Using Nuclear Spectroscopy in Cased Hole Wells to Estimate Petrophysical Properties and Hydrocarbon Saturation in South American Freshwater Sands

Abstract

Most of the mature South American oil and natural gas fields are represented by formations with moderate porosity, low resistivity, heavy oil, and general anisotropic behavior of other petrophysical properties. This leads to complex decision making when attempting to identify zones of interest with good reservoir qualities in addition to hydrocarbon saturation. These mature fields offer a unique challenge in that the petrophysical analysis needs to rely on data from newly drilled cased hole wellbores, necessitating numerous corrections. In this challenging environment, very few technologies can provide the data required for a precise petrophysical assessment. For this reason, a geochemical spectroscopy tool was employed with measurements that included elemental concentrations, the sigma formation cross section, and a direct measurement of the carbon concentration in the surrounding formation. The resulting data and innovative interpretation provided an accurate assessment of formation mineralogy including clay types, porosity, and hydrocarbon saturation.

These reservoirs, which have been in production for decades, contain relatively fresh water (approximately 2-5 kppm NaCl), moderate porosities (9 to 20 p.u.), and lithologies dominated by sand or sand/shale. The primary objective of the logging program was to incorporate nuclear spectroscopy applications to evaluate and characterize the zones of interest aligned with historical production data. In addition to the geochemical spectroscopy tool, the logging program also included gamma ray, spectral gamma ray, neutron porosity, and density. This suite of tools supplied the measurements required to characterize the formation through the determination of mineralogy, porosity, and hydrocarbon saturation.

The mineralogical model was based upon quartz, feldspar, pyrite, a highly conductive mineral, and numerous clays including illite, kaolinite, chlorite, and smectite. Open hole information from nearby wells was also incorporated into the petrophysical interpretation using normalization procedures and prediction analytics. Since this was a cased hole logging program, and calcium was a significant component of the cement, a cement-mimicking mineral was constructed based upon calcium oxide. This provided important quality control information regarding the condition of the borehole and cement placement, as very little calcite was present in the subsurface formations of the field.

Once the formation mineralogy, porosity, and matrix density were computed, the hydrocarbon saturation was calculated using two approaches: excess carbon and the ratio of carbon to oxygen elemental yields. The final interpretation provided key information, not only for the drilling campaign, but also for the workover planning and quantification of OOIP (original oil in place). Field examples are provided to demonstrate the complete workflow from the design of the logging program to the specialized interpretation methods and final deliverables.