In the Dual Porosity Compensated Neutron log (CNT-G) a radioactive source mounted on the sonde emits fast neutrons which are scattered and slowed down by collisions with the nuclei in the formation. Whenever they reach the "thermal" energy level they are captured by the nuclei of atoms such as hydrogen, chlorine and silicon, and gamma rays of capture are emitted. Two pairs of detectors measure both epithermal (intermediate) and thermal (slow) neutrons. The epithermal detectors are spaced closer to the source than the thermal detectors in order to maintain a good statistical precision in the count rates. A new data processing method utilizing the individual count rates rather than their ratios is used to derive porosity. This technique minimizes the environmental effects on the response of the epithermal detectors due to their closer spacing from the source and provides better porosity measurements in shaly formations.

Applications

Porosity.

In reservoir engineering its importance is quite evident; in the study of the volcanic rocks that make up the upper oceanic crust, a good in-situ porosity measurement is most important to the correct understanding of the crustal structure. First, because it samples both the small-scale (microcrack, vesicle) porosity seen in the cores and the large-scale fractures not sampled by drilling, and secondly because other properties such as density, seismic velocity, and permeability, depend strictly on porosity variations and on the geometry of the pore space. In the presence of clays or hydrous alteration minerals a correction is required to account for the presence of bound water.

Lithologic determination.

Because the hydrogen measured by the tool is present not only as free water but also as bound water in clay minerals, the porosity curve, often combined with the density log, can be used to detect shaly intervals, or minerals such as gypsum, which has a high hydrogen index due to its water of crystallization. Conversely, the neutron curve can be used to identify anhydrite and salt layers (which are both characterized by low neutron readings and by high and low bulk density readings respectively).

Limitations

Environmental Effects.

Eccentralization of the tool by a bow spring is the most important requirement to obtain reliable porosity measurements. This is not routinely performed in ODP boreholes because of the increased risk of getting the tool stuck in the drill pipe. The lack of contact of the tool with the borehole wall during the recording results in the attenuation of the formation signal by the borehole fluid and, in turn, the overestimate of the true porosity of the formation.

Hole size also affects the neutron log response; the formation signal, particularly for the epithermal count rates, tends to be masked by the borehole signal with increasing hole size.

In liquid-filled holes the influence of the borehole fluid depends on its salinity - chlorine is a strong neutron absorber - and density: the addition of weighting additives such as barite will yield a lower porosity reading.

In the Ocean Drilling Program, the neutron tool is sometimes recorded through the drilling pipe and the bottom hole assembly. Because iron is a strong neutron absorber, the effect will be of an increased porosity reading, depending on the thickness of the pipe.

Depth of Investigation and Vertical Resolution.

The depth of penetration of the neutrons is inversely related to the porosity of the formation, but also depends on the source-detector spacing. In general we can say that for porosity ranging from 0 to 30 % the depth of investigation varies from 2 ft (61 cm) to about 6 in (15 cm). The vertical resolution is 1.5 ft (46 cm).

Log Presentation

The CNT-G is recorded in linear porosity units for a particular lithology (limestone, sandstone, dolomite). The thermal porosity curve (NPHI or TNPH) is usually displayed. When CNT-G is run in combination with the lithodensity and spectral gamma ray tool the neutron and density curves are usually displayed in the same track with Gamma Ray and Caliper curves in a separate track.

Tool Specifications

Temperature Rating	400¡ F (205¡ C)
Pressure Rating	20 kpsi (13.8kPa)
Tool Diameter	3 3/8 in (9.21 cm)
Tool Length	16.6 ft (5.06 m)
Sampling Interval	6 in (15.24 cm)
Max. Logging Speed	1,800 ft/hr
Vertical Resolution	1.5 ft (46 cm)
Depth of Investigation	see "Limitations"