Many methods of calculating water saturation require knowing the chloride concentration in formation water. Chlorides have a strong effect on water properties, and they impact saturation estimates that are based on resistivity, dielectric dispersion, or thermal neutron absorption. Here we introduce a new direct quantitative measurement of formation chlorine from nuclear spectroscopy, enabling a continuous log of water salinity within a limited radial depth.
Neutron capture spectroscopy is sensitive to chlorine and is a natural fit for measuring its concentration, except that the spectrum contains chlorine from both the formation and borehole. The borehole chlorine background can be large and is highly variable. Historical efforts to derive water salinity from spectroscopy have relied on ratios of chlorine and hydrogen, which are affected by the borehole and hydrocarbons. The direct use of chlorine provides a more reliable basis for salinity interpretation after isolating its formation signal. We partition the borehole and formation components of chlorine via two unique spectral standards. The contrast between the two standards arises from differences in gamma ray scattering based on their point of origin. The shape of the borehole chlorine standard must be adjusted along depth to account for environmentally dependent scattering, which we achieve with a continuously varying function of borehole and formation properties. The algorithm is derived from 129 laboratory measurements and 2,995 numerical simulations spanning a diverse range of conditions. The remaining signal is converted into a log of formation chlorine concentration.
In combination with total porosity, chlorine concentration sets a minimum value for water salinity. Adding an organic carbon measurement enables the simultaneous estimation of water volume and salinity. Chlorine concentration can also be combined with a selected water salinity to compute a water volume for comparison with other methods. Finally, chlorine concentration enables calculation of a maximum expected sigma, which can identify the presence of excess thermal absorbers in the matrix.
The systematic uncertainty on the chlorine concentration ranges from 0.03 to 0.07 wt%, depending on borehole size. The resulting salinity accuracy is inversely proportional to porosity. A potential limitation of the measurement is its depth of investigation, reaching 8 to 10 in. for 90% of the signal. The chlorine concentration is sensitive to filtrate or connate water, depending on formation permeability and invading fluids.
We first present the technique to measure formation chlorine, supported by modeling, laboratory data, and core-log comparisons. We then propose petrophysical workflows to interpret the chlorine concentration.
