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Stanford University, Stanford, California
Corresponding author: M. A. Gutierrez, mario@pangea.stanford.edu
| The first 20% of the full text of this article appears below. |
La Cira-Infantas oil field (LCI) in the Middle Magdalena Valley Basin of Colombia has estimated original oil in place of 3700 million barrels. However, cumulative production of more than 1700 wells has not exceeded 724 million barrels. The recovery factor is about 20%.
About 55% of LCI wells were drilled before the development of borehole logging. Dickey (1992) wrote of this area: "Before electric logging, it was impossible to see where the individual pay sands were by examination of the cuttings. Consequently, many sands were shut off behind pipe." Thus, in mature oil fields like LCI with low recovery factor, well recompletion guided by new cased-hole logging is economically attractive because moderate investment may reveal bypassed oil-filled intervals.
Diverse logging techniques have been used to evaluate reservoir properties behind the casing. Applications based on cased-hole logging include evaluation of porosity, lithology, and hydrocarbon saturation. Cased-hole logs play a particularly important role in detecting and evaluating bypassed hydrocarbon intervals. Logging techniques varying from borehole gravimeter and conventional gamma-ray logs to advanced nuclear tools have been used to identify bypassed reserves. Nuclear tools—including pulsed neutron capture, pulsed neutron spectral, and high-resolution gamma-ray spectroscopy—are the primary methods for detecting bypassed oil behind the casing. However, data quality can be negatively affected by borehole environmental factors, the limited depth of investigation, and reservoir characteristics such as shaliness, radioactive zones, low porosity, brine salinity, and complex lithology.
Cased-hole acoustic techniques developed in the last few years are attractive alternatives to nuclear methods. Elastic waves generated by full-waveform acoustic tools sample formation behind the casing and can detect the pore fluid and assess porosity. However, only a relevant rock physics model allows reliable interpretation of full-waveform acoustic data in terms of reservoir parameters. In this case study we show how theoretical rock physics can link full-waveform
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