Abstract: Combining NMR and Density Logs for Petrophysical Analysis in Gas Bearing Formations
FREEDMAN, R., CHANH CAO MINH and GREG GUBELIN, Schlumberger Sugar Land Product Center; THAL MCGINNESS and BOB TERRY, Amoco; JUSTIN FREEMAN, Shell E&P Technology; DAVID RAWLENCE, Woodside Offshore Petroleum
Summary
A new well logging method of gas detection and quantification
that combines the total CMR porosity (TCMR) and
density porosity (DPHI) measurements has been developed. It is
applicable to gas reservoirs with gas near the wellbore. The method
provides new petrophysical equations for (1) total formation
porosity corrected for gas effect on both measurements and (2) the
volume of gas near the borehole. The “TCMR-DPHI gas-corrected
total porosity” is a new petrophysical measurement.
Gas-corrected porosities from the new TCMR-DPHI technique can be
used in conjunction with resistivities from deep-reading electrical
tools to compute accurate gas saturations and consequently more
accurate estimates of gas reserves.
In zones that have gas near the borehole, TCMR underestimates total porosity because of two effects: (1) low hydrogen index of gas and (2) insufficient polarization of gas because of its long T1 relaxation time. On the other hand, in the presence of gas, DPHI overestimates the total porosity because the low density of the gas reduces the measured formation bulk density. Thus, zones with gas near the wellbore can be identified by the separation between the DPHI and TCMR logs. The new method is analogous to use of the neutron-density log crossover as a means of gas detection. The neutron log, however, usually reads too high in shaly sands and other environments because of the effects of thermal neutron absorbers, whereas the TCMR log is not affected by these environmental effects.
To be able to unambiguously attribute the separation between DPHI and TCMR logs solely to the gas effect, the wait time for the logging sequence must be sufficiently long to polarize the liquids (e.g., formation water, mud filtrate); however, the method does not require that the gas phase be polarized.
The advantages include (1) faster logging in many environments because the gas does not have to be polarized, (2) robust gas evaluation because the separation in porosity is accentuated by the opposite effect of gas on the DPHI and TCMR logs, (3) total porosity corrected for gas effect and (4) simple interpretation analogous to the familiar neutron-density gas detection.
The TCMR-DPHI equations for (1) gas-corrected total porosity and
(2) volume of gas are derived from the petrophysical response
equations for total Nuclear
Magnetic Resonance (NMR) porosity and
formation bulk density. The effects of uncertainties in the inputs
to these equations on the computed values of the gas-corrected
total porosities and volumes of gas are studied to elucidate the
robustness of and uncertainties in the outputs.
The TCMR-DPHI gas analysis is applied to field logs from commercial gas wells. The TCMR-DPHI gas-corrected total porosity logs are compared to neutron-density logs and to porosity measurements on conventional core plugs.
Mark of Schlumberger
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah