Monday, January 21, 2019

Neutron Porosity Logging Revisited

A downhole accelerator, updated source-to-detector spacings, and greater detector efficiency are key features of the new IPL Integrated Porosity Lithology tool that enhance the accuracy of formation evaluation, especially neutron porosity measurements.

The IPL tool's reduced sensitivity to clays and dolomitazion, and improved vertical resolution make it ideal for evaluating thin beds, shaly sands, mixed-lithology carbonates and micaceous sands. Epithermal neutron porosity measurements- not sensitive to fluid salinities- can be made without a chemical source, and they can be corrected for tool standoff.

The tool consists of three sondes:
  • The Hostile Environment Natural Gamma Ray Sonde (HNGS) determines thorium, uranium and potassium concentrations with better accuracy, precision and reduced borehole effects than previous tools. It can log twice as fast, thanks to bismuth germanate crystal scintillation detectors, which are more efficient than conventional sodium iodide detectors.
  •  The Accelarator Porosity Sonde (APS) measures epithermal neutron ratio porosities, the formation capture cross section (sigma), and the epithermal neutron slowing-down time using one thermal and four epithermal neutron detectors. The pulsed 14-million electron volt (MeV) accelerator generates neutrons with enough energy to allow epithermal neutron detection and improves wellsite safety by eliminating the need for a radioactive source.
  • The Litho-Density sonde measures compensated bulk density and photoelectric factor. Magnetic shielding for the photomultiplier tubes and high-speed electronics improve the stability of the measurement.

The most innovative part of the IPL tool string is the APS sonde, in which accelartor replaces the chemical neutron source, and the detector geometry is altered to reduce lithology effects, increase sensitivity to gas in shaly reservoirs and reduce borehole effects. It also provides a neutron porosity measurement with vertical resolution comparable to density and resistivity measurements. Understanding the impact of the improvements requires a brief discussion of the neutron porosity measurement.

Traditional neutron porosity tools, such as the CNL Compensated Neutron Log tool, emit 4.5-MeV average energy neutrons from a radioactive source. These neutrons are detected after losing energy in billiard-ball type elastic collisions with formation nuclei. Generally, the count rate at neutron detector is inversely proportional to the amount of hydrogen in the formation. When the hydrogen content is high, many neutrons are slowed and captured- the count rate will be low and porosity high. When the hydrogen content is low, fewer neutrons are absorbed and more reach the detector. The count rate will be high and porosity low.

 Conventional neutron porosity devices measure the ratio of neutrons counted by a detector close to the neutron source (the near detector) to those counted by one farther from the source (the far detector). This near-to-far ratio is less sensitive to environmental effects than the count rate from a single detector. The ratio is converted to porosity using laboratory calibrations and expressed on logs in porosity units (p.u) for a limestone matrix.

Interpretation can be complicated by three factors: formation atom density, clays and gas. An increase in formation atom density, which relates to the matrix density of the formation, increases neutron scattering. This reduces the number of neutrons reaching the detector and elevates the measured porosity. In clays, the additional hydrogen content of hydroxyls increases the apparent porosity reading. The combined boost in porosity readings from these two factors is called the shale effect. Another phenomenon, the gas or excavation effect, caused reduced or even negatif porosity readings. It occurs when some pore space contains gas, which contributes far less hydrogen to scatter neutrons than does water. Consequently, the count rate is higher and measured porosity lower. 

There are two types of neutron porosity detectors- epithermal and thermal- named for the energy levels of the neutrons they detect. An epithermal detector counts neutrons with energies from a few tenths  of an eV to approximately 10 eV; a thermal detector counts neutrons with energies around 0.025 eV. Thermal neutron detectors have higher count rates, and so better counting statistics, than epithermal detectors. However, elements in the formation such as chlorine or boron can capture thermal neutrons, causing an inflated apparent porosity reading by lowering count rates. Epithermal neutrons, on the other hand, will not be captured, so an epithermal porosity sonde gives truer readings. The challenge in epithermal neutron porosity detection has been to develop a source that produces enough high-energy neutrons to ensure statistically meaningful count rates.

