Quantifying HIRES Nuclear Tool Resolution in Thinly Bedded Reservoirs Using Time Series Analysis
- W.S. Dodge Sr. (Esso Australia Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Asia Pacific Oil and Gas Conference, 7-10 November, Melbourne, Australia
- Publication Date
- Document Type
- Conference Paper
- 1994. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 3.3.2 Borehole Imaging and Wellbore Seismic, 5.6.1 Open hole/cased hole log analysis
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To assess the benefits of high resolution nuclear logs run in the Gippsland Basin, time series analysis has been used to quantify vertical resolution and random noise. Time series analysis involved taking the Fourier transform of the measurement signal which was then analysed in the frequency domain. The frequency response characteristics of the signal were used to compute the vertical resolution of the measurement based upon signal and noise coherence. The signal to noise ratio of the measurements yielded information about the random broadband noise inherent in nuclear measurements.
HIRES data provided by service companies showed a significant vertical resolution enhancement relative to standard acquisition data but at the expense of greater random noise. The enhanced resolution processing algorithms used by one service company improved the intrinsic resolution of HIRES bulk density and thermal neutron porosity data, while the other company's processing showed enhancement for the bulk density measurement but not the thermal neutron porosity. Broadband noise is a significant tradeoff to using data with enhanced resolution processing for both the bulk density and neutron porosity measurements.
The Gippsland basin offshore the south-east coast of Australia is well known for it's prolific sandstone reservoirs. Wells easily produce at rates up to 10000 STB/D from the massively clean upper Latrobe multi-darcy permeability marine and non-marine facies. These reservoirs are now undergoing depletion and there is a real business need to characterise the poor quality thinly bedded oil bearing reservoirs that were bypassed in original field development.
In 1992 high resolution (HIRES) gamma-gamma density and thermal neutron nuclear data was acquired from 4 exploration wells in the Gippsland Basin. Additionally, during the same period, another wireline company logged the first infill development drilling programme in the Gippsland Basin using HIRES logging.
HIRES logging provides increased vertical resolution to better characterise porosity, fluid saturations and mineralogic properties which ultimately translates into improved quantification of hydrocarbon volumes. The benefits of HIRES logging must be quantified to justify the expense of slower logging speeds, additional processing costs and increased data storage requirements.
HIRES logging involves acquiring data that is sampled more frequently in depth than standard data acquisition. The increased sampling in some cases improves vertical resolution which is the ability to resolve thinner bed units. The tradeoffs are slower logging speeds and 3 to 6 times more data per log interval. Service companies offer HIRES enhanced resolution processing (ERP) products (Galford, 1986, Flaum, 1987, Smith, 1990) which are claimed to enhance vertical resolution beyond the intrinsic resolution of the base measurement.
Several examples of gamma-gamma bulk density and thermal neutron porosity data are shown as a basis for this comparison of HIRES data acquisition, quality and processing. The HIRES data is compared directly to resistivity borehole images and core photographs which illustrate the accuracy of HIRES data to detect thin beds and bed boundaries.
Time Series Analysis (TSA) is a quantitative method used to evaluate the precision of HIRES data.
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