Compensated Neutron Dual Porosity Tool (CNT-G*)

Description

The CNT-G tool employed a chemical source (Am-Be) to bombard the borehole and the formation with fast neutrons (4.5 MeV) and two pairs of sensors to detect the number of neutrons (count rates) in the epithermal (100 eV – 0.1 eV) and thermal energy range (<0.025 eV). In the scattering process the neutrons interacted elastically with the atoms in the formation, were slowed down, and lost part of their kinetic energy with each collision; upon reaching the thermal energy level, they were absorbed by the surrounding nuclei. The amount of energy lost by the neutrons depended on the relative mass of the nuclei with which they interacted Because the greatest energy loss occurs during the collision with hydrogen atoms – which have a mass almost equal to that of neutrons – the slowing down and capture processes were mainly controlled by the hydrogen concentration in the formation. By taking the ratio of count rates at each pair of detector, a measurement of the porosity of the formation — compensated for the borehole parameters — was provided.

The response at the thermal detectors could be greatly affected by elements with a large thermal neutron capture cross section, such as chlorine, boron, gadolinium, and samarium; these are usually present in very small quantities in the borehole fluid or in clay or alteration minerals, yet they can cause the porosity of the formation to be overestimated. The epithermal detectors, instead, were less sensitive to these neutron absorbers and provided a more reliable measurement of the true porosity of the formation over clay-rich intervals. Because the epithermal neutron count rate is about one order of magnitude less than that for the thermal neutrons, the detectors were placed closer to the source in order to improve statistical variations.

The CNT-G was deployed extensively during the Ocean Drilling Program.

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 second, 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.

Lithology. Because the hydrogen measured by the tool is present not only as free water but 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 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).

Environmental Effects

Eccentralization of the tool by a bow spring could be very helpful in obtaining reliable porosity measurements. 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 likely overestimation of the true porosity of the formation. In the majority of ODP holes, however, the CNT-G was run without an eccentralizer.

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 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 was sometimes recorded through the drilling pipe and the bottom hole assembly. Because iron is a strong neutron absorber, the effect was an increased porosity reading, depending on the thickness of the pipes.

Log Presentation

The CNT-G provided an epithermal (ENPH) and thermal neutron porosity (NPHI) measurement. The porosity curves were presented either in decimal units or in percents along with the bulk density.

Tool Specifications

Measurement Specifications

The CNT-G was typically run in combination with the density and gamma ray tools.

* ®trademark of Schlumberger