Boron: Tracking A Trace Element

Our research shows that boron in the rock man Trix occrs in quantities large enough to affect seriously log analysis in several formations along the Texas Gulf Coast. Examples of log-derived quantities affected by boron content include porosity (compensated neutron log), gas indicator (crossover of neutron and density porosity curves), shale indicator (neutron-density crossplots used in shaley sand interpretations), water salinity and saturation (pulsed neutron logs), and elemental analysis (derived from energy spectrum of gamma rays of capture). Our study of cores from the Frio Formation indicated no obvious geographic correlation. There was a clear trend toward higher boron content in Frio shales and shaley sands than in relatively clean sands. Boron also occurs in significant amounts in mud constituents such as bentonite, barite, and lignosulfonate. Boron also occurs in Gulf Coast Frio formation waters, but this does not directly affect thermal neutron logs or pulsed neutron logs run soon after drilling, because these logs respond mainly to conditions close to the well, within the invaded zone, where little or no formation water is present. It may be possible to correlate boron in the formation water with boron in the rock, but a lot more data would be needed to establish such a correlation. A reliable epithermal neutron log would go a long way toward solving the boron problem, but epithermal logs to date have been too sensitive to borehole environment. An approach suggested recently by D. V. Ellis et al. offers some hope that the problem could be solved by simultaneous use of data from the compensated neutron and pulsed neutron logs, which are affected differently by the boron content of the formation. Again, more data are needed to validate this approach.