# How do you find the porosity of a well log?

Asked By: Rosalva Dahms | Last Updated: 2nd February, 2020
Category: science geology
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CNL (compensated neutron) logs: also called neutron logs, determine porosity by assuming that the reservoir pore spaces are filled with either water or oil and then measuring the amount of hydrogen atoms (neutrons) in the pores. These logs underestimate the porosity of rocks that contain gas.

Beside this, how do you calculate the effective porosity of a log?

Total porosity minus clay-bound water (CBW). Log effective porosity. In essence, total porosity minus shale water, where solid minerals and the volume of shale (Vsh) constitute the matrix (non-effective porosity) and the remaining volume constitutes the effective porosity.

Similarly, what is porosity log? Neutron logs are porosity logs that measure the hydrogen ion concentration in a formation. In clean formations (i.e. shale-free) where the porosity is filled with water or oil, the neutron log measures liquid-filled porosity. Neutrons are created from a chemical source in the neutron logging tool.

Keeping this in view, how do you find the porosity of a density log?

Obtaining porosities from a density log

1. Φ = porosity.
2. ρma = matrix density (see table below)
3. ρb = formation bulk density (log value)
4. ρf = density of the fluid saturating the rock immediately surrounding the borehole—usually mud filtrate (use 1.0 for freshwater and 1.1 for saltwater mud)

How do you determine porosity?

There are two basic equations used to calculate porosity. The first equation uses the total volume and the volume of the void. Porosity = (Volume of Voids / Total Volume) x 100%. The second equation uses the total volume and the volume of the solid.

### What is the difference between porosity and effective porosity?

Effective porosity excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains. Total porosity is the total void space in the rock whether or not it contributes to fluid flow. In log interpretation, effective porosity means the total porosity less the clay-bound water.

### What is residual porosity?

Typically, the residual porosity amounts to a value around 3%–4%. This means porosity is closed and an additional hot isostatic pressing (HIP) process can be applied without capsulation, resulting in practically 100% dense material. However, possible impurity uptake has to be considered.

### What is matrix porosity?

matrix porosity. [′mā·triks p?′räs·?d·ē] (geology) Core-sample porosity determined from a small sample of the core, in contrast to total porosity, where the whole core is used.

### What is the porosity of sand?

Typical values of soil porosity for different soils
Description USCS Porosity [-]
Coarse sand (SW) 0.43
Fine sand (SW) 0.46
Poorly graded sands, gravelly sands, with little or no fines SP 0.43
Silty sands SM 0.49

### How are porosity and permeability related?

Porosity is a measure of how much of a rock is open space. This space can be between grains or within cracks or cavities of the rock. Permeability is a measure of the ease with which a fluid (water in this case) can move through a porous rock.

### What is effective porosity in geology?

Effective porosity is that portion of the total void space of a porous material that is capable of transmitting a fluid. Total porosity is the ratio of the total void volume to the total bulk volume.

### How do you calculate porosity of sandstone?

Porosity is the percentage of void space in a rock. Porosity is the percentage of void space in a rock. It is defined as the ratio of the volume of the voids or pore space divided by the total volume.

Grain density
1. Dolomite, ρ = 2.8–3.1 g/cm3
2. Shales, ρ = 2.65–2.8 g/cm3
3. Sandstones, ρ = 2.65–2.80 g/cm3

### Which logging tool can detect porosity directly?

Nuclear magnetic resonance. Nuclear magnetic resonance (NMR) logging uses the NMR response of a formation to directly determine its porosity and permeability, providing a continuous record along the length of the borehole. The chief application of the NMR tool is to determine moveable fluid volume (BVM) of a rock.

### What do resistivity logs measure?

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.

### What does a neutron density log measure?

9.2. 6 Neutron Log. The neutron logging tool measures the amount of neutrons scattered (post-1960s tools) or GRs emitted (older tools) from the formation after it is exposed to a neutron source. The neutrons are emitted from a radioactive source that is held against the borehole wall, as shown in Figure 9.2.

### What does sonic log measure?

Sonic logs measure the travel time of sound through the rock, recorded in microseconds per foot or per meter (abbreviated as usec/ft or usec/m, sometimes us/ft or us/m). The tool emits a sound pulse about once or twice per second from a transmitter.

### How does sonic log calculate porosity?

An empirical equation is used for calculating porosity in sandstones in which adjacent shale values (Δtsh) exceed 100 μsec/ft (Eq. 6):
1. Δtsh = specific acoustic transit time in adjacent shales (μsec/ft),
2. 100 = acoustic transit time in compacted shales (μsec/ft)

### Why is gamma rays high in shale?

In particular, shales usually emit more gamma rays than other sedimentary rocks, such as sandstone, gypsum, salt, coal, dolomite, or limestone because radioactive potassium is a common component in their clay content, and because the cation exchange capacity of clay causes them to adsorb uranium and thorium.

### What is cycle skipping in sonic log?

cycle skip. An anomalously high transit time in a log, such as a continuous velocity log, observable as a spike on the log, commonly caused by the presence of fractures, gas, unconsolidated formations, aerated drilling mud and enlarged boreholes.

2.65 g/cm3

### What is the density of the log?

The density log usually labelled 'RHOB' measures the density of the borehole and the rocks penetrated by the drill bit. The unit for density is gram per cubic centimetre. The logs (sonic and density) measurements are usually made every 6 in.

### How do you calculate bulk density?

Bulk density, or dry bulk density, is a property of soil obtained by dividing the mass of solids in the soil by the total volume. Wet bulk density, on the other hand, is obtained by dividing the mass of water plus the mass of solids by the total volume.