Neutron-density log

MWD Technology 901. Directional Drilling Parameters 954. Safety Parameters 961. LWD Technology 971. Gamma and Ray Logs 971. Resistivity Logs 974. Neutron-Density Logs 985. 

A problem with the early MWD mud pulse systems was the very slow rate of data transmission. Several minutes were needed to transmit one set of directional data. Anadrill working with a Mobil patent [100] developed in the early 1980s a continuous wave system with a much faster data rate. It became possible to transmit many more drilling data, and also to transmit logging data making LWD possible. Today, as many as 16 parameters can be transmitted in 16 s. The dream of the early pioneers has been more than fulfilled since azimuth, inclination, tool face, downhole weight-on-bit, downhole torque, shocks, caliper, resistivity, gamma ray, neutron, density, Pe, sonic and more can be transmitted in realtime to the rig floor and the main office. 

A typical set of logs recorded while drilling is shown in Figure 4-280. The wireline caliper is shown in the gamma ray track. Displayed on this attachment are gamma ray, R curve, Pe curve, neutron and density curve. The delta-rho curve is the quality curve check for the density log. 

The true porosity <1> is determined with the neutron-density Pe logs. R is generally given by the deep investigation resistivity curve. R equals R, in the water formations. It increases rapidly in hydrocarbon saturated formations. 

Figure 4-301 shows sample neutron-density Pe log over the same interval. Figure 4-302 shows sample MWD resistivity (left) and wireline dual induction (right) for the same interval. 

Porosity and Lithology Determination from Litho-Density Log and CNL Compensated Neutron Log... 

Figure 4-303. Porosity and lithology determination from the Litho-Density logs and the CNL Compensated neutron log. Courtesy Schlumberger.) ( trade mark of Schlumberger.)...

The Vsh and R h figures can be obtained from induction-, neutron- and formation-density logs by cross-plot techniques described in Schlumberger Well Surveying Corp. (1978) and references cited therein. 

Porosity and permeability core analysis data for the sampled well were made available to the authors by Elf (99 data points from the interval under investigation). Core porosity data have an uncertainty of less than 0.5%, which arises from the variable amount of stress relaxation following withdrawal of the core from the subsurface. Analytical errors are insignificant. Sonic transit time, neutron density, density and other wireline data recorded at 5 cm intervals by petrophysical logging methods were also made available by Elf These data were used to derive porosity and mineral proportions using methods outlined by Doveton (1994) and Hearst Nelson (1985). 

The signals from the sonic transit time, neutron density and density logs can be integrated and resolved for three mineral types and total porosity using three algorithms relating each separate log... 

Neutron density recorded by log (porosity units) Neutron density of mineral X (porosity units) Neutron density of fluid in pore space (porosity units)... 

Table 2. Petrophysical response characteristics of quartz, dolomite, shale and the pore fluid as used to calculate the mineralogy from neutron density, sonic transit time and density logs...

Sonic transit time, neutron density and density log data for the cored interval are presented as functions of depth in Fig. 5. The same data are cross-plotted in Fig. 6 with the positions of the three minerals added. Equations (l)-(4) can be solved for porosity plus three solid-grain components. The logs have been converted into fractional porosity and the fractional quantities of quartz, dolomite and shale. The rock was thus assumed to consist of three minerals quartz (all silica minerals and feldspar), dolomite (all carbonate minerals) and shale (all clay minerals). Each group of minerals has approximately uniform responses to the three wireline logging tools. Petrographic analysis shows that the quartz/feldspar ratio is greater than about three (Table 3), suggesting that the assumption about the quartz component is... 

