Despite its value and importance to oilfield development and reservoir management, carbon/oxygen (CO) logs are commonly associated with significant challenges that are either related to the wellbore logging environment and/or the physics of the measurement. Shallow depth of investigation is considered the greatest challenge related to the nature of the pulsed-neutron (PN) measurement. It can imply a high degree of uncertainty on the measurement and consequently the calculated water saturation, affecting the true assessment of the reservoir fluids’ saturations, especially in challenging logging environments. In this paper we introduce and prove an innovative approach to increase the depth of investigation of the PN measurement.
Currently, all PN logging tools use an electric pulsed neutron generator (PNG), or "particle accelerator" or Minitron, to probe downhole formations with 14 MeV neutrons and record the returning gamma ray signal at a shallow depth of investigation (DOI), which is generally in the range of 7 inches for C/O measurement and 12 inches for sigma measurement. In this new approach, we introduce the idea of increasing DOI of the measured gamma rays through increasing the energy level of the neutrons emitted by a PNG. To prove the concept, a computer modeling and simulation study was conducted using Monte Carlo N-Particle (MCNP) for a pulsed-neutron logging tool to determine DOI for neutron energies higher than 14 MeV.
The study involved five different combinations of borehole and formation fluids. Each involved a "block" of 24 MCNP calculations. The 24 calculations inside each block represented the 24 possible combinations of 3 neutron energies (14, 20, 40 MeV), two gamma ray spectral types (inelastic, capture), and four detectors. Data simulation shows that the DOI rises substantially with energy for all tested detectors. Where the enhancement in DOI with the increase in neutron energy is more prolific in case of the inelastic measurement compared to the capture measurement. And of course the deeper the detector (further from the source) the better the DOI, although this can compromise the precision of the measurement. Yet with the recent technology advancements mainly in PNG (producing more neutron population) and GR detector technology (higher and faster count rates), this shall enhance the precision of the measurement and enable us to acquire both accurate and precise measurements at deeper detectors.
This patented, innovative approach shall significantly reduce and possibly eliminate one of the main reasons behind the uncertainty of reservoir saturation monitoring using PN logs, which is shallow depth of investigation of the measurement. Having a PNG that can produce neutrons at higher energy levels compared to current industry standard shall allow a deeper, more accurate and a representative evaluation of the reservoir.
