Part 1 of this Blog went over the density tool, how it gets porosity values and what it can be used for. In the case of the Neutron tool, along with providing another porosity value, it is used in conjunction with the Density tool as a gas indicator and lithology identifier.
The Neutron tool measures the hydrogen content of the formation. This hydrogen content is correlated into a formation porosity using the index of water equal to 1.
What does the hydrogen content of a formation give you?
When paired with a Density Tool, can aid in the detection of a gas vs. oil bearing formation
Calculate porosity for your Water Saturation calculation
Since it is casing compensated, provides a porosity for formations behind the casing to surface (typically an ERCB requirement)
How does the Neutron tool work?
With the use of an Americium Beryllium radioactive source, inserted into the tool, the decay process mapped out below shows how neutrons are emitted into the formation.
The steps then begin:
The neutrons from the source are emitted at a “fast” energy level, >100 keV, and create interactions with the nucleus’ in the formation, causing them to slow down to a speed that the neutron tool detectors can use
The thermal neutrons are then received back at the helium detectors and a ratio of the detectors is used to measure in CPS (counts per second)
The ratio is used to compensate the tool for borehole affects and differing source strength
The CPS are used to determine the porosity via tool calibration
Gas and Shale Effects of the Neutron Tool
In the event of a gas or a shale formation the Neutron tool provides a falsely low and an expected high reading from these respectively.
In a gassy formation, because the hydrogen content of gas is less than that of water, the Neutron tool will provide porosity readings lower than actual formation porosity. For example, in a clastic rock such as sandstone, with the curves plotted on a sandstone matrix, the Neutron and Density curve will actually cross when in a gassy formation.
Note: This crossing may also occur if your plotted matrix is limestone and you are actually in a clastic formation.
Due to the amounts of bound water in shales, a higher percentage of the neutrons from the source are slowed down, and in turn captured by the detector. From this, a higher count rate is detected, which correlates to a higher porosity reading. This increase in porosity is not seen by the Density tool, providing you with a pronounced spread between the two curves when in a shale.
Calibration and why it is necessary:
As mentioned in Part 1 of this blog, calibrations are very important to ensure the conversion from CPS to electron density is correct. In the case of the Neutron tool, the calibration is done to generate a detector ratio / porosity relationship.
What 2 things affect the quality of the density data?
Mud system – Elements such as chlorine in the mud system (aka salty mud), will in fact capture the emitted neutrons before they reach the detectors, causing an error in the tools response
Logging Speed – Being a statistical tool, like the Density, the quality will be affected by logging speeds
Neutron Tool Differences
Since the LWT Neutron Tool is safely located within the drill string, porosity can be corrected for the casing/drill pipe size, using industry standard charts. There are no differences between Cordax's tool vs any typical Neutron tool on the market.