IMPROVED POROSITY MEASUREMENT SENSITIVITY

WITH A PULSED NEUTRON GENERATOR

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 coefficient D 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 information?

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 accurately computed.

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

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.