Stand-Alone Cased Hole Logging Accurately Distinguishes New Gas Pay from Tight Zones

Published: 06/16/2017

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The search for new pay without openhole logs

The operator of a producing well in North Texas wanted to identify new pay zones to complete. However, no openhole logs had been run prior to completion. The well was drilled with an 8 3/4-in bit and completed with 5 1/2-in 20-lbm/ft casing. No tubing is present in the water-filled well.

Cased hole reservoir evaluation that does not need openhole data

Pulsar multifunction spectroscopy service provides a complete, stand-alone petrophysical volumetric interpretation in cased wells. This is possible through the combination of spectroscopically quantified mineralogy and lithology with traditional cased hole pulsed neutron logs and the new FNXS measurement. FNXS definitively differentiates gas-filled porosity from liquid-filled zones and tight formations because the fast neutron inelastic scattering response is not dominated by any elements. This results in the FNXS values for rock matrix and water falling in the same range, which focuses the measurement's sensitivity on variation in gas content.

Differentiation of gas-filled porosity from tight zones

The well was logged at 1,000 ft/h in GSH-lithology mode, which simultaneously acquires data for gas, sigma, and hydrocarbon index (GSH) in addition to elemental spectroscopy in a single pass.

Gamma ray, sigma, and thermal neutron porosity (TPHI) from Pulsar service (Tracks 1 through 3, respectively) show several clean zones that could potentially be tight gas reservoirs. The near/far and near/deep burst ratios in Track 5, which are the gas indicators for conventional cased hole pulsed neutron logging, flag all the clean zones as gas bearing. However, only the uppermost zone at approximately X,790–X,800 ft is identified as gas bearing by the FNXS measurement, which is shown with a cutoff set for 0-pu quartz at 6.8 m–1 to account for the low-porosity lithology. The dry-weight elemental concentrations of calcium and silicon in Track 6 confirm that this interval comprises interbedded sandstones, limestones, and shales.

The petrophysical interpretation shown in the right two tracks was generated using Pulsar service's sigma, TPHI, FNXS, gamma ray, and dry-weight elements in a weighted linear solver. The lack of openhole logging was not an impediment to obtaining a full analysis because Pulsar service does not need that data input.

Graph: Stand-Alone Cased Hole Logging Accurately Distinguishes New Gas Pay from Tight Zones
Although conventional cased hole logging identified several zones as potential gas-bearing reservoirs (Track 5), FNXS confirmed that only the uppermost zone has gas-filled porosity, differentiating it from the other tight zones.
United States, North America, Onshore

Challenge: Find new pay in a cased well that was not logged before it was completed.

Solution: Run Pulsar multifunction spectroscopy service, which incorporates the new fast neutron cross section (FNXS) measurement that overcomes the biases of conventional cased hole pulsed neutron logs.

Results: Definitively identified gas-filled porosity zone within tight formations that previously would have been incorrectly flagged as also gas bearing.

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