[20/03/2007] NUCLEAR TOOLS GENERAL

One of the most important parameters required for evaluation of a potential oil or gas reservoir is porosity. Porosity is defined as the pore volume per unit volume of formation. In addition to the resistivity measurements discussed in the previous section, knowledge of the amount of pore space within the formation is required for accurate formation evaluation.

Several different types of tools have been developed to evaluate formation porosity compensated density, litho-density with Pe measurement compensated neutron, and BHC sonic tools. Although all four respond to formation porosity, each responds in a different manner. Therefore, combinations of two or three types of porosity tools provide much more information about porosity, lithology, and rock properties. The combination of density and neutron tools can distinguish between oil and gas in the pore space in known lithologies. The combination of litho-density and compensated neutron log remove ambiguity about gas effect or lithology effect in unknown lithologies.

The combination of density and sonic data is used to produce synthetic seismic traces so that seismic data may be correlated to the well log data and depth points assigned to that well in the seismic data view.

The data from the resistivity logs and several porosity logs can be combined so that complex analytical computations can be completed giving you a rapid and accurate evaluation of your well on site. The following descriptions are of tools used for porosity determination.

THEORY OF COMPENSATED NEUTRON (CNT)

The CNT uses a radioactive source to bombard the formation with high-energy neutrons. The high-energy neutrons are slowed to thermal energy by the formation. For the most part, this change in energy level is caused by collision with hydrogen atoms. Two proportional detectors count the slow (thermal) neutrons deflected back to the tool. The use of two detectors provides compensation to overcome many of the limited of other neutron tools. As a result, the CNT is less influenced by borehole effects.

Tool readings depend mostly on the hydrogen content of the formation - a function of the amount of water and / or hydrocarbons in the pore space.

Since gas has less hydrogen per unit volume than either oil or water, the CNT differentiates between gas zones and those filled with water and / or oil. This characteristic makes the CNT an excellent gas indicator when its readings are compared with those of other porosity tools.

The CNT is used only in liquid filled open or cased-holes.

The principal uses of the CNT are :

> For porosity analysis, to provide data for computation of water solution.
> For lithology, identification when combined with other logs.
> To aid gas determination when combined with other porosity logs. As a shale volume indicator.

COMPENSATED NEUTRON (9073)

The Compensated Neutron Tool (CNT-9073) in combination  with other sensors at same tool package.

The Compensated Neutron tool contains a two detector, neutron system which uses a 5.0 Curie, Am241Be source to record neutron porosity of the formation. Additionally, the tool also records short and long neutron counts, natural gamma and a deep guard resistivity.

Transmission of the data to the surface is accomplished with the use of a  digital telemetry which encodes the data for transmission and time multiplexes the signals with other sensors.

Specifications
 

Offset Parameter

Operation Parameter

THEORY OF LITHO DENSITY (LDT)

The Litho Densty Tools provides measurements of formation density, and in addition, it also treasured the photoelectric absorption index expressed a photoelectric factor (PEF) of the rock

As a result the PEF measurement depends predominately on the atomic number of the material composing the formation. Consequently, PEF is closely related to the Iithology of the formation and allows better evaluations in intervals of unknom Iithology and complex lithology

The LDT is normally run in combination with the CNT and GR to provide determination of

>> Effective Porosity
>> Lithology
>> Gas Zones
>> Mineral identification

COMPENSATED DENSITY (CDT-9039)

The compensated Density TooI measures the density of the formations panetrated by the borehole. The term 'bulk density is used because the density measured includes the density of the fluid in the pore spaces as well as the grain density of the rock The CDT utilizes two defectors to compensate bulk denstiy mesurements for mud-cake and hole rugosity. A compensation curve is distalayed on the log to indicate the amount of correction applied to the recorded bulk density The density tool is held against one side of the borehole by a spring, which also served as a caliper. The CDT is an excellent for measuring porosity for any gas or liquid Filled uncased hole.

The 9039 tool contains a two detector, focused density system using a 1Ci Cesium 137 source, to record apparent bulk density of the formation.

Transmission of the data to the surface is accomplished with the use of a  digital telemetry which encodes the data for transmission and time multiplexes the signals with other sensors.

Specifications
 

Offset Parameter

Operation Parameter

The principal uses of the CDT are :

• For porosity analysis, to provide data for water saturation calculations

• For Iithology identification when combined with other porosty logs

• To aid in gas determination when refrained with a neutron log

• Provides a caliper measurement