Geology IN_ Petrophysics_ The study of physical and chemical rock properties.pdf

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17/11/2014 Geology IN: Petrophysics: The study of physical and chemical rock properties.

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Petrophysics: The study of physical and chemical rock properties.

Is the study of physical and chemical rock properties and their interactions with fluids.A major application of

petrophysics is in studying reservoirs for the hydrocarbon industry. Petrophysicists are employed to help reservoir

engineers and geoscientists understand the rock properties of the reservoir, particularly how pores in the subsurface

are interconnected, controlling the accumulation and migration of hydrocarbons. Some of the key properties

studied in petrophysics are lithology, porosity, water saturation, permeability and density.

Petrophysics

Akey aspect of petrophysics is measuring and evaluating these rock properties by acquiring well lo g measurements

- in which a string of measurement tools are inserted in the borehole, core measurements - in which rock samples

are retrieved from subsurface, and seismic measurements. These studies are then combined with geo logical and

geophysical studies and reservoir engineering to give a complete picture of the reservoir.

While most petrophysicists work in the hydrocarbon 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.

Petrophysical studies are used by petroleum engineering, geology, mineralogy, exploration geophysics and other

related studies.
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Most petrophysicists are employed to compute what are commonly called conventional (or reservoir) petrophysical

properties. These are:

A description of the rock's physical characteristics, such as grain size, composition and texture. By

studying the lithology of local geological outcrops and core samples, geoscientists can use a combination of log

measurements, such as natural gamma, neutron, density and resistivity, to determine the lithology down the well.

The percentage of a given volume of rock that is pore space and can therefore contain fluids. This

is typically calculated using data from an instrument that measures the reaction of the rock to bombardment by

neutrons or by gamma rays but can also be derived from sonic and NMR logging.

The fraction of the pore space occupied by water. This is typically calculated using data from

an instrument that measures the resistivity of the rock and is known by the symbol .

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.

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 Earths 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.

Conventional petrophysical properties

Lithology:

Porosity:

Water saturation:

The quantity of fluid (usually hydrocarbon) that can flow through a rock as a function of time and

pressure, related to how interconnected the pores are. Formation testing is so far the only tool that can directly

measure a rock formation's permeability down a well.

Permeability:

Thickness

Reservoir models

Methods of analysis

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 thewells drilled in a field.

Coring


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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.

is used as a relatively inexpensive method to obtain petrophysical properties downhole.Measurement tools are conveyed downhole using either wireline or LWD method.Well Logging

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.
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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 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.

GEO ExPro MagazineGeoscience Explained! Geology, Geophysics & Reservoir Management
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Geology IN_ Petrophysics_ The study of physical and chemical rock properties.pdf