porosity and permeability of a rocks

Porosity: 

Porosity indicates the amount of water that rock or soil can hold.

In the upper few kilometres of the Earth, bedrock and soil contain small cracks that are filled with air or          ground water, these open spaces is called the porosity of rocks or soil.The porosity of sand and gravel is typically high because of the tiny clay particles.

Permeability:

Permeability is the ability of rock or soil to transmit fluids which may be liquid(water) or gas through the interconnected spaces between the particles. Water can flow rapidly through material with high permeability. Most materials with high porosity also have high permeability,it depends on how well the pores are connected and on pore size.

The connections between pores affect permeability         

because the pores, no matter how large, must be

connected for water to flow through rock or soil.

The permeability of many other rocks depends on the density of fractures in the rock

How to Determine the Porosity and Permeability:

A. Porosity

It can be measured in lab as well as in the field.

Lab Measurement

It includes variety of methods but few are most important:

1.Thin Section

In thin section it can either be measured visible estimation of pore spaces or point counting under the microscope, can be inaccurate.

2.Volumetric Method (core/plugs)

In this method two volume out of three (bulk volume, pore volume and matrix volume) is required to determine porosity.

Pore volume can be calculated by saturating the core sample with water/liquid.

The bulk volume can be calculated easily from the dimensions of the core sample, if irregular water displacement technique is applied.

If the pore space volume and bulk volume is known then apply direct method, use the formula.

The matrix volume is calculated when the density of mineral in a rock is know along with the weight of sample. Then subtract the matrix volume from bulk volume.

3.Helium Method/Ideal gas Method

The most correct method of measuring porosity is the helium expansion method.

A dried sample is placed in a chamber of known volume and the pressure is measured with and without the sample, the volume of the gas is kept constant. The difference in pressure is the pore volume

Field Measurements

The field measurements include the interpretation of various petrophysical logs, which are obtained within the field.

There are various types of geophysical logs but most important which are used in formation evaluation w.r.t porosity are as: Sonic Logs, Neutron Logs and Density Logs.

None of these logs measure porosity directly (some calculation involved).

1. Sonic Logs

A log that measures interval transit time (At) of a sound wave travelling through the formation along the axis of the borehole

The acoustic pulse from a transmitter is to detemine at two or more receivers. The time of the first detection of the transmitted pulse at each receiver is handle to produce At

The At is the transit time of the wave front over one foot of formation and is the reciprocal of the velocity

Interval transit time is both depends on porosity and lithology.

Sonic log is usually displayed in track 2 or 3

Units: psec/ft, psec/m Uses

Porosity Identification

Lithological Identification

Synthetic Seismogram.

2. Neutron Logs

Neutron logs measure the hydrogen content in a formation. In clean, shale free formations, where the porosity is filled with water or oil, the neutron log measures liquid filled porosity(^N, PHIN, NPHI).

Neutrons are emitted from a chemical source (americium-beryllium mixture). At collision with the formation nuclei, the neutrons loses energy. With enough collisions, the neutron is absorbed and a gamma ray is emitted.

Since a neutron is a little bit heavier than a proton, the element which closely approximates the mass of a neutron is hydrogen. In  collisions between the neutron & hydrogen the average energy transfers to the hydrogen nucleusis about lA that of the energy originally contained in the neutron. Where as, if the scattering nucleus was oxygen (mass16 amu) the neutron would retain77% of its energy.

Materials with large hydrogen content like water or hydrocarbons
become very important for slowing down neutrons. Since hydrogen
in a porous formation is intensely in the fluid-filled pores,
energy loss can be compared to the formation’s porosity.

Neutron curves commonly displayed in track 2 or 3

Unit: Porosity Unit (P.U), decimal, fraction or Percentage.

Uses

Porosity determination

Lithology Identification

3. Density Logs

A continuous record of a formations bulk density: overall density of a rock including solid matrix and fluid enclosed in the pores (matrix and porosity)

The logging technique of the density is a bombardment of medium to high energy 0.2 to 2 Mev r-rays and measurement of their attenuation b/w the tool source and detectors

Attenuation is a function of Compton scattering;

in dense formation; Compton scattering is extreme, few detectable r-ray reach the tools detectors

while in less dense formation: the r-ray detection is much high

the standard density tool have a source of r-ray, which is usually radio-cesium, which emits rays 662 Kev, radio-cobalt can also be used and the two detectors near and far that records the scattering effects.

Neutron are commonly recorded in track 2 or 3

Unit: Porosity unit (p.u), v/v decimal, fraction and percentages

Formula Formation bulk density (pb) is the function of matrix 

density (pma), porosity and fluid density (pf)

The matrix density and fluid density need to be known.

B. Permeability

The permeability can be determined by Darcy’s Law in the lab (core plugs)

There is no direct method in the field to determine the permeability from logs

Reservoir Rock Pressure

The fluid present in pore of reservoir rock are under certain degree of pressure, called as reservoir rock pressure or fluid pressure or formation pressure or bottom hole pressure.Pressure is the force apply per unit area by the fluids against the walls of the reservoir rock.

Unit: psi (pound per square inch)

Types of reservoir rock pressure

·         Static bottom hole pressure

·         Flowing pressure

·         Differential pressure

·         Original pressure

Static Bottom Hole Pressure

When a producing well is shut down, the reservoir pressure begin to rise, until reached final stage where pressure is maximum called as static bottom hole pressure.

This pressure is lower than the original reservoir pressure (natural pressure)

The difference between the shut down pressure and original pressure is the decline in reservoir pressure.

Flowing Pressure: pressure while the well is producing.

Differential Pressure: difference between flowing pressure and

static pressure

Original Pressure: pressure of reservoir rock before any production.

Reasons for Reservoir Rock Pressure

·         Hydrostatic Pressure

·         Lithostatic Pressure

1. Hydrostatic Pressure

Water, which are at rest within sediments, exerts pressure on rock same in all directions

Depends upon the density

Hydrostatic pressure = height of water column x density x gravity Hydrostatic Pressure gradient

Depth increases; pressure increases, normally the hydrostatic gradient ranges from 0.434 psi/foot in pure water to 0.5 psi/foot in brine solution

2. Lithostatic Pressure

Pressure exerted by overburden of rock

The lithostatic pressure and hydrostatic pressure are inter-related

Abnormal Hydrostatic Pressure

This is the deviation from the normal pressure gradient, depend upon certain geologic factors

It was first named as “geopressure” replaced by over pressure and under pressure terminology, these pressure can only be possible when the reservoir is sealed in impermeable layer called as pressure compartment.

Over pressure: when there is deviation in pressure higher then normal estimated reservoir pressure. This is caused by uplift, Rock compaction, increase in heat flow.

Under pressure: when there is deviation in pressure lower then normal estimated pressure of reservoir. This is caused by burial, decrease in heat flow.