. These pore space is filled with bounded water by definition.
Key equations:
(1-Φe-Vsh) + Vsh + Φe*Swe + Φe*(1-Swe) = 1
Sandstone matrix volume + shale volume + water volume in sand + hydrocarbon volume in sand = total rock volume
Φe = ΦT – Vsh *Φsh
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
deposits and oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
or natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
reservoirs. Petrophysicists in the oil and gas industry typically are employed in helping the engineers and other geoscientists understand the rock properties of the reservoir. Petrophysicists evaluate the reservoir rock properties by employing well log measurements, in which a string of measurement tools are inserted in the borehole, core
Coring
Coring happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium conditions. This causes the exterior of the material to harden faster than the interior. Coring causes the exterior layers to retain more of the higher melting temperature element...
measurements, in which rock samples are retrieved from subsurface, and sometimes seismic measurements, and combining them with geology and geophysics.
Petrophysical studies are used by petroleum engineering
Petroleum engineering
Petroleum engineering is an engineering discipline concerned with the activities related to the production of hydrocarbons, which can be either crude oil or natural gas. Subsurface activities are deemed to fall within the upstream sector of the oil and gas industry, which are the activities of...
, geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
, exploration geophysics
Exploration geophysics
Exploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface Earth, in order to detect or infer the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, groundwater reservoirs, and other geological...
and other related studies.
Some of the key properties studied in petrophysics are lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
, porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
, water saturation, permeability, density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, solid mechanics
Solid mechanics
Solid mechanics is the branch of mechanics, physics, and mathematics that concerns the behavior of solid matter under external actions . It is part of a broader study known as continuum mechanics. One of the most common practical applications of solid mechanics is the Euler-Bernoulli beam equation...
, magnetization
Nuclear magnetic resonance in porous media
Nuclear magnetic resonance in porous media covers the application of using NMR as a tool to study the structure of porous media and various processes occurring in them. This technique allows the determination of characteristics such as the porosity and pore size distribution, the permeability, the...
, electrical conductivity, thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
and radioactivity.
While most petrophysicists work in the oil and gas industry, some also work in the mining and water resource industries. The properties measured or computed fall into three broad categories: conventional petrophysical properties, rock mechanical properties, and ore quality.
Conventional petrophysical properties
Most petrophysicists are employed to compute what are commonly called conventional (or reservoir) petrophysical properties. These are:Lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
: What type of rock is it. When combined with local geology and core study, geoscientists can use log measurements such as natural gamma
Gamma ray logging
Gamma ray logging is a method of measuring naturally occurring gamma radiation to characterize the rock or sediment in a borehole. It is sometimes used in mineral exploration and water-well drilling, but most commonly for formation evaluation in oil and gas well drilling . Different types of...
, neutron
Formation evaluation neutron porosity
In the field of formation evaluation, porosity is one of the key measurements to quantify oil and gas reserves. Neutron porosity measurement employs a neutron source to measure the hydrogen index in a reservoir, which is directly related to porosity...
, density
Density logging
Density logging is a well logging tool determining rock bulk density along a wellbore. This is the overall density of a rock including solid matrix and the fluid enclosed in pores. Geologically, bulk density is a function of the density of the minerals forming a rock Density logging is a well...
, Photoelectric, resistivity
Resistivity logging
Resistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current...
or their combination to determine the lithology downhole.
Porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
:
The amount of pore (or fluid occupied) space in the rock. This is typically measured using an instrument that measures the reaction of the rock to bombardment by neutronsFormation evaluation neutron porosity
In the field of formation evaluation, porosity is one of the key measurements to quantify oil and gas reserves. Neutron porosity measurement employs a neutron source to measure the hydrogen index in a reservoir, which is directly related to porosity...
or by gamma rays
Density logging
Density logging is a well logging tool determining rock bulk density along a wellbore. This is the overall density of a rock including solid matrix and the fluid enclosed in pores. Geologically, bulk density is a function of the density of the minerals forming a rock Density logging is a well...