The APS sonde combines the best of both epithermal and thermal neutron techniques by using an accelarator. The accelerator emits eight times as many neutrons with three times as much energy as the conventional logging source. The increased neutron population makes epithermal neutron detection possible without compromising counting statistics.

Like previous epithermal neutron porosity tools, the APS sonde contains near and far epithermal detectors. It has two additional epithermal detectors, called the epithermal array, and a thermal detector.

It has two additional epithermal detectors, called the epithermal array, and a thermal detector. 

The main function of the APS sonde is to measure the formation hydrogen content with minimal influence from the formation atom density.  Adding and rearranging detectors reduce the tool's sensitivity to formation atom density. A plot of epithermal neutron population versus source-to-detector spacing shows how hydrogen content and formation atom density affect count rates. In the lower part of the figure, for a fixed formation atom density, the epithermal neutron population decreases when porosity increases. In the upper part of the figure, for fixed hydrogen content and a relatively large distance from the source, the epithermal neutron population decreases as the formation atom density increases. 

At short source-to-detector spacings, the effect is reversed because neutron backscattering dominates. At intermediate source-to-detector spacings, in the crossover zone, the detector is not sensitive to formation atom density. Characterizing the epithermal neutron population in this way guided the placement of the near, array and far detectors shown at the bottom of the figure.

In practice, the near-to-array measurement, which has a vertical resolution of 1 ft [30 cm] , is used to determine formation porosity. The near-to-far measurement, which exhibits greater shale and gas effects, gives a response similar to that of the CNL tool. When the density measurement is not used in clean formations, comparing the two responses identifies  the gas effect on the near-to-far reading, and flags gas-bearing beds. In shaly formations, the additional boost in apparent near-to-far porosity caused by the increased atom density of clay mineral is used to improve the evaluation of clays.

The epithermal array detectors are used to monitor and correct the effects of tool standoff. The thermal detector  determines sigma by detecting neutrons rather than gamma rays, as with conventional pulsed neutron tools. This, as well as detector shielding from the borehole, improves vertical resolution and provides a sigma value relatively free from borehole effects. 

A log example from Rogers County, Oklahoma, USA shows how the IPL tool string improves formation evaluation in shaly sand reservoirs. The left track contains sigma and tool standoff from the APS sonde and the uranium-free gamma ray log from the HNGS sonde. The right track contains the near-to-array and near-to-far porosities; the bulk density and long-spaced photoelectric effect from the Litho-Density tool; and for comparison,a CNL thermal neutron porosity log made during a separate run. In the left track, the IPL sigma curve shows good correlations with the uranium-free gamma ray curve at bed boundaries. The computed tool standoff reads close to zero over the section, indicating good tool string eccentralization and borehole integrity.

The near-to-array porosity and gamma ray logs indicate a gradual upward decrease in clay across two shale intervals, from 730 to 685 ft and 668 to 655 ft. The near-to-far porosity log reads 4 to 8 p.u. higher than the near-to-array reading in the shales, which is to be expected because of the clay's increased atom density. 

Across the same intervals, the CNL thermal neutron porosity log reads higher than the near-to-array porosity (an epithermal measurement) because of thermal absorbers in the formation and increased formation atom density. The effect of thermal neutron abosrbers is quantified by the APT Accelerator Porosity Tool formation sigma measurement, which is 26 to 40 capture units (c.u.) over the shale intervals. A correction factor of about -6 p.u. must be applied to the CNL porosity curve to account for the additional thermal absorption, using published sandstone charts. After such a correction, the CNL porosity would be close to the APS near-to-far porosity.

A second log example, recorded in clean sands, shows how the APS sonde can detect gas without the use of a radioactive source. This is especially desirable for wells in which mudcake or borehole rugosity compromise the quality of shallow density measurements, or where tool sticking is a problem.

The IPL curves shown in the right track are density, the near-to-array neutron porosity plotted on a limestone compatible scale and the near-to-far neutron porosity. The difference between the porosity measurements (blue shading) corresponds to the gas effect and correlates well with the conventional density-neutron separation indicating gas (pink shading). In shaly sands, this technique does not work because the increased formation atom density of shales obscures the gas effect from the far detector. Instead, an APS porosity-sonic overlay, which also correlates with a neutron-density overlay, can identify gas.

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