Downhole Neutron-Induced Spectroscopy Element and Mineral Estimates Compared to a Ring-Tested Core Reference
- Yngve Bolstad Johansen (AkerBP) | Olav-Magnar Nes (AkerBP) | Harish Datir (Schlumberger) | Paul R. Craddock (Schlumberger / Schlumberger-Doll Research Center) | Lalitha Venkataramanan (Schlumberger / Schlumberger-Doll Research Center)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 60th Annual Logging Symposium, 15-19 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
- 3 in the last 30 days
- 115 since 2007
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Use of advanced measurements such as neutron-induced spectroscopy can be highly beneficial when characterizing complex net. It infers dry-weight elements from inelastic- and capture-neutron gamma-ray spectra. Subsequently, mineral fractions are estimated using the dry-weight elements as input to multimineral log analysis or other element-to-mineral transforms. There are many advantages knowing the accurate mineral fractions when doing formation evaluation. Petrophysical outputs such as volume of shale, net-to-gross ratio, porosity, and permeability are improved with reduced uncertainty in mineral fractions. Other potential benefits are improved pore pressure predictions in shales, and better inputs to fluid substitutions and rock mechanical models.
The motivation for this study was to derive and compare dry-weight element and mineral fractions measured downhole to dry-weight element and mineral fraction measurements on core, for a mineralogically complex formation in the Norwegian North Sea. The goals were to: (1) Assess the accuracy of dry-weight element and mineral fractions measured on core, which typically are assumed to be the ground-truth against which to validate logs; (2) Compare dry-weight element fractions derived from downhole neutron-induced spectroscopy, to core measurements; (3) Derive log-based estimates of dry-weight mineral fractions without use of core results to constrain or optimize the log interpretations (4) Validate that high-fidelity neutron-induced spectroscopy logging with multimineral inversions can provide accurate, and robust mineral fraction logs over multiple wells.
Key results from this study are three-fold: (1) Core data can suffer from errors which depend on core preparation and analysis, and is not always the definite ground-truth; (2) Core-log comparisons illustrate that chemical element concentrations can be accurately determined by neutron-induced spectroscopy in complex formations; (3) High-fidelity neutron-induced spectroscopy logs with using of existing multimineral inversion or newer experimental transforms can produce accurate dry-weight mineral fraction logs, in mineralogically complex formations at the Norwegian North Sea.
Many routine calculations used to characterize rocks and reservoirs, are based on simple assumptions with regards to mineral abundance and how these are distributed within the sediments. For many formations, these assumptions are sufficient to complete the evaluation with a manageable level of uncertainty. For other more complex formations, the impact can be much larger (Johansen et al., 2018). In these complex cases, failure to characterize formation mineralogy can lead to erroneous rock property estimates at an early stage in the evaluation process. This is unfortunate as this error amplifies towards the end of the workflow and ultimately can result in significant errors in estimates of oil in place and reserves. Accurate mineral fraction logs are important to reduce such uncertainties. Different methods can be used to estimate dry-weight mineral fractions. Some methods are applied downhole such as neutron-induced spectroscopy (w/spectral gamma ray), while other methods such as X-ray fluorescence (XRF) and X-ray diffraction (XRD) are applied on core in the laboratory.
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