. Sonic wave speed
Sonic logging
Sonic logging shows a formation’s interval transit time, designated Dt. It is a measure of a formation’s capacity to transmit sound waves. Geologically, this capacity varies with lithology and rock textures, notably porosity....
and NMR
Nuclear magnetic resonance in porous media
Nuclear magnetic resonance in porous media covers the application of using NMR as a tool to study the structure of porous media and various processes occurring in them. This technique allows the determination of characteristics such as the porosity and pore size distribution, the permeability, the...
logs are also measured to derive rock porosity.
Water saturation:
The fraction of the pore space occupied by water. This is typically measured using an instrument that measures the resistivityResistivity logging
Resistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current...
of the rock.
Permeability
Permeability (fluid)
Permeability in fluid mechanics and the earth sciences is a measure of the ability of a porous material to allow fluids to pass through it.- Units :...
: The quantity of fluid (usually hydrocarbon) that can flow from the rock as a function of time and pressure. Formation testing
Drill Stem Test
A drill stem test is a procedure for isolating and testing the surrounding geological formation through the drill stem. The test is a measurement of pressure behavior at the drill stem and is a valuable way to obtain important sampling information on the formation fluid and to establish the...
is so far the only direct downhole permeability measurement. In case of its absence, which is common in most cases, permeability estimation is derived from other measurements, such as porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
, NMR
Nuclear magnetic resonance in porous media
Nuclear magnetic resonance in porous media covers the application of using NMR as a tool to study the structure of porous media and various processes occurring in them. This technique allows the determination of characteristics such as the porosity and pore size distribution, the permeability, the...
, sonic
Sonic logging
Sonic logging shows a formation’s interval transit time, designated Dt. It is a measure of a formation’s capacity to transmit sound waves. Geologically, this capacity varies with lithology and rock textures, notably porosity....
, by empirical correlations.
Thickness of rock with enough permeability to deliver fluids to a well bore. This property is often called “Net reservoir rock.” In the oil and gas industry, another quantity “Net Pay” is computed which is the thickness of rock that can deliver hydrocarbons to the well bore at a profitable rate.
Reservoir models are built upon their measured and derived properties to estimate the amount of hydrocarbon present in the reservoir, the rate at which that hydrocarbon can be produced to the Earth’s surface through wellbores and the fluid flow in rocks. In the water resource industry, similar models are used to compute how much water can be produced to the surface over long periods of time, without depleting the aquifer.
Rock mechanical properties
Some petrophysicists use acoustic and density measurements of rocks to compute their mechanical properties and strength. They measure the compressional (P) wave velocity of sound through the rock and the shear (S) wave velocity and use these with the density of the rock to compute:The rocks compressive strength which is the compressive stress that causes a rock to fail.
The rocks flexibility,
the relationship between stress and deformation for a rock. Converted-wave analysis
Converted-wave analysis
During seismic exploration, P-waves penetrate down into the earth. When a P-wave hits an interface , it can reflect upwards as an S-wave . Other P-wave to S-wave conversions can occur, but the down-up conversion is the primary focus...
is also used to determine subsurface lithology and porosity.
These measurements are useful to design programs to drill wells that produce oil and gas. The measurements are also used to design dams, roads, foundations for buildings and many other large construction projects. They can also be used to help interpret seismic signals from the Earth, either man-made seismic signals or those from earthquakes.
Ore quality
Bore holes can be drilled into ore bodies (for example coal seams or gold ore) and either rock samples taken to determine the ore or coal quality at each bore hole location or the wells can be wireline logged to make measurements that can be used to infer quality. Some petrophysicists do this sort of analysis. The information is mapped and used to make mine development plans.Methods of analysis
CoringCoring
Coring happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium conditions. This causes the exterior of the material to harden faster than the interior. Coring causes the exterior layers to retain more of the higher melting temperature element...
and core analysis is a direct measurement of petrophysical properties. In the petroleum industry rock samples are retrieved from subsurface and measured by core labs of oil company or some commercial core measurement service companies. This process is time consuming and expensive, thus can not be applied to all the wells drilled in a field.
