Neutron porosity

Because the neutron tool responds to hydrogen it can be used to differentiate between gas and liquids (oil or water) in the formation. A specific volume of gas will contain a lot fewer hydrogen atoms than the same volume of oil or water (at the same pressure), and therefore in a gas bearing reservoir the neutron porosity (which assumes the tool is... 

Yes, the curve increases sharply in the main zone at 8425 ft. Oil from 8425 to 8400 ft. Gas above 8,400 ft. Gas is indicated by a density porosity larger than the neutron porosity. 

For the neutron porosity measurement, fast neutrons are emitted from a 7.5-curie (Ci) americium-beryllium (Am-Be) source. The quantities of hydrogen in the formation, in the form of water or oil-filled porosity as well as crystallization water in the rock if any, primarily control the rate at which the neutrons slow down to epithermal and thermal energies. Neutrons are detected in near- and far-spacing detectors, located laterally above the source. Ratio processing is used for borehole compensation. 

The caliper is used to correct the density and neutron porosity measurements for borehole effects and also can be used as a borehole stability indicator. Figure 4-289 shows an example of comparison between the MWD ultrasonic caliper and the four-arm wireline caliper run five days later. 

With the lithology matching the log scale, and assuming the formation fully invaded by mud filtrate, a neutron porosity and a density porosity can be determined. 

If the neutron porosity is low and the density porosity is high, the formation contains gas and the true porosity can be determined with charts. 

A high neutron porosity and low density porosity occur in the shale zones 2256-2274 and 2356-2388 ft. In the clean zone, around 2280-2290 ft, the cleanest part reads = 18% and = 13%. Plotting the point in the CNL chart of Figure 4-303, the rock matrix appears to be a dolomitized limestone and the true porosity is 17%. 

CNLcor, NEUTRON POROSITY INDEX (pu) (Apparent Limestone Porosity)... 

Neutron. The neutron porosity values recorded with most tools match closely the wireline thermal neutron logs in the lower porosity ranges (under 25 porosity units). In high porosity zones, the LWD neutron porosities lie between thermal and epithermal wireline values. 

Neutron porosity For the neutron log, the hydrate response is nearly the same as liquid water. Hydrate is 6% higher than fresh water, somewhat more than that for salt water. Both differences... 

In both fields, integration of core data with wireline logs shows that carbonate-cemented zones produce marked increases in bulk density and resistivity readings, and are characterized by much lower neutron porosity values and sonic travel times than vertically adjacent reservoir zones that lack significant carbonate cement. 

Thus, we can conclude that elastic scattering, as the most common interaction, is dominated by hydrogen (H). Hydrogen content controls the strength of elastic scattering this is the background for correlation with water content or neutron porosity . But the other elements also interact with neutron radiation this results in a matrix effect for neutron porosity determination. 

Because in reservoir rocks, the dominant elastic scattering is controlled mainly by hydrogen, such a system gives information about the hydrogen concentration , which is controlled by water or hydrocarbons in the formation. This has created the term neutron porosity . The following section will discuss the conditions of neutron-derived porosity. 

If the neutron response of the rock components is expressed as neutron porosity then pure limestone (calcite) has and freshwater has... 

TABLE 5.16 Neutron Response of Some Rock Neutron Porosity Components, Expressed as ... 

Shale indicated by high neutron porosity and gamma Limestone, S = 1 shows exact porosity... 

Sandstone, S = 1 shows lower Neutron porosity Dolomite, S = 1 shows higher Neutron porosity Snndstone shows lower porosity (gas effect)... 

Shale indicated by high neutron porosity and gamma... 

Neutron porosity is an important component for crossplot techniques and combined mineralogy-porosity calculation in order to give an estimate for matrix composition and porosity (see Section 5.6). 

COMBINATION OF GAMMA-GAMMA DENSITY AND NEUTRON POROSITY—MULTIPLE POROSITY METHODS... 

Given are measured input parameters (for example density, neutron porosity, velocity of elastic waves). 

Porosity is a key parameter for reservoir studies. Porosity can be calculated from gamma-gamma-density and neutron porosity if the matrix and fluid properties are known. This is particularly problematic for reservoir rocks with a composite matrix. A typical example is a carbonate as a mixture of calcite and dolomite then the equations for the measured magnitudes are ... 

Shale content shifts the points from the sand line to the right (neutron porosity increases) it shifts apparently towards the dolomite. 

Gas-bearing zones shift the points to the left (neutron porosity decreases) it shifts toward low neutron porosity. 

FIGURE 5.19 Calculated neutron-density crossplot x-axis gives the neutron porosity for a limestone-calibrated tool y-axis gives the bulk density. Lines are plotted for sandstone/quartz (matrix density 2.65 g/cm, matrix neutron response —0.03) limestone (matrix density 2.71 g/cm, matrix neutron response 0.00) dolomite (matrix density 2.86 g/cm, matrix neutron response +0.04) freshwater (density 1.00 g/cm, neutron response 1.00). 

A Bulk density p = 2.540 g cm Neutron porosity =0.100 Pure limestone with porosity < = 0.100... 

C Bulk density p = 2.454gcm Neutron porosity =0.205 mixture of dolomite and limestone with a higher limestone content porosity = 0.190... 

This solution uses measured gamma-gamma density and neutron porosity as input. If the formation is composed of calcite, dolomite and fluid, the set of the linear equations (5.25), (5.26) can be solved for the unknown volume fractions including porosity. 

NPHI— neutron-porosity log (limestone-calibrated neutronlog),... 

Allen, L.S., Mills, W.R., Desai, K.P., Caldwell, R.I., 1972. Some features of dual-spaced neutron porosity logging. In 13-th Annual Logging Symposium Transactions, Society of Professional Log Analysts (SPWLA). Paper P. 

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