ABSTRACT
Many methods of calculating water saturation require knowing the chloride concentration in formation water. The chlorides have a strong effect on the properties of water, and they impact saturation estimates that are based on resistivity, dielectric dispersion, or thermal neutron absorption cross section. In this work, we introduce a new, direct, quantitative measurement of formation chlorine from nuclear spectroscopy, which enables a continuous log of formation water salinity within a limited radial depth.
Neutron-capture spectroscopy is sensitive to the presence of chlorine and would be a natural fit for measuring chlorine concentration, if not for the fact that the spectrum contains chlorine gamma rays from both the formation and borehole. The borehole chlorine background can be large and is highly variable from well to well and along depth. Historical efforts to derive water salinity from spectroscopy have relied on ratios of chlorine and hydrogen, which suffer from the presence of the borehole yield and from the hydrogen signal of hydrocarbons. A more reliable basis for salinity interpretation is provided by the direct use of chlorine, once its formation signal has been isolated. We partition the borehole and formation components of chlorine via two unique spectral standards. The contrast between the two standards arises from gamma rays undergoing different amounts of scattering based on their point of origin. The shape of the borehole chlorine standard must be dynamically adjusted along well depth to account for environmentally dependent gamma ray scattering. We represent the borehole standard as a linear combination of two components, with a ratio that is a continuously variable function of borehole size, borehole fluid density, and neutron transport in the formation. The algorithm is derived from a combination of 115 laboratory measurements and 2435 simulated measurements. The modeled database spans a diverse range of lithology, porosity, borehole size, and fluids, and is used to validate the treatment of borehole chlorine. The formation standard describes the remaining chlorine signal, and its yield is readily converted into a log of formation chlorine concentration.
The chlorine concentration is useful for multiple petrophysical workflows. 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 water salinity. Chlorine concentration can also be combined with a selected water salinity to compute an apparent water volume for comparison with other methods. Finally, chlorine concentration enables calculation of a maximum expected Sigma, which can be compared with the bulk Sigma log to identify excess thermal absorbers in the matrix.
A potential limitation of the measurement is its radial depth of investigation (DOI), which is limited to 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 workflows to interpret the formation chlorine concentration in terms of water salinity.
