The biggest disappointment for using 14-Mev neutron
generators for dual-spaced neutron logging by the ratio method was its very
poor porosity measurement sensitivity.
The clearest expression of this problem could be seen from
the ratio-porosity transform graph: for the neutron generator source it rolled
over at about 50-60 pu, making quantitative work in high porosity formations
difficult and calibrations in a 100% water bath impossible.This feature was present in both continuous
and pulsed modes of generator operation.This was particularly unfortunate in view of the many other excellent
uses of pulsed neutron generators to measure carbon / oxygen ratios, silicon / calcium
ratios, and formation capture cross section (SIGMA).
Would it be possible to improve the porosity measurement
resolution for a neutron generator based system, while maintaining its dynamic
range?The answer was provided by US Patent 3,818,225: directly measure
the thermal neutron diffusion coefficientD and use it to measure formation
porosity with good resolution and sensitivity throughout the full range of
porosities from 0% to 100%! In this
case, the neutron generator must be operated in pulsed mode.
From a physics point of view, for a pulsed neutron generator
operating at 1 KHz, after about 300 microseconds, two neutron processes remain
– diffusion and capture.Why not
directly measure the diffusion coefficient and extract all available
In today’s market, Chappell Hill Logging goes well beyond
the scope of this earlier work in a number of very significant ways.For example, detection of thermal capture gamma rays in place of thermal
neutrons utilizes higher count rates with better statistical precision.Moreover, Chappell Hill Logging employs short, equal time sampling(10 microseconds/sample) for all
detectors permitting direct use of powerful standard digital signal processing
and non-linear least squares methods to extract the maximum possible
information from each detector in real time.Moreover, certain key issues related to time averaging and integration,
both required for wide time gates, are avoided.
Of course, measurement of the formation capture cross
section SIGMA remains the primary objective for Chappell Hill Logging.However, by directly measuring the thermal
neutron diffusion coefficient, SIGMA can be immediately corrected for
diffusion.Also, since accurate porosity
values are provided, reservoir volumetrics and water saturations are also more
Another advantage is that D is independent of formation
salinity, a prediction that can easily and directly be verified from the
nuclear micro geophysical model LVPM.This means that porosity values need not be corrected for formation SIGMA
values!The thermal neutron diffusion
coefficient D is related to SIGMA by the equation
Interrelationship Between Thermal Neutron Parameters
where L is the thermal neutron diffusion length.Although L and SIGMA are strong functions of
formation salinity, D is essentially independent of salinity!
The figure below again shows the results from the LVPM model in
which formation porosity is plotted versus the thermal neutron diffusion
coefficient.The curves shown are for
the standard lime, sand, and dolomite formations.The logging software uses polynomial
expansions to compute porosity, given the diffusion coefficient.
Not provided at this time are the details whereby the
thermal neutron diffusion coefficient is measured or how it is used to correct for diffusion – this
is done to protect the competitive advantages Chappell Hill Logging is
seeking to exploit.Suffice it to say
that the same physical model is used as taught in US Patent 3,818,225, save that a multitude of 10
microsecond time gates are now used in place of just two wider gates for each
detector.Also, more sophisticated
mathematical methods are employed to extract the thermal neutron diffusion
coefficient in real time.