Petrophysics

Petrophysics ("petro" is Latin for "rock" and "physics" is the study of nature) 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 the analysis of well logs in which a tool is inserted in the borehole, and the properties of the rock are measured.

Petrophysics is a sub-field of Geophysics. Petrophysical studies are utilised by Geology, Mineralogy, Exploration geophysics and other related studies.

Some of the properties studied in petrophysics are Porosity, Density, Magnetization, Electrical conductivity, Solid mechanics, 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.

Conventional Petrophysical Properties

Most petrophysicists are employed to compute what are commonly called conventional (or reservoir) petrophysical properties. These are:

"Porosity:" $phi$ 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.

"Water Saturation:" $S\_w$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.

"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.

These measurements and computed properties are used to compute the amount of oil or gas present in a well bore and the rate at which that hydrocarbon can be produced to the Earth’s surface. In the water resource industry, the values are used to compute how much water can be produced to the surface.

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.

These measurements are useful to design programs to drill wells into the Earth and to design 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

Obtaining rock samples from a well bore (a process called coring) is expensive and rare in the petroleum industry. So core samples which contain very precise and complex information are used to calibrate the less expensive wireline logging measurements which are taken over nearly all portions of all wells.

An example of wireline logs is shown in Figure 1. The left hand “track” or graph, 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. 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 next track over in the plot records the depth below surface in feet, so these rocks are 11,900 feet below the surface of earth. In the next track, to the right, 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 next track, to the right, 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 next 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 water. Again the magenta line includes the water that is permanently bound to the rock, the line inside that one, in black, is the fraction of the rock containing either water or oil that can move, or be “produced.”

The final 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.

Petrophysical Rock Model and definitions of common terms

"Symbols and Definitions:"

ΦT - Total porosity (PHIT), this is usually computed from the density of the rock, which can be measured with an instrument dropped into the bore hole.

Sw - Total water saturation, the fraction of the pore space occupied by water.

ΦE - Effective shale corrected porosity, the water bound to the rock is removed from PHIT to compute this value.

Swe - Effective shale corrected Sw, exclusive of shale bound water.

Qvn - Shale bound water saturation, sometimes called Swb. Swb or Qvn (Juhasz, Normalized Qv – The Key to Shaly Sand Evaluation Using the Waxman-Smits Equation in the Absence of Core Data, SPWLA twenty-second Annual Logging Symposium, June, 1981, page 23) is equal to Vsh*Φsh/Φ.

Vsh - Volume of shale. This includes medium to very fine silt plus clay and the shale bound water.

Key equations:

(1-ΦE)(1-Sh) + Sh(1-ΦE) + (ΦE*Swe) + [ΦE*(1-Swe)] = 1

Sandstone + shale + water + hydrocarbon = 1

Or, in another form:

Matrix rock + shale +bound water + water + hydrocarbon = 1

(1-Φ)(1-Vsh) + Vsh(1-Φ) + (Φ*Swb) + (ΦE*Swe) + [ΦE*(1-Swe)] = 1

Vsh = Sh(1-ΦE) (Sh is the volume of shale as a % of rock matrix, Vsh is shale as a % of all of the rock)

ΦE = ΦT – ΦT *Swb (PHIE is the porosity in the sand portion of the rock)

Qvn = Vsh*Phish/ΦT

ee also

* Formation evaluation