The Accelerator Porosity Sonde (APS) uses a powerful electronic neutron source instead of a chemical source. The large neutron source yield allows epithermal neutron measurements and detector shielding, resulting in porosity values that ate less influenced by environmental conditions. Five detectors provide information for porosity evaluation, gas detection, shale evaluation, improved vertical resolution, and borehole corrections.

With the APS, more accurate porosities are determined by counting neutrons in five detectors, four epithermal and one thermal, and by using a pulsed source of high-energy neutrons. This new configuration provides several advantages with respect to the older tool (CNT-G). The new source is safer and has a higher neutron yield, allowing for epithermal measurements that are less sensitive to lithologic effects on neutron porosity, such as those resulting from the presence of shales and dolomites. a higher neutron yield, combined with better wall contact due to an additional eccentralizing bow-spring, reduces the effects of borehole irregularities. In addition, the pulsing of the neutron source provides for two new measurements: (1) an epithermal neutron slowdown time that is used to evaluate the tool standoff from the borehole wall and correct the porosity measurement; and (2) a formation thermal neutron cross-section (sigma) that is a useful indicator for the presence of elements of high thermal neutron capture cross-section, such as B, Cl, and rare earth elements commonly associated with shales and possibly dolomites. The APS provides two epithermal porosity measurements: one is corrected for borehole conditions and tool standoff and is nearly independent of lithology effects, whereas the other is similar to that obtained from the CNT-G.

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).

Enviromental Effects

Eccentralization of the tool by a bow spring is the most important requirement to obtain reliable porosity measurements. The Triple Combo string utilizes an in-line eccentralizer to accomplish consistent contact with the borehole wall. The eccentralizer is vital in preventing poor contact of the tool with the borehole wall which can lead to 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 ODP, 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.

Log Presentation

The APS is recorded in linear porosity units for a particular lithology (limestone, sandstone, dolomite). The Near/Array Limestone Porosity Corrected (APLC) is usually displayed. When APS is run in combination with the lithodensity and spectral gamma ray tool the porosity and density curves are usually displayed in the same track with Gamma Ray and Caliper curves in a separate track.

Specifications

Temperature Rating	350° F (175° C)
Pressure Rating	20 kps
Tool Diameter	3 5/8 in (9.2 cm)
Tool Length	13 ft (3.96 m)
Sampling Interval	6 in (15.24 cm)
Max. Logging Speed	1,800 ft/hr
Vertical Resolution	2 in (5.08 cm)