Well logging
Well logging, also known as borehole logging is the practice of making a detailed record of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface or on physical measurements made by instruments lowered into the hole...
is used as a relatively inexpensive method to obtain petrophysical properties downhole. Measurement tools are conveyed downhole using either wireline or LWD
Logging While Drilling
Logging While Drilling is a technique of conveying well logging tools into the well borehole downhole as part of the bottom hole assembly ....
method.
An example of wireline logs is shown in Figure 1. The first “track”, shows the natural gamma radiation level of the rock. The gamma radiation level “log” shows increasing radiation to the right and decreasing radiation to the left. The rocks emitting less radiation have more yellow shading. The detector is very sensitive and the amount of radiation is very low. In clastic rock formations, rocks that have smaller amounts of radiation are more likely to be coarser grained and have more pore space, rocks with higher amounts of radiation are more likely to have finer grains and less pore space (Poupon, Clavier, Dumanoir, Gaymard, and Misk, Journal of Petroleum Technology, July, 1970, page 868).
The second track over in the plot records the depth below the reference point which is usually the Kelly bush or rotary table in feet, so these rocks are 11,900 feet below the surface of earth.
In the third track, the electrical resistivity of the rock is presented. The water in this rock is salty and the salt in the water causes the water to be electrically conductive such that lower resistivity is caused by increasing water saturation and decreasing hydrocarbon saturation (Brown, A Mathematical Comparison of Common Saturation Equations, SPWLA twenty-seventh annual logging symposium, June, 1986, paper T, appendix 1).
The fourth track, shows the computed water saturation, both as “total” water (including the water bound to the rock) in magenta and the “effective water” or water that is free to flow in black. Both quantities are given as a fraction of the total pore space.
The fifth track shows the fraction of the total rock that is pore space, filled with fluids. The display of the pore space is divided into green for oil and blue for movable water. The black line shows the fraction of the pore space which contains either water or oil that can move, or be “produced.” In addition to the what is included in blackline, the magenta line includes the water that is permanently bound to the rock.
The last track is a representation of the solid portion of the rock. The yellow pattern represents the fraction of the rock (excluding fluids) that is composed of coarser grained sandstone. The gray pattern represents the fraction of rock that is composed of finer grained “shale.” The sandstone is the part of the rock that contains the producible hydrocarbons and water.
Rock volumetric model for shaly sand formation
Symbols and Definitions:1. Shale is composed of silt, clay and their bounded water which will not flow.
2. Hydrocarbon are stored only in pore space in sand matrix.
ΦT - Total porosity (PHIT), which includes the pore space in sand and shale.
Sw - Total water saturation, the fraction of the pore space occupied by water.
Φe - Effective shale corrected porosity which includes only the pore space in sand. The pore space in shale which is filled with bounded water is excluded.
Swe - Effective shale corrected water saturation. The volumetric fraction of Φe which is occupied by water.
Vsh - Volumetric fraction of shale. This includes medium to very fine silt plus clay and the shale bound water.
Φsh - Shale porosity. Volumetric fraction of pore space in shale. These pore space is filled with bounded water by definition.
Key equations:
(1-Φe-Vsh) + Vsh + Φe*Swe + Φe*(1-Swe) = 1
Sandstone matrix volume + shale volume + water volume in sand + hydrocarbon volume in sand = total rock volume
Φe = ΦT – Vsh *Φsh
See also
- Formation evaluationFormation evaluationIn petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum. Essentially, it is the process of "recognizing a commercial well when you drill one"....
- Gardner's relationGardner's relationGardner's relation, or Gardner's equation, named after G. H. F. Gardner and L. W. Gardner, is an empirically derived equation that relates seismic P-wave velocity to the bulk density of the lithology in which the wave travels...
- Archie's law
- Society of Petrophysicists and Well Log AnalystsSociety of Petrophysicists and Well Log AnalystsThe Society of Petrophysicists and Well Log Analysts is a non-profit collective of scientists involved in petrophysics and the extraction of gas, oil, and minerals through earth sciences...