Neutron LogNeutron Log

NEUTRON LOGNEUTRON LOG

• Logging tool emits high energy neutrons into formation

• Neutrons collide with nuclei of formation’s atom

• Neutrons lose energy (velocity) with each collision

• Most energy lost when colliding with a hydrogen atom nucleus

• Log records porosity based on neutrons captured by formation

NEUTRON TOOL PRINCIPLENEUTRON TOOL PRINCIPLE

• Detects neutrons from the source which have been scattered back by the formation

•The neutron tool employs a dual detector design to compensate for mudcake, lithology, etc.

•Still, corrections are required for the NPHI values

NOTICE : The tool is pressed against the borehole wall to eliminate mud effects

NEUTRON TOOL PRINCIPLENEUTRON TOOL PRINCIPLE

LIFE OF A NEUTRONLIFE OF A NEUTRON

• Neutrons emitted from source

• Neutrons interact with Hydrogen in formation

• Neutrons loose energy

• Neutrons are absorbed or reflected back to

detectors

– High counts = Low porosity

– Low counts = High porosity

LIFE OF A NEUTRONLIFE OF A NEUTRON

• Collisions cause neutrons to lose energy

• Energy loss due mainly to hydrogen

• Therefore tool measures amount of hydrogen in formation, ie., water

NEUTRON TOOL DESIGNNEUTRON TOOL DESIGN

• Both detectors placed in long spacing zone

• Count rates at the detectors are inversely proportional to formation porosity

• Near/Far Ratio proportional to porosity

• Ratio compensates for mudcake

• Source AmBe 15-20Cu 5MeV neutrons

NEUTRON MEASUREMENTNEUTRON MEASUREMENT

• Uses

• Porosity

• Lithology

• Curve N

Pe

b N

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

POROSITY FROM NEUTRON LOGPOROSITY FROM NEUTRON LOG

NeutronLog

CNLLC

0.45 -0.15

FACTORS AFFECTINGFACTORS AFFECTINGPOROSITY LOG RESPONSESPOROSITY LOG RESPONSES

• Lithology of formation

• Fluid content

0425.002.1 log NN

)( clayclaycorr NxVNN

ENVIRONMENTAL EFFECTS ON NPHIENVIRONMENTAL EFFECTS ON NPHI

Lithology Effects

• The tool responds mostly to the presence of hydrogen, but different minerals will slow neutrons to different degrees

•Therefore, the Neutron tool reads different values for NPHI in different formations of the same porosity

•This must be taken into account for the NPHI curve

On a limestone scale, the NPHI is 20%. However the formation is a sandstone. What is the true porosity?

Answer : 25%.

NEUTRON LOG INTERPRETATION NEUTRON LOG INTERPRETATION

• Ratio converted to apparent porosity, N

• Many environmental effects

• Assumes

• Matrix

• Usually LS

• Sometimes SS

• Water-filled

• Charts POR-12-16

• Chart varies with tool

GAS EFFECTS ON NPHIGAS EFFECTS ON NPHI

• Gas has a lower hydrogen concentration than oil or water due to it’s lower density

• A neutron tool will interpret gas to be water occupying a smaller volume; a smaller volume means a smaller porosity

• Hence in gas zones, the neutron tool reads lower porosity than the actual

• This gas effect requires correction

Notice the NPHI reading is less across a gas zone as compared to what it was in an oil zone.

GASEFFECT

NEUTRON LOG INTERPRETATIONNEUTRON LOG INTERPRETATION• Reads deeper than density

• More affected by virgin zone fluid• Gas effect

• Gas lowers H concentration, lowers apparent porosity

Gas effect

SHALE EFFECTS ON NPHISHALE EFFECTS ON NPHI

• Shale has bound water in it’s lattice structure

• This water is immobile and does not represent any EFFECTIVE porosity

• However, the neutron tool responds to the presence of hydrogen

• Since there is a substantial amount of water in shales, the neutron tool reads a high NPHI

• Shaliness has an effect on the neutron tool and this requires correction

High NPHI across shales

On a limestone scale, it reads actual porosity in limes

In sands, it reads a different porosity from the actual

SHALEEFFECT

NEUTRON LOG INTERPRETATIONNEUTRON LOG INTERPRETATION

• Shale effect

• Responds to bound water

Shale effect