- Formation evaluation neutron porosity
In the field of formation evaluation, porosity is one of the key measurements to quantify an oil and gas reserves. Neutron porosity measurement employs neutron source to measure hydrogen index in a reservoir, which is directly related to porosity. Hydrogen Index (HI) of a material is defined as the ratio of the concentration of hydrogen atoms per cm3 in the material, to that of pure water at 75oF. As hydrogen atom is present in both water and oil filled reservoir, measurement of its amount allow estimation of the amount of liquid-filled porosity.
Neutrons are typically emitted by a chemical source such as Americium Berillium (Am-Be) or Plutonium Berrilium (Pu-Be), or generated by electronic neutron generators such as minitron. Fast neutrons are emitted by these sources with energy ranges from 4 MeV to 14 MeV, and inelastically interact with matters. Once slowed down to 2 MeV, they start to scatter elastically and slow down further until the neutrons reached thermal energy level of about 0.025 eV. When thermal neutrons are absorbed, gamma rays are emitted. Suitable detector, positioned at certain distance from the source, can measure either epithermal neutron population, thermal neutron population, or the gamma ray emitted after the capture.
Mechanics of elastic collisions predict that the maximum energy transfer happened when collisions happens between two particles having equal mass. Consequently, neutron slowing down is most affected by hydrogen atoms (H) because it has the mass very close to a neutron. As hydrogen is fundamentally associated to water and oil present in the pore space, measurement of neutron population within the investigated volume is directly linked to porosity.
Determination of porosity is one of the most important uses of neutron porosity log. Corrections for lithology, borehole parameters, and others are necessary for accurate porosity determination as follow:
# Borehole size
# Borehole salinity
# Borehole temperature and pressure
# Mud cake
# Mud weight
# Formation salinity
# Tool standoff from borehole wall
Subject to various assumptions and corrections, values of apparent porosity can be derived from any neutron log. One can not underestimate the slowing down of neutrons by other elements even if they are less effective. Certain effects, such as lithology, clay content, and amount and type of hydrocarbon, can be recognized and corrected for, only if additional porosity information is available, for example from sonic and/or density log. Any interpretation of neutron log alone should be undertaken with a realization of the uncertainties involved.
Effect of light hydrocarbon and gas
The quantitative response of neutron tool to gas or light hydrocarbon depends primarily on hydrogen index and "excavation effect". The hydrogen index can be estimated from the composition and density of the hydrocarbon
Given a fixed volume, gas has considerably lower hydrogen concentration. When pore spaces in the rock are excavated and replaced with gas, the formation has smaller neutron-slowing characteristic, hence the terms "Excavation Effect". If this effect is ignored, neutron log will read the porosity value too low. This characteristic allow neutron porosity log to be used with other porosity logs (such as density log) to detect gas zones and identify gas-liquid contacts.
Neutron tools are based on measurement of neutron cloud of different energy level within the investigated volume. Epithermal-neutron tools measure epithermal neutron density with energy level between 100eV and 0.1eV in the formation. Thermal-neutron tools only measure the population of neutrons with a thermal energy level, and Neutron-gamma tools measure the intensity of gamma flux generated by thermal neutron capture. The tools usually have two detectors (or more) with different spacings from the source to produce ratio of count rates, which theoretically reduce borehole effects.
Helium-3 (He-3) filled proportional counter is the most common epithermal and thermal neutron detector. Helium has a high neutron capture cross section and produce the following reaction when interacts with neutron.
3H + 1n → 1H + 3H + 764keV energy
To boost the charge produced by the interaction between Helium and Neutron, a high voltage is applied to the anode of the counter. An operating high voltage is chosen to give enough gain for counting purposes. Most Helium-3 counter use a quench gas to stabilize high voltage performance and prevent run-away.
* John T. Dewan, "Open-Hole Nuclear Logging - State of the Art" - SPWLA Twenty-Seventh Annual Logging Symposium, June 9-13 1986.
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