Published: 11/02/2012
Published: 11/02/2012
An operator needed to determine saturations in a lithologically complex shale gas reservoir with multiple clay types in addition to quartz, calcite, dolomite, and pyrite. The formation water resistivity was not known, which made the evaluation of organic carbon (kerogen) and gas and oil volumes difficult when based on traditional porosity and resistivity techniques.
Dielectric Scanner multifrequency dielectric dispersion service makes high-resolution measurements of permittivity and conductivity at multiple depths of investigation and four frequencies to provide dielectric dispersion at a 1-in [2.54-cm] vertical resolution. Because there is a large difference in the permittivity of water from that of rock matrix or hydrocarbons, the resulting determination of water-filled porosity is insensitive to salinity.
The total hydrocarbon volume of the reservoir is obtained as the difference between thewater volume from Dielectric Scanner service and an independently measured total porosity, which is commonly provided by nuclear porosity measurements from the Platform Express integrated wireline logging tool. Accurate determination of the water volume is also a key input to detailed petrophysical analysis for the quantification of total porosity, total organic carbon (TOC), and oil and gas volumes.
The reservoir porosity model illustrates how a comparison of Dielectric Scanner service's water-filled porosity with total porosity from conventional neutron-density logs quickly identifies the hydrocarbon-bearing intervals and provides the basis for calculating the total water saturation. For wells drilled with OBM, Dielectric Scanner service's water-filled porosity yields the irreducible water saturation for the formation.
The total hydrocarbon volume was easily calculated for the gas shale reservoir by comparison of the total porosity with Dielectric Scanner service's water-filled porosity (Track 6 on left-hand log). The salinity-independent total water saturation calculated from these curves is shown in Track 3, and the calculated water salinity from Dielectric Scanner service is shown in Track 2.
Water-filled porosity and formation water salinity computed from Dielectric Scanner multifrequency measurements are valuable inputs to the petrophysical analysis of this unconventional reservoir, greatly improving the accuracy of the determination of saturation volume, TOC, and oil and gas saturations. The detailed Shale Gas Advisor petrophysical analysis shown on the right-hand log provided previously unavailable understanding of the reservoir.
Challenge: Determine water volume for the petrophysical evaluation of a gas shale unconventional reservoir, where complex lithology and unknown water salinity render conventional porosity and resistivity analysis ambiguous.
Solution: Run Dielectric Scanner multifrequency dielectric dispersion service to accurately determine water volume and provide critical inputs for detailed petrophysical evaluation.
Results: Accurately calculated water-filled porosity for comparison with total porosity to provide a salinity-independent determination of irreducible water saturation in a gas shale reservoir drilled with an oil-base mud (OBM) system.