Fig. 17. Wireline log characteristics, smoothed acoustic impedance curve and 3D seismic response over the Upper Angel Formation at Angel-2. Note that the major dolomite-cemented zones (black bars) are identifiable on the basis of neutron, density, resistivity and sonic log profiles. The zones appear as discrete layers at this location, with a cumulative thickness of 164 m, and are not fully cemented but contain some residual porosity. The dolomite-cemented zones occur both above and below the gas-water contact (GWC). The smoothed acoustic impedance curve shows that the zones produce a visible seismic response which is mappable. For an example of a line through the 3D seismic volume see Ryan-Grigor Schulz-Rojahn (1995 their Fig. 10a,b).

The MES relies on a superposition of a given number of canonical events, each of them being defined by a neutron irradiation on the 56Fe seed nuclei during a time f rr at a constant temperature T and a constant neutron density Nn. In contrast to the canonical model, no hypothesis is made concerning any particular distribution of the neutron exposures. Only a set of canonical events that are considered as astrophysically plausible is selected a priori. We adopt here about 500 s-process canonical events covering ranges of astrophysical conditions that are identified as relevant by the canonical model, that is, 1.5 x 108 < T < 4 x 108 K, 7.5 < log./Vn[cm 3] < 10, and 40 chosen t rr-values,... 

Fig. 2. Gamma-ray, neutron and density logs from the East Ford Unit 41 R well. The neutron log shows a gas effeet in the lower Ramsey 1 sandstone below the uppermost calcite-cemented layer.

Density, neutron, NMR logs if dry clay parameters used to derive porosity... 

Shale content Preferred are shale content calculations based on gamma-log and/or neutron-density combination. 

Porosity. Two porosity logs are run today (1994) namely, neutron and density. Soon the sonic will also be available. 

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. 

Well logging Electrical surveys resistivity conductivity shale formation factor salinity variations Interval transit time Bulk density Hydrogen index Thermal neutron capture cross section Nuclear magnetic resonance Downhole gravity data After drilling... 

Porosity data can be obtained from laboratory measurements on sidewall samples and core samples from boreholes (e.g. Monicard, 1981), from geophysical well logs (sonic, density and neutron logs Serra, 1987) or from seismic data. 

Fluid saturation, which defi nes oil-water contact, was determined using wireline log interpretation and RFT data. Shaliness was obtained through density/neutron log data and the salinity of the formation water was analysed from RFT sampled water. Additional parameters used in the calculation, such as saturation and cementation exponents (n and m, respectively), and tortuosity coefficient (a), were measured in the laboratory. 

Fig. 8. Continuously cored interval with high recovery (100%) of the Namorado Sandstone composed of numerous individual turbidite layers. Observe that density, neutron and sonic logs clearly show decreasing porosity values below the oil-water interface. Arrows indicate the strongly cemented sandstones. Note also that type II calcite commonly occurs in the oil zone, whereas type III dominantly appears below the oil-water contact.

Density, neutron and sonic logs show a distinct shift below the oil-water interface identifying reduced porosity because the frequency of calcite-cemented beds is greater below than above the interface. We also note that type II calcite occurs mostly above the oil-water contact, whereas type III calcite appears below this limit (Fig. 8). Abreu et al. (1992) suggested that the interval below the water-oil interface was continuously cemented by calcite after oil had filled the crest structure. 

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. 

Nuclear logging using gamma radiation, to measure the bulk density, and neutron radiation, to measure water content, are techniques used both at sea and ashore. Preiss (1968) and Richards and Chaney (1997) describe how these techniques may be used for marine sediments both on core samples and in situ. These in-situ methods are particularly apph-cable to near-surface sediments which can be extremely porous and tend to suffer the greatest amount of disturbance when sampled. It is difficult using these techniques to obtain accuracies better than 1%, due to the problems of cahbrating the instruments with specimens of different chemical and mineralogical compositions. 

If all the parameters on the right side are known with confidence, the porosity can be determined. This method was applied successfully in some field applications. Chin et al. (1986) gave an invasion porosity log that showed both qualitative and quantitative agreement with conventional neutron and density porosity logs. In the paper, the authors determined the asterisked properties of the mud using API filtration tests and obtained q-(t) from standard resistivity... 

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