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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.
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.
Abstract The Alta field in the Barents Sea was discovered in 2014. The reservoir formation is primarily carbonate rocks with high formation water salinity. Extensive waterflooding processes have led to an approximately 200-m rise of water level. The complexities and uncertainties regarding imbibition, current free water level, and pseudo fluid contacts within the field translate into uncertainty in the hydrocarbon volume estimation. Initial, triple-combo-based petrophysical evaluations have already been updated using advanced log measurements, as reported in an earlier publication. The evaluation is now consolidated by using two new techniques relying on advanced spectroscopy logging and combination with dielectric dispersion logging. Their objective is to further reduce the uncertainty in water saturation associated with variable apparent water salinity. The present contribution proposes a workflow that relies on two novel techniques. The first technique is a direct quantitative measurement of formation chlorine concentration from nuclear spectroscopy, which helps resolve the formation's apparent water salinity and provides a way to calibrate formation matrix sigma. The second technique relies on the existing combined inversion of dielectric dispersion and formation sigma, including explicitly invasion effects. This second technique benefits from the first technique's insight to adjust sigma interpretation and provide bounds for possible salinity variations. The workflow provides robust flushed and unflushed zone salinities, here the most uncertain and variable parameter, combined with accurate estimations of virgin and residual hydrocarbon saturations. The quantification of dielectric textural parameters describing how the water is shaped inside the formation is also improved, contributing to the improvement of virgin zone hydrocarbon saturation estimation.
ABSTRACT New-generation nuclear spectroscopy logging tools can provide downhole mineralogy and total organic carbon measurements in both open and cased wells. This technology is made possible by a combination of advanced physical measurements, data processing, and petrophysical interpretation. There is an emerging need to educate petrophysicists, core analysts, geologists, and other earth scientists on this technology to further expand its application, a task that requires a collective effort of nuclear experts in both logging and operating companies. Here we demonstrate such an effort from an operator's perspective using a case study in a carbonate reservoir. The downhole mineralogy is successfully determined despite extremely challenging logging conditions, namely a large borehole of 17.5 inch in diameter and mud salinity of 130 parts per thousand (ppk) in an upper open-hole section, and logging through 7" casing in a lower section. To justify the logging program in these extreme conditions, Monte Carlo nuclear modeling is applied during the pre-job planning process to optimize logging parameters and mitigate potential unfavorable effects of a large borehole and high salinity. During and after data acquisition, detailed data quality control with a complete set of raw and intermediate processing data helps to identify additional corrections needed for metal debris in the cased-hole section. The elemental dry weights are finally incorporated into multimineral analysis, improving the accuracy of mineralogy determination over the traditional method based on gamma ray-neutron-density-sonic logs, and enabling the formation evaluation through casing. This case study is used to demonstrate best practices of nuclear spectroscopy logging and interpretation, including accurate pre-job planning, in-depth data quality control using complete raw and intermediate processing data, customized environmental corrections, and appropriate mineral models being applied. Collaborative work between logging and operating companies is critical towards expanding the operating envelope of new logging technology and advancing the general knowledge of the industry.
Reeder, Stacy Lynn (Schlumberger) | Kleinberg, Robert L. (Schlumberger) | Vissapragada, Badarinadh (Schlumberger) | Machlus, Malka (Schlumberger) | Herron, Michael M. (Schlumberger) | Burnham, Alan (American Oil Shale LLC) | Allix, Pierre (Total Exploration and Production)
Historically, well-logging and interpretation workflows have been developed mainly for use in porous and permeable reservoir formations and are not commonly used to evaluate source rocks or unconventional reservoirs. Instead, the evaluation of oil shales, such as the organicrich deposits of the Green River Formation, has relied primarily on expensive and inefficient core analyses, such as the Fischer assay, and simple log interpretation. With the potential oil equivalent in place exceeding a trillion barrels, there is a need for detailed characterization of these oil shale deposits using high-resolution well logs.
We have logged two Green River Formation wells using combinations of standard and advanced logging techniques. This program was supported by extensive core analysis, including Fischer assay and thorough mineralogical and chemical analyses. Methods of determining kerogen content from log responses were developed along with multiple methods of estimating a continuous log of formation-water salinity. We developed methods for quantitatively evaluating these Green River Formation oil shales by integrating standard logs with more advanced logging measurements including nuclear magnetic resonance, elemental capture and inelastic spectroscopy, and dielectric dispersion. The results and the petrophysically derived multimineral model are validated by the core measurements and then applied to a nearby well.