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Case Study: Identifying Bypassed Producible Gas in a Complex Completion: Dual 3 1/2-in Tubing in 8 1/2-in Wellbore

New FNXS measurement from Pulsar multifunction spectroscopy service differentiates gas-bearing thin sands from very low porosity limestone in a single cased hole run

Challenge: Accurately identify producible hydrocarbons in interbedded sand, shale, limestone, and coal for recompletion of a well with dual 3 1/2-in tubing strings.

Solution: Obtain a full petrophysical volumetric interpretation, including differentiation of gas-filled porosity from tight zones, from a single run of Pulsar multifunction spectroscopy service.

Result: Identified potentially producible gas zones as recompletion targets without any bias introduced by the unusual completion.

Complex completion and lithology for assessing recompletion

A producing well in Southeast Asia with an unusual dual-tubing completion needed to be logged to identify potential producible zones for recompletion targets. The 8 1/2-in borehole had been completed with two strings of 3 1/2-in tubing. Each tubing string had its own perforations in different intervals, and both tubings were being produced. In addition to the nonstandard tubing configuration, the formation lithology also posed logging challenges because it comprises interbedded sand, shale, limestone, and coal.

Optimized pulsed neutron logging service that differentiates gas-filled porosity

New Pulsar multifunction spectroscopy service introduces the unique fast neutron cross section (FNXS) measurement to definitively differentiate gas-filled porosity from liquid-filled zones and tight formations. The fast neutron inelastic scattering response used to calculate FNXS is not dominated by particular elements, which is the case for conventional neutron logging in cased holes. With its measured values for rock matrix and water in the same range, FNXS is insensitive to variation in liquid-filled porosity but highly sensitive to variation in gas-filled porosity. The default corrections are readily adjusted to account for nonstandard wellbore environments.

In addition to the FNXS measurement, a single run of Pulsar service delivers a complete stand-alone petrophysical volumetric interpretation incorporating high-fidelity mineralogy and lithology for cased holes.

Identification of gas-filled porosity zones for recompletion targets

Both tubings were filled with light hydrocarbon, and the longer of the two was selected for logging. The initial crossplot of FNXS versus thermal neutron porosity (TPHI) acquired by Pulsar service shows the influence of the completion and fluid content. The standard correction for FNXS for the same 8 1/2-in bit size includes only the usual single string of 3 1/2-in 9.3-lbm/ft tubing and water in the wellbore. The resulting standard-corrected FNXS measurement clearly plots too low.

An additional offset correction calculated to account for the light hydrocarbon in the tubing and the dual-tubing configuration made the corrected FNXS close to the theoretical value in the shale zones. This provided FNXS values that are much more consistent with the sandstone and limestone envelopes for gas and water, as would be expected from the varied lithology of low-porosity limestone interbedded with shale, thin sands, and coal in the log (Track 10).


Download: Identifying Bypassed Producible Gas in a Complex Completion: Dual 3 1/2-in Tubing in 8 1/2-in Wellbore (0.44 MB PDF)

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Identifying Bypassed Producible Gas in a Complex Completion: Dual 3 1/2-in Tubing in 8 1/2-in Wellbore
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Multifunction spectroscopy service pairs multiple detectors with a high-output pulsed neutron generator to provide stand-alone cased hole formation evaluation and reservoir monitoring with openhole logging quality. Visit Pulsar Spectroscopy Service page