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